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Motors, Part 10

Motors, Part 10Arguably, the most significant way motor circuits differ from other circuits is the job of protecting against overcurrent (also called overload) is (usually) separated from the job of protecting against short-circuit and ground fault protection.

With any other type of electrical circuit, the overcurrent protection device (OCPD) does both jobs. With motor circuits, separate overload protection devices open the circuit upon overload and the OCPD does the other two jobs.

Overcurrent is a small magnitude increase over normal current. For example, a motor drives a conveyor belt. If several rollers jam, the motor needs 55A instead of 50A to drive that belt. As more rollers jam, more current is needed and soon you’re into an overload situation. The motor overloads open, but the branch circuit OCPD does not.

But if the motor shorts a winding to its case, the OCPD opens.

Why this job split? A motor draws much more current to start than it does to run (for the typical industrial motor used in manufacturing, it’s on the order of five times the run current). This inrush current makes it necessary to size the branch or feeder OCPD too large to properly protect against overload.

A common method of providing overload protection is via thermal strips installed in the motor starter. Not surprisingly, these are called “overloads.” These (and other types of overload devices for motor circuits) protect the motor, not the circuit.

Part III of Article 430 provides the requirements for motor overload protection.

« Part 9 |Part 11 »|Source: Mark Lamendola | Mindconnection