Page 177 - HVAC Pump Handbook

Rishel_07.qxd 20/4/06 6:29 PM Page 174

Pump Drivers and Variable-Speed Drives

174 HVAC Pumps and Their Performance

the pump manufacturers for operation with 50-Hz motors at induction
motor speeds of around 960 and 1450 rev/min, compared with 1150 and
1750 rev/min for such motors operating on 60-Hz power.
Almost all electric motors in the HVAC field are induction-type. Large
cooling tower or condenser motors over 1000 hp may be synchronous-
type, running at 1200 and 1800 rev/min. Wound rotor and direct-current
motors offer few advantages for most of these pump applications.

7.2.1 Electric motor power characteristics
All electric motors are designed to operate with some variation in the
electric power supplied to them. Following are some general rules for
the variation in power characteristics:
1. Voltage variation must be limited to 10 percent at rated frequency.
2. Frequency variation must be limited to 5 percent at rated voltage.
3. Combined variation of voltage and frequency must be limited to
the arithmetic sum of 10 percent.
4. On polyphase motors, the voltage imbalance should be kept within
1 percent between phases. Table 7.1 describes the great decrease in
motor rating caused by greater imbalances. Balancing of loads
between phases in a building becomes a necessity to ensure maxi-
mum output from the polyphase electric motors.

TABLE 7.1 Derating Factors Due to Voltage
Imbalance for Polyphase Motors
Percent voltage imbalance Derating factor
0.0 1.00
0.5 1.00
1.0 1.00
1.5 0.97
2.0 0.95
2.5 0.93
3.0 0.89
3.5 0.85
4.0 0.82
4.5 0.78
5.0 0.76
SOURCE: AC Motor Selection and Application
Guide, Bulletin GET-6812B, General Electric
Company, Fort Wayne, Ind., 1993, p. 3; used with
permission.

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.