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Our previous issue left off with a prelude to the requirements for motor controllers that are autotransformer starters. Picking up from there, let's look at the NEC requirements.
First, it must be able to perform the three basic functions required of all motor controllers:
Additionally, an autotransformer controller have these control positions [430.82(B)]:
Two of the three basic functions of motor controllers involve turning it off. For this reason, an autotransformer controller can’t be capable of resting in the starting position [430.82(B)]; this would render the overload device inoperative (the controller would keep restarting the motor).
Another variety of motor controller is the rheostat controller. Like autotransformer motor controllers, they also must perform those three basic functions (two of which involve turning the motor off). If you look at the two NEC requirements for these [430.82(C)], you find they are all about not interfering with the ability to shut the motor off.
If you source motor controllers from a reputable distributor, you don’t have to worry about these requirements because the distributor will carry only controllers that comply with 430.82.