Posted by MarkB_MI on December 31, 2020 at 03:22:42 from (174.230.6.146):
In Reply to: Horsepower change posted by marcusmerritt on December 30, 2020 at 14:16:01:
Well, there are a couple of things going on. First off, there's the rated power of the motor versus the claimed "maximum power" on the product on which it's installed. Many manufacturers grossly exaggerate the latter and the motors they use aren't marked with the rated horsepower. Horsepower ratings for table saws and air compressors are often totally fictitious. Although the rated horsepower isn't marked, it's pretty easy to derive from rated current and voltage; multiply current and voltage to get the volt-amps, then multiply the VA by the power factor to get the rated power in watts. If the power factor isn't marked, assume 0.7. Then divide the watts by 746 to get the horsepower. My air compressor has a decal that says "6.5 max hp", yet the actual horsepower is closer to 3. Yes, this should be against the law but it isn't.
The other thing is that any induction motor will put out as much power as is demanded, up until it stalls out. This is different from internal combustion engines, which only put out so much torque and slow down if the demand for torque is greater than what the engine can produce. This means the motor manufacturer can honestly claim a horsepower rating greater than what might be prudent for motor operation at a 100 percent duty cycle. The motor will get hot at the rated power, but it can produce that power nonetheless.
There are various SAE standards defining exactly how manufacturers rate the horsepower of internal combustion engines. I'm not aware of similar industry standards for electric motors.
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