Motor Space heater 4160v fault question

[Mila15]

We have a situation where a motor heater is part of 4160v compressor control circuit. We have a standard that any critical control circuit should be supplied with UPS. Question is, if there is a fault in motor winding (4160v), can fault current find it's way into 120v heater circuit (UPS)? Is heater embedded in such a way to make this possible?

 

[waross]

All the heaters that I have seen and serviced were in the end bell. None were embedded in the windings.
Also, the heater is normally cycled off when the generator is in operation.

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

 

[edison123]

Normally, MV motors have space heaters fixed in the stator frame below the end winding with no physical contact with the stator winding. So, a winding fault should not affect such space heaters.

But I have also seen a few motors with fiberglass covered space heaters wrapped around the end winding (which is a bad design in my opinion). So, when an end winding fault occurs, there is a possibility that such a wrap-around space heater will suffer some collateral physical damage.

Regardless of the type of space heaters used, the space heater supply must be off when the motor is running and so any winding fault should not affect your 120 V supply.

BTW, why do you have a space heater on UPS?

Muthu

http://www.edison.co.in

 

[Mila15]

Thanks all. Edison123, the space heater for this application is part of equipment control circuit, and we use UPS power on all major control circuits. I think even if space heater is switched off, it's only switched from high side (not neutral) and if there is a short it might find it's way back to UPS. Unless it's positioned far from winding like you guys said. I think motor vendors should take that into consideration (I hope).

 

[edison123]

Space heater is not such a critical requirement that it has to be clubbed with motor control system. You could use a two pole breaker to completely isolate the space heater, if you want to be sure.

Muthu

http://www.edison.co.in

 

[davidbeach]

Given the presence of UPS power I'm going to guess that you've also got some form of emergency/standby generation. In that case the generator power is much more appropriate for the heater. There's no harm in the heater being powered down for the time it takes the generator to come up to speed. That will free up UPS capacity and get the UPS circuit out of the motor housing. That and also the two pole breaker.

 

[waross]

Given that the heater is switched off automatically, I would go with a two pole relay rather than a two pole breaker.
Many anti-condensation heaters have a current monitor alarm that alarms if the heater fails. I imagine that is why it is part of the control scheme.
It will be the alarm function which needs the UPS.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Bill

An over current relay monitored by the control system could provide both isolation and less power drain from UPS. I have seen mostly only a fuse protection for the space heater.

Muthu

http://www.edison.co.in

 

[waross]

I am not talking about protection Muthu.
I have seen connections to heaters fail, I have seen failures due to extreme corrosion of stranded conductors supplying heaters.
However, anti condensation heaters are typically supplied with 1/2 rated voltage. 240 Volt heaters fed with 120 Volts is common.
I have seen monitoring circuits that alarm on loss of current to the heater.
When a heater alarm went off in the little island utility, I would get a phone call and be on the next flight to the island.
Yes it can happen but I personally never had an over-current trip on an anti condensation heater. All the failures that I have repaired were loss of current trips.

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