Page 204 - HVAC Pump Handbook

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Pump Drivers and Variable-Speed Drives

Pump Drivers and Variable-Speed Drives 201

6. Power supply. Variable-frequency drives are like any other com-
mercial power equipment in that they are capable of withstanding the
acceptable variations of commercial power that were defined earlier
in this chapter. Likewise, they may not be able to operate properly
with harmonic distortions in the power supply that exceed the accept-
able percentages established by the IEEE.
7. Number of drives. The simplest arrangement for HVAC pumps is
one drive for each pump that is to be variable speed. More than one
pump can be operated on a single drive. Usually, the cost of transition
equipment to add motors to an operating drive is so expensive that it
is cheaper to purchase multiple drives instead. Also, a multiplicity of
drives provides better standby capability. Almost without exception in
the HVAC industry, a variable-speed drive is furnished for every
pump that is to be variable speed.

7.3.6 Variable-speed drive accessories
and requirements
Following are popular accessories and requirements for variable-
frequency drives:

1. Certification. Variable-frequency drives are often part of pumping
systems that bear the label of various approving agencies such as
Underwriters Laboratories (UL), Electric Testing Laboratories
(ETL), or in Canada, the Canadian Standards Association (CSA).
Care must be taken in evaluating these drives and their accessories
when compliance is required with such standards.
For example, some drive manufacturers have a certain approval
on their basic drives, but they do not have such approval on the
drive’s accessories or enclosures. It is imperative that such approval
be ascertained to avoid expensive field approvals or changes.
2. Bypass starters. Bypass starters were required for variable-frequency
drives when they were new and untried. These drives have
become so reliable and low in cost that standby starters are not
needed on most HVAC installations. Their cost has remained high
so that there are other economical means to achieve standby capa-
bility. Often, adding a standby drive or even an additional pump,
motor, and drive offer a better solution. It is the decision of the
system designer to determine the best solution for a particular
installation. If a standby starter is determined to be the optimum
solution for standby power, it should be designed around the fol-
lowing recommendations.
a. Configuration. Bypass starters are often used to keep a pump in
operation during failure of the variable-speed drive. The designer

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