Horsepower to watts

[Bill Dufour]

Everything I can find says that one hp is 745 watts.
So 240volts at 3.1 amps should be one hp.

So why is my one hp pump motor 240 volts at nameplate 7.2 amps? I can see some loss of efficiency but over 50%. What am I not understanding here?
Bill D

[Frank Pratt]

Most of that is down to inefficiency, but some is because with a lower power factor, the current peak is slightly out of phase with peak voltage. in an inductive circuit, VxA= VA, not watts.

[Bob Jones 5443]

That's probably 7.2 full-load amps on the nameplate.

[Doug Garson]

Is it a dual voltage motor? Could that be the 110V amperage?

[Bill Dufour]

It may be a California thing. Pool motors over one HP must be two speed or variable speed. So maybe they just call it one hp even if it is more.
Bill D

the 120 amps is 14.4

[David L Morse]

Frank's answer is the right one: Input current = Output HP x 750 / Input Voltage / Efficiency / Power Factor.

In general, the smaller the motor the lower the efficiency.

Here's a random 1 HP motor:
https://sawmillcreek.org/attachment.php?attachmentid=457877&d=1621425035

So, similar current rating to the motor in question. Note the 67% efficiency and 70% power factor. .67 x .7 = .47 so, yes, the apparent input power is more than double the actual output power. (real power (W) = apparent power (VA) times power factor)

The chart came from this Manufacturer's publication (https://www.grainger.com/ec/pdf/24C182%20_1.pdf). If you really want to nerd out on motor characteristics the tables and charts at the end of the document will be interesting.

[ray grundhoefer]

Many years ago when I was going to school for electrical it was 746 watts = 1 horepower. But yeah it doesn't work out that way in real life for the reasons listed above.

[Mike Henderson]

Many years ago when I was going to school for electrical it was 746 watts = 1 horsepower. But yeah it doesn't work out that way in real life for the reasons listed above.

I think the exact number is 745.7 Watts per Horsepower. I round to 750 because that's accurate enough for my use. But you still have to take efficiency and power factor into account.

For a 3 to 5 HP motor, I've been using 1,000 to 1,200 VA per HP but I may have to reevaluate that.

Mike

[David L Morse]

I think the exact number is 745.7 Watts per Horsepower. I round to 750 because that's accurate enough for my use. But you still have to take efficiency and power factor into account.

For a 3 to 5 HP motor, I've been using 1,000 to 1,200 VA per HP but I may have to reevaluate that.

Mike


Many years ago when I was going to school for electrical it was 746 watts = 1 horepower. But yeah it doesn't work out that way in real life for the reasons listed above.

You're both right. The exact definition of horsepower depends upon context. There are several values in use. Like Mike, I find that 750 W is close enough for practical stuff. That value is also what's commonly used for converting motor ratings between metric and US conventional units.

fwiw, the unit of horsepower was invented by a guy named Watt as a marketing gimmick for his steam engines.

[Tom Bender]

I used 750 watts when calculating a/c loads. In that way it is accurate, but not for sizing wiring to a motor based on HP.