View Full Version : Missing .7 hp?

QSIMDO

02-10-2008, 01:50 PM

I just bought a Leeson 3~ motor allegedly rated at 2 hp.

Yeah, well...220 volts, 5.2 amps, 85.5 efficiency rating / 746 = 1.3 hp, no?

Do I have a beef with the company who sold it or did I, yet again, screw up a calculation?

davidh

02-10-2008, 02:01 PM

SAME REASON I LAUGH WHEN i see a 5hp compressor motor thats the size of a heater motor in a car. . . (not quite) the rating is peak hp for a very short period of time. . my 3 hp lathe motor is nearly the size of a mac truck. . . .

fasto

02-10-2008, 02:19 PM

I just bought a Leeson 3~ motor allegedly rated at 2 hp.

Yeah, well...220 volts, 5.2 amps, 85.5 efficiency rating / 746 = 1.3 hp, no?

Do I have a beef with the company who sold it or did I, yet again, screw up a calculation?

That formula is for single-phase motors. Three phase:

volts * 1.73 * amps = watts, watts / efficiency * 746 gives HP, I get 2.2 HP for your motor, sounds about right.

I just checked a few motors around this shop, 2hp 208V 3-ph is 6A on all of them.

BobWarfield

02-10-2008, 03:15 PM

Len, that Baldor I just got off eBay is supposed to be a 3HP. It's spec'd for 7.6 amps at 220V 3 phase.

Sure sounds like Leeson is taking some liberties!

Best,

BW

barts

02-10-2008, 03:55 PM

Your motor's current ratings are very similar to those below:

http://surpluscenter.com/sort.asp?numrec=20&lastrec=40&sort=1&keyword=MB3D&catname=electric&UID=2008021014424354

Remember that AC power depends on phase angles between voltage and current; they' re not going to be all the same.

= Bart

QSIMDO

02-10-2008, 04:07 PM

Len, that Baldor I just got off eBay is supposed to be a 3HP. It's spec'd for 7.6 amps at 220V 3 phase.

Sure sounds like Leeson is taking some liberties!

Best,

BW

Bob,

Let's go with fasto and others.

I like their answers!

http://www.veris.com/calculator.asp

Now if I can stop obsessing about this freakin' motor....I'll go clean the shop.

They rate them by using the max voltage including start up voltage roughly 1 1/2 times as running voltage.

Isn't it on this forum that someone posted that CLASSIC picture of the armature the removed from a cheap Cumming bargain grinder. The grinder was rated 1/2 hp (notice I used small letters for small horse power :) and the armature was about the size of small can of soup.

Hal

Forrest Addy

02-10-2008, 06:20 PM

You guys pay attention to Fasto. You can't use single phase motor formulae on three phase motors. In three phase motors you have an additional factor when calculating backwards from nameplate data to obtain motor HP. It's motor Volts, times full load Amps, times power factor as a decimal, times efficiency, times square root of 3, divided by 746. There's more but these dadum fields account for the larger components of the problem.

For the original example: 230 x 5.2 x 0.8 (typical value) x 85.5% x 1.73 / 746 = 1.89. Something's not quite right. NEMA rated motor develop full nameplate mechanical HP when run and loaded to nameplate ratings.

Someone feel like working this out?

fasto

02-10-2008, 06:41 PM

You guys pay attention to Fasto. You can't use single phase motor formulae on three phase motors. In three phase motors you have an additional factor when calculating backwards from nameplate data to obtain motor HP. It's motor Volts, times full load Amps, times power factor as a decimal, times efficiency, times square root of 3, divided by 746. There's more but these dadum fields account for the larger components of the problem.

For the original example: 230 x 5.2 x 0.8 (typical value) x 85.5% x 1.73 / 746 = 1.89. Something's not quite right. NEMA rated motor develop full nameplate mechanical HP when run and loaded to nameplate ratings.

Someone feel like working this out?

You've got it. Almost.

230V * 5.2A * 1.73 * 85.5% * 0.84 / 746 = 2.0 Hp

Power factor of a *fully loaded* induction motor is about 0.84.

*Unloaded* is about 0.62.