Uneven Poles in Synchronous Motor Rotor

Wow, knowing almost nothing about high-current electricity, I can say this: I would say that, with that size motor, "uneven poles" is quite possible. I would say that the problem is that the rotor assembly's center of rotation is not the axis of the assembly; the rotor is defective. Where was the rotor made?

You'll probably get good vibrations right before the magic smoke is released.

Can you confirm your suspicions?

You can't expect expert help without some real analysis.

You should NOT expect help with paying for it locally, or from your motor supplier.

Miracles cost money.

- Rotor Made in FRANCE by GE

- I am just want to know, what will happen if we have uneven (3 North & 1 South) poles in synchronous motor?

- Motor reaching full speed after putting excitation also and pf remains negative at -0.03 & vibration after excitation switched on

This is out of my area of competence.

But a negative power factor is not normal.

I hear my mother calling me.

https://en.wikipedia.org/wiki/Power_factor

http://www.ccontrolsys.com/w/Negative_Power_Values

Looks like there is an indication for trouble!

Sure sounds like shorted windings. GA

What you are describing sounds as if you are slipping poles. That is an abnormal and dangerous situation with a synchronous motor. Stop using it now and call in an expert on synchronous machines. This is not something that an inexperienced person can handle with free advice over the Internet. It needs a. Expert on site who can investigate it thoroughly. Do so immediately.

You do not give enough information, is this a new motor or a rewind why would you suspect uneven poles?

If the motor runs normally in asynchronous mode and starts vibrating in synchronous mode, then the issue is likely to be the rotor circuit it could be a power diode or a rotor winding fault call in an expert.

This is a brand new motor and first time we are doing the testing, after smooth starting of motor as a induction motor and applying excitation at near to sync. Speed, huge vibration comes. We are not able to increase pf from negative -0.03 to up to decrease current

We have tested exciter panel which is working fine. We have tested exciter rectifier which is also working fine and then we have tested rotor poles by applying low voltage high DC current to check the pole formation at pole shoe. We checked poles by campus at pole shoes and we found 3 poles as a "North Pole" & 1 Pole as a "South Pole".

Now I want to know, is the uneven poles which we suspected by testing, what will be the behavior of motor in these case to confirm our doubt of uneven poles in rotor

Regards,

T.D.

What did you use to check the polarity of the poles, a Boy Scout compass?? You have not done the first test that a professional would do, checking the integrity of the rotor winding! Do you know the winding parameters, basics like the factory test parameters, cold resistance, inductance, and insulation values, etc. My money is on an improperly wound, faulted, and/or connected field winding.

Call in people who know what they're doing before your next "Gee, let's fire up this sucker and see what it can do..." test destroys the bearings, forces the rotor into the stator, and/or tears the motor free of its mountings. I see a Darwin award in your future or maybe a pink slip...

We have already performed, Rotor Winding Resistance and IR test and found ok

Your problem has all the signatures of a magnetic problem so let's concentrate on the physical aspects of the rotor.

Is this a round rotor or salient pole, does it have amortisseur (damper) windings/bars, how was the rotor shipped, have you checked the retaining rings, torque of pole shoe mounting bolts, proper fitment of the rotating diodes, straps, insulators, magnetic centering of the rotor, thrust bearing misalignment, what axis is the vibration the greatest, vertical or horizontal mounting, what is the nature of the load, steady/pulsing, have you used a gaussmeter to verify the uniformity of the external field, etc., etc.

Remember that a rotor at rest may test perfectly and come up to 95% synchronous speed perfectly, but when the MMF from the field combines with that of the stator the forces can shift components enough to throw it out of balance, and that you cannot easily measure the electrical condition of the rotor to check for winding shift and subsequent insulation failure, shorts turn to turn, or to ground when in motion, and they often revert to normal at rest.

I agree that rotor behavior changes with speed. Therefore manufacturers conduct impedance test from standstill to rated speed and back to standstill. OP should have this data (or should ask for this data) and perform this test (hope this is not brush less excited generator).

