Specifications

15
If the frequency applied to the motor is increased while the voltage remains constant, torque
capability will decrease as speed increases. This will cause the horsepower capability of the
motor to remain approximately constant. Motors run in this mode when operated above base
speed, where drive output voltage is limited by the input line voltage. This operating range
is known as the “constant horsepower” range. The typical maximum range for constant
horsepower is about 2.3 to 1 (60 to 140 Hz). The diagram below depicts the characteristics
of a typical AC induction motor with a 60 Hz base speed.
6.1.1 VARIABLE TORQUE VS. CONSTANT TORQUE
Variable frequency drives, and the loads they are applied to, can generally be divided into
two groups: constant torque and variable torque. Constant torque loads include: vibrating
conveyors, punch presses, rock crushers, machine tools, and just about every other
application that is not considered variable torque. Variable torque loads include centrifugal
pumps and fans, which make up the majority of HVAC applications.
Variable torque loads are governed by the affinity laws, which define the relationships
between speed, flow, torque and horsepower. The following diagram illustrates these
relationships:
Warning
Consult motor manufacturer before operating motor and/or drive equipment above
base speed.
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