Page 205 - HVAC Pump Handbook

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Pump Drivers and Variable-Speed Drives

202 HVAC Pumps and Their Performance

must determine how critical the standby operation of a pump is
during drive failure. The designer should not just indiscriminately
put a bypass starter on every drive.
The designer also must realize that the use of bypass starters
forces the pump to full speed when operating with the bypass
starter. On high-pump-head applications, it may be necessary
for the operator to adjust the pump discharge manual valve to
alleviate some of the pressure on the water system.
Facilities with critical operations such as hospitals, computer
centers, and research facilities should have consideration for
these starters. It is doubtful that commercial buildings or edu-
cational facilities require them. It is the responsibility of the
designer to make this decision.
Bypass starters must be designed properly. First, the decision
must be made as to whether across-the-line starting is accept-
able or whether some form of reduced-voltage starting is required.
When this decision is under consideration, if reduced-voltage
starting is required, the type used must have the same motor
wiring as that required by the variable-speed drive. Usually,
this eliminates part-winding or wye delta types of starters on
460-V power service. The solid-state starter is proving to be a
very acceptable means of accomplishing reduced-voltage
starting.
The arrangement of the bypass starter is critical to ensure
that adequate safety is provided to the operator or service per-
sonnel. Figure 7.16b describes a bypass starter arrangement
that provides some safety for operators. Although it is recom-
mended that the variable-speed drive be serviced with both the
drive and bypass starter depowered, it is recognized that this is
impossible on some facilities. Only qualified electricians should
service this equipment. As indicated in the previous section, it
is imperative that the bypass starter, its enclosure, and the
arrangement be approved by the same standard as the basic
drive.
b. Starting. Transferring pump operation from a variable-speed
drive to a bypass starter can be accomplished two ways, manu-
ally and automatically by transferring the pump motor from
the drive to the bypass starter and returning the pump to full
speed.
(1) Manual transition. Under this procedure, the operator rec-
ognizes a failure because the pump is usually stopped on
drive failure. The power is transferred manually from the
drive to the bypass starter, and the pump is returned to

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