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MOTOR CONTROL FUNDAMENTALS

• Published by Square D
• Reprint permission granted

0.0 INTRODUCTION

The intent of this manual is to familiarize those who use it with terms and concepts which are fundamental to an understanding of motor control equipment and its applications. Since the manual is not intended to serve as an engineering text, the material covered will be general in nature. Study of the definitions, symbols, diagrams and illustrations, however, will give the student a sound background in the language and basic principles associated with motor control products.

NOTE

To help in understanding the language of motor control, cross-references are provided. The figures in brackets following a word or phrase refer to the paragraph where that word or phrase is explained. Paragraph 1.2 makes reference to motor torque (3.4). Paragraph 3.4 explains what is meant by "torque."

As a supplement to this manual, a separate set of questions on the material is available. These questions are meant to reinforce the basic principles covered, and can also serve to direct the student to specific areas in which he might profit from additional review.

0.1 SCOPE

Since over 90% of all motors are used on AC, DC motors and their control will not be discussed. Wound rotor motors and AC commutator motors have only a limited application and are also not included. The squirrel cage is the most widely used motor. Therefore, its control is the induction motor subject of this manual. The use of high voltages (2400, 4800 and higher) introduces requirements which are additional to those for 600 volt equipment, and although the basic principles are unchanged, these additional requirements are not covered here.

The subject will be dealt with by first establishing guidelines for the selection of motor control equipment and then defining some basic motor control terms. The protective function of motor control is then discussed, followed by manual and magnetic control. The component parts of the standard magnetic starter are reviewed, and electrical diagrams are introduced.

Contactors and variations of magnetic starters are next examined, and the manual concludes with a review of relays, timers and pilot devices.

1.0 SELECTION OF MOTOR CONTROL

The motor, machine and motor controller (2.0) are interrelated and need to be considered as a package when choosing a specific device for a particular application. In general, five basic factors influence the selection of a controller:

1.1 ELECTRICAL SERVICE

Establish whether the service is DIRECT (DC) or ALTERNATING CURRENT (AC). If AC, determine the number of phases and frequency, in addition to the voltage.

1.2 MOTOR

The motor should be matched to the electrical service, and correctly sized for the machine load (horsepower rating). Other considerations include motor speed (3.3) and torque (3.4). To select proper protection for the motor, its FULL LOAD CURRENT rating (3.1), service factor (3.8) and time rating (3.7) must be known.

1.3 OPERATING CHARACTERISTICS OF CONTROLLER

The fundamental job of a motor controller is to start and stop the motor, and protect the motor, machine operator.

The controller might also be called on to provide supplementary functions, which could include reversing, jogging or inching (3.9), plugging (3.10) operating at several speeds or at reduced levels of current and motor torque.

1.4 ENVIRONMENT

Controller enclosures (4.0) serve to provide protection for operating personnel by preventing accidental contact with live parts. In certain applications, the controller itself must be protected from a variety of environmental conditions which might include:

• Water, rain, snow or sleet
• Dirt or non-combustible dust
• Cutting oils, coolants or lubricants

Both personnel and property require protection in environments made hazardous by the presence of explosive gases or combustible dusts.

1.5 NATIONAL CODES (5.0) AND STANDARDS

Motor control equipment is designed to meet the provisions of the National Electrical Code (N.E.C.). Code sections applying to industrial control devices are Article 430 on motors and motor controllers and Article 500 on hazardous locations.

The 1970 Occupational Safety and Health Act, as amended in 1972, requires that each employer furnish employment free from recognized hazards likely to cause serious harm. Provisions of the act are strictly enforced by inspection.

Standards established by the National Electrical Manufacturers Association (NEMA) assist users in the proper selection of control equipment. NEMA Standards provide practical information concerning construction, test, performance and manufacture of motor control devices such as starters, relays and contactors.

One of the organizations which actually tests for conformity to national codes and standards is Underwriters Laboratories, (UL). Equipment tested and approved by UL is listed in an annual publication, which is kept current by means of bimonthly supplements which reflect the latest additions and deletions.

2.0 MOTOR CONTROLLER

A motor controller will include some or all of the following functions: starting, stopping, overload protection (8.1), overcurrent protection (7.0), reversing, changing speed, jogging (3.9), plugging (3.10), sequence control (3.11), pilot light indication. The controller can also provide the control for auxiliary equipment such as brakes, clutches, solenoids, heaters and signals. A motor controller may be used to control a single motor or a group of motors.

2.1 STARTER

The terms "starter" and "controller" mean practically the same thing. Strictly speaking a "starter" is the simplest form of controller and is capable of starting and stopping the motor and providing it with overload protection (8.1).

Motor controllers can be simple or complex. Both the small fractional horsepower manual starter (9.1), above, and the special control panel, right, qualify as motor controllers.

3.0 AC SQUIRREL CAGE MOTOR

The work horse of industry is the ac squirrel cage motor. Of the thousands of motors used today in general applications, the vast majority are of the squirrel cage type. Squirrel cage motors are simple in construction and operation — merely connect three power lines to the motor and it will run.

The squirrel cage motor gets its name because of its rotor construction, which resembles a squirrel cage and has no wire winding.

3.1 FULL LOAD CURRENT (FLC)

The current required to produce full load torque (3.4) at rated speed (3.3)

3.2 LOCKED ROTOR CURRENT (LRC)

During the acceleration period at the moment a motor is started, it draws a high current called the "inrush" current. The inrush current when the motor is connected directly to the line (so that full line voltage is applied to the motor), is called the locked rotor or stalled rotor current. The locked rotor current can be from 4 to IO times the motor full load current. The vast majority of motors have a LRC of about 6 times FLC, and therefore this figure is generally used. The "6 times" value is often expressed as 600% of FLC.

3.3 MOTOR SPEED

The speed of a squirrel cage motor depends on the number of poles of the motor's winding. On 60 cycles, a 2 pole motor runs at about 3450 RPM, a 4 pole at 1725 RPM, a 6 pole at 1150 RPM. Motor nameplates are usually marked with actual full load speeds but frequently motors are referred to by their "synchronous speeds" — 3600, 1800, and 1200 RPM respectively.

3.4 TORQUE

Torque is the "turning" or "twisting" force of the motor and is usually measured in pound-feet. Except when the motor is accelerating up to speed, the torque is related to the motor HP by the formula:

The torque of a 25 HP motor, running at 1725 RPM would be computed as follows:

If 90 pound-feet were required to drive a particular load, the above motor would be overloaded and would draw a current in excess of Full Load Current.

3.5 AMBIENT TEMPERATURE

The temperature of the air where a piece of equipment is situated is called the ambient temperature. Most controllers are of the enclosed type and the ambient temperature is the temperature of the air outside the enclosure, not inside. Similarly if a motor is said to be in an ambient temperature of 30° C. (86° F.), this is the temperature of the air outside the motor, not inside. Per NEMA Standards, both controllers and motors are subject to a 40° C. (104° F.) ambient temperature limit.

3.6 TEMPERATURE RISE

Current passing through the windings of a motor results in an increase in the motor temperature. The difference between the winding temperature of the motor when running and the ambient temperature (3.5) is called the temperature rise.

The temperature rise produced at full load is not harmful provided the motor ambient temperature does not exceed, 40° C (104° F.)

Higher temperature caused by increased current or higher ambient temperatures produces a deteriorating effect on motor insulation and lubrication. An old "rule of thumb" states that for each increase of 101 above the rated temperature motor life is cut in half.

3.7 TIME (DUTY) RATING

Most motors have a continuous duty rating permitting indefinite operation at rated load.

