I'm putting together an electric power unit for a customer, and have come across something that has me confused. I have a 3 HP, 3 phase, 460v electric motor as a prime mover, but the current draw doesn't seem to make sense.

The label says it draws 3.9 amps at full load. According to my calculations, 3.9 amps at 460 volts is only 2.4 horsepower. What gives? Is there something different about how 3 phase electrical components are measured?

Wow, nobody? Peter, no input?

Josh;

You have probably figured out I can't help but I can offer a forum site that can.

Try the PLCS forum here:

http://www.plctalk.net/

Also there are several other sites for electrical questions.

It turns out, each phase has its own voltage. You take the square root of the number of phases, in this case 3, then divide that into the line voltage.

3 phase divisor is 1.732 (square root of 3).
line voltage ÷ 1.732 = phase voltage
460 ÷ 1.732 = 265.6 volts.
Electrical horsepower = I × E ÷ 746
3.9 amps × 265.6 volts ÷ 745 = 1.389 horsepower
3 phases × 1.389 horsepower = 4.17 horsepower
I'm told you must now factor in "power factor," which is 0.9, as well as motor efficiency, which will give you around 3 horsepower as listed.