The symptom is consistent with the winding on one rotor limb being energised to oppose - instead of add to - the magnetisation of the winding/limb directly opposite.

Going back to the question in the last part of the original post.

The rotor may be imagined as two bar magnets crossed to make a + ; set so a stationary winding will see poles in order N-S-N-S as the rotor makes one turn.

If the bar magnets are of unequal strength, one can imagine the 50 Hz induced in the stator windings being imperfect with unbalanced currents and torques.

The weaker magnet can be considered as one of the correct ampere-turns, combined with another small magnet of opposing ampere-turns [or as an electro-magnet in which some of the turns are shorted-out by a fault].

The whole can be seen as a correct rotor with a "pirate" magnet [two poles N & S] added - which "pirate" will behave as a two pole rotor in a four pole machine and induce 25 Hz voltages in the stator in addition to the correct 50 Hz voltages.

SHOCKHISTAN writes that he has known wrong-connected windings from this maker, which could make one bar of the cross effectively not energised - a two pole rotor alone in a 4 pole machine.

The sketch shows the rotor with one winding energised in the wrong direction.

3 north poles and 1 south on the outside.

67 model

What does the Factory Acceptance Test [FAT] Report say on the topic?

• The equipment should not have been shipped before a satisfactory FAT has taken place, as it is at the vendor's workshop that the easiest solutions to any issues may be found!
• A piece of equipment this size should have been accompanied by vendor commissioning service as part of the package, on account of purchaser unfamiliarity. It then becomes the vendor's responsibility to ensure customer satisfaction with the as-installed equipment.

It sounds as though a vendor service call-out is the way forward.

It is definitely case of shorted turns in one of the poles. Do you have impedance measurement data (at manufacturers works)? If yes, you should repeat the test and compare the result to confirm that rotor pole turns are shorted. Some times this test is done at higher frequency for higher accuracy. But power frequency data is also given. You may like to look into test report of test done at works before dispatch of the machine.

Brand new. You should be talking to the manufacturer.

Have you paid the vendor yet?

A Brand New 17.5 MW Synchronous motor, the manufacturer should be all over it. Anything you do could compromise the warranty, this is a no brainer call them in now.

Synchronous motors " synchronize" the stator rotating magnetic field to the static DC rotor magnetic field.

Each "north pole" from the stator fields attracts a "south pole" in the rotor and each "south pole" from the stator attracts a "north pole" from the rotor.

If one rotor pole is reversed a strong repulsion force from the stator field is developed, this force fighting against the attraction of the correct polarity poles. Synchronization is never achieved and severe destructive vibration developed.

It is possible that one of the coil windings has been connected in "reverse" polarity.

I have observed this in more than one motor when I worked for GE Service and the symptoms can definitely mimic the operating condition you are experiencing.

Coil winding assemblies are usually wound in the same direction of rotation with the coil-to-coil connections reversed top-to-bottom around the rotor or stator (if wound type) to create the North-South-North-South .... poles.

Example:

T-Lead to the bottom of the first coil, out the top of the coil and connected to the top of the next coil then out the bottom of that coil connected to the bottom of the next coil and so on around the motor.

The condition is fatal and I strongly suggest you get GE field service on site to repair the motor before you attempt another start.

It is also possible that the rectifier/DC supply has failed and is passing AC voltage to the rotor.

Either way you really need to get GE field service on site to identify and address the issue.

It is possible to check the winding magnetic poles by applying a low voltage DC source to the stator coil leads and/or the rotor coil leads, then pass a handheld magnet under each coil center to determine the number of poles and the polarity.

However, this method requires removal of the rotor from the stator which is labor intensive and should be reserved for experienced, competent large apparatus technical repair personnel.

To check the rotor excitation voltage for AC you can lift/remove the brushes, defeat the starter/contactor from closing, then apply the DC exciter and measure/analyze the voltage waveform at the starter for amplitude and quality.

Hope this is useful to you.

Good luck and always stay safe.

This article about torsional vibration of a 12 MW synchronous motor at start may be

interesting :

http://www.ge-mcs.com/download/orbit-archives/1996-2000/March%201997/397swan.pdf