Intermittent duty ratings are based on a fixed operating time (5, 15, 30, 60 minutes) after which the motor must be allowed to cool.

3.8 MOTOR SERVICE FACTOR

If the motor manufacturer has given a motor a "service factor," it means that the motor can be allowed to develop more than its rated or nameplate HP, without causing undue deterioration of the insulation. The service factor is a margin of safety. If, for example, a 10 HP motor has a service factor of 1.15, the motor can be allowed to develop 11.5 HP. The service factor depends on the motor design.

3.9 JOGGING (INCHING)

Jogging describes the repeated starting and stopping of a motor at frequent intervals for short periods of time. A motor would be jogged when a piece of driven equipment has to be positioned fairly closely — e.g. when positioning the table of a horizontal boring mill during set-up. If jogging is to occur more frequently than 5 times per minute, NEMA standards require that the starter be derated.

A NEMA Size I starter (see par. 10.21 on ratings) has a normal duty rating of 7'h HP at 230 V., polyphase. On jogging applications, this same starter has a maximum rating of 3 HP.

3.10 PLUGGING

When a motor running in one direction is momentarily reconnected to reverse the direction, it will be brought to rest very rapidly. This is referred to as "plugging&." If a motor is plugged more than 5 times per minute, derating of the controller is necessary, due to the heating of the contacts.

Plugging can only be used if the driven machine and its load will not be damaged by the reversal of the motor torque.

3.11 SEQUENCE (INTERLOCKED) CONTROL

Many processes require a number of separate motors which must be started and stopped in a definite sequence, as in a system of conveyors. When starting up, the delivery conveyor must start first with the other conveyors starting in sequence, to avoid a pile up of material. When shutting down, the reverse sequence must be followed with time delays between the shutdowns (except for emergency stops) so that no material is left on the conveyors. This is an example of a simple sequence control. Separate starters could be used but it is common to build a special controller which incorporates starters for each drive, timers, control relays, etc.

4.0 ENCLOSURES

NEMA and other organizations have established standards of enclosure construction for control equipment. In general, equipment would be enclosed for one or more of the following reasons:

1. Prevent accidental contact with live parts
2. Protect the control from harmful environmental conditions
3. Prevent explosion or fires which might result from the electrical arc caused by the control

Common types of enclosures per NEMA classification numbers are:

NEMA I — GENERAL PURPOSE NEMA 4 — WATERTIGHT

The general purpose enclosure is intended primarily to prevent accidental contact with the enclosed apparatus. It is suitable for general purpose applications indoors where it is not exposed to unusual service conditions. A NEMA I enclosure serves as protection against dust and light indirect splashing, but is not dusttight.

NEMA 3 — DUSTTIGHT, RAINTIGHT

This enclosure is intended to provide suitable protection against specified weather hazards. A NEMA 3 enclosure is suitable for application outdoors, on ship docks, canal and construction work, and for application in subways and tunnels. It is also sleet-resistant.

NEMA 3R — RAINPROOF, SLEET RESISTANT

This enclosure protects against interference in operation of the contained equipment due to rain, and resists damage from exposure to sleet. It is designed with conduit hubs and external mounting, as well as drainage provisions.

NEMA 4 — WATERTIGHT

A watertight enclosure is designed to meet the hose test described in the following note: "Enclosures shall be tested by subjection to a stream of water. A hose with a one inch nozzle shall be used and shall deliver at least 65 gallons per minute. The water shall be directed on the enclosure from a distance of not less than 10 feet and for a period of five minutes. During this period it may be directed in any one or more directions as desired. There shall be no leakage of water into the enclosure under these conditions."

A NEMA 4 enclosure is suitable for applications outdoors on ship docks and in dairies, breweries, etc.

NEMA 4X — WATERTIGHT, CORROSION-RESISTANT

These enclosures are generally constructed along the lines of NEMA 4 enclosures except they are made of a material that is highly resistant to corrosion. For this reason, they are ideal in applications such as paper mills, meat packing, fertilizer and chemical plants where contaminants would ordinarily destroy a steel enclosure over a period of time.

NEMA 7 — HAZARDOUS LOCATIONS — CLASS I

These enclosures are designed to meet the application requirements of the National Electrical Code for Class I hazardous locations. In this type of equipment, the circuit interruption occurs in air.

"Class I locations are those in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures."

NEMA 9 HAZARDOUS LOCATIONS — CLASS II

These enclosures are designed to meet the application requirements of the National Electrical Code for Class II hazardous locations.

"Class II locations are those which are hazardous because of the presence of combustible dust."

The letter or letters following the type number indicates the particular group or groups of hazardous locations (as defined in the National Electrical Code) for which the enclosure is designed. The designation is incomplete without a suffix letter or letters.

NEMA 12 — INDUSTRIAL USE

The NEMA 12 enclosure is designed for use in those industries where it is desired to exclude such materials as dust, lint, fibers and flyings, oil see page or coolant see page. There are no conduit openings or knockouts in the enclosure, and mounting is by means of flanges or mounting feet.

NEMA 13 — OILTIGHT, DUSTTIGHT

NEMA 13 enclosures are generally of cast construction, gasketed to permit use in the same environments as NEMA 12 devices. The essential difference is that, due to its cast housing, a conduit entry is provided as an integral part of the NEMA 13 enclosure, and mounting is by means of blind holes, rather than mounting brackets.

5.0 CODE

The National Electrical Code deals with the installation of equipment and is primarily concerned with safety — the prevention of injury and fire hazard to persons and property arising from the use of electricity. It is adopted on a local basis, sometimes incorporating minor changes or interpretations. NEC rules and provisions are enforced by governmental bodies exercising legal jurisdiction over electrical installations and used by insurance inspectors. Minimum safety standards are thus assured.

6.0 PROTECTION OF THE MOTOR

Motors can be damaged, or their effective life reduced, when subjected to a continuous current only slightly higher than their FULL LOAD CURRENT rating (3.1), times the Service Factor (3.8).

NOTE: Motors are designed to handle "INRUSH" or "LOCKED ROTOR CURRENTS" (3.2) without excessive temperature rise, provided the accelerating time is not too long nor the duty cycle (see "JOGGING") (3.9) too frequent.

Damage to insulation and windings of the motor can also be sustained on extremely high currents of short duration, as found in "grounds" and "short circuit."

All currents in excess of Full Load Current can be classified as overcurrents. In general, however, a distinction is made based on the magnitude of the overcurrent and equipment to be protected.

An overcurrent up to Locked Rotor Current is usually the result of a mechanical overload (8.0) on the motor. The subject of protection against this type of overcurrent is covered in Article 430 (Part C) of the National Electrical Code entitled "MOTOR RUNNING OVERCURRENT (OVERLOAD) PROTECTION." IN THIS MANUAL THE DESIGNATION "MOTOR RUNNING OVERCURRENT (OVER LOAD) PROTECTION" WILL BE SHORTENED SIMPLY TO "OVERLOAD PROTECTION&," and will define protection against overcurrents not exceeding Locked Rotor Current.

Overcurrents due to short circuits or grounds are much higher than Locked Rotor Currents. Equipment used to protect against damage due to this type of overcurrent must not only protect the motor, but also the branch circuit conductors and the motor controller. Provisions for the protective equipment are specified in Article 430 under Part D entitled "MOTOR-BRANCH-CIRCUIT SHORT CIRCUIT and GROUND FAULT PROTECTION." IN THIS MANUAL THE ABOVE TITLE WILL BE SHORTENED SIMPLY TO "OVERCURRENT PROTECTION," and will designate protection against high overcurrents as would typically be encountered in short circuit or grounds.

Motor OVERLOAD protection differs from OVERCURRENT protection, and each will be separately covered in succeeding paragraphs.

