Below is a case study 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 case study was considered true and correct at the time of publication, and has been amended slightly to improve readability.

A large 8000 horsepower, 12.3 kV, 200 RPM synchronous motor, which was operating a reciprocating compressor at a chemical plant, tripped on a 'short-circuit' warning during start-up in 65 milliseconds while using a General Electric Multilin. The motor was tested using a number of different technologies (including surge testing), but a fault could not be found.

The All-Test IV Pro 2000 was then used to investigate the winding and to isolate the fault, and the initial testing provided the following results...

These readings clearly indicated a fault and in order to investigate further, additional readings were taken with the rotor in different positions. Several readings were also taken with the motor neutral opened, with one phase tested at a time.

As observed (additional testing was performed to confirm the finding), there are most likely rotor faults, since the fault readings follow the re-positioning of the rotor. In addition, the impedance and inductance readings do not follow each other, which indicates that the windings are in poor condition (earlier confirmed using partial discharge testing).

Based upon how quickly the motor protection had tripped, the initial conclusion was that there was either a catastrophic stator winding failure or a problem with one of the secondary circuits. Regardless, it was thought that there were definite rotor faults. This particular motor would start with the rotor field circuits shorted, using them to assist the amortisseur windings to produce enough torque during start-up.

Once the motor had been partially disassembled, further testing revealed a catastrophic failure in the rotor fields and the rotor field circuit open. This would cause the rotor bar resistance to increase dramatically, magnetically saturating the amortisseur winding, which would result in an immediate and rapid increase in current. The control instrument detected this sharp current increase as a short. The actual stator windings were tested and found to be poor but operable.

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.