Three Motor Diagnostics Case Studies (3.3, 6.6 and 11 KV Motors)

Case Study 1:

Motor rating: 3.3 KV, 560 KW.

Problem: Tripping on startup with fuse failures

Investigation: Motor had 03 leads coming out. Client did insulation resistance testing and results were fine. They carried out winding resistance and results were within limits. motor was still tripping and they were soon to run out of fuses.

We tested the motor with MCA (AT5) and it should phase to phase short(s). Motor was sent to workshop which confirmed the results and motor was rewound.

Lesson learnt: MCA is a useful tool. it works by checking the balances of winding resistance, impedances, phase angles, and Current-to-Frequency ratio as well as LV Tan delta for the motor winding. It can also detect broken rotor bars in induction motors. Its an offline test that takes 5 minutes or so.

Case Study 2:

Motor ratings: 6.6 KV, 250 KW

Problem: Routine Testing in shutdown

Testing Results: IR/PI values for this motor was greater than 3 which was good. MCA showed no abnormality. Tan delta tip up was however, 8% well above the expected value of <1% for these size and kind of motors and so was the starting value of the tan delta at 0.2% of Uo. It was concluded that motor has contamination. IR being DC is very responsive to moisture so it was also mentioned that perhaps contamination is not due to moisture.

Upon opening the motor with our recommendation, motor winding was found contaminated with grease from bearing (at least 1/8th of the winding) and iron rust at bottom half of the winding. They cleaned the motor with solvent and air. Complete cleaning with dry ice blasting was planned for next outage.

Lesson learnt:

Excessive greasing is dangerous for motors. It is a well known fact that more bearings fail due to over greasing than under greasing. It was suspected that somebody pumped many strokes on stand still cold motor. Plant maintenance team has now bought SDT 270 Ultrasound kit for Condition based lubrication. Another thing is that if service provider has the capability of understanding what the overall test results are saying, he can easily reach to right conclusion.

Case Study 3:

Motor ratings: 11 KV, 8 MW

Problem: Routine Condition Assessment Tests

Test Results: The motor was found with 2% + tan delta at 0.2 PU and tan delta tip greater than 3%. Partial Discharge was also higher for new machine and PD inception voltage was also equal to or less than 0.5 PU which was not good. Motor was new and hence we advised the client about poor quality of manufacturing (improper curing as most probable reason and poor workmanship as well). We recommended the client to install online monitoring system on the motor asap.

Motor failed in service.

Lesson Learnt: Infant mortality was defined as something very important by Nowlan and Heap in their thesis for united decades ago but it still holds true. Many equipment fail pretty soon after being taken into service due to poor quality of manufacturing or installation, commissioning and initial maintenance. Establish baseline for new equipment as soon as you get an opportunity and track the performance.

Final Suggestions for maintenance engineers:

For LV motors, consider MCA and Signature Analysis (ESA) for troubleshooting as well as condition assessment.

For MV motors upto 6.3 KV, MCA, ESA with Tangent Delta Tip up cover almost all key failure modes of the machines.

For MV motor 6.6 KV to 11 KV (13.8 KV in North Americas) add Partial Discharge testing to condition assessment.