7.0 "OVERCURRENT PROTECTION"

The function of the OVERCURRENT protective device is to protect the motor branch circuit conductors, control apparatus and motor from short circuits or grounds. The protective devices commonly used to sense and clear overcurrents are thermal of carrying the starting current of the motor but the device setting shall not exceed 250% of full load current with no code letter on the motor, or from 150 to 250% of full load current depending upon the code letter of the motor. Where the value is not sufficient to carry the starting current, it may be increased, but shall in no case exceed 400% of the motor full load current.

The National Electrical Code requires (with a few exceptions) a means to disconnect the motor and controller from the line, in addition to an OVERCURRENT protective device to clear short circuit faults. The circuit breaker illustrated below incorporates fault protection and disconnect in one basic device. When the OVERCURRENT protection is provided by fuses, a disconnect switch is required, and the switch and fuses are generally combined.

8.0 OVERLOADS

A motor has no intelligence and will attempt to drive any load, even if excessive. Exclusive of "Inrush" or "Locked Rotor Current" when accelerating, the current drawn by the motor when running is proportional to the load, varying from a "no load current (approximately 40% of FLC) to the Full Load current rating stamped on the motor name-plate. When the load exceeds the torque (3.4) rating of the motor, it draws higher than Full Load Current (3.1) and the condition is described as an OVERLOAD. The maximum overload exists under Locked Rotor (3.2) conditions, in which the load is so excessive that the motor stalls of fails to start, and as a consequence draws continual Inrush(LRC) Current.

OVERLOADS can be electrical, as well as mechanical, in origin. Single phasing of a polyphase motor, or low line voltage, are examples of electrical overloads.

8.1 OVERLOAD PROTECTION

The effect of an OVERLOAD is a rise in temperature in the motor windings. The larger the OVERLOAD, the more quickly the temperature will increase to a point damaging to the insulation and lubrication of the motor. An inverse relationship, therefore, exists between current and time — the higher the current, the shorter the time before motor damage or "burn out" can occur.

All overloads shorten motor life by deteriorating the insulation. Relatively small overloads of short duration cause little damage, but if sustained, could be just as harmful as overloads of greater magnitude. The relationship between the magnitude (percent of full load) and duration (time in minutes) of an overload is illustrated by the Motor Heating.

The ideal OVERLOAD PROTECTION for a motor is an element with current sensing properties very similar to the heating curve of the motor, which would act to open the motor circuit when Full Load Current is exceeded. The operation of the protective device should be such that the motor is allowed to carry harmless overloads, but is quickly removed from the line when an overload has persisted too long.

8.2 OVERLOAD PROTECTION — FUSES

Fuses are not designed to provide overload protection. Their basic function is to protect against short circuits (OVERCURRENTS). Motors draw a high INRUSH current (generally 6 times the normal FLC), when starting. Single element fuses have no way of distinguishing between this temporary and harmless inrush current and a damaging OVERLOAD. Thus, a fuse chosen on the basis of motor FLC would blow every time the motor started. On the other hand, if a fuse were chosen large enough to pass the Starting or Inrush Current, it would not protect the motor against small, harmful overloads which might occur later.

Dual element or time delay fuses can provide motor OVERLOAD protection, but suffer the disadvantage of being non-renewable and must be replaced.

8.3 OVERLOAD PROTECTION — OVERLOAD RELAYS

The OVERLOAD RELAY is the heart of motor protection. Like the dual element fuse, the OVERLOAD RELAY has inverse trip time characteristics, permitting it to hold in during the accelerating period (when Inrush Current is drawn), yet providing protection on small OVERLOADS above FLC when the motor is running. Unlike the fuse, the OVERLOAD RELAY IS RENEWABLE and can withstand repeated trip and reset cycles without need of replacement. It should be emphasized that the OVERLOAD RELAY does not provide short circuit (7.0) protection this is the function of overcurrent protective equipment like fuses and circuit breakers.

The OVERLOAD RELAY consists of a current sensing unit connected in the line to the motor, plus a mechanism, actuated by the sensing unit, which serves to directly or indirectly break the circuit. In a manual starter, (9.0) an overload trips a mechanical latch causing the starter contacts to open and disconnect the motor from the line. In magnetic starters (10.0), an overload opens a set of contacts within the overload relay itself. These contacts are wired in series with the starter coil in the control circuit (10.30) of the magnetic starter. Breaking the coil circuit causes the starter contacts to open, disconnecting the motor from the line.

OVERLOAD RELAYS can be classified as being either Thermal (8.4, 8.5) or Magnetic (8.9). Magnetic overload relays react only to current excesses, and are not affected by temperature. As the name implies, Thermal Overload Relays rely on the rising temperatures caused by the overload current to trip the overload mechanism. Thermal Overload Relays can be further subdivided into two types — Melting Alloy and Bimetallic.

8.4 MELTING ALLOY THERMAL OVERLOAD RELAY

In these overload relays (also referred to as "solder pot relays") the motor current passes through a small heater winding. Under overload conditions the heat causes a special solder to melt, allowing a ratchet wheel to spin free, opening the contacts. When this occurs the relay is said to "trip." To obtain appropriate tripping current for motors of different sizes, or different full load currents, a range of thermal units (heaters) is available. The heater coil and solder pot are combined in a one piece, non-tamperable unit. The heat transfer characteristic and the accuracy of the unit cannot be accidentally changed, as is possible when the heater is a separate component. Melting alloy thermal overload relays are "hand reset," thus after they trip they must be reset by a deliberate hand operation. A reset button is usually mounted on the cover of enclosed starters. Thermal units are rated in amperes and are selected on the basis of motor full load current (3.1) not HP.

8.5 BIMETALLIC THERMAL OVERLOAD RELAY

Bimetallic thermal overload relays employ a U-shape bimetal strip, associated with a current carrying heater element. When an overload occurs, the heat will cause the bimetal to deflect and open a contact. Different heaters give different trip points. In addition, most relays are adjustable over a range of 85% to II 5% of the nominal heater rating. These relays are field convertible from hand reset to automatic reset and vice-versa. On automatic reset, the relay contacts, after tripping, will automatically reclose when the relay has cooled down. This is an advantage when the relays are inaccessible. However, automatic reset overload relays should not normally be used with 2-wire control (14.0).

With this arrangement, when the overload relay contacts reclose after an overload relay trip, the motor will restart, and, unless the cause of the overload has been removed, the overload relay will trip again. This cycle will repeat and eventually the motor will burn out due to the accumulated heat from the repeated inrush current. More important is the possibility of danger to personnel. The unexpected restarting of a machine may find the operator or maintenance man in a hazardous situation, as he attempts to find out why his machine has stopped.

8.51 AMBIENT COMPENSATION

Ambient-compensated bimetallic overload relays were designed for one particular situation; that is, when the motor is at a constant temperature and the controller is located separately in a varying temperature. In this case, if a standard thermal overload relay were used, it would not trip consistently at the same level of motor current if the controller temperature changed. This thermal overload relay is always affected by the surrounding temperature. Single Pole Three Pole To compensate for the temperature variations the controller may see, an ambient-compensated overload relay is applied. Its trip point is not affected by temperature and it performs consistently at the same value of current.

8.6 THERMAL OVERLOAD RELAY TRIP CHARACTERISTICS

Melting alloy (8.4) and bimetallic (8.5) overload relays are designed to approximate the heat actually generated in the motor. As the motor temperature increases, so does the temperature of the Thermal Unit. The motor and relay heating curves show this relationship. No matter how high the current drawn, the overload relay will provide protection, yet the relay will not trip out unnecessarily.

