[brian_]

So here goes, someone throws a motor on the bench and ask what can it do? All we have is nameplate data that reads:


Operating Voltage = 48 vdc
Load Torque = 5.237 ft-lbs
Load Speed = 4150 Rpm
Load Current = 88 amps
Peak HP = 10
Continuous HP = 4.14

First thing it tells me I would be using 48 volts x 88 amps = 4224 watts. From that if I divide 4224 watts / 4.14 HP = 1020 w/hp efficiency...

That's the inverse of efficiency, plus a unit conversion constant... but I think you know that. It looks like you're calculating an "electrical power to produce mechanical power" factor, in watts/horsepower. As for actual efficiency:

input: 48 volts x 88 amps = 4224 watts (as above)
output: 4.14 hp = 4.14 x 746 = 3088 watts
efficiency = output / input = 3088 W / 4224 W = 73%

That's not good, but I don't know if any better can be expected.

The "Load Torque" and "Load Speed" do correspond to the listed "Continuous HP".

 

[brian_]

"Watts per horsepower" is not a "bizarre declaration of efficiency." It is a units conversion equal to 746w/hp, always.

The units conversion factor is certainly 746 W/hp. What Sunking calculated and presented as "w/hp efficiency" was a combination of the inverse of efficiency and the units conversion factor, which I agree (as I suggested earlier) is bizarre.

To recap:

First thing it tells me I would be using 48 volts x 88 amps = 4224 watts. From that if I divide 4224 watts / 4.14 HP = 1020 w/hp efficiency.

input: 48 volts x 88 amps = 4224 watts (as above)
output: 4.14 hp = 4.14 x 746 = 3088 watts
efficiency = output / input = 3088 W / 4224 W = 73%

The value of 1020 is simply the product of the inverse of efficiency (1/0.73) and the conversion factor (746 W/hp). It is the amount of input power needed to produce an output, in units of input watts and output horsepower.

 

[brian_]

Motors are rated either in Output Mechanical Horse Power, or Input Electrical Power.

They are rated in output (mechanical) power, and there is of course a corresponding input (electrical) power for the rated conditions. Input power will be expressed in watts or a multiple (kW); you are accustomed to seeing output power expressed in horsepower (which is traditional and still used, especially in North America), but it would be in watts or a multiple (kW) for anyone working in consistent S.I. units.

I did a quick search for euro motor specs, and the first thing I found was a catalog for the Marathon Electric Motors IEC IE3 Motor Range, which shows (in the Technical Data charts starting on page 12) output power in kW (that's kW only, there is no mention of horsepower).

You might also be interested in the EU's ecodesign requirements for electric motors, which consistently uses the proper power units (kW, not hp) for motor output.

A lot of people are hung up on "horsepower", reading a lot of meaning into the use of this unit which is often not justified. It is not limited to mechanical power, and is not the only power unit which is appropriate for mechanical power.