Keeping the medium voltage motor warm by applying 400V to the winding

[krisys]

Have anyone come across the method of keeping the medium voltage (say 11kV or 6.6kV) motor warm, by applying the low voltage to its terminals?

By applying the low voltage to its terminals, the current is directly passing thru the conductor and generating heat. Also, the shaft rotates slowly (say, 0.5 rmp), thus preventing the damage to the bearing and avoiding the shaft bending problem.

This is better than heating through the space heater, if you are keeping it for long term preservation.

Any experience or view?

 

[itsmoked]

I've never heard of the motors spinning while using a low voltage heating scheme. Aren't they frequently DC to prevent humming and any motion?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dogzzz]

A little different experience .
Keeping motor warm : Normally we have 1 year gap between design stage and commissioning stage for offshore vessels , thus before commissioning the motors and generators are being dried out .
-A low voltage 3 phase AC is applied , low enough so that the shaft cannot be rotated or in other words current do not reach to a value which can rotate the shaft.
-The applied current is switched On and Off at regular intervals and the insulation resistance is recorded .
-The insulation resistance decreases initially with rise in temp and continues to fall until a steady temp is reached
-When the machine becomes dry , the insulation resistance comes back high to normal levels.
If you plot a graph it shall become like as shown below .

Keeping generators warm : - We short circuit all the phases to earth at a low voltage such that stator windings do not exceed FLA.
- This earthing point connection which is provided by the switch gear vendor should not be used .
-this method is only possible if you are using a generator whose field you can control , manually , thus
the method is not successful for brush-less generators.

 

[edison123]

We can run many MV motors in our shop just with 415 V though they may take some time to get to full speed. The current, once it reaches the full speed, is so low and it will not keep the windings warm.

Aren't MV motors supposed to come with built-in space heaters, which is much easier way to keep the windings warm? You can always retrofit space heaters and they require only a few hundreds of watts.

Muthu

http://www.edison.co.in

 

[krisys]

dogzzz (Electrical)
It is a novel idea!

I have personally seen the 11kV, 3000kW motor when supplied from 415V the current was about 35A. The stator was standstill initially for sometimes when we were near the motor(say about 5 to 10 minutes).
After an hour when we came back to the location, we have noticed that the motor was slowly turning. May be 0.5 to 1 rotation in a minute!

The motor was decoupled. Someone tried to stop the shaft by holding tight. It even stopped!

By the way it is not a magic. The torque developed was negligibly small. It would be just enough to create a torque to overcome the frictional resistance.

 

[krisys]

edison123 (Electrical),


Yes, I agree. You should not allow the motor to accelerate to full speed. If you feel it is accelerating, lock the rotor, so that the locked rotor condition will exist and the motor will get warmed up.

If the shaft is rotating at ultra low speed (couple of rpm), then the locked rotor condition is still existing.

 

[electricpete]

DC is typically used for winding heating because you can create the same current with a lot lower voltage on dc than ac (dc doesn't have to overcome inductive reactance.)

As I understand you are hoping to apply three phase low voltage to slowly rotate the machine?

It's not a standard approach. Some things to think about.
1 If bearings are sleeve bearings, forget about it… you don't want to subject them to continuous slow rotation.
2 I would think rotation would be somewhat unpredictable.
3 We'd like to be able to make sure things don't overheat…. bearing in mind that it takes far less current to overheat quasi stationary machine (no self cooling from rotor motion) than a machine rotating at normal speed. The stator is probably not a concern but the rotor brings some questions to mind:
3A rotor would be continuously subject to deep bar effect with highest heating at the top of the bar.
3B rotor iron would be continuously subject to low magnitude high frequency flux. Rotor iron problems that would not cause a problem at normal speed (low slip) might cause a problem at continuous low speed (high slip).
3C We probably don't have any ability to monitor local temperature of rotor (the best we can do is perhaps check shaft extension from time to time as indirect indicator).
3D I suspect most people have a gut feel that the amount of heat input is low enough that it's not going to hurt a sturdy squirrel cage rotor... but how reliable is that gut feel considering you are about to do an experiment that may have never been done before. What about potential effects of abnormal temperature distribution (even if not excessive) over a long period of time. If you are the first to do this experiment, you may become famous/infamous for being the first to discover these potential effects... perhaps only after the machine has been put in service. It may well be fine but I would not be ready to do it at my plant without some more thought.


=====================================
(2B)+(2B)' ?

 

[krisys]

electricpete (Electrical)
Appreciated for the detailed perspective! It is thought provoking. Now the discussion is entering into a very interesting stage!

The manufacturer should be able to give some input on this uneven heating up due to locked rotor. I agree that the degree of uneven heating is very critical and risky. But in my view it is still worth pursuing. If the unevenness of heating is within the acceptable limit, probably this method of heating would be better than the space heater heating.

As I understand, the space heaters can prevent condensation. But it cannot dry out the winding, if the winding has already absorbed some moisture. The switching on the space heater would create a hot zone (barrier) between the outside ambient and the motor winding insulation. Thus, the trapped moisture would further deteriorate the winding insulation at an elevated temperature.

Any thought?

 

[waross]

Define the problem.
Do you want to dry the windings or prevent the windings from getting damp in the first place?
To avoid condensation the internal iron must be kept above the dew point. This does not take much heat.
Incidently condensation may occur at fairly high temperatures. Condensation in a motor typically occurs when the temperature and the dew point are rising. If the dew point rises above the temperature of the internal iron or copper, you have instant condensation. In the tropics, near the ocean, this can occur at 80 degrees or 90 degrees above.
50 Watts or 100 Watts would protect a 600kW machine against condensation.
It takes a lot more heat to dry out windings.
We had the 600 kW machines soaked with water during a hurricane. The anti condensation heaters were no use at all. I dried one machine with current through the windings. Took about 200 Amps for a couple of days. 200 Amps was about 28% of full load current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Drying out heating and anti-condensation heating are two entirely different things. The first one needs winding temperatures closer to 90 to 95 deg C and second one needs that the winding temps be above 3 to 5 deg C above the ambient.

We dry out MV motor stator windings with 400 V with the rotor pulled out, not with the rotor in.

In my 30 odd years, I have not come across a single case of windings damaged by space heaters.

Muthu

http://www.edison.co.in