AC Motor Control Circuits

The circuit shown here provides two-direction control (forward and reverse) for a three-phase electric motor:

Explain how the reversal of motor direction is accomplished with two different motor starters, M1 and M2. Also, explain why there is only one set of overload heaters instead of two (one for forward and one for reverse). Finally, explain the purpose of the normally-closed contacts in series with each starter coil.

The starter and overload heater assembly for an industrial electric motor is often located quite a distance from the motor itself, inside a room referred to as a motor control center, or MCC:

Since it is impossible for a technician to be in two places at once, it is often necessary to perform diagnostic checks on a malfunctioning electric motor from the MCC where the technician has access to all the control circuitry.

One such diagnostic check is line current, to detect the presence of an open motor winding. If a three-phase motor winding fails open, the motor will not run as it should. This is called single-phasing. A good way to check for this condition is to use a clamp-on (inductive) ammeter to check line current on all three lines while the starter is energized. This may be done at any location where there is physical access to the motor power conductors.

Suppose, though, you are working on a job site where single-phasing is suspected and you do not have a clamp-on ammeter with you. All you have is a DMM (digital multimeter), which does not have the ability to safely measure the motor’s current. You are about to head back to the shop to get a clamp-on ammeter when a more experienced technician suggests an alternate test. He takes your DMM, sets it to the AC millivolt range, then connects the test probes to either side of each overload heater element, one heater at a time like this:

Across each overload heater element he measures about 20 mV AC with the starter engaged. From this he determines that the motor is not single-phasing, but is drawing approximately equal current on all three phases.

Explain how this diagnostic check works, and why this determination can be made. Also describe what limitations this diagnostic procedure has, and how a clamp-on ammeter really is the best way to measure motor line current.

A popular strategy for AC induction motor control is the use of variable frequency drive units, or VFDs. Explain what varying the frequency of power to an AC induction motor accomplishes, and why this might be advantageous.