Page 206 - HVAC Pump Handbook

Rishel_07.qxd 20/4/06 6:29 PM Page 203

Pump Drivers and Variable-Speed Drives

Pump Drivers and Variable-Speed Drives 203

full speed. This procedure has several advantages: (a) the
operator can inspect the equipment and ensure that it is
acceptable to return the pump to duty, and (b) the operator
can adjust the discharge valve on the pump to avoid the
imposition of excessive water pressures on the system.
This is the recommended procedure for utilizing standby
starters with variable-speed drives. The use of multiple
pumps in parallel provides the best method of handling
variable-speed drive failure.
(2) Automatic transition with stopped pump. If the water sys-
tem cannot accept a stopped pump until an operator can
transfer the pump to a bypass starter, the pump motor can
be transferred automatically. The pump-control system,
upon sensing a drive failure, transfers the pump motor to its
standby starter and starts the pump. This has disadvan-
tages, since there is no visual inspection during the transi-
tion, and there may be a possibility that the pump operat-
ing at full speed may overpressure the water system.
3. Drive enclosures. Standard NEMA 1 enclosures are adequate for most
indoor HVAC installations of variable-speed drives (see Fig. 7.16b).
Figure 7.16b describes a typical variable-speed drive assembly
with control center and bypass starters. There is seldom any need
for closed, nonventilated enclosures such as NEMA 3, 3R, or 4.
Special ambient conditions such as the presence of water may
require these enclosures. For hazardous locations, NEMA 7 or 9
enclosures may be required. Local and insurance codes will dictate
the use of these special types of enclosures.
If there is a need for a nonventilated enclosure for use with internal
air-conditioners or ducted cooling air, the Nema 12 type is usually the
best enclosure (see Fig. 7.16a).
4. Instrumentation. All drives should have at least the following
instrumentation.
a. Ammeter for supply power
b. Percent speed meter
c. Hand-off-automatic switch
d. Manual speed potentiometer
e. Common fault alarm
Some drives have a number of diagnostic indications and proce-
dures that replace the common fault alarm. Others provide addi-
tional information digitally.
5. Control. Variable-speed drives can be furnished with internal
microprocessors for controlling the speed of pumps. However,
pump speed control is only part of the pump-control algorithm.

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.