Notes and answers to question from Albert Lo from Sydney Water

From Imrtwiki

Issues

1. Motors with Variable Speed Drive

Problem: These days organisations have been changing Direct On line (DOL) starters to VSD starters to cater for their applications. Although they have their good points, but there are some major issues with their operations.

When starting up the motor, VSD applied step up frequencies to drive the motor to its rated capacity and step down frequencies to slow down the motor to a full stop. This step up and step down operations causes current induction in the windings/ rotor and this in turn passes down from the bearings to the stator and returns. Eventually over a period of time, the bearing fails. The workgroup also stated that even when a sealed bearing is used, it causes voltage to build up in the motor which leads to failure to the motor. The recommended solution to the problem is install carbon brushes on the motor shaft and shunt it to ground as a way of discharging the inducted current.

2. Condition Monitoring on Bearings

Problem: Condition Monitoring techniques such as vibration analysis, are used to determine integrity of the bearings in the motors. But not many people in the companies or organisations would take any notice of or understand of the results of these diagnoses. Some reasons are due to lack of expertise in this field, not properly trained, could not afford the time to look at the results or leave it to the contractors to do the analysis and interpret the results. Not all but some contractors would manipulate the results and this would mislead the customers who did not have a clue on condition monitoring process.

3. Condition Monitoring using Thermography

Peter of Onesteel Corporation claimed that thermography as a condition monitoring tool used on crane motor in the extreme environment (Area not suitable for working in) in the steel works is quite good. It is used to detect hot spots in the motor especially in the hash environment. To use thermography, it must be used when the motor on equipment to be needed in operational condition and not in a off mode condition (switch off).

To support this comments, Bruce (Sydney Water) mentioned that thermography would be useful if the equipment such as the transformers and in the circuit breakers, starters, motors & etc, fully energised operational condition, not in the cold or ‘off’ condition.

4. Partial Discharge Test on Large Motors

Noel of Blue Circle Pty recommended partial discharge testing on at least 3.3KV motors give a good result in relation to the condition of the motor.

5. High Efficiency Motors

Currently in new capital projects mentioned by the workgroups, it seems that for a new motor installation, it must be a high efficiency type (95% - 97%) of motor as stated in the current Australian Standards.

6. Lubrication on the motor’s bearings

According to Richard of Rio Tinto, bearings tend to lose about (5% - 10%) of its solvent (lubrication) each month. Also he mentioned that to do a proper cleaning of the bearings (Large types), the bearings have to be dismantled, cleaned from inside to outside of the bearing casing, use of a new lubricant instead of the old one. With this methodology, this results in no damage to the motor shaft and rarely have bearing problems. Recommended lubricant is Caltex RPM grease.

7. Life Span of the Seal Bearings

Seal bearings are normally used on motors with less than 4 Hp. It is replaced once every 4 to 5 years.

8. Prevention of build up current in the Motor

Chris from Hurter Water mentioned in his pumping operations, to prevent water hammer in the pipes after the pumps ceases pumping water upstream, he uses Variable Speed Drive (VSD) starter to drive up the motor to a desired speed and them transfer it over to online operation.

9. Good feedbacks from the workgroup on motors

• Toshiba Motor – High Efficiency, shorter delivery time for purchasing new motors, expensive than TECO.
• TECO Motor – High Efficiency, longer delivery time for purchasing new motors, cheaper, good performance (depends on the models).
• Reliance Motor – Fits wells with Variable Frequency Drives.
• CMG Motor – Good if Australian Made, but not overseas.
• WEG Motor – Performance varies as manufactured differently in different factories in Brazil, cannot determine which factory in Brazil produces the Best WEG Motor.

10. Cleaning of Motors

Some workgroup members have experience in using steam cleaning, pressurized cleaning or warm soapy water to remove the oil stains, carbons or etc in the motor. For motor brushes, they normally use vacuum to suck the dust off the motor brushes instead of compressed air/ blowers as they tend to push the dust into the armature and the windings.