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relationship between amps and HP

KzooRichie's picture


I noticed in the review of minis in the tools and workshop issue the both the Steel City and the Rikon had 1/2 hp motors. One used 3 amps and the other 6.6 (can't remember which was which). It was always my understanding that amps and HP were directly related and if one was higher so was the other. How can two motors be so vastly different in amps but both be 1/2 HP?

RickL's picture

(post #84338, reply #1 of 11)

Were they both rated at the same voltage? That would account for the difference or it's a misprint.

ptu's picture

(post #84338, reply #2 of 11)

You are right, horsepower and amps are related. one horsepower equals 750 watts. At 120 volts, 3amps =360 watts,1/2 horsepower; 6.6amps=792watts, or more than 1 horsepower. So the 3 amp is probably more accurate, but it makes one wonder what the other one is measuring.

HowardAcheson's picture

(post #84338, reply #3 of 11)

>> It was always my understanding that amps and HP were directly related and if one was higher so was the other.

Not directly true. It relates to it but is not directly related.

The formula you are referring to is for a theoretical situation. The oft quoted formula of 1HP=746 watts is not how real motor horsepower is measured. Horsepower is measured on a dynomometer controlling temperature in the motor. The formula is for a theoretical situation which does not take into account efficiency, service factor and other factors. Fine for physics but not for real world motors.

Howie.........


Edited 11/20/2006 2:10 pm ET by HowardAcheson

Howie.........
RickChristopherson's picture

(post #84338, reply #4 of 11)

Sorry Howard. You know I don’t normally disagree with you, but there is a necessary correction in this case.

There is nothing theoretical in the 746 watts = 1 hp. This is a conversion factor, similar to converting inches to millimeters. As with any conversion factors, it is absolute.

The reason why the two motors mentioned above have different current draws is because one of them (6-amp) is at 120 volts and the other is at 240 volts. They are still both 1/2 hp.

The amount of power delivered at the shaft of the motor is absolutely related to the amount of electrical power given to the motor. This is the principle of Conservation of Energy where energy can neither be created nor destroyed. All of the electrical power given to the motor can be mathematically accounted for (in real life, not just theory) and the power of the output known.

As a matter of fact, a dynamometer is typically a motor/generator running in reverse, where mechanical power is applied to the shaft of the motor/generator, and the horsepower is calculated by recording the resulting current, voltage, and frequency of the electrical side.

The proper equation for the conversion is:
HP = I x V x pf x eff / 746

Where:
I = Current
V = Voltage
pf = powerfactor (time shift in the signals)
eff = Efficiency (which includes heat generation, windage losses, and magnetic losses)
746 = convertion from units of horsepower to units of watts.


Edited 11/20/2006 6:21 pm ET by RickChristopherson

MikeHennessy's picture

(post #84338, reply #8 of 11)

In a perfect world, you would be correct. Amps consumed would relate directly to power delivered at the shaft. Similarly, we could measure a car's horsepower by simply measuring miles driven per gallon of gas consumed. Alas, we do not live in such a place. In our world, there are things to consider such as friction and efficiency of design, both of which will take some of that electrical energy and convert it into something other than HP at the shaft. Heat, sound and vibration are the first that come to mind. These not insignificant factors will vary depending on the motor's design and the conversion factor will be accurate only in the case of a perfectly efficient engine. BTW, when you find one, you might want to think about using it to run that perpetual motion machine.  ;-)


Mike Hennessy
Pittsburgh, PA

Mike Hennessy
Pittsburgh, PA
Everything fits, until you put glue on it.

RickChristopherson's picture

(post #84338, reply #9 of 11)

Mike, you didn’t look at the equations very carefully, because all of the real-world factors you mention are covered by the equation.

People think that this is “theoretical” because it is something that they don’t understand, and therefore it is not real. When a motor is designed, all of these parameters are calculated and known. Some manufacturers even publish the full load efficiency and powerfactor. However, just because they are not published outside a company does not mean they are not known within the company.

Many motors will state their powerfactor right on the motor nameplate because it is a figure that commercial facilities sometimes need (too long to explain why). Without knowing the PF, a good approximation is 0.8, and within reason, this approximates the majority of motors. The efficiency is rarely published, but again, most motors fall within the 80% efficiency range. These values don’t vary significantly from motor to motor because there are no new huge breakthroughs in motor design that are going to set one company drastically different than another.
==============
Oh by the way as long as I am making a posting. "Service Factor" and Powerfactor are two entirely different things. The service factor relates to how well the motor will perform beyond its rated power. The powerfactor is an electrical property for all electrical systems (not just motors) to provide the phase angle between current and voltage (short answer).

MikeHennessy's picture

(post #84338, reply #10 of 11)

Rick said: "Mike, you didn’t look at the equations very carefully, because all of the real-world factors you mention are covered by the equation."


Ahhhh! You want me to look at the equations! Fair 'nuff. ;-)


You are correct, oh mathmatically gifted one. <G> The equations do allow for all that extra "stuff" that robs power from the final output. I think we're on the same page here, or at least on the same chapter. That said, amps is still only an approximation (albeit a close one) of HP at the shaft unless you measure and calculate the extraneous losses for your particular application. That being true, and since I don't carry a dynamometer around with me, I always look at the current draw rather than the HP ratings since the approximation is a close indication of power delivered and probably way better than a manufacturer's HP claims, since there are a bunch of ways HP can be measured giving wildly differing results.


Mike Hennessy
Pittsburgh, PA

Mike Hennessy
Pittsburgh, PA
Everything fits, until you put glue on it.

RickChristopherson's picture

(post #84338, reply #11 of 11)

Mike Said: I think we're on the same page here, or at least on the same chapter.

Yes, in reading your latest posting, I believe we are on the same page.

One thing I realized later that I should point out about the equation posted above is that it is the simplified version specifically used for calculating the horsepower at full rated load. The equation doesn't really change, but the variables become complex instead of being simple constants; much the same as Ohms Law becomes more complex when dealing with time varying (i.e. AC power) singals. As a matter of fact, the equation above is from Ohms law, but to make it simpler, we extract the PF and eff from the current and voltage variables.

The powerfactor will actually vary depending on how much load is applied to the motor. When the motor is idling without load, the powerfactor drops to horendously low values, and that is why an idling motor still draws a large amount of current for virtually no work performed. The efficiency is a function of rpm and current.

forestgirl's picture

(post #84338, reply #5 of 11)

"The formula is for a theoretical situation which does not take into account efficiency, service factor and other factors."  That statement fits with what I've been told when looking at various motors.  The quality of the motor (windings, etc., I guess?) affect how many amps it draws....

forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-) 

forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-) 

cadiddlehopper's picture

(post #84338, reply #7 of 11)

"Quality" isn't quite the correct description since it could indicate both durability and energy conversion efficiency. No electric motor is 100% efficient. If a motor draws 6 Amperes, claiming that it is a 1-HP motor is somewhat stretching the truth as we really want to hear it. Rather than claiming HP, it would be more honest and useful to customers for manufacturers to state current draw, efficiency, and service factor.

Cadiddlehopper

jako17's picture

(post #84338, reply #6 of 11)

Some manufacturers quote peak horse power .That is the hp developed at the point of stalling the motor.Just about useless in the real world.Remember all the balony over compressors and craftsman routers