You have a couple of options. HP is speed times torque. Whatever motor you use, you can let it float just a little and measure the rotational force "torque" on a weight scale at the rpm. A big enough motor and you can apply a DC braking voltage to the AC coils. Or, you can fudge a little and just generate a voltage with it and measure the power generated. You will have to get rid of 4KW of heat.
Or, in accordance with commercial standards where 4 or 5 HP is available from a normal 120VAC outlet (~1.8kw), measure the peak stall torque and (separately) the maximum redline RPM, and use these two values to calculate mythical HP.
Haven't you guys seen the various consumer products that claim more HP than is available from the standard 120 VAC outlet that they plug into ? Things like exercise equipment, air compressors, etc.
"5 HP motor" (sic) running from a normal 120 VAC, 15 Amp outlet.
Yes, I've seen some vacuum cleaner motors claiming 4 or 5 HP from a 120V 15Amp circuit. Is this a peak power representation or continuous duty. If continuous duty, how's that possible to produce 4 -5 HP from 120V @ 15A? It would seem to me that even with PF of 1 and eff of 100% the max you could get out of 120V @ 15A is (IxExPFxEFF)/746 => (120 x 15 x 1 x 1)/746 = 2.41 HP. Real world performance would be even lower because actual efficiency would not be 100% and PF would be less than 1.
Back to the thread, using an AC generator for a dyno -- the measurement of output power from the generator is the tricky part. "Normal" watt/amp meters can be fooled by the varying frequency output that you will be generating (unless you only plan to operate at the motor/generator synchronous speed -- 3600 rpm for most?). Not sure that the voltage won't vary with speed as well? You'll need to measure voltage/current with high speed o-scope or DAQ card. Putting a 1 to 4-kw variable load onto the generator should be fairly easy: go buy a bunch of lamp sockets at the hardware store, along with a supply of a range of bulbs of varying wattages. Wire all the sockets onto a board, and add/subtract load by screwing in or removing lamps. Remember, though, you may not be able to measure output power by summing the lamp wattages.
btrueblood: You should be able to use "true" RMS meters and multiply the readings, providing you keep the loads resistive. Don't know how resistive tungsten bulbs are.
And yes, I remember the first time I saw a SIX HORSEPOWER Sears air compressor. Noting the motor was about a third smaller than the 3HP on my home brew. Then I looked in the cooling ports to see that the motor actually filled only the front thrird of the the actual motor housing to boot!
You don't need to measure power out put from the alternator to measure the HP.
The tricky thing is to be able to apply a good load to the alternator. What you do is mount it just like a commerical dyno is mounted. And read the torque output. Either with a load cell or scale against a torque arm.
