Motors Failing - 480 Volt, Three Wire Service 1

[Robert789]

I work for an electric cooperative. We have a customer who tells me that he has had to replace a three-phase pump motor 5 or 6 times within the last year. The pump is supplied by a 480 volt delta service (three phase, three wire).

I have not seen the motor yet (its down in a well), nor was the customer able to give me much information about it. Our serviceman tells me that he can find no problems on our side of the service entrance.

I have suggested to the customer that the original motor might have failed for some reason, and then been replaced with a motor which has wye connected windings. Am I correct in thinking that only a motor with delta connected windings will work properly on a three wire service? I am thus far unable to find any information in textbook or electronic form that will confirm this. Any helpful comments would be appreciated.

 

[buzzp]

I dont believe the internal winding configuration of the motor will mandate delta or Y be used on the service entrance. It is more likely related to such things as:
motor over sized for the application- this sounds like a
submersible so there needs to be adequate fluid
flow to keep the motor cool
motor undersized- normal running amps are higher than the
FLA (full load amps) on the motor nameplate
cabling problems- if this is a deep well and there was no
calculations done as to the voltage drop of the
cable
Incoming power- This could be low or high voltage
problems and doesnt necessarily have to be low or
high all the time. large load fluctuations could
cause temporary low or high voltage conditions.
Unbalanced voltage can cause over current
problems as well.
Something that you should ask him to do is measure all three line currents and make note of the phases. Also, measure the line voltage loaded and unloaded. If the running amps are compared to the nameplate amps this will be a good indicator as to if the motor is sized properly. Don't count out voltage problems because at the time they were measured they appeared to be ok. Also, does he have any protection on this motor besides an overload? If you post the data above then I could be more specific as to the nature of the problem. Thanks

 

[dydt]

the supply voltage does not require a motor to be delta or wye connected. as long as the voltage and phases are matched.

assumming that there is no problem on your end,i.e., severe voltage fluctuations and quality of power supply, your customer should check the following..

are they over loading the motor?
is it the right size for the job ?
any water or contaminants going inside the motor?
are they using any overload protection?
if there is, is the overload coordinates with the size of the motor?
did they monitor the current drawn by the motor and compared it with the nameplate rating?

i dont think there is something with the power you are supplying, otherwise all motors connected to your grid will have the same problem like this.

dydt

 

[dpc]

I'll add a couple of things:

The motor internal winding is almost certainly connected delta, but this is not really an issue one way or the other. Virtually all 480V motor stators are delta connected and can be used on either wye or delta systems.

"Ungrounded" 480V delta systems are notorious for insulation failures in motors due to high voltage to ground. This can occur due to low level arcing faults and repetitive arc restrikes. Motor insulation is the weakest part of the system and will fail first for high voltages.

Have you checked grounding of the **primary** side of the service and do you experience frequent lightning strikes in the area?

Such a frequent failure rate makes me wonder about voltage transients, ferro-resosance, switching/lightning surges, etc.

Is this the only load on this service?

dpc

 

[TheDOG]

I would be checking the overload settings. Submersible pumps generally require a class 5 overload setting (6 x FLC for 5 secs) - but you will have to check with the supplier.

 

[busbar]

Assuming the motors have failed insulation to ground, I'll bet dpc has hit the nail on the head, if there is no intentional grounding at the serving transformer; i.e., either ungrounded delta or ungrounded wye (or even ungrounded-T for lower-kVA, single-can banks.)

Transient overvoltages riding on the ungrounded secondaries of transformers with medium-voltage primaries can be real common, extra expensive and positively maddening. Because they can be short-lived and not sustained, they can be tough to find with ordinary meters. [Once had a 1kV-rated meter display >1800V because of this problem.] This overvoltage is passed to everything else common to the transformer secondary. The problem most likely is initiated by a combination of switching and a high degree of primary-to-secondary capacitive coupling, and/or excess leakage. The fix for this is a {~~250-ohm ~~500w} resistor connecting the transformer XO bushing to the local grounding electrode, which should also be carefully verified as common to the meter/starter enclosures and the motor frame. If there is no wye-point access on the serving transformer, it can be created by an auxiliary grounding transformer.

Related problems and cures have been discussed at etips in: Thread238-3370 Thread238-6870 Thread237-7403 Thread238-9005 Thread238-10998 thread238-16620

 

[BJC]

Is the well runnign dry? If the pump periodically takes the water level down to low it could be loosing cooling. Submersabe pumps have to be under water. He could be drawing the water level down and just burning it up. Look for changes in water usage.

 

[Robert789]

Thanks for the replies.

The transformer bank is open wye - open delta, with no ground on the secondary side. Our practice is to corner ground all delta services now, but this installation has been in place for quite a while. I have asked one of our crews to go by and correct this.

Additionally, I stopped by the site and found the motor nameplate information on a sticker in the control panel. It is a 7.5 HP (5.5 KW) unit with a service factor of 1.15 and has delta connected windings. However, this is the only load attached to the service, and a review of billing information from the past year indicates a peak demand each month of between 8.0 and 9.9 KW. If all of this is going to the motor, then it appears to be overloaded by up to 80%.

Robert

 

[busbar]

If not already done, checks of running phase currents, and running and idle phase-to-phase and phase-to-ground voltages are in order. Excessively high, low or unbalanced ø-ø or excess ø-grd voltages can quickly damage stator insulation, especially in submersible motors with their inherently high flux densities. Are the overload relay/heaters properly sized for the connected load? Has the customer verified that his protective equipment is operating, and within the manufacturer’s tolerances? It’s normal to fix ‘undertripping’ motor-overload relays, but ‘overtripping’ relays are often unintentionally not questioned because folks don’t recognize the damage that can follow. With multiple failures, it may be time to closely review these items.

 

[kassad]

well rgmaxwell
the motor is undersized. A service factor of 1.15 means that the motor can be overloaded to 115% more than the rated HP. For a peak demand of 9.9KW you need to use a 15 HP (11 KW) motor. A motor smaller than 11 KW would be 7.5 KW and with a service factor of 1.15 the max overloading will be 8.625 KW and you also have to use some margin for the pump. So i think that using a motor of 11 KW would take care of the problem.

kassad