I have a question regarding electric motor ratings. I recognize that manufacturers don't all spec their motors the same, so here is what I typically do to get a rough idea of HP rating:

conversion: 1 HP(electric) = 746 Watts

Name Plate: Voltage = 230V (RMS)

Full Load Current Draw = xx.x AMPS

I multiply 230V by the full load current draw to get the power, then divide by 746 to get my approximate HP rating. I must say, it does seem to give "ballpark" estimates. However, comparing this HP rating only occasionally gets me "real close" to the HP rating on the motor and almost NEVER close enough to be considered equal, regardless of whether it's a single phase or 3 phase motor. "Regular" motors typically over-estimate the HP rating by my standard (my calculated rating is typically between 0.75 and up to 2-3 HP lower than nameplate rating when avoiding the crap "peak HP" rating so prevalent, particularly with Sears), whereas air compressor motors are typically underestimated (my calculated HP rating is higher than the nameplate HP rating)

Does anyone know how an honest manufacturer tests and specifies these figures, or do you think it the difference between the power input (which is what my calculation computes) and power output?

btw, the 5HP motor on the rotary phase converter I built draws ~62amps peak when I start it up (probably less than 0.2 seconds) -- that would make it about a 19HP motor according to "Sears Standards". It draws only 2 amps when up to speed and under no load.

conversion: 1 HP(electric) = 746 Watts

Name Plate: Voltage = 230V (RMS)

Full Load Current Draw = xx.x AMPS

I multiply 230V by the full load current draw to get the power, then divide by 746 to get my approximate HP rating. I must say, it does seem to give "ballpark" estimates. However, comparing this HP rating only occasionally gets me "real close" to the HP rating on the motor and almost NEVER close enough to be considered equal, regardless of whether it's a single phase or 3 phase motor. "Regular" motors typically over-estimate the HP rating by my standard (my calculated rating is typically between 0.75 and up to 2-3 HP lower than nameplate rating when avoiding the crap "peak HP" rating so prevalent, particularly with Sears), whereas air compressor motors are typically underestimated (my calculated HP rating is higher than the nameplate HP rating)

Does anyone know how an honest manufacturer tests and specifies these figures, or do you think it the difference between the power input (which is what my calculation computes) and power output?

btw, the 5HP motor on the rotary phase converter I built draws ~62amps peak when I start it up (probably less than 0.2 seconds) -- that would make it about a 19HP motor according to "Sears Standards". It draws only 2 amps when up to speed and under no load.

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