1000 HP,Synchronous Motor 1

[aolalde]

I have a Synchronous Motor coupled to a Ball Mill by clutch. It starts uncoupled and after it synchronizes the clutch engages the load.
Nameplate and other data and parameters are:
1000HP, PF 1.0, 4160V, 60HZ, 180 rpm, 60°C Rise Stator, 70°C Rise Rotor, Excitation 125V, kVA 800, 111 Amps armature, 125 V field, Exc. Amps 158, frame 9404, GE.
180 coils, 40 poles, 1.5 coils per group, 2 circuits wye. Lap winding.
We cut out 2 coils, each located in different phases.

Q. What is the expected performance of this motor under full load ( 1000HP) and this unbalanced winding condition? Temperature rise, Currents, Torques and Vibration.

 

[aolalde]

The coils were caped because they developed a short circuit to ground and the production requirements do not allow time for rewinding.

 

[jraef]

I was just about to ask. I'll be interested to know this as well. I was involved in a similar situation and never thought to "cap" the coil windings. Interesting solution.

"Venditori de oleum-vipera non vigere excordis populi"

 

[edison123]

with 2 ckt wye, may be you should cut out same no. of coils from each of the parallel ckt of each phase to prevent circulating current.

To minimize uneven mag pull, cut out coils should be as much diametrically opposite as possible.

Because of reduced turns, flux density and the no-load current will increase. Higher flux density will increase core temp and generally a higher winding temp also occurs. The winding RTD's located in the cut out coils will not read the acutal winding temp and should not be used for temp monitoring.

In my 20 years as a rewinder, I have also done some coil bypassing in an emergency situation but I have found that the cutomers let them ride in the same state until disastorus failure followed shortly.

If this failure had resulted in severe core damage, cutting out coils may not help since the damaged core area may reach very high temp and by conduction, heat will flow to healthy area and damage the coils in that zone.

 

[electricpete]

Aolalde mentions maintaining balance.

When we cut out a coil in a single circuit wye, the shop recommended to cut out one from each coil for electrical symmetry among phases.

edison points out need to maintain electrical symmetry between wyes, so maybe you should cut out one in each phase of each of the two wyes (6 total)? Does it make a difference if there is equalizer between neutral of the two wyes? I'm thinking and not sure at the moment.

As edison mentioned, sounds like good idea to keep mechanical symmetry when selecting coils to cut to the extent possible to minimize u.m.p.

One would think with a fractional slot winding 1.5 coils per group, it would be far better to cut out a coil from a group with 2 coils (leaving 1) than a group with 1 coil (leaving 0). In fact I'm pretty sure you'd be asking for trouble to cut out a coil from a single-coil group.

aolalde - can you send an email to the address listed on my profile? I'm not all that familiar with this subject but I do have some detailed info in pdf format.

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[edison123]

pete,

If possible, mail me your article.

thx.

kuamr

 

[jbartos]

Suggestion: I agree with postings suggesting a removal of pertinent windings to achieve the symmetry thus avoiding vibrations, noise, etc. If possible, reduce the motor terminal voltage in a proportion to the removed number of turns in windings. This will approximately equal the no-load current of the healthy motor. Beside this, it will prevent any overheating due to the larger magnetization current. The torque will be somewhat reduced since the motor terminal voltage was reduced. Normally, motors tend to be HP rated higher than required by the motor shaft load. In that case the motor may be loaded just about right. If this is not feasible, do not reduce the voltage; however, increased cooling would be beneficial.

 

[aolalde]

Thanks for all your help especially to electric-Pete.
We decided to leave this motor running with only two coils from different phases removed; this was due to the short number of coils per group. It has half of the groups with only one coil. Attempts to balance the circuits will result in dangerous loose of the pole array and saturation since the voltage can not be reduced.
The motor is running with satisfactory performance since we reduced the load to 87%.
The temperature is normal, the currents are around 15% unbalance and vibration has increased slightly. We hope the motor will be rewind in the next outage.

 

[jbartos]

Question to the previous posting: Please, how was the load reduced to 87%?

 

[aolalde]

Load was reduced by reducing the balls weight.

 

[jbartos]

Comment on the original posting marked ///\\Q. What is the expected performance of this motor under full load ( 1000HP) and this unbalanced winding condition? Temperature rise, Currents, Torques and Vibration.
///It would be better to have this modified motor simulated by some motor manufacturer for available output parameters. Visit

http://www.tecowestinghouse.com/Products/Custom_Engineered/Synchr_Spec.html

and send them specification\\\

 

[aolalde]

JB.
This is a motor manufactured by GE.
Will Westinghouse be interested and take responsability to calculate the performance of a modified competitor motor?

 

[jbartos]

Comment on the previous posting: It does not hurt to ask a question. Of course, GE branch(es) may be preferred or already contacted.
GE Company, GE Industrial Control Systems
2901-T E Lake Rd.
Erie, PA 16531 0001
USA
Tel: 814-875-2234

http://www.ge.com

Or call: 800-626-2004

 

[aolalde]

We just finished the new winding of this motor. Coils and winding materials were ordered in advance and the motor was re-wound at operating site in 4 days, this reduced substantially the money loss when the mill is out of production.
The motor work with unbalanced phases for around 40 days and did not show signs of potential failure. Any way it was rewound to prevent a catastrophic failure.

Many thanks for all the help and opinions provided.

 

[Skogsgurra]

Good to have the feed-back, aolalde.

You did not say if the motor performs to specifications now. I assume it does.

 

[electricpete]

Thanks for the feedback. Sounds like a critical machine has been handled with very minimal impact.

My gut would have said cutting out one out of 2 coils in a group would be big trouble. So much for my intuition.

4 days for a full rewind of this big slow-speed motor at customer's site? Impressive.

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[light1]

What was a GE recommendation, if I may ask?

 

[aolalde]

Skogsgurra (Electrical
Yes, now the motor performs to specifications. It is driving 110 amperes per line at 4160 Volts, 1.0 PF.

light1 (Electrical)
They had to make a complete test and evaluation, we did not have time for that.

 

[aolalde]

EPete.
The Analysis of performance of synchronous machines operating with unbalanced armature windings, provided helped a lot. Thanks.