Friday, 9 November 2012

LV Motor Maintenance

Motor maintenance are split into Predictive and Preventive.

1. Motor bearing lubrication. Typically motor frame size D180 inclusive and below are sealed for life bearings. Anything larger are greased as frequent as every 1000hrs. Refer to their respective manual for accurate greasing requirements.

2. Cooling fins and fans clean up. Frequency depends on operating environment. In generally clean environment, 4-8 yearly is acceptable. Increase accordingly to its operating environment. Note that in particularly post OH/ outage, plant will have bits and pieces of wrapping papers and plastics. That tend to get suck onto the ventilation guard of the fans blocking up cooling path. Thus I recommend performing this work post OH.

3. Alignment check should be perform on annual or 2 yearly basis particularly when new as the foundation sinks into place. It can later be extended to 4 yearly or as required based on VA.

4. VA is to be performed monthly, 3 monthly or 6 monthly depending on criticality of the motor. That should also pick up a range of other failure modes.

5. Terminals and glands is to be inspected 2 yearly in extreme processing operating environment, 5 or 10 yearly in a clean factory environment.

6. Thermography to be carried out monthly to check for anomalies and hot spot on motor and connections.

1. Measurement and trending of insulation resistance. For motor rated below 1000V, it is measured with a 500Vdc megger. (TECO manual extract)

2. In accordance with IEEE-43 clause 9.3 standards at the time of writing, the formula to calculate an acceptable limit is:

                      R => [ (Rated Voltage (V)/ 1000) + 1 ] x 10MegOhm

3. I recommend performing Motor Circuit Analysis (MCA) commencing with 2 years interval until sufficient data for trending (3-5 points), before extending to 4 years interval or more. It is a quick easy NDT that gives fairly good reliable indication of motor health whether it will survive another 2 or 4 years, or till your next available replacement opportunity.

4. On HV motors, I also recommend installing partial discharge monitoring equipment. Especially critical motors, it is better to go online PD monitoring. Trending data should indicate the deterioration rate of PD. Maintenance personnel should start taking concern and planning shall the PD deteriorates at a rate of double every 6 months or any anomalies.

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