*
For additional assistance, please contact Toshiba Adjustable Speed Drive Marketing Dept. at (800) 872-
2192
67
27
Overspeed
Introduction
As the user changes motor RPM by commanding different output frequencies, the G3 varies its output
voltage in order to maintain a constant volts per hertz ratio. The torque a motor develops is proportional to
this ratio:
T
mtr
~ Voltage (volts)
Frequency (Hz)
On a 460 volt drive, the drive outputs 460 volts at 60 Hz; the motor can develop its rated torque. The drive
outputs approximately 230 volts at 30 Hz to allow the motor to develop its rated torque at half speed.
OUTPUT FREQUENCY (HZ)
Item 2,
BASE FREQUENCY #1
(typically 60 Hz)
Rated Torque, Rated Voltage
Volts (and HP)
Torque
Many users would like to run motors at speeds higher than the motor design speed. The G3 has no
problem outputting up to 400 Hz (contact Inverter Marketing for information on 800 Hz operation). Over
60 Hz, however, the G3 cannot “make” voltage to keep the V/Hz ratio constant (voltage becomes fixed at
the line value of 460 volts). Therefore, as frequency increases, the torque that can be developed by the
motor decreases. This guideline will discuss a motor’s torque producing ability at frequencies greater than
its design frequency.
Test Results
The above volts/Hz relationship could lead one to estimate torque available in overspeed by an inverse
relation. For instance to estimate continuous torque at 90 Hz,
T
90
= T
60
X (60/90) = 67% T
60
