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post #1 of (permalink) Old 03-07-2005, 11:27 AM Thread Starter
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Electric motors - peak vs continuous hp?

Since so many shop items are now rated by peak horsepower, such as compressors, chop saws, and even shop vacs, what is peak horsepower all about? Why is it able to exceed the calculated power which the item draws from the outlet? Where does this added "power" come from? What does peak horsepower tell us about a compressor or other item that isn't subject to unpredictable stalling forces, like a chopsaw or tablesaw where we load it down intermittently?
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post #2 of (permalink) Old 03-07-2005, 02:37 PM
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Re: Electric motors - peak vs continuous hp?

Maybe this will help [img]images/graemlins/grin.gif[/img],,, and again maybe not. [img]images/graemlins/headspin.gif[/img]

Horspower/Torque/ Peak HP
Horsepower and Torque: A Practical Explanation
Probably the best for your question.
How Horse Power Works
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post #3 of (permalink) Old 03-07-2005, 07:03 PM
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Re: Electric motors - peak vs continuous hp?

In terms of electrical motors the peak horsepower is what the motor can put out, but not for very long or it will overheat.

A (regular) horsepower rating is one that it can put out 'till the cows come home. Often you'll see a temperature increase associated with it - 60*C temperature rise is fairly common.

I think the peak horsepower rating is mainly to fool you into thinking that you're getting more than you really are. [img]images/graemlins/mad.gif[/img]
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post #4 of (permalink) Old 03-08-2005, 09:17 AM
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Re: Electric motors - peak vs continuous hp?

What Jim Lou said.

Peak HP when purchasing power tools is pure garbage. If looking at compressors for example, compare [email protected] & at 90 psi. That is the only real relavent comparison. Peak HP is the inrush current that lasts for just a split second. Any longer and it would blow the circuit breaker. Max usable HP from a compressor, vaccum, etc. is about 2 hp on a 15 amp breaker. Ever notice the lights dim when you turn on some tool, or appliance first kicks on. That is the inrush current or max hp drawn to start the motor turning. A compressor designed to operate on household 110V 15A breaker, will begin filling the tank and probably only use 1/2 hp. As the tank fills up, and the compressor has to work harder to pump against a higher head, current draw (HP) goes up. The compressor probably switches off near the rated 15A draw (2 hp) but not over it. A circuit breaker will handle the extra Amp draw for a very short time, because very little heat is developed in such a short time. Let it stay on for even a second, and the ciruit will blow.

This is why in industry, a whole other industry has developed to start large motors. Large motors will use capacitors, electronics, variable speed drives and various other techniques to start up. Without them, electric systems could not be designed economicaly to handle the startup current required by these big motors.
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