Submersible Motor Deterioration

Need more information:

Are these new motors? Been in service for long time? Recently rewound? All same brand of motors? Change from a previous brand or model?

Have the motors been failing in operation, or is it just your test values decreasing?

Voltage fluctuation shouldn't affect the insulation resistance in that way. Low or high voltage could cause heating; phase imbalances would be worse for the motors (you didn't say if all 3 phases tend to fluctuate in synchronism).

More likely reason for insulation resistance to decrease is moisture or other contamination. How well sealed ARE the motors?

Hi PeterT : Many thanks for your reply. There are 3 new same brand motors all with drop in Ohms , but they have not failed because if this . 1st motor down to earth - 150mm2 x 350m cable damage with water ingress ; 2nd motor - Grid power surge , still running but tripping often on low voltage; 3rd values dropping but operational .

2 phases balanced , but 3rd 5-10Volt variance, Motors are clean water insulated . Maybe as you say moisture contamination!! as 1st cable , 2nd cable is also damaged with moisture ingress ???

The 150mm2 cable is Red Line but used in Mine Shaft installation .

Regards Dave

you have cable problems. Any splices in the cable?

If you have a unit that has failed (unit 1?), remove the cable from the motor about 6 inches above the connection to the motor and test motor & cable separately. If there is a splice, then cut that out and test motor, splice and cable.

Does insulation resistance go down when insulation is hot?

Is motor hot after running 12 hours?

Hi Carl - On commissioning in Jan, the cable did warm up , but have not checked the resistance to heat reading ! what would happen if so ?

The motors are submersible (Bore Hole) and generally submerged min 6.0m below water level . Water at ambient 24deg C . The installation is down a mine shaft approx 3m x 5m. Depth of Shaft 500m .

Regards Dave

The water temperature should be recorded at the end of pumping occasionally because over-time it increases and affects the IR value. Even water level should be measured occasionally before and after pumping to see how deep the pump is immersed in water at the end of pumping to see whether water level is dropping.

Yes, your unstable grid voltage is probably much worse than you realize. That range of variation is 12.7% and most designs are intended to stay within 10% variation of nominal line voltage.

If you are using a conventional meter to measure these fluctuations, it is likely that your situation is worse because most meters are designed to work with a well shaped sinusoid of 50 to 60 Hz. A grid that supplies a lot of DC power supplies may suffer from odd harmonic distortion.

Low voltage and odd harmonic distortion will contribute to overheating. High voltage will contribute to insulation breakdown resulting in short circuited windings. Perhaps you would do well to invest in an inverter grade motor and some line filters.

Agreed !! Our National grid is severely overloaded and a 20% variation is not uncommon with no recourse !!! Would such variations cause the insulation breakdown ?

I have suggested voltage regulators & monitoring equipment to analysis further. Thank you for the input !! Dave

20% variation of voltage in itself, would not cause the insulation to fail, but it does warm up the motor to an undesirable level, "voltage drop - increase in current". That does have the effect of shortening the motor/pump run life.

Most manufacturers only recommend a variation of no more than 10%

I have send you a private message.

I would check to make sure the motor is operating within temp range limits....that the motor is sized properly.....that the motor/pump is not being overloaded....That the maintenance has been properly performed.....would record voltage and current usage during regular day of work...any failed units have forensic analysis done...

Thank you , Pump & motor are selected correctly and the pump is now operating at BEP , & believe installed correctly . The maintenance team have been diligent todate . I have suggested that voltage regulators be installed & upgraded protection panel be installed to monitor all parameters . Regards Dave

Insulation resistance should be measured at the same temperature before and after running the motor,otherwise a correction factor should be applied.

No not really.

how deep is this motor/pump, what is the operating temp? Are you using a DOL switchboard or VSD? Is it a true ESP motor or something adapted? who made it? do you have data sheets for this motor? what about pump curves and motor loads? Is it new or used? where was it used before? any data from previous use?

Yes I know your head is spinning from the questions, but I am wondering why you have continually tested the motor AND cable. Did you suspect a problem BEFORE installation, hence the testing?

It is not recommended by ANY pump manufacturer to continually test the insulation of motor and cables, as it shorten the run life.

As for power fluctuations...your motor controller (do you have one?) should be set up to control/protect the motor.

I just answered a question for Saudi Aramco on the same subject.. what is an acceptable valve.

any reading above ZERO, if the pump runs and your test is not a short to earth, and it starts.. let it run!

Folks test ESP's and think they are only testing the cable or the motor, never both! What about the splices, and the other components in the electrical circuit that can and do fail?

Maybe your motor is degrading/failing.. your continual "Meggar" testing has not helped. Leave it run until it fails! Then buy a new one!

The project was to dewater a flooded mine, 500m depth, Pumps selected 160m3/hr @ 300mH, BEP. Units are genuine new largebore 10" submersible units , motor manufactured in Turkey . Actually have limited data on the motors . Controller is a standard Star Delta panel but have recommended an upgrade to monitor / protect more parameters .

I selected & commissioned the units, but can not answer as to why the client is continually testing the motor & cable (Maybe overly cautious) . Due to the limited resources we are faced in this country all are very cautious .

Agreed on letting the units run .

Thank you for your comments most appreciated & valued .

Regards Dave