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 50 horsepower, 2600 RPM, delta connected electric motor was installed and rotation checked on a generator cooling pump motor. When the motor was brought on-line, there was an 11% (p-p) current unbalance with a less than 0.5% (p-p) voltage unbalance. The motor had a 120 Hz vibration (electrical) and an excessive operating temperature, although the peak current was under the nameplate full load value.

An All-Test Pro 31 was used to determine the phase unbalance, with results of 000, -016 and -016 (% unbalance) phase to phase when the rotor was shifted to the peak unbalance on each phase. Two consecutive motors, which were the same model with similar serial numbers, were then selected for review and tested using both the All-Test IV Pro 2000 and individual bridges of different manufacture.

The unbalance was found to be striking, and related to the unbalanced current, vibration and overheating of the motor. Many possibilities were explored, ranging from power quality to calibration of the test equipment, but these all proved satisfactory.

When interviewed, the manufacturer noted that process changes were made at a particular location for larger concentric wound machines. In a motor of this size and speed, the first set of concentric coils (one phase) curls under the following phases, reducing the equipment's winding appearance and mechanical strength.

To combat this issue, the manufacturer made the decision to significantly increase the size of the first set of coils in their automated process (first phase), which also happens to be the furthest from the rotor. This allows the coil ends to appear, without having to make post-winding modifications to the coils. No dynamometer testing, full load testing or otherwise was performed on the motor design, other than an applied voltage impedance test, which 'met design requirements'.

During the interview, a statement was made that any equivalent manufactured motor would exhibit the same results. As such, several other motors of the same frame size, but from different manufacturers, were evaluated. Similar findings were not found.

The result of this study concluded that this particular motor design was not acceptable to the user for this critical application. As a result of these findings, the power plant now performs Motor Circuit Analysis (MCA) testing on all new critical motors, checking for winding and rotor defects that may impact equipment reliability.

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.