Table of Contents
Advertisement
A 3 HP 460 volt G3 was used to run a 3 HP TEFC 4 pole EQP3 at 60, 90, 120, and 150 Hz. At rated motor
current, the following maximum continuous torques were developed:
Hz
Actual % Full-Load Torque
60
90
120
150
As motor RPM increases:
The torque required to overcome bearing friction increases at the same rate.
The torque required to move the motor's fan increases as the square of the speed change.
Above 60 Hz, the motor's breakdown (maximum) torque decreases as the square of the speed change.
The motor's impedance increases at the same rate the frequency increases. As the impedance
increases, the current decreases (because more voltage isn't available), thereby decreasing the
available torque.
For this particular motor, at 150 Hz it appears that the breakdown torque (which decreases as the square of
speed increase) has "crossed" over the full-load torque and is now the limiting factor for torque
development.
Recommendations:
On applications requiring output frequencies up to 90 Hz, derate the motor's rated full-load torque by
the ratio of base frequency to maximum operating frequency
On applications requiring output frequencies greater than 90 Hz, derate the motor's full-load torque by
the square of the ratio of base frequency to maximum operating frequency.
Check with motor manufacturer concerning overspeed operation. Rotor balance and bearing life
are of concern when overspeeding a motor.
Example
The following are calculations to determine if test motor can develop rated HP at 300 Hz.
Breakdown torque is the maximum amount of torque a motor can develop for a short time before it stalls.
The breakdown torque for this 3 HP motor is 33.6 ft-lbs.
Take 80% of this value to prevent operation on the edge of a stall:
0.8 x 33.6 = 27 ft-lbs
Derate motor torque at 300 Hz:
27 ft-lbs x (60/300)
RPM of motor at 300 Hz:
1800 x (300/60) = 9000 RPM
For additional assistance, please contact Toshiba Adjustable Speed Drive Marketing Dept. at (800) 872-
182
97
56
36
2
= 1.08 ft-lbs
Predicted % Full-Load Torque
100
67
50
40
2192
68
Advertisement
Table of Contents
