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Motor Controllers, Part 5

Motor Controllers, Part 5In Part 4, we looked at two of the four methods for determining the correct rating for a motor controller.

The third method is one you can use for stationary motors of 2HP or less and 300V or less [430.83(C)]. You can use a general use switch or a general use snap switch to control the motor:

  1. General use switch. But it must have an ampere rating at least twice the Full Load Current (FLC) of the motor.
  2. General use snap switch. But only on ac circuits, and the switch must be suitable for only ac (it can’t be general-use ac-dc). Plus, the motor FLC can’t be more than 80% of the ampere rating of the switch.

The fourth method applies to torque motors. In these applications, the controller must have a continuous-duty, FLC rating at least that of the motor nameplate rating [430.83(D)].

Section 430.83 ends with one last requirement, but it’s not a method. It applies to whatever method you’re using. Basically, it says the voltage rating of the circuit can’t exceed the voltage rating of the controller [430.83(E)].

Now, here’s another fact about controllers. An overcurrent protection device (OCPD), such as a circuit breaker, must open all conductors of the circuit. But that’s not a requirement for a motor controller [430.84]. Why the difference? The controller need only stop the motor, while the OCPD must de-energize the circuit.

« Part 4 | Source: Mark Lamendola | Mindconnection