Electric motor amps vs hp

[yukon cornelius]

Can someone help with amps vs HP. I have an electric motor with no HP rating but list 2.9 amps. I am looking at replacements and there is a huge variance. Some 1/3 HP are 2.8 amps and some are 1.5 amps. Same with 1\2 HP.  Do I try to match HP or amps. Each have the same rpm range

[Magicman]

The difference could be in the efficiency ratio:  Amps vs HP

[PineNut]

I would have to make some estimates but this is what I would come up with.

With 1 hp = 746 watts, the 1/3 hp would be 249 watts   ( 746 * 1/3 = 249 )

Converted to amps at 120 volts it would be about 2.1 amps  (  249 / 120  =   2.1).

But this is at 100% efficiency. I would estimate the efficiency to be maybe 80%.
That would make it about 2.6 amps   (   2.1  / .8   = 2.6 )

This would put it in the range of 2.8 to 2.9 amps for 1/3 hp. (Maybe my efficiency is too high.) That 1.5 amps is some ones dream unless it was a 240 volt motor. Then it would be reasonable

[muggs]

My book doesn't  start until 1/6 hp. It rates that at 4.4 amps on 115 V. This motor is running on 115V right? The reason for your confusion is that some of these builders just plain lie about HP. I think you are better off to go with amps.   Muggs :

[Gary_C]

Go with the amps. You need a motor that is capable of running at the same rated volts, RPM's, and at least as many amps.

[TKehl]

Match Amps more than likely.  Really need to know the Voltage, but I think PineNut nailed it above.

Double the voltage (110->220) = half the amperage. (2.8->1.4). 

The real world is not a perfect match, but close enough for this.


Side note, if you want it to drop in, make sure the frame size matches as well.  That way your shaft shaft and bolt locations will drop right into the same setup.  Most fractional motors will be a 56 frame, but there are variations within 56 (foot versus flange mount) and some are just flat different frames.

Second side note, is this an open or enclosed motor?  Mostly matters if this will be in a wet or dusty environment.

[Kbeitz]

This all changes if it's a 220 volt motor or 3 phase.

[yukon cornelius]

Thanks everyone! I had left out that it was 208/230 1 pH. I had estimated its efficiency to be 50% or less by its age. Mid 80s window unit. VERY large unit, 30,000 btu. I was able to use some of the calculations you shared and ended up settling in on 65% efficient 1\3 HP 2.3 amp  motor. It seems to be a great fit and is up and running. I shall sleep good tonight with minimal sticking to the sheets. Thanks for the assistance.