Variable Torque vs. Constant Torque: Ratings and Programming
Copyright © 2004 Francis J. Martino
A typical rating for a VFD is 15 HP, 21 Amps with a constant torque load and
that same drive is also rated 20 HP, 27 Amps when used with a variable
When operating a 20 HP motor on a variable torque load, the assumption is
that the load will require an average speed and torque that are less than 100%
If the load operates at 90% of maximum speed then the load will be 0.90 cubed
of the maximum or, 0.9 x 0.9 x 0.9 = .729 of maximum output as per the affinity
curve of a pump or fan.
Thus, it may be assumed that the motor will average .729 x 20 HP = 14.58 HP.
The operation of the motor with a variable torque load is then found to be
within the rating and heat dissipation capability of the drive.
With a high static head, as is discussed in Elusive Energy Savings: Centrifugal
Pumps and Variable Speed Drives - Part I, the pump system curve will not follow
the cubed function of the affinity curve. The application will require a higher
average speed and torque that may exceed the rated heat dissipation of the drive.
The drive temperature will then be elevated.
However, the extra heat will not shorten the lifetime of the drive so as to cause
it to fail during the warranty period.
Such are the ratings based on the affinity curves.
Based on the pump system curve:
If the system curve has a high static head and the system curve crosses to the
left of the affinity curve, as shown in the above referenced article, then the
motor will be overloaded by virtue of the squared function of the variable torque
programming causing the motor to be starved for volts. (Note that the variable torque
programming provided by the drive will be a square function while the pump
affinity curve is a cubed function.)
See also the second paragraph of footnote 3 of:
Energy Savings: Solid-State Reduced Voltage Starters vs. VFDs
The motor may well be sized properly for the application but the drive is in need
of being programmed for constant torque.
If the drive is overheating it is because of a high static head and, therefore,
the drive should have been selected on the basis of constant torque.
If the motor is overheating it is because the motor is operating with excessive
slip due to the low voltage that is caused by the variable torque programming.
The drive should be programmed for constant torque.