Motor Failure under single phasing 2

[prc]

A star connected induction motor, 2.2 kW 400 V 3 phase supply, integrally connected to an oil pump,failed with over current.Oil is circulating through the stator of winding,therby getting cooling.

Instead of a no -load current of 1.8-2.2 A, motor took 3.6,1.8 and 5.52 A current in three lines.Full load current 5 A and the motor is protected by a 6A MCB.

On inspection, stator overhang area is burnt.What can be the probable cause of failure? Vendor says it is due to single phasing and overvoltage of 2.5 kV might have come which caused sparking from stator to ground.Difficult to accept that loss of a phase can cause such failure.Can the experts explain the probable failure mode?

 

[ScottyUK]

What's the problem with believing single phasing as a cause? Is there something apparent on the motor which leads you to suspect this isn't the cause? A motor which is already rotating may continue to rotate if it loses a phase. A pump would be a good candidate because it would tend to slow down and deload until the limited torque available from single phase supply could maintain motion. It would be running at a high slip which would cause some additional heating in the rotor.

Why isn't the motor protected by a thermal overload relay? MCBs are a poor match for a motor damage curve, which is one reason why they aren't normally used for this duty.


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If we learn from our mistakes I'm getting a great education!

 

[prc]

Scotty, loss of one phase is the conclusion by the manufacturer.Will the overhang of the stator get burnt by loss of one phase out of three?There is no heating found on cast rotor.This is a cooler pump used in transformers.What type of protection is recommended for such pump motor?

 

[electricpete]

If the motor has been single-phased, you will see stator overheating on 2/3 phases (if it's a wye) or 1/3 phases (if it's a delta). The endwinding is the easiest place to see the damage. There are some pictures of that somewhere...I'll see if I can dig that up. (By the way do you have any pictures?)

It's true that unbalanced voltage poses some unique challenges for rotor due to high frequency and skin effect of the reverse rotating field. That produces heat in the rotor which can also end up raising stator temperature. Additionally, the unbalanced voltage produces negative torque which must be overcome by additional positive torque, so the motor draws more stator current to drive the same load. The rotor tends to be a lot more beefier and it's not surprising to me that the stator gets overheated and not the rotor.

MCC-fed motors typically include a combination starter with thermal overload protection. I have seen the transformer fans and pumps are usually not fed by an MCC and instead simply protected by long-time protection built into the breaker in the control cabinet, which as Scotty mentioned will not be as closely matched. Unbalance and open-phase is tricky to protect against to begin with since the amount of heating (including rotor effects) may not be truly reflected in stator current magnitude.

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(2B)+(2B)' ?

 

[electricpete]

Some photo's of stator damage apparent from the endwinding for various types of faults:

http://www.firstelectricmotor.com/motor failure.htm

http://www.compsys.com/drknow/aplpa...14510be34cdc29a0852567ec005ad246!OpenDocument

Hopefully you can see what I mean that single phasing can be evident from the endwinding.

Of course single-phasing can be an initator outside the motor from the power system, or it can be result of motor internal failure. You mentioned spike to 2.5kv...not sure how that fits in but certainly can be the beginning of many types of failures.

I think it would be tough for anyone here to judge the failure cause without a lot more info: pictures, test results, conditions at time of failure, power system grounding, etc. I would tend to trust the people that looked at the winding (assuming they were provided sufficient info about the failure) unless you had other evidence to contradict what they said. But certainly doesn't hurt to talk about it and more info will give better responses.

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(2B)+(2B)' ?

 

[ScottyUK]

What type of protection is recommended? I would suggest just a standard motor starter - nothing unusual about a circulating pump.

I'll guess this is an OFAF or ODAF transformer, and that the pump is energised as a second or third cooling means after the transformer operates in ONAN and ONAF modes at lower temperatures. Presumably the motor is switched by a contactor, in which case it would be a trivial exercise to add a standard thermal overload relay. Most transformer pumps would be quite happy with a Class 10 overload because they run up to full speed quickly. Upstream of the contactor would typically either be a breaker or a fuse, providing short-circuit protection in addition to protecting the wiring within the compartment against overload.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

A lot of motor overload relays include unbalance protection as a standard feature. In may designs, the unbalance protection is capable of detecting and tripping on unbalanced currents that are less than full load current.

Bill
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"Why not the best?"
Jimmy Carter

 

[prc]

Thank you pete and scotty.Iam attaching the photos of failed stator.

 

[waross]

Difficult to accept that loss of a phase can cause such failure.

Accept it.
There are two failure modes.
On single phase, the motor has about 1/3 the torque. As a result it slows down a little. But when it slows down, it slides down the torque:current curve. One phase must now supply enough current to develop the torque to drive the load. If the load is much more than 1/3 of full load, the energized winding may fail.
This failure mode occurs when the phase lost is limited to the motor circuit.
If an incoming phase is lost and the motor is connected to a panel with other non-motor loads, then the motor acts as an induction generator and attempts to supply the loads on the missing phase. If "B" phase is lost, you will be supplying loads on both "A-B" and "B-C". So expect the backfeed current on the motor "A" phase to be about the sum of the backfed currents on "A" and "C".
This failure mode may cause motor failure even with no mechanical load on the motor.
This effect may be useful if the current is within the thermal capacity of the motor.
That's how a rotary phase converter works.

Bill
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"Why not the best?"
Jimmy Carter