Motor Failure During Starting

Do you actually have a motor failure or is this theoretical failure modes when employing reduced voltage starting?

Ask a question that makes sense.

"What causes motor failure during reduced voltage starting"

Does the motor FAIL, never to be used again?

Does the thermal OL trip?

Breaker trip?

I have many references, all quite good, thank you.

davechow,

Give the forum COMPLETE motor information and how the motor failed.

More information: better answer.

Motor current is highest when a motor starts. Once it gets up to speed, it develops "back emf" which limits the current draw. If the voltage is too low, the motor never gets up to speed (or may not start at all under load) and continues to draw large current until the windings overheat.

https://en.wikipedia.org/wiki/Counter-electromotive_force

The synchronous motor is 10,000 hp @ 12.47kV. It has failed during starting using 30kVA autotransformer 80% tap. PF= 1.0, H = 3s, 3600rpm, motor relay has tripped on differential protection due to current rise in phases.

Synchronous motors only operate at synchronous speeds, in your case 3600 rpm. They have to be spun up by various methods as described in the link below.

http://electricalengineeringtutorials.com/synchronous-motor-starting-methods/

Which of these methods does your motor use?

damper windings

What we've got here is... failure to communicate.

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Sounds like maybe you had a transformer arc or short...it needs to be checked...

http://www.electrical4u.com/differential-protection-of-transformer-differential-relays/

Have you checked insulation resistance (meggering)?

http://www.netaworld.org/sites/default/files/public/neta-journals/NWFA06_Gregorec.pdf

In simple arithmatic a 30kva transformer at 12.47kv will deliver 2.4amps - which to my mind is not enough to run a 10,000hp motor.......

autotransformer

I doubt your motor has itself failed. If motor did fail then either a mechanical failure (bearing wear, obstruction) or damaged stator windings will give you tell-tale sounds or smells to indicate why it failed. It is far more likely the mechanical loading is over sized for use with this under sized voltage.

There is always a possibility that this is just an ESO condition.

As others have said, you need to define "failure"; i.e., the failure mode. Did the motor fail to reach synchronous speed, accelerate the load, operate as designed, carry the load, etc., or did the motor fail electrically or mechanically, bearings seize, windings overheat/burnout, high vibration, protective relaying operated unexpectedly, shaft snap, motor come off its mounting, etc.

We have plenty of references for those events and more, but more information is required from you.

I doubt you meant to say it was a 30kVA transformer on a 10,000HP motor. If that was correct, you need to stop whatever you are doing, slowly back away and don't touch anything else until a qualified professional arrives...

One of the possibilities is the required motor protective equipment is either incorrectly set, inadequate for the job, or completely absent. Another is that there is some form of shortcoming within the motor itself.

However, these are symptoms of earlier problems further upstream. The cause of them could be any combination of:

• The capabilities of the Designer are not up to the required standard
• The capabilities of the Installer are not up to the required standard
• The capabilities of the Maintainer are not up to the required standard
• The organisation's training is not up to the required standard
• The organisation's equipment reception, storage and mechanical handling procedures and the personnel that carry out these functions are not up to the required standard
• The equipment supplier's quality control is not up to the required standard
• The equipment's commissioning was not carried out to the required standard
• The organisation's Management is not up to the required standard
• The organisation's Human Resources personnel are not up to the required standard
• "Acts of God", such as, and not limited to, fire, storm, flood, tempest, extremes of temperature and humidity, earthquake, building collapse, and so-on
• Acts of war - how about riot, or has the motor been used for target practice recently by friend or foe, (rhetorical question - NNTR)? Etc., etc.
• Acts of commercial competition, such as, and not limited to, sabotage, dumping of sub-standard product, etc., etc.
• The motor having previously been used for peaceful purposes for something other than for what was intended, such as, and not limited to, a battering ram, as ballast weight for a tower crane or a boat, to prop open a door, as an anchor for a boat or aircraft, etc., etc.
• etc., etc.

The reference material is the motor itself and its history, none of which can be seen from here.

Good luck with the investigation.

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Homework!!

Your relay should tell you which motor phase appears to be (or is) faulted. The autotransformer should be outside of the differential relay CTs.

For summing CT differential protection, we assume no one has changed out current transformers or relay taps since the unit was successfully started, assuming it has run at one time, i.e. this is not the initial startup. Different CTs saturate at different current levels, so the current you measure going into one phase may not appear to be the same current coming out of the phase, especially during starting.

Check your CT & circuit resistance to make sure you have no loose connections or open windings. A good relay tester will have the equipment to do a primary injection test, worth the effort to test the relay, as opposed to tearing down the motor.

The above does not apply where the leads for each winding are brought through the same current transformer.

Failing that, you need to look at the motor, and decide if the insulation has been compromised. At a cost of perhaps $400,000, exclude other possibilities first. As Rixter notes earlier, excessive starting time could be a root cause. Besides the current that heats up the stator, leading to insulation failure to ground or another phase, you also add heat to the rotor with extended start times. However, the motor should already be protected for that, and should not allow for more than a few starts per hour.

If the differential relay is operating correctly, you should be able to measure/test the motor, and find the fault severe enough to trip the relay. Some differential protection with summing CTs will add time delay to prevent false trips during starting. A reduced voltage starter just adds more time & stress to the motor.

Sorry, It is a 30MVA autotransformer. The failure is in stator insulation winding. Mainly in phases A & C.

OK, new answer, commensurate with the amount of information given...

Stuff happens...

So did it fail turn to turn or turn to ground, and how did you determine the cause of the failure?

Ask the company you are going to contract to get the motor repaired, they may have an analysis service or could perhaps give you a guess based on actually seeing the damage.

It goes without saying that you should not attempt to repair the motor yourself.