Technical » A Primer - Electric Motor Horsepower Ratings

Release Date: 6/20/2012

The Numbers Game:

 

A Primer on Single-Phase A.C. Electric Motor Horsepower Ratings

 

Kevin Brady, Minneapolis, Minnesota 

 

DISCLAIMER:

The reader assumes all responsibility and liability associated with the hazards of working with power tools. The author has no control over how a reader will act as a result of obtaining information in this article. The author shall not be responsible for any errors or omissions that may be present in this article. Accordingly, the author shall assume no liability for any action or inaction of a reader.

This article is directed toward a hobbyist audience and is not intended for application in a commercial, institutional, or industrial setting. Commercial shops are generally governed by a complex set of worker safety regulations, such as those mandated by OSHA. Satisfying the compliance of such regulations is beyond the scope of this article.

 

YOU CAN HANG A SIGN ON A PIG SAYING IT'S A HORSE...BUT IT'S STILL JUST A PIG.

 

This old saw applies to both animals and advertising alike. And while the distinctions between a pig and a horse should be painfully obvious, attempting to differentiate the true horsepower of an alternating current (A.C.) electric motor from its advertised power is not easily accomplished. No doubt you have noticed a great deal of hype concerning these horsepower claims. Power tools and other shop devices are no exception. Advertising claims, touting huge horsepower ratings in seemingly modest equipment, are commonplace. Just because the ads or the packaging says it, does that make it so? What does this really translate to in regards to power? Let's get to the bottom of it.

 

DO THE MATH.

 

Since most people find mathematics to be about as fascinating as watching a guy making a weiner dog out of balloons, I'll keep the calculations simple. One horsepower (HP) equals 746 watts. If a motor is rated at a true 1HP, that means it delivers 746 watts of mechanical power. Since single-phase motors (the kind usually found in consumer power tools) are never 100% efficient in converting electrical energy to mechanical energy, the amount of electrical power consumed by the motor is considerably higher than the mechanical power delivered. In fact, losses from heat, friction, etc., mean that a typical single-phase motor is, at best, around 60-70% efficient. A figure closer to 60% is more realistic for the small induction motors typically found in many home-shop power tools. That means a genuine 1HP motor requires roughly 1250W or more of electrical power to deliver its rated power. In short, you will need at least 10 amps of current at 125V, or 5-plus amps at 250V to realistically deliver a true one horsepower from your motor. This is a good rule of thumb to keep in mind.

 

Most portable power tools, and many of the cheaper stationary tools, use universal motors. Unlike induction motors, the speed of which is dependent on A.C. frequency and the number of poles, universal motors are capable of much higher speeds, and variable speeds. Universal motors are generally more

 

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