Current versus load for split phase motor

[Jerry224]

I want to adjust the load (a fan) on a new split phase motor (3/4 hp, 120 v, 60 hz) so as to absorb rated power. I planned to adjust pully ratio until I achieved the rated FLA of 11.1 amps. Imagine my surprise to find the no-load current was about 12 amps and the current did not change appreciably when a moderate load was applied. What's going on? How do I decide how much load to apply? Is something wrong with this motor? (It starts and runs OK and sounds normal.)

Thanks for any advice.

Jerry Jackson
San Antonio

 

[itsmoked]

As a motor is loaded less its power factor gets worse. The current you are measuring is both the current contributions from reactive AND actual power. As the motor is loaded more the balance of these two will shift. To you it looks like nothing is changing but in fact you are getting more work out of the motor. Go ahead and mess with the pulley ratios until you hit FLA. It could be a ways from where you are now.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

I agree. Single phase motors (especially split phase type and shaded pole type) can have very lower power factor and efficiency compared to three phase.

This means you have a a relatively high reactive load and resistive loss load even at no-load. The increase in resistive component due to mechanical loading can be relatively small compared to the no-load components.

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[electricpete]

I didn't mean to imply that losses due to inefficiency are constant as load varies. Some are some aren't. Reactive power demand is relatively constant as load varies. And reactive component is effective at masking resistive component when the resistive component is much smaller.

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