Effects on Motor Running at Half Rated Volage? 1

[Laplacian]

Background:
Plant has four identical motors with the following nameplate data:
Manufactured by GE with Aluminum rotor bars
2500HP
4000V
1.15SF
Frame 8511Z
Code G
1185 rpm

3 of these 4 motors were later re-tapped to run at 2300V with 6 leads brought to the terminal box instead of the original 3 leads.

Situation:
One of the motors operating in 2300V service experienced failed bearings and a galled rotor shaft. Maintenance personnel installed the spare motor that had not been converted for dual voltage service. The motor tripped on overload three times; the third trip emitted some smoke.

All electrical tests yield good results. There is evidence of one particular stator coil overheating (slightly darker orange than all other windings) and the paint is blistered off the rotor.

My question is what exactly occurred in the motor (both stator and rotor) due to operating at half voltage? I know what happens plus or minus 15% of nominal voltage based on graphs published in reference manuals. What do these curves do for +/- 50% nominal voltage? I have found all relevant equations regarding stator currrents for varying voltage, but can't find general equations for stator currents.

I'd appreciate any technical information for use in determining re-winding the motor or accepting it with the visible signs of overheating.

 

[motorsdirect]

I would imagine, that if your motor was working on load,that if you only supplied half voltage,it would never get up to speed, hence it would be working very hard with little or no cooling, and overloading as it would be trying to get up to its synchronous speed(almost a stalled situation)...Hence the windings would cook themselves!!

Jeff

http://www.motors-direct.co.uk

 

[Skogsgurra]

Yes. Motorsdirect is correct. The peak torque is only one fourth of normal peak torque. So the motor will probably run at a low speed (=enormous rotor losses) if it runs at all. In both cases you will have lots and lots of heat in both rotor and stator. I think that it was a foolish thing to do.

 

[edison123]

I assume when you say

"3 of these 4 motors were later re-tapped to run at 2300V with 6 leads brought to the terminal box instead of the original 3 leads."

You mean the original 4000 V star connected stator (3 leads) was rewired to run at 2300 V delta connection (6 leads). This was done because your incomer supply voltage to motor was changed from 4000 V to 2300 V for some reason ?

If so, why only 3 motors and not the 4th one was reconnected to run at 2300 V?

And yes. If you connected a 4000 V motor to 2300 V supply, assuming that your motor is running near full load, the motor starting torque will only be 1/4th and your running current will be very high (3 O/L trips confirm that).

Change this replaced motor winding connection to 2300 V before you restart.

 

[jraef]

Like edison123, I have more questions about your question than I do answers. Why did you have the motors re-tapped? And what did they do to re-tap them (this is not a standard proceedure if the motor was not designed for it in the first place). If it was 4000V and they "split" the windings, you would end up with a 2000V winding pattern. 4000V windings are usually used on things like 4160V deep well pumps or other loads where the motor is a long distance away from the power source, so that voltage drop has less effect on the motor. If you split that winding to 2000V, and connected the motor to a 2300V source relatively close to the motor, you would have a serious overvoltage problem. 2300V is 15% higher than the motor winding rating if that is what you did, and in many areas, 2300V is the "utilization voltage", but the voltage supplied by the utility is often 2400V, so then you would be 20% over the 2000V rating.

Lots of "what-ifs" here.

"Venditori de oleum-vipera non vigere excordis populi"

 

[mc5w]

First of all, operating a 4,000 volt motor on 2,300 volts would be like the wye phase of a wye delta starter which would cause the motor to develop 1/3 of normal torque and cause the motor to draw 1/3 of normal locked rotor current. If you leave the motor in that state for too long it will trip the motor overloads. Wye delta starting is only intended to get hte motor up to 1/4 or 1/3 speed where it actually has a chance of developing enough countervoltage to withstand full voltage without knockingn out the lights.

When I was working for a company that had 2 plants about 2 or 3 miles apart, somebody replaced the single phase motor in a 120 volt fan without bothering to check the taps on the new motor. What happened was that the internal motor overload cycled on and off. Almost ALL replacement dual voltage single phase motors are connected at the factory for the higher voltage on the basis that the wrong voltage will trip out the overload and the lack of performance will be obvious.

Now, if you apply TWICE the voltage of the connected tap, then the motor will almost immediately smoke.

Mike Cole, mc5w@earthlink.net

 

[busbar]

If they haven't been evaluated for rating and proper operation already, the running-overcurrent/motor-overload relays need examination.

 

[Laplacian]

Sorry for all the confusion. A little more background:
2 of the motors were running extruders, the other two were spare. The extruders were upgraded to 3500HP and the two 2500HP motors were moved to different extruders to upgrade them from 1500HP to 2500HP.

One spare motor and the two in service motors were reconnected as 2300/4000V. The orignal service was 4160V; however they were moved to a location with existing 2400V switchgear/starters. The remaining 2500HP spare can still be used as a reduced capacity replacement for the 3500HP load should one fail.

These motors are ~ 25 years old and the reconnection work was done in 1988. This is nothing new to our local motor shop. The motors have been very reliable as bearings are the only failures to date on them.

I don't know the original winding connection; right now they are dual voltage wye/delta with NEMA 6-lead connections in the terminal box. The motor shop added the dual voltage nameplate under the original mfg. nameplate when the modification was done. Looking at the stator end-turn windings, it is apparent that the lead joints were cut, reconnected and re-tapped/varnished and that it was not a factory job.

 

[aolalde]

Getting back to your original post:

“My question is what exactly occurred in the motor (both stator and rotor) due to operating at half voltage? I know what happens plus or minus 15% of nominal voltage based on graphs published in reference manuals. What do these curves do for +/- 50% nominal voltage? I have found all relevant equations regarding stator currrents for varying voltage, but can't find general equations for stator currents. “

The voltage applied is 1/1.732 not one half; any way the speed torque curve is reduced to 1/3 of the design torque. The Stator Current curve before the breakdown torque is reduced to 1/3 too.

For instance at locked rotor, the current is Iln= 1/3*6*Ifl = 2*Ifl (two times the full load nameplate current) this current will not drop significantly until the motor reaches the breakdown torque peak.
Since the speed torque curve decays to 1/3 most probably the motor will stay struggling to accelerate unsuccessfully. The rotor current follows the stator current proportionally so you will expect the rotor handling double of the full load current too.

Under this condition the winding losses ( stator and rotor) are increased up to 4 times the normal loss at full load, following the second power of the increased current , Winding-loss= R*(2*I)^2

That is why your windings overheated. Darkening of the winding means an elevated temperature above the insulation rating. The winding life expectancies are halved by each 10 °C over the insulation class. 100 °C excess temperature means the life is reduced to 1/(2^10) of the expected life (1/1024).
If the insulation tests OK I will work that motor (at the correct voltage supply) but schedule a close rewinding.