8.7 THERMAL OVERLOAD RELAY SELECTION

Motor full load current (3.1), the type of motor, and the possible difference in ambient temperature (3.5) between the motor and the controller must all be taken into account when choosing overload relay thermal units or overload heaters. Motors of the same HP and speed do not all have the same full load current (FLC). Always refer to the motor nameplate for the FLC — do not use a published table. These tables of motor FLC's show the average or normal FLC's and the FLC of the motor in question may be quite different, particularly with the small single phase motors. Thermal Unit selection tables are published on the basis of continuous duty motors (3.7), with 1.15 service factor (3.8), operating under normal conditions. The tables are shown in the catalog and also appear on the inside of the door or cover of the controller. These selections will properly protect the motor and allow the motor to develop its full HP, allowing for the service factor, if the ambient temperature is the same at the motor as at the controller. If the temperatures are not the same, or if the motor Service Factor is less than 1.15, a special procedure is required to select the proper thermal unit.

8.8 ALARM CONTACTS

Standard overload relay contacts are closed under normal is sometimes required to indicate when a motor has stopped conditions and open when the relay trips. An alarm signal due to an overload trip. Also, with some machines, particularly those associated with continuous processing, it may be required to signal an overload condition, rather than have the motor and process stop automatically. This is done by fitting the overload relay with a set of contacts which close when the relay trips, so completing the alarm circuit. These contacts are called "alarm contacts."

8.9 MAGNETIC OVERLOAD RELAY

A magnetic overload relay has a movable magnetic core inside a coil which carries the motor current. The flux set up inside the coil pulls the core upwards. When the core rises far enough (determined by the current and the position of the core) it trips a set of contacts on the top of the relay. The movement of the core is slowed by a piston working in an oil filled dash pot (similar to a shock absorber) mounted below the coil. This produces an inverse-time characteristic. The effective tripping current is adjusted by moving the core on a threaded rod. The tripping time is varied by uncovering oil bypass holes in the piston. Because of the time and current adjustments, the magnetic overload relay is sometimes used to protect motors having long accelerating times or unusual duty cycles. (The instantaneous trip magnetic overload relay is similar but has no oil filled dash pot.)

9.0 MANUAL STARTER

A manual starter is a motor controller whose contact mechanism is operated by a mechanical linkage from a toggle handle or push button which is in turn operated by hand. A thermal unit and direct acting overload mechanism provides motor running overload protection. Basically, a manual starter is an "ON-OFF" switch with overload relays.

Manual starters are generally used on small machine tools, fans and blowers, pumps, compressors, and conveyors. They are the lowest cost of all motor starters, have a simple mechanism, and provide quiet operation with no ac magnet hum. Moving a handle, or pushing the START button, closes the contacts which remain closed until the handle is moved to "off," or the STOP button is pushed, or the overload relay thermal units trips.

Manual starters are of the "Fractional HP" type (9.1) or the "Integral HP" type (9.3) and usually provide across the line starting (16.0). Standard manual starters cannot provide low voltage protection (15.1) or low voltage release (15.0). If power fails the contacts remain closed, and the motor will restart when power returns. This is an advantage for pumps, fans, compressors, oil burners, etc., but for other applications it can be a disadvantage and can even be dangerous to personnel or equipment. For manual starters with low voltage protection.

Here is a practical example in the application of motor control. The manual starter as shown at the right or another maintained contact (14.3) device should not be used in an application of this type where the machine or operator will be endangered if power fails and returns without warning. For dangerous applications such as this dough mixer, a magnetic starter and momentary contact pilot devices (14.4) giving 3-wire control (13.1) or a manual starter with low voltage protection (9.4) should be used for safety purposes.

9.1 FRACTIONAL HORSEPOWER (FHP) MANUAL STARTER

FHP manual starters are designed to control and provide overload protection for motors of 1 HP or less on 115 or 230 volts single phase. They are available in single and two pole versions and are operated by a toggle handle on the front. When a serious overload occurs, the thermal unit "trips" to open the starter contacts, disconnecting the motor from the line. The contacts cannot be reclosed until the overload relay has been reset by moving the handle to the full OFF position, after allowing about two minutes for the thermal unit to cool. The open type starter will fit into a standard outlet box and can be used witha standard flush plate.

9.2 MANUAL MOTOR STARTING SWITCHES

Manual motor starting switches provide on-off control of single phase or 3 phase ac motors where overload protection is not required, or is separately provided. Two or 3 pole switches are available with ratings up to 10 hp, 600 volts, 3 phase. The continuous current rating is 30 amperes at 250 volts maximum and 20 amperes at 600 volts maximum.

The toggle operation of the manual switch is similar to the FHP starter, and typical applications of the switch include small machine tools, pumps, fans, conveyors, and other electrical machinery which have separate motor protection. They are particularly suited to switch non-motor loads, such as resistance heaters.

9.3 INTEGRAL HORSEPOWER MANUAL STARTER

The Integral Horsepower manual starter is available in 2 and 3 pole versions, to control single phase motors up to 5 HP and polyphase motors up to 10 HP respectively.

The two pole starters have one overload relay and 3 pole starters usually have 3 overload relays. When an overload relay trips, the starter mechanism unlatches, opening the contacts to stop the motor. The contacts cannot be reclosed until the starter mechanism has been reset by pressing the STOP button or moving the handle to the RESET position, after allowing time for the thermal unit to cool.

9.4 MANUAL STARTER WITH LOW VOLTAGE PROTECTION

Integral Horsepower manual starters with Low Voltage Protection (LVP) prevent automatic start-up of motors after a power loss. This is accomplished with a continuous- duty solenoid which is energized whenever the line-side voltage is present. If the line voltage is lost or disconnected, the solenoid de-energizes, opening the starter contacts. The contacts will not automatically is restored. To close the contacts, the device must be close when the line voltage manually reset. This manual starter will not function unless the line terminals are energized.

Typical applications include conveyors, grinders, metal-working machinery, mixers, woodworking machinery and wherever standards require Low Voltage Protection.

10.0 MAGNETIC CONTROL

A high percentage of applications require the controller to be capable of operation from remote locations, or to provide automatic operation in response to signals from pilot devices such as thermostats, pressure or float switches, limit switches, etc. Low voltage release (15.0) or protection (15.1) might also be desired. Manual starters cannot provide this type of control, and therefore magnetic starters are used.

The operating principle which distinguishes a magnetic from a manual starter is the use of an electromagnet. The electromagnet consists of a coil of wire placed on an iron core, as shown in the illustration below, right. When current flows through the coil, the iron of the magnet becomes magnetized, attracting the iron bar, called the armature. To this extent (both will attract the iron bar) the electromagnet can be compared to the permanent magnet shown on the left.

The field of the permanent magnet, however, will hold the armature against the pole faces of the magnet indefinitely, and the armature could not be dropped out except by physically pulling it away. In the electromagnet, interrupting the current flow through the coil of wire causes the armature to drop out due to the presence of an air gap (10.5) in the magnetic circuit.

With manual control, the starter must be mounted so that it is easily accessible to the operator. With magnetic control, the pushbutton stations or other pilot devices can be mounted anywhere on the machine, and connected by control wiring into the coil circuit of the remotely mounted starter.

10.1 MAGNET FRAME AND ARMATURE ASSEMBLIES

In the construction of a magnetic controller (2.0), the armature (10.4) is mechanically connected to a set of contacts, so that when the armature moves to its closed position, the contacts also close. The drawings show several magnet and armature assemblies in elementary form. When the coil has been energized, and the armature has moved to the closed position, the controller is said to be "picked up," and the armature "seated" or "scaled-in."

