Advantages of Motor Space Heating and Thermister Protection?

[dodgeybro]

Hi all,

Just started a new job on greenfield project. Design is a mess and more than half the motors have arrived without motor space heaters. We are operating in potentially subzero conditions, but relatively low humidity. Obviously motor heaters are recommended but can anyone point me to any definitive study on the advantages on motor longevity of space heaters vs not? Is it worth the cost of retrofitting space heaters, and by how long will it prolong the life of a motor. I've had a trawl through some of the manufacture web sites without any luck so far.

I've also always understood motor space heaters as a device to prevent condensation, by keeping temperature above condensation temp. Do they have any additional advantage in cold conditions to reduce thermal shock? Again any study data would be appreciated.

Also can anyone clarify my understanding of the function of PTC thermistors. I understand that they prevent motor overheating in the event of high torque starting, or operators trying to repeatedly restart. They are unlikely to have any advantage in the case of a catastrophic insulation failure I believe. Surely smart relays with thermal modelling would be just as effective and cheaper than labour and material costs of running additional cabling.

Thanks for any feedback in advance.

 

[GoldDigger]

Surely smart relays with thermal modelling would be just as effective and cheaper than labour and material costs of running additional cabling.

Unless the thermal modelling is being done at a controller which is at a different ambient temperature from the motor it is protecting. That, IMHO, would be where actual motor temperature sensing would have a potential advantage.


On the other hand, that alone might not be enough, since there would be a time lag between the rise in temperature of the inner part of the motor winding and the corresponding rise of temperature where the thermistor is located. I believe, for example, that at least one of the temp sensors in a transformer which has sensing is buried in the middle of the windings during manufacture.

 

[petersonra]

Obviously motor heaters are recommended but can anyone point me to any definitive study on the advantages on motor longevity of space heaters vs not? Is it worth the cost of retrofitting space heaters, and by how long will it prolong the life of a motor. I've had a trawl through some of the manufacture web sites without any luck so far.

I've also always understood motor space heaters as a device to prevent condensation, by keeping temperature above condensation temp. Do they have any additional advantage in cold conditions to reduce thermal shock? Again any study data would be appreciated.

I doubt you will find much. They used to be pretty common but I think over time people realized that they were one of those things that probably served no real function most of the time so people mostly stopped using them.

I never heard of any study showing that there is any significant thermal shock involved in motors at normal ambient temperatures.

It would not surprise me to find that modern motors are more resistant to whatever negative effects of condensation there might be.

Also can anyone clarify my understanding of the function of PTC thermistors. I understand that they prevent motor overheating in the event of high torque starting, or operators trying to repeatedly restart. They are unlikely to have any advantage in the case of a catastrophic insulation failure I believe. Surely smart relays with thermal modelling would be just as effective and cheaper than labour and material costs of running additional cabling.

Thermistors are there to read the actual temperature inside the windings of a motor. They do not replace the thermal modeling that standard overloads use. They have nothing to do with detecting insulation failure, only temperature, although I suppose that high enough temperatures would eventually result in insulation failure.

Personally, I am not convinced either is especially desirable except in some very limited cases, especially with <600V motors. I think they are more common in MV motors.

Incidentally, if you really want space heaters and none were provided there are devices that we used to use that introduced a small DC current in the motor coils to generate some heat. I am not sure that it is possible to add the winding heaters to an existing motor as they glued to the end of the windings, IIRC.

 

[Jraef]

Two "giants" of the motor industry, Richard Nailen .PE. and Edward Cowern, P.E. have both written a little on the subject, The "Cowern papers" are now owned by Baldor (who he worked for) and available on line, the part on motor heating is at the very end:
www.baldor.com/pdf/manuals/PR2525.pdf

Richard Nailen's work was for EASA, the Electrical Apparatus Service Assoc. through their magazine "Electrical Apparatus", where he was the Editor. So it is often distributed by member companies, here is one I found:
www.brithinee.com/knowledge/Space_Heaters.htm?

Not mentioned in either unfortunately are the less important side benefits of keeping bearing lubricant flowing properly at start-up, and helping to prevent icing that may interfere with starting. I used to do some kiln dryer circulating fan motor systems for lumber mills in the Pacific NW and British Columbia. Unloading and reloading the wood into the outdoor kilns took a couple of hours, so in the winter time the steam coming out the door would sometimes form ice on the motor shafts which occasionally was enough to cause the breakers to trip when the motors were re-started. We put in some of the retrofit motor winding heaters that use the existing motor power wiring and it cured that problem. The motor absolutely lasted longer too, which is why we put them in. The ice prevention was just a pleasant surprise.

RTD sensors added to a Motor Protection Relay will enhance the protection by biasing the thermal model with real world feedback, but are not generally considered better than a current based model. In Europe they sometimes will use thermistors in lieu of motor thermal OL relays on LV motors, but the motors are usually built with them inside already whether you use them or not, and IEC motors are not built with any "Service Factor" like NEMA design motors are, so it's more critical for them. Best practice here on NEMA design LV motors is to supplement the thermal OL with thermistors or RTDs when the motor is in a high ambient, like maybe an oven circulating fan, a hot water pump or a boiler blower, but not as a replacement for a standard OL relay.

 

[templdl]

Hi all,

Just started a new job on greenfield project. Design is a mess and more than half the motors have arrived without motor space heaters. We are operating in potentially subzero conditions, but relatively low humidity. Obviously motor heaters are recommended but can anyone point me to any definitive study on the advantages on motor longevity of space heaters vs not? Is it worth the cost of retrofitting space heaters, and by how long will it prolong the life of a motor. I've had a trawl through some of the manufacture web sites without any luck so far.

I've also always understood motor space heaters as a device to prevent condensation, by keeping temperature above condensation temp. Do they have any additional advantage in cold conditions to reduce thermal shock? Again any study data would be appreciated.

Also can anyone clarify my understanding of the function of PTC thermistors. I understand that they prevent motor overheating in the event of high torque starting, or operators trying to repeatedly restart. They are unlikely to have any advantage in the case of a catastrophic insulation failure I believe. Surely smart relays with thermal modelling would be just as effective and cheaper than labour and material costs of running additional cabling.

Thanks for any feedback in advance.

I'll respond on behalf of the space heaters. The other guys are addressing the thermister/RTD subject.
The space heaters are used to provide warmth when the motor is not run for a long enough period of time for moisture to condense occur in the motor keeping the windings dry. When the motor is off the space heater is turned on and turn off again when the motor starts.
Another method to keep the motor warm is by applying a DC current through one on the windings while the motor is off which heats the motor from within the windings. I sold one job with this feature which in my personal opinion is superior to space heaters by space heaters are commonly 120vac controled with a relay and an often times can be added in the field.
The method of heating the motor via a winding with DC you are depending upon the DC voltage source and the resistance of that winding to provide heat.
With with space heaters or heating the motor internally adding a thermostat to defeat the heat when heating is not required is recommended.