356 Electrical Power Systems Technology
2 πNT
hp = ————
33, 000
NT
= ———
5252
where:
HP = the horsepower rating,
2 π = a constant,
N = the speed of the motor in revolutions per minute (rpm), and
T = the torque developed by the motor in foot-pounds.
Sample Problem: Power of a Motor
The mechanical power output of a motor is dependent upon the
speed of rotation and the torque produced. Power output is expressed as:
n × T
P = ———
9.55
where:
P = mechanical power in Watts,
n = speed of rotation in revolutions per minute (r/min),
T = torque in newtons per meter (N/m), and
9.55 = a constant equal to 30/pi.
Given: a motor rotating at 3,600 r/min produces a torque of 5N/m.
Find: the power output developed by the motor.
Solution:
n x T
P = ——
9.55
3,500 × 5
= ————
9.55
P = 1,885 watts
W
power in HP = ——
746