Industrial Motor Control diagram

The beginner and overload heater assembly for a commercial electric engine is frequently found further from motor itself, inside a space named a motor control center, or MCC:

As it is impossible for a specialist to be in two locations at the same time, it is necessary to do diagnostic inspections on a malfunctioning electric engine through the MCC where specialist features accessibility all the control circuitry.

One particular diagnostic check is line current, to detect the existence of an available engine winding. If a three-phase motor winding fails available, the engine won't operate since it should. This can be known as single-phasing. A great way to look for this condition is by using a clamp-on (inductive) ammeter to check on range current on all three lines whilst starter is energized. This might be done at any location where there's actual usage of the motor power conductors.

Assume, though, you are focusing on employment web site where single-phasing is suspected and you also don't have a clamp-on ammeter with you. All you have is a DMM (electronic multimeter), which won't have the ability to properly measure the motor’s present. You're about to return to the store to obtain a clamp-on ammeter whenever an even more experienced professional shows another test. He takes your DMM, establishes it into AC millivolt range, then links the test probes to either part of each overload heater factor, one heater at any given time such as this:

Across each overload heater element he steps about 20 mV AC using starter engaged. Using this he determines the motor isn't single-phasing, it is drawing approximately equal current on all three stages.

Explain how this diagnostic check works, and just why this determination could be made. In addition describe exactly what limits this diagnostic process has actually, and exactly how a clamp-on ammeter in fact is the simplest way to determine engine line existing.

Each overload heater factor possesses a tiny bit of electric weight, the key for this diagnostic process. Needless to say, the measurement acquired is purely qualitative, perhaps not quantitative as a clamp-on ammeter would give.

Follow-up question number 1: what sort of result might occur with this diagnostic check if the engine were certainly single-phasing considering among the overload heating units a failure available?

Follow-up question no. 2: the other causes could there be for a three-phase motor “single-phasing” other than an engine winding unsuccessful available?

I have tried personally this diagnostic check over and over again to troubleshooting a single-phasing electric engine. Its amazing exactly what sorts of diagnostic inspections you could do with a high-quality DMM and a sound knowledge of electrical principle!