# Synchronous motor windings as heaters



## Lone Crapshooter (Nov 8, 2008)

After 34 years of doing industrial work I am beginning to work on synchronous motors. The motors that I work on are 40's vintage 2400 volt . 

To heat the motors in the off cycle they have a low voltage high current transformer that feeds a bridge rectifier that feeds DC to the stator winding of the motor. There is a heater contactor that is interlocked with the main contactor that kills the DC to the stator when the main contactor pulls in.
DC is used to keep the motor from acting like a transformer and feeding voltage back in to the rotor field supply circuit. 

My question is with all of the components associated with this DC heater circuit would it not be a better solution to have place 120 or 240 volt AC restive elements in the motor enclosure rather than use the motor stator windings for the motor heater or is there something I am not seeing here. 

Thanks LC


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## backstay (Feb 3, 2011)

Isn't the DC for the heat the same as the field? That would be why, it was already there. No other circuit to add.


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## Lone Crapshooter (Nov 8, 2008)

No this is totally a separate power supply from the DC field supply. 
The transformers are rated like 38 volts at 120 amps them that is feed into a bridge rectifier then into a vacuum contactor that is wired to 2 of the 3 stator or T leades.

LC


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## scotch (Oct 17, 2013)

Pretty common on the older large motors in a damp-wet environment....used to be used on large hermetic compressors in refrigeration as well in 50's and 60's ...just caught the tail end of all that stuff in my apprenticeship . Last one I saw was in 70's....a 11 Megawatt steam turbo genset from the UK with water cooling thru' the windings .When it first came online it went Bang ! Then the LV to the windings was the factory fix to keep it dry.


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## frenchelectrican (Mar 15, 2007)

Lone Crapshooter said:


> My question is with all of the components associated with this DC heater circuit would it not be a better solution to have place 120 or 240 volt AC restive elements in the motor enclosure rather than use the motor stator windings for the motor heater or is there something I am not seeing here.
> 
> Thanks LC


There is a reason why it used the DC for winding to keep the whole thing warm instead of local spots from AC heaters because I have see some peoples try to use the AC heaters in few spots and it was not the best result due the moisture basically flashover once you kick the main winding voltage on. ditto with the rotors too they run small DC current to keep the coils warm too.


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## Big John (May 23, 2010)

The only caution I have with this, is that I've seen poor brush connections end up slowly burning flat spots in the rings where they sit in the same spot for long periods of time.

Definitely causes accelerated damage when the field is put back online.

This controlled heater circuit probably won't have that problem. What I think usually causes it is leaving the excitation applied to heat the winding, and selecting too high a current. 

If done carefully it's a much more effective heater than using resistance elements.


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## Lone Crapshooter (Nov 8, 2008)

Only the stator winding is used as a heater.


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## Big John (May 23, 2010)

My mistake, I've never seen that method. 

I'll stand by my last assessment, though: Using the winding as a heater guarantees far better heat distribution and much less likelihood spot condensation, especially for large, open frame motors.


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## telsa (May 22, 2015)

Crazy me...

After 70 years of success -- I don't re-design nothing.


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## JRaef (Mar 23, 2009)

Yes, there is separate equipment in the control panel, but they use the motor leads for the heating circuit, so no added conduit out to the motor for the space heaters, plus the better heat distribution.

A-B used to sell* a device called a "Bulletin 1410 Motor Winding Heater", it did exactly that same thing for LV motors, but automatically sensed the AC being applied and turned itself off. Can't do that with MV though, hence the contactor to isolate it.

* They discontinued it a few years ago, not enough sales because nobody knew about it.I think maybe Motortronics still sells a version though.


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## MechanicalDVR (Dec 29, 2007)

scotch said:


> Pretty common on the older large motors in a damp-wet environment....used to be used on large hermetic compressors in refrigeration as well in 50's and 60's ...just caught the tail end of all that stuff in my apprenticeship . Last one I saw was in 70's....a 11 Megawatt steam turbo genset from the UK with water cooling thru' the windings .When it first came online it went Bang ! Then the LV to the windings was the factory fix to keep it dry.


I believe you mean semi-hermetic compressors, hermetic compressors have external heaters.


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