CLAPPER TYPE — The armature is hinged. As it pivots to seal in, the movable contacts close against the stationary contacts.

VERTICAL ACTION — The action is a straight line motion with the armature and contacts being guided so that they move in a vertical plane.

HORIZONTAL ACTION — Both armature and contacts move in a straight line through a horizontal plane.

BELL CRANK — A bell crank lever transforms the vertical action of the armature into a horizontal contact motion. The shock of armature pickup is not transmitted to the contacts, resulting in minimum contact bounce and longer contact life.

10.2 MAGNETIC CIRCUIT

The magnetic circuit of a controller consists of the magnet assembly, the coil and the armature. It is so named from a comparison with an electrical circuit. The coil and the current flowing in it cause magnetic flux to be set up through the iron in a similar manner to a voltage causing current to flow through a system of conductors. The changing magnetic flux produced by alternating currents results in a temperature rise in the magnetic circuit. The heating effect is reduced by laminating the magnet assembly and armature.

10.3 MAGNET ASSEMBLY

The magnet assembly is the stationary part of the magnetic circuit. The coil is supported by and surrounds part of the magnet assembly in order to induce magnetic flux into the magnetic circuit.

10.4 ARMATURE

The armature is the moving part of the magnetic circuit(10.2). When it has been attracted into its sealed-in position, it completes the magnetic circuit. To provide maximum pull (to close the contacts) and to help insure quietness, the faces of the armature and the magnet assembly are ground to a very close tolerance.

10.5 AIR GAP

When a controller's armature (10.4) has sealed-in, it is held closely against the magnet assembly. However, a small gap is always deliberately left in the iron circuit. When the coil is de-energized, some magnetic flux (residual magnetism) always remains — and if it were not for the gap in the iron circuit, the residual magnetism might be sufficient to hold the armature in the sealed-in position.

10.6 SHADING COIL

A shading coil is a single turn of conducting material (generally copper or aluminum) mounted in the face of the magnet assembly or armature. The alternating main magnetic flux induces currents in the shading coil and these currents set up auxiliary magnetic flux which is out of phase from the main flux. The auxiliary flux produces a magnetic pull out of phase from the pull due to the main flux and this keeps the armature sealed-in when the main flux falls to zero (which occurs 120 times per second with 60 cycles ac). Without the shading coil, the armature would tend to open each time the main flux goes through zero. Excessive noise, wear on the magnet faces, and heat would result.

10.10 MAGNET COIL — INRUSH AND SEALED CURRENTS

The magnet coil has many turns of insulated copper wire wound on a spool. Most coils are protected by an epoxy molding which makes them very resistant to mechanical damage.

When the controller is in the open position there is a large air gap (not to be confused with the built-in air gap (10.5) in the magnet circuit (10.2) since the armature is at its furthest distance from the magnet. The impedance of the coil (which in ac magnetic circuits is the property to limit or resist current flow) is relatively low, due to the air gap, so that when the coil is energized, it draws a fairly high current. As the armature moves closer to the magnet assembly, the air gap is progressively reduced, and with it, the coil current, until the armature has sealed in. The final current is referred to as the sealed current. The inrush current is approximately 6 to 10 times the sealed current. The ratio varies with individual designs. After the controller has been energized for some time, the coil will become hot. This will cause the coil current to fall to approximately 80% of its value when cold.

AC magnet coils should never be connected in series. If one device were to seal-in ahead of the other (quite likely, if the devices are not identical, and a possibility even if they are) the increased circuit impedance will reduce the coil current so that the "slow" device will not pick up or, having picked up, will not seal. AC coils should be connected in parallel.

10. 11 MAGNET COIL — INRUSH AND SEALED CURRENT RATINGS

Magnet coil data is usually given in volts-amperes (volts times amperes or VA). For example, given a magnetic starter whose coils are rated at 600 VA inrush and 60 VA sealed, the inrush current of a 120 volt coil is 600/120 or 5 amperes, and the sealed current is 60/120 or .5 amperes. The same starter with a 480 volt coil will only draw 600/480 or 1.25 amperes inrush and 60/480 or .125 amperes sealed.

10.12 PICK-UP VOLTAGE

The minimum control voltage which will cause the armature to start to move is called the pick-up voltage.

10.13 SEAL-IN VOLTAGE

The seal-in voltage is the minimum control voltage required to cause the armature to seat against the pole faces of the magnet. On devices using a vertical action magnet and armature (IO. I), the seal-in voltage is higher than the pick-up voltage to provide additional magnetic pull to insure good contact pressure.

Control devices using the bell-crank armature and magnet arrangement (10.1) are unique in that they have different force characteristics. Devices using this operating principle are designed to have a lower seal-in voltage than pick-up voltage. Contact life is extended, and contact damage under abnormal voltage conditions is reduced, for if the voltage is sufficient to pick-up, it is also high enough to seat the armature.

10.14 DROP-OUT VOLTAGE

If the control voltage is reduced sufficiently, the controller will open. The voltage at which this happens is called the drop-out voltage. It is somewhat lower than the seal-in voltage.

10.15 MAGNET COIL — VOLTAGE VARIATIONS

NEMA Standards require that the magnetic device operate properly at varying control voltages from a high of 110% to a low of 85% of rated coil voltage. This range, established by coil design, insures that the coil will withstand given temperature rises at voltages up to 10% over rated voltage, and that the armature will pick up and seal in, even though the voltage may drop to 15% under the nominal rating.

10.16 EFFECTS OF VOLTAGE VARIATION — VOLTAGE TOO HIGH

If the voltage applied to the coil is too high, the coil will draw more than its designed current. Excessive heat will be produced and will cause early failure of the coil insulation. The magnetic pull will be too high, which will cause the armature to slam home with excessive force. The magnet faces will wear rapidly, leading to a shortened life for the controller. In addition, contact bounce may be excessive, resulting in reduced contact life.

10.17 EFFECTS OF VOLTAGE VARIATION — VOLTAGE TOO LOW

Low control voltage produces low coil currents and reduced magnetic pull. On devices with vertical action assemblies (10.1) if the voltage is greater than pick-up voltage (10.12) but less than seal-in voltage (10.13), the controller may pick up but will not seal. With this condition, the coil current will not fall to the scaled value. As the coil is not designed to carry continuously a current greater than its sealed current, it will quickly get very hot and burn out. The armature (10.4) will also chatter. In addition to the noise, wear on the magnet faces results.

In both vertical action and bell-crank construction (10.1), if the armature does not seal, the contacts will not close with adequate pressure. Excessive heat, with arcing and possible welding of the contacts, will occur as the controller attempts to carry current with insufficient contact pressure.

10.18 AC HUM

All ac devices which incorporate a magnetic effect produce a characteristic hum. This hum or noise is due mainly to the changing magnetic pull (as the flux changes) inducing mechanical vibrations. Contactors, starters and relays could become excessively noisy as a result of some of the following operating conditions:

• Broken shading coil (10.6)
• Operating voltage too low (see para. 10.17)
• Wrong Coil
• Misalignment between the armature and magnet assembly — the armature is then unable to seat properly
• Dirt, rust, filings etc. on the magnet faces the armature is unable to seal in completely
• Jamming or binding of moving parts (contacts, springs, guides, yoke bars) so that full travel of the armature is prevented
• Incorrect mounting of the controller, as on a thin piece of plywood fastened to a wall, for example, so that a "sounding board" effect is produced

10.20 MAGNETIC STARTER — POWER CIRCUIT

The power circuit of a starter includes the stationary and movable contacts, and the thermal unit or heater portion of the overload relay (8.3) assembly. The number of contacts (or "poles") is determined by the electrical service. In a 3 phase, 3 wire system, for example, a 3 pole starter is required.

