Below are some examples of good and bad motors (identified using the All-Test equipment we supply) that we took from our Motor Circuit Analysis Book. This book, which was written by Dr. Howard W. Penrose, Ph.D., shares many instances where Motor Circuit Analysis (MCA) was applied to find a variety of faults.

Please note that the following examples were considered true and correct at the time of publication, and have been amended slightly to improve readability.

The following results are from a good, rebuilt subway traction motor...

Several key points can be determined from the above readings...

• All of the readings are very balanced, which represents a good motor
• Inductance readings are zero, which is a result of a very low inductance reading (less than 1 mH)
• Resistance readings of traction motors are very low, often in the low milli-ohm range

The following is a Yaskawa spindle motor that tested good on a test bench...

The following brushless DC servo motor tested bad on a test bench.

Notice the slight variation in resistance, and the significant changes in impedance and inductance. The phase angle (a difference of over five points) compared to I/F (balanced) indicates a coil-to-coil fault.

The following motor was found to have seized bearings, and had been submerged during operation (one concern was whether the motor was salvageable)...

The following points can be made from the above results...

• All of the readings, except for impedance and inductance, are fairly balanced
• The steep angle found as the difference between impedance and inductance is due to leakage to ground from contamination that has affected the insulation
• The small 'shift' in phase angle and I/F indicates that some minimal damage may have occurred due to contamination - however, the windings may be salvageable

The following servo stator was found to have a turn-to-turn short during bench testing...

Of particular interest is the resistance, which is balanced outside the range of a standard ohmmeter. The phase angle and I/F readings are also significantly different, and when both readings are out of range, the most common cause is a turn-to-turn short.

The following motor was tested from a Motor Control Centre (MCC)...

Further analysis determined that the fault was due to shorted windings in the motor. Although the motor was still operating, it would occasionally trip overloads at 20% full load of operation. Several key points can be determined from the above readings...

• All unbalances point toward a severe winding fault
• Phase angle and I/F deviations indicate turn faults
• Impedance to inductance relationship indicates winding contamination

All this suggests that the winding is failing due to winding contamination, and will require an immediate decision to be made as to whether the motor is repaired or replaced.

Just because an electric motor is brand-new, it cannot be taken for granted that faults do not exist. The following motor was tested at a manufacturer's booth during an exhibition...

The unbalanced I/F readings indicate a phase fault within the electric motor. New electric motors will have occasional phase unbalances, with winding faults rare and rotor casting void faults being more common.

If you have a success story that you would like to share with us and fellow readers, or if you would like to discuss any of this further, then please do not hesitate to get in touch.