Page 180 - HVAC Pump Handbook

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Pump Drivers and Variable-Speed Drives

Pump Drivers and Variable-Speed Drives 177

7.2.5 Torque and horsepower
Torque is the turning force acting through a radius and is rated in
pound feet. Horsepower is the rate of doing work and is rated in foot
pounds per minute. Thus

1 hp 33,000 (ft lb)/min (7.2)
There are several torques for an induction motor, and they are
described in Fig. 7.1 for torque, speed, and motor current curves.
Locked-rotor torque is also called starting torque. This is the torque
that the motor can develop when at rest or zero speed. Pull-up torque
is the minimum torque developed by the motor as it runs from zero to
full-load speed.
Since most centrifugal pumps in the HVAC field are variable-
torque machines, where the torque varies as the cube of the speed, the
locked-rotor or pull-up torque developed by NEMA design B motors is
adequate for most of these pumps. The only torque required of the

Figure 7.1 Electric motor performance curves. (From AC Motor
Selection and Application Guide, Bulletin GET-6812B, General
Electric Company, Fort Wayne, Ind., 1993, p. 5.)

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