10.21 MAGNETIC STARTERS — NEMA SIZES AND RATINGS

Power circuit contacts handle the MOTOR LOAD. The ability of the contacts to carry the Full Load Current (3.1) without exceeding a rated temperature rise, and their isolation from adjacent contacts, corresponds to NEMA Standards established to categorize the NEMA Size of the starter. The starter must also be capable of interrupting the motor circuit under Locked Rotor Current (3.2) conditions.

To be suitable for a given motor application, the magnetic starter selected should equal or exceed the motor Horse- power and Full Load Current ratings. Example: Motor to be controlled has a 50 hp rating, the service is 230 volts, polyphase, and the motor Full Load Current is 125 amps; reference to the table (Pg. 15) will show that a NEMA Size 4 starter would be required for normal motor duty. If the motor were to be used for jogging (3.9) or plugging duty, (3. 1 0) a NEMA Size 5 starter should be chosen.

10.30 MAGNETIC STARTER — CONTROL CIRCUIT

The circuit to the magnet coil which causes a magnetic starter to pick-up and drop-out is distinct from the power circuit (10.20). Although the power circuit can be single phase or polyphase, the coil circuit is always a single phase circuit. Elements of a coil circuit include the following:

• The Magnet Coil
• The contact(s) of the overload relay assembly (8.3)
• A momentary (14.4) or maintained (14.3) contact pilot device, such as a push button station, pressure, temperature, liquid level or limit switch, etc.
• In lieu of a pilot device, the contact(s) of a relay (22.0) or timer (23.0).
• An auxiliary contact on the starter, designated as a holding circuit interlock (14.0), which is required in certain control schemes.

The coil circuit is generally identified as the CONTROL CIRCUIT, and contacts in the CONTROL CIRCUIT handle the COIL LOAD.

The inter-wiring shown in the illustration above (except Wiring Diagram B) covers only the control circuit wiring provided by the factory (see Common Control 17.0). Per NEMA Standards, the single phase Control Circuit is conventionally wired between Line I and Line 2. As review of Wiring Diagram A will show, the control circuit is connected to the single phase circuit at Line 2, but like a lamp plug with only one prong, there is no control circuit connection to Line 1.

Wiring Diagram B illustrates that the control circuit is table) completed by the additional wiring of a pilot device between terminal 3, on the auxiliary contact, and terminal 1 (Line 1) on the starter.

10.31 MAGNETIC STARTERS — CONTROL CIRCUIT CURRENTS

Although the power circuit and control circuit voltage may be the same (see Common Control 17.0), the current drawn by the motor in the Power Circuit (see paragraph 10.21) is much higher than that drawn by the coil in the control circuit.

Pilot devices and contacts of timers and relays used in control circuits are therefore not generally Horsepower rated, and the current rating is low, compared to a starter or contactor (18.0).

Inrush and Sealed Currents (10.10, 10.11) of a control circuit can be determined by reference to a magnet coil A standard duty push button (25.0) with a rating of 15 amps Inrush, 1.5 amps Normal (Sealed) Current at 240 volts 60 hertz can satisfactorily be used to control the coil circuit of a 3 pole NEMA Size 3 starter or contactor, which has an Inrush Current of 2.9 amps (700VA/24OV) and a Sealed Current of .2 amps (46VA/24OV).

As a comparison of the differences in current, the Power Circuit contacts of the above starter may be controlling a 30 HP Polyphase motor, drawing a Full Load Current of 78 amps.

11.0 WIRING DIAGRAM

By superimposing the Control Circuit Diagram (A) in paragraph 10.30 over the Power Circuit Diagram in paragraph 10.20, a composite picture of all points of connection on the magnetic starter can be obtained. The combined illustration is identified as a WIRING DIAGRAM and it shows, as closely as possible, the actual location of all of the component parts of the starter. The dotted lines represent Power Circuit connections made to the starter by the user.

Since wiring connections and terminal markings are shown, this type of diagram is helpful when wiring the starter, or tracing wires when troubleshooting. Note that bold lines denote the Power Circuit, and thin lines are used to show the Control Circuit. Conventionally, in ac magnetic equipment, black wires are used in Power Circuits and red wiring is used for Control Circuits.

12.0 ELEMENTARY DIAGRAM

The elementary diagram gives a fast, easily understood picture of the circuit. The devices and components are not shown in their actual positions. All the control circuit components are shown as directly as possible, between a pair of vertical lines, representing the control power supply. The arrangement of the components is designed to show the sequence of operation of the devices, and helps in understanding how the circuit operates. The effect of operating various interlocks (14.1) control devices etc. can be readily seen — this helps in trouble shooting, particularly with the more complex controllers. This form of electrical diagram is sometimes referred to as a "schematic" or "line" diagram.

13.0 TWO WIRE CONTROL

In the wiring and elementary diagrams shown, two wires connect the control device (which could be a thermostat, float switch, limit switch or other maintained contact device) to the magnetic starter. When the contacts of the control device close, they complete the coil circuit of the starter, causing it to pick up and connect the motor to the lines. When the control device contacts open, the starter is de-energized, stopping the motor.

Two wire control provides low voltage release (I 5.0), but not low voltage protection (15.1). Wired as illustrated, the starter will function automatically in response to the direction of the control device, without the attention of an operator.

The dotted portion shown in the elementary diagram represents the holding circuit interlock (14.0) furnished on the starter, but not used in 2-wire control. For greater simplicity, this portion is omitted from the conventional 2-wire elementary diagram.

13.1 THREE WIRE CONTROL

A 3-wire control circuit uses momentary contact (14.4) Start-Stop buttons and a holding circuit interlock (14.0), wired in parallel with the Start button, to maintain the circuit.

Pressing the N.O. Start button completes the circuit to the coil. The power circuit contacts in Lines 1, 2 and 3 close, completing the circuit to the motor, and the holding circuit contact (mechanically linked with the power contacts) also closes. Once the starter has picked up, the Start button can be released, as the now closed interlock contact provides an alternate current path around the reopened Start contact.

Pressing the N.C. Stop button will open the circuit to the coil, causing the starter to drop out. An overload condition, which causes the overload contact to open, a power failure, or a drop in voltage to less than the seal-in (10.13) value, would also de-energize the starter. When the starter drops out, the interlock contact re-opens, and both current paths to the coil, through the start button and the interlock, are now open.

Since 3 wires from the push button station are connected into the starter — at points l(Ll), 2 and 3 — this wiring scheme is commonly referred to as 3-wire control.

14.0 HOLDING CIRCUIT INTERLOCK

The holding circuit interlock is a normally open (N.O.) auxiliary contact provided on standard magnetic starters and contactors. It closes when the coil is energized to form a holding circuit for the starter after the "start" button has been released (see paragraph 13.1, 3-wire control). As a matter of economics, vertical action (10.1) contactors and starters in the smaller NEMA sizes (Size 0, Size 1) have a holding interlock which is physically the same size as the power contacts.

14.1 ELECTRICAL INTERLOCKS

In addition to the main or power contacts (see 10.21), which carry the motor current, and the holding circuit interlock (14.0), a starter can be provided with externally attached auxiliary contacts, commonly called electrical interlocks. Interlocks are rated to carry only control circuit currents, not motor currents. N.O. and N.C. versions are available. Among a wide variety of applications, interlocks can be used to control other magnetic devices where sequence operation is desired; to electrically prevent another controller from being energized at the same time (see Reversing Starters 19.0); and to make and break circuits to indicating or alarm devices such as pilot lights, bells or other signals.

Electrical interlocks are packaged in kit form, and can be easily added in the field.

