In 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:
- General use switch. But it must have an ampere rating at least twice the Full Load Current (FLC) of the motor.
- 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.