Chuck Yung
EASA Technical Support Specialist
You may have experienced this situation before: Your best customer just called to say he has a plant down and is in desperate need of a 100 HP, 6-pole, 460-volt motor. And he needs it now!
You check your stock and find one that's pretty close. The frame is right, but it's a 125 HP unit at 575 volts. What can you do? You know that the motor will run at 460 volts, but how much horsepower will it produce?
This formula should simplify those calculations, and help you bail out that customer:
HP1 x (V2/V1)2 = HP2
Where:
HP1 = Rated horsepower
V1 = Rated voltage
HP2 = Horsepower at line voltage
V2 = Line voltage
To follow through with our example, that 125 HP motor rated 575 volts would produce only 80 HP when operated at 460 volts, or 87 HP operated on 480 volts.
On the other hand, a 150 HP motor should work:
150 x (460/575)2 = 96 HP
Verifying the customer's actual line voltage would make a big difference, too. For example, operating the same motor at 480 volts produces an additional 9
HP in this case: 150 x (480/575)2 = 105 HP
Those 20 volts make quite a difference. Don't assume as many do that the ratio of HP to voltage is linear. That would result in a failed motor and an unhappy customer!
You might also use the formula to drive home the point you've been trying to make about low line voltage. This applies especially to construction/portable plants where long distances from distribution to the motor are involved. The derate factor can be calculated for each specific situation. When considering motors running at or near full load, it may shed light on that "problem motor" that keeps failing.
