Heating 3 phase Motors by single phasing


Thread Starter

Thomas D.

Our plant has for steam kilns for drying lumber. We have 11 fans per kiln, each driven by a 15 hp open frame 480v/ 3phase motor. We often suffer motor failures and it is highly suspected that we are getting moisture build-up in the windings. i have been given a suggestion that when the fans are not running for a period of time, we simple run 115vac through 2 of the phases (single phasing without turning the motor). I have been told that 115 may be too much voltage and 24 vac might be better. I can hook the necessary relays up through a PLC and insure when the heaters come on and off, so that is no problem. I simply have no experience in this and was wondering if someone out there may have a recommendation. Is this a common practice? What voltages are typically used? What type of current draw would I expect? Any help would be greatly appreciated.
Rockwell has a product invented by a friend of mine. It's called "Motor Winding Heater" Bulletin 1410. The device mounts in the Motor Control circuit and is used to keep motors dry, not dry motors out - The device is single phase and uses the motor phase voltage conditioned by an SCR. The device comes on when the motor is de-energized by the starter. My friend is John Holmquist with Weyerhaeuser - he invented this back in the 1980's - licensed the idea to Allen-Bradley. John is 75 y.o. and still works for Weyerhaeuser and is still very active in the forest products industry.

Brian Rode Engineered Handling Inc.

Here is a thought how about replacing them with TEFC motors as long as you don't hose them down the moisture problem should go away. good luck

Crucius, Wesley

No. Common practice would be to use a sealed motor! In lieu of replacing 44 motors (for X 11), you might try this trick, but it seems quite risky and somewhat wasteful in terms of power consumption (I would guess you'd need to burn around 100 watts per motor to have any impact). What about cycling the fans off and on, maybe one at a time for short periods... If the problem is simply high ambient humidity, this might keep the motors a little dryer, although, if the problem is due to condensation resulting from heating and cooling cycles, this may actually aggravate the problem...

Bouchard, James [CPCCA]

This is an approach that has been used for many years especially on larger high voltage motors. The idea is to pass a current that is less than the full load current through the motor winding to heat it and dry it out. I have seen it done using a welding machine ( on a 4160 volt motor ) they just adjusted the current to 90 % of the full load current ( and checked it with a clamp on ammeter ). They then measured the insulation resistance every 4 hours until it got the to the required value. So in your case you would have to set up a test rig with a variable transformer to see what voltage gives say 90% of the full load current and then arrange to supply it. I would have difficulty though with a permanent installation of this type since it leaves power on the motor at all times and could be considered a safety hazard. You could look at several other ways to keep the moisture out. Use TEFC ( Totally Enclosed Fan Cooled ) motors which keeps the ambient air away from the motor. In years back we had a lot of problems with open motors in applications where we had moisture and this fixed most of the problems. Another approach is to use motors with an epoxy dip on the windings, as it is more resistant to moisture. Talk to your rewind shop about that. A final approach is to install a heater in the motor housing to keep it warm and avoid condensation. Hope this helps James Bouchard
Hello... A long time ago in an other industry (as in different then the one I am employed in at the moment) we had similar issues with large roof mounted refrigeration condenser motors. We considered heating the motors by using the windings as a resistance element... but we did an estimate and the cost of the power needed to make any difference on a motor of its size (125 Hp) was more then we were willing to pay. Another option was required... what we ended up doing, and this worked great, was when a motor failed due to moisture penetration and required a re-wind we sent it out for such and ordered the re-wind shop to "epoxy encapsulate" the winding. That was the end of our troubles and may be a good answer for you. On large frame motors it is an expensive proposition to do this... but cheaper in the long run then wasted watts or re-wind costs. On a 15 Hp motor like yours it should cost very little. Best Regards... Rick Kelly