Page 189 - HVAC Pump Handbook

Rishel_07.qxd 20/4/06 6:29 PM Page 186

Pump Drivers and Variable-Speed Drives

186 HVAC Pumps and Their Performance

of head and a 75-hp motor is fully loaded at the pump’s selection point.
If the pump can carry out to (operate at) capacities greater than 2000
gal/min, the brake horsepower required will exceed 75 hp, and the
motor will be overloaded. For long life and efficient operation, a 100-hp
motor should be selected for this pump. It is recommended that the
specification for electric motors for pumps include the statement that
the motor will not be overloaded beyond its nameplate rating when the
pump operates at any point on its head-flow curve.
The above recommended procedure is necessary for all constant-
speed pumps. Fortunately, motors equipped with variable-frequency
drives can be sized to the brake horsepower required at the pump
design condition. The drive can be sized by the maximum motor horse-
power at this design condition.
A simple control technology is now available that eliminates the
normal oversizing of pump motors and drives. This is the development
of the “running limit” that prevents the motor from overloading the
drive. For example, in Fig. 7.3 if we must comply with the requirement

RUNNING LIMIT OF VARIABLE
SPEED/DRIVE FORCES PUMP
TO FOLLOW THE HORSEPOWER
CURVE, NOT THE PUMP CURVE.

200 25 HP 30 HP 60 65
12.8" 40 HP
70
175 20 HP 75 80 50 HP
12.0" 15 HP 84 84 80 75 70
TOTAL HEAD IN FEET 125 11.2"
150

10.4" 10 HP
9.6"
100
8.8"
75
50
25

0
5 6 10 20
28 27 23 13
0 200 400 600 800 1000 1200 1400
GALLONS PER MINUTE
Figure 7.3 Running limit for variable frequency drives.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.