MOTORS ACTING UP?
 
Q: This might be a question that is almost out of date in today’s era of ECM motors. I've seen factory-installed, hopelessly undersized, 1/6 HP shaded pole motors running a large blower, at nowhere near synchronous speed yet adequate enough for low air flow requirements, running with no problems for years. But, if you replace this with a PSC motor of the same HP, it will overheat. I understand this; the PSC is much more efficient. It slips less and therefore overheats the system. If you bump up the motor's HP to a 1/3 HP PSC motor, everything works — a much higher airflow for less electricity.

But, if the motor happens to be a multi-speed 1/3 HP, or a multi-HP, you can run on a lower 1/6 HP tap with a lot of slippage, without overheating! Why? I've never had a good answer for this one. Settle my years of wondering so that I might die happy and not spend eternity wondering about the answer! Thanks!
— Greg Blackman, AirCrafters Heating & Cooling

A: Thanks for the question, Greg. I went straight to our engineers for this stumper. This is what GE's lead motor designer from HVAC Motors Engineering, GE Commercial Motors by Regal Beloit, had to say:

“All 1/6 HP motors are not created equal. The SP motor would have had more material to begin with than the PSC motor; therefore, even though it was less efficient, it was able to dissipate the watts loss. If the SP motor is designed to run at 900 RPM, and the PSC motor is designed to run at 1080 RPM, then the PSC motor and the SP motor can have close to the same efficiency if the PSC motor runs at 900 RPM. The 1/3 HP motor would have more mass, and when it is run on lower speeds, it could dissipate the heat easier than a 1/6 HP PSC motor could."

“We use various rotor resistances to get the peak efficiency at the design point specified by the OEM. For a non-ECM motor, the efficiency can vary greatly when you move away from the intended operating speed. I know you can replace an SP motor with a PSC motor only if they are designed to operate at about the same RPM point.”

Those engineer guys really know their stuff! Good thing, because now you won't have to spend eternity wondering about motors. And because I used Greg’s question in this column, he gets FREE STUFF. Do you have an HVAC/R question? Send it to me, and if I use it, you can get FREE STUFF.

James.Flynn@totaline.com

Jim Flynn is training manager for Totaline sales centers and has 35 years of experience in HVAC/R. Every month, we challenge readers to Stump Jim Flynn with a question about Totaline sales centers and the industry. Contact him at James.Flynn@totaline.com.

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