AC Tech MC1000 series - Page 15

13
CONSTANT TORQUE
CONSTANT HP
TORQUE
HORSEPOWER
HORSEPOWER
TORQUE
FREQUENCY (Hz)
TORQUE (%)
20 40 60 80 100 120
150
130
110
90
70
50
30
10
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 diagram below
illustrates these relationships:
WARNING!
Consult motor manufacturer before operating motor and/or driven equipment
above base speed.