14.2 CONTROL DEVICE (PILOT DEVICE)

A device which is operated by some non-electrical means (such as the movement of a lever), and which has contacts Vertical Action in the control circuit of a starter, is called a "control device" Operation of the control device will control the starter and hence the motor. Typical control devices are control stations(25.0), limit switches (27.0), foot switches (26.0), pressure switches (28.0), float switches (29.0). The control device may be of the maintained contact (14.3) or momentary contact (14.4) type. Some control devices have a HP rating, and are used to directly control small motors through the operation of their contacts. When used in this way, separate overload protection (such as a manual starter) normally should be provided, as the control device does not usually incorporate overload protection.

14.3 MAINTAINED CONTACT

A maintained contact control device is one which when operated will cause a set of contacts to open (or close) and stay open (or closed) until a deliberate reverse operation occurs. A conventional thermostat is a typical maintained contact device. Maintained contact control devices are used with 2-wire control (I 3.0).

14.4 MOMENTARY CONTACT

A standard pushbutton is a typical momentary contact control device. Pushing the button will cause NO contacts to close and NC contacts to open. When the button is released, the contacts revert to their original states. Momentary con- tact devices are used with 3-wire control (13.1) or jogging service.

15.0 LOW VOLTAGE (UNDER VOLTAGE) RELEASE

By the nature of its control circuit connections, a 2-wire control scheme provides "Low Voltage Release." The term describes a condition in which a reduction or loss of voltage will stop the motor, but in which motor operation will automatically resume as soon as power is restored.

If the 2-wire control device in the diagram in paragraph 13.0 is closed, a power failure or drop in voltage below the seal-in value (10.13) will cause the starter to drop out, but as soon as power is restored, or the voltage returns to a level high enough to pick up and seal (10.17), the starter contacts will re-close and the motor will again run. This is an advantage in applications involving unattended pumps, refrigeration processes, ventilating fans, etc. In many applications, however, the unexpected restarting of a motor after power failure is undesirable, as in a process where a number of motors must be restarted, or operations performed, in a prescribed sequence. In some applications, the automatic restart presents the possibility of danger to personnel, damage to machinery or to work in process.

If protection from the effects of a low voltage condition is required, the 2-wire control scheme is not suitable, and 3 wire control, which provides the desired protection (15.1) should be used.

15.1 LOW VOLTAGE (UNDER VOLTAGE) PROTECTION

The 3-wire control scheme described in paragraph 13.1 provides "Low Voltage Protection." In both 2 and 3-wire control, the starter will drop out and the motor will stop in response to a low voltage condition or power failure.

When power is restored, however, the starter connected for 3-wire control will not pick up, as the reopened holding circuit contact and the N.O. Start button contact prevent current flow to the coil. To restart the motor after a power failure, the low voltage protection offered by 3-wire control requires that the Start button be depressed. A deliberate action must be performed, insuring greater safety than that provided by 2-wire control.

Manual starters with low voltage protection (9.4) offer this same type of protection.

16.0 FULL VOLTAGE (ACROSS THE LINE) STARTER

As the name implies, a full voltage or across-the-line starter directly connects the motor to the lines. The starter can be either manual or magnetic.

A motor connected in this fashion draws full Inrush Current (3.2) and develops maximum starting torque (3.4) so that it accelerates the load to full speed in the shortest possible time. Across-the-line starting can be used wherever this high inrush current and starting torque are not objectionable.

With some loads the high starting torque will damage belts, gears and couplings and material being processed. High inrush current can produce line voltage dips which cause lamp flicker and disturbances to other loads. Lower starting currents and torques are therefore often required, and are achieved by reduced voltage starting.

17.0 COMMON CONTROL

The coil circuit (10.30) of a magnetic starter or contactor is distinct from the power circuit. The coil circuit could be connected to ANY single phase source of power, and the controller would be operable, provided the coil voltage and frequency match the service to which it is connected.

When the control circuit is tied back to Lines 1 & 2 of the starter, the voltage of the control circuit is always the same as the power circuit voltage and the term COMMON CONTROL is used to describe the relationship. Other variations include separate control (17.2) and control through a Control Circuit Transformer (17.1).

17.1 CONTROL CIRCUIT TRANSFORMER

It is sometimes desirable to operate pushbuttons or other control circuit devices at some voltage lower than the motor voltage. In diagram A on Page 23, a single phase control transformer (with dual voltage 240-480 volt primary, 120 volt secondary) has its 480 volt primary connected to the 480 volt 3-phase, 3-wire service brought into the starter.

Note, however, that the control circuit is now connected to the 120 volt secondary of the transformer, rather than being connected to Lines 1 & 2, as in Common Control (17.0).

The coil voltage is therefore 120 volts, and the pushbuttons or other control devices operate at the same voltage level. A fuse is often used to protect the control circuit, and it is common practice to ground one side of the transformer secondary.

17.2 SEPARATE CONTROL

Control of a power circuit by a lower control circuit voltage can also be obtained by connecting the coil circuit to a separate control voltage source, rather than to a transformer secondary (see paragraph 17.1).

The term used to describe this wiring arrangement is "Separate Control." As is evident from diagram B, Page 23, the coil rating must match the control source voltage, but the power circuit can be any voltage (up to a 600 volt maximum.)

18.0 CONTACTOR

The general classification of "CONTACTOR" covers a type of electromagnetic apparatus designed to handle relatively high currents. A special form of contactor exists for lighting load applications, and will be separately covered (see Paragraph 18.1).

The conventional contactor is identical in appearance, construction and current carrying ability to the equivalent NEMA size magnetic starter. The magnet assembly and coil, contacts, holding circuit interlock and other structural features are the same.

The significant difference is that the contactor does not provide overload protection. Contactors, therefore, are used to switch high current, non-motor loads, or are used in motor circuits if overload protection is separately provided. A typical application of the latter is in a Reversing Starter (19.0).

18.1 LIGHTING CONTACTOR

Filament type lamps (tungsten, infra-red, quartz) have inrush currents of approximately 15-17 times the normal operating currents. Standard motor control contactors (18.0) must be derated if used to control this type of load, to prevent welding of the contacts on the high initial current.

Per the rating table, a NEMA Size I contactor has a continuous current rating of 27 amperes, but if used to switch certain lighting loads, must be derated to 15 amperes. The standard contactor, however, need not be derated for resistance heating or fluorescent lamp loads, which do not impose as high an inrush current.

Lighting contactors differ from standard contactors in that the contact tip material is a silver tungsten carbide which resists welding on high initial currents. A holding circuit interlock is not normally provided, since this type of contactor is frequently controlled by a 2-wire (13.0) pilot device such as a time clock or photo-electric relay.

Unlike standard contactors, lighting contactors are not HP rated or categorized by NEMA size, but are designated by ampere ratings (20, 30,60, 100,200, 300 amperes). It should be noted that Lighting Contactors are specialized in their application, and should not be used on motor loads.

18.2 MECHANICALLY HELD CONTACTS

In a conventional contactor, current flow through the coil the contacts in a switched position. (N.O. contacts will be creates a magnetic pull to seal in the armature and maintain held closed, N.C. will be held open). Because the contactor action is dependent on the current flow through the coil, the contactor is described as ELECTRICALLY HELD. As soon as the coil is de-energized, the contacts will revert to their initial position.

MECHANICALLY HELD versions of contactors, (and relays 22.0), are also available. The action is accomplished through use of two coils and a latching mechanism. Energizing one coil (latch coil) through a momentary signal causes the contacts to switch, and a mechanical latch holds the contacts in this position, even though the initiating signal is removed, and the coil is de-energized. To restore the contacts to their initial position, a second coil (unlatch coil) is momentarily energized.

