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.

During an on-site visit, it was noted that six of twelve motors on a baghouse at a steel manufacturer had been routinely failing since the initial installation eighteen months prior. The initial observation was that the 150 horsepower motors would trip immediately upon start-up, particularly on warm days or if they were restarted immediately after operating.

Over time, each of the six motors ended up with winding faults, termed as 'lightning strikes' by the repair facilities. The other six motors exhibited none of the same results.

The fan motors were set-up as two groups of six immediately next to each other. All components used were from the same manufacturer and installed roughly at the same time. The six motors were each fed from two transformers, which in turn were fed from the same bus. Engineering firms, repair centres and consultants were all presented with the opportunity to resolve this issue.

The motors were evaluated and the cables tested conductor to conductor with megohm meters and DC high-potential test sets. In addition to this, power quality measurements were made and operation charts recorded. No faults could be found.

Out of curiosity, Motor Circuit Analysis (MCA) was applied. The first application was on a motor that was being removed because it would not start from the Motor Control Centre (MCC). It was assumed that the winding had faulted but, from the results below, the motor appeared completely balanced.

Testing was then performed from the MCC to the motor, in which there was a disconnect at the motor. In a few instances, the simple resistance was not repeatable and it was determined that the cables ran through a common cable tray to the motors. This resulted in a mutual inductance between the energised and de-energised equipment, so a series of manual AC tests were performed.

On one of the motors, a phase-to-phase impedance of 1500 Ohms at 800 Hz was detected. This was exceeding high, as they were averaging just 18 Ohms at 800 Hz on the other motors. It was quickly discovered that the disconnect had been open between the starter and the motor. The cable was isolated from the starter and the disconnect, and the value remained roughly the same.

Each of the cables were then tested (on the side that the motors were failing), and the values fell between 1200 Ohms and 3500 Ohms at 800 Hz. The low readings were found on the cables that had a load just before they were disconnected. These results could not be duplicated on the other group of six motors.

When queried further, it was found that there was a very interesting specification for how the cable was to be installed. This very precise engineering specification stated that the cable had to be unrolled prior to being run. It appeared that the cables on the side that the motors were failing on had been strung across a road and run over prior to installation by a variety of heavy vehicles. This had resulted in damage between conductors that showed up during start-up, and that gradually worsened as the defects broke down.

The condition of the cable from wire to wire was not seen in any DC measurement. In the case of the motor that was about to be replaced because of a suspected winding fault, it was found that the cable had finally shorted. Use of MCA in this instance took just approximately one hour.

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.