Page 181 - HVAC Pump Handbook

Rishel_07.qxd 20/4/06 6:29 PM Page 178

Pump Drivers and Variable-Speed Drives

178 HVAC Pumps and Their Performance

motors for these pumps is the inertia of the motor rotor and the pump
2
rotating element. This is called the starting-load inertia WR for the
2
pump shaft and impeller and WK for the motor rotor and shaft. This
inertia is low enough for most centrifugal pumps used in this field
that it is of no concern. On very large condenser pumps with large-
diameter impellers, it may be necessary to check the inertia, particularly
when reduced-voltage types of starters with reduced starting torques
are used.
Full-load torque is the torque required to produce the rated horse-
power of the motor at full-load speed in pound feet. The equation for
this torque is
bhp 5250
Full-load torque, lb ft (7.3)
max.rev/min
where bhp maximum brake horsepower
rev/min maximum speed

7.2.6 Motor currents
Figure 7.1 describes a typical motor current curve for a NEMA design
B motor. The locked-rotor current is the maximum steady-state cur-
rent that the motor will draw at 0 rev/min with rated voltage and fre-
quency applied to the power terminals. NEMA has developed a set of
code letters for various locked-rotor amperes for electric motors.
These code letters run from A to V. The code letter for most HVAC
pump motors is G, which indicates that the locked-rotor current is
approximately 650 percent of the full-load current of the motor. This
code letter appears on the nameplate of every NEMA-rated motor.
Full-load current is the steady-state current of a motor operating at
full-load torque with rated voltage and frequency applied to the power
terminals of the motor.
There may be several reasons for reducing the locked-rotor or start-
ing current. The electric utility may require it or the electrical distrib-
ution of the building may be better served with reduced starting cur-
rents. A number of different devices are available for this, and they
are known as reduced-voltage starters.
1. Autotransformer. This method provides several taps to adjust the
starting voltage.

Starting voltage at motor, % Line current, % Starting torque, %
80 64 64
65 42 42
50 25 25

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