MECHANICALLY HELD contactors and relays are used where the slight hum (10.18) of an electrically held device would be objectionable, as in auditoriums, hospitals, churches, etc.

19.0 REVERSING STARTER

Reversing the direction of motor shaft rotation is often required. Three phase squirrel cage motors can be reversed by reconnecting any two of the three line connections to the motor. By interwiring two contactors, an electromagnetic method of making the reconnection can be obtained.

As seen in the power circuit (right), the contacts (F) of the Forward contactor, when closed, connect Lines 1, 2 and 3 to motor terminals TI, T2 and T3 respectively. As long as the Forward contacts are closed, mechanical and electrical interlocks (1 4. 1) prevent the Reverse contactor from being energized.

When the Forward contactor is de-energized, the second contactor can be picked up, closing its contacts (R) which reconnect the lines to the motor. Note that by running through the Reverse contacts, Line I is connected to motor terminal T3, and Line 3 is connected to motor terminal TI. The motor will now run in the opposite direction. Whether operating through either the Forward or Reverse contactor, the power connections are run through an overload relay assembly, which provides motor overload protection. A magnetic reversing starter, therefore, consists of a starter and contactor, suitably interwired, with electrical and mechanical interlocking to prevent the coil of both units from being energized at the same time. Manual Reversing Starters (employing two manual starters) are also available. As in the magnetic version, the Forward and Reverse switching mechanisms are mechanically interlocked, but since coils are not used in the manually operated equipment, electrical interlocks are not furnished.

20.0 COMBINATION STARTER

A Combination Starter is so named since it combines a disconnect means, which might incorporate a short circuit protective device, (see 7.0, Overcurrent Protection), and a magnetic starter in one enclosure. Compared with a separately mounted disconnect and starter, the Combination Starter takes up less space, requires less time to install and wire, and provides greater safety.

Safety to personnel. is assured because the door is mechanically interlocked, so that it cannot be opened without first opening the disconnect. Combination starters can be furnished with circuit breakers or fuses to provide overcurrent protection, and are available in non-reversing and reversing versions.

21.0 RELAY - CONTACTOR COMPARISON

A control relay is an electromagnetic device, similar in operating characteristics to a contactor. The contactor, however, is generally employed to switch power circuits, or relatively high current loads.

Relays, with few exceptions, are used in control circuits, and consequently their lower ratings (I 5 amperes maximum at 600 volts) reflect the reduced current levels at which they operate.

Contactors generally have from one to five poles. Although normally open and normally closed contacts can be provided, the great majority of applications use the normally open contact configuration, and there is little, if any, conversion of contact operation in the field.

As compared to contactors, it is not uncommon to find relays used in applications requiring 10 or 12 poles per device, with various combinations of normally open and normally closed contacts. In addition, some relays have convertible contacts, permitting changes to be made in the field from N.O. to N.C. operation, or vice versa, without requiring kits or additional components.

22.0 CONTROL RELAYS

A relay is an electromagnetic device whose contacts are used in control circuits of magnetic starters, contactors, solenoids, timers and other relays. Relays are generally used to amplify the contact capability or multiply the switching functions of a pilot device. Diagram B represents a VOLTAGE AMPLIFICATION. A condition may exist in which the voltage rating of the temperature switch is too low to permit its direct use in a starter control circuit operating at some higher voltage. In this application, the coil of the interposing relay and the pilot device are wired to a low voltage source of power compatible with the rating of the pilot device. The relay contact, with its higher voltage rating, is then used to control the operation of the starter.

Diagrams A & B demonstrate how a relay amplifies contact capacity. Diagram A represents a CURRENT AMPLIFICATION. Relay and starter coil voltages are the same (220 volts), but the ampere rating of the temperature switch is too low to handle the current drawn by the starter coil (M). A relay is interposed between the temperature switch and starter coil. The current drawn by the relay coil (CR) is within the rating of the temperature switch, and the relay contact (CR) has a rating adequate for the current drawn by the starter coil.

22.1 RELAY VARIATIONS

Relays differ in voltage ratings (I 50, 300, 600 volts), number of contacts, contact convertability, physical size, and in attachments to provide accessory functions such as mechanical latching (I 8.2) and timing.

In selecting a relay for a particular application, one of the first steps should be a determination of the control voltage at which the relay will operate. Once the voltage is known, the relays which have the necessary contact rating can be further reviewed, and a selection made, on the basis of the number of contacts and other characteristics needed.

Relays are commonly used in complex controllers to provide the logic or "brains" to set up and initiate the proper sequencing and control of a number of interrelated operations.

23.0 TIMERS AND TIMING RELAYS

A pneumatic timer or timing relay is similar to a control relay, except that certain of its contacts are designed to operate at a pre-set time interval after the coil is energized or de-energized. A delay on energization is also referred to as "On Delay." A time delay on de-energization is also called "Off Delay."

A timed function is useful in applications such as the lubricating system of a large machine, in which a small oil pump must deliver lubricant to the bearings of the main motor for a set period of time before the main motor starts.

In pneumatic timers, the timing is accomplished by the transfer of air through a restricted orifice. The amount of restriction is controlled by an adjustable needle valve, permitting changes to be made in the timing period.

24.0 DRUM SWITCH

A drum switch is a manually operated 3 position 3 pole switch which carries a HP rating and is used for manual reversing of single or 3 phase motors. Drum switches are available in several sizes, and can be spring-return-to-off (momentary contact) or maintained contact. Separate overload protection, by manual or magnetic starters, must usually be provided, as drum switches do not include this feature.

25.0 CONTROL STATION (PUSH BUTTON STATION)

A control station may contain push buttons, selector switches and pilot lights. Push buttons may be momentary or maintained contact. Selector switches are usually maintained contact, or can be spring return to give monetary contact operation.

Standard duty stations will handles the coil currents of contactors up to Size 4. Heavy duty stations have higher contact ratings and provide greater flexibility through a wider variety of operators, and interchangeability of units.

26.0 FOOT SWITCH

A foot switch is a control device operated by a foot pedal used where the process or machine requires that the operator have both hands free. Foot switches usually have momentary contacts but are available with latches which enable them to be used as maintained contact devices.

27.0 LIMIT SWITCH

A limit switch is a control device which converts mechanical motion into an electrical control signal. Its main function is to limit movement, usually by opening a control circuit when the limit of travel is reached. Limit switches may be momentary contact (spring return) or maintained contact types. Among other applications, Limit Switches can be used to start, stop, reverse, slow down, speed up or recycle machine operations.

27.1 SNAP SWITCH

Snap switches for motor control purposes are enclosed, precision switches require low operating forces and have a high repeat accuracy. They are used as interlocks, and as the switch mechanism for control devices such as precision limit switches, and pressure switches. They are available also with integral operators for use as compact limit switches, door operated interlocks, etc. Single pole double throw and 2 pole double throw versions are available.

28.0 PRESSURE SWITCH

The control of pumps, air compressors, welding machines, lube systems, and machine tools requires control devices which respond to the pressure of a medium such as water, air or oil. The control device which does this is a pressure switch. It has a set of contacts which are operated by the movement of a piston, bellows or diaphragm against a set of springs. The spring pressure determines the pressures at which the switch closes and opens its contacts.

29.0 FLOAT SWITCH

When a pump motor must be started and stopped according to changes in water (or other liquid) level in a tank or sump, a float switch is used. This is a control device whose contacts are controlled by movement of a rod or chain and counterweight, fitted with a float. For closed tank applications, the movement of a float arm is transmitted through a bellows seal to the contact mechanism.