Page 188 - HVAC Pump Handbook

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Pump Drivers and Variable-Speed Drives

Pump Drivers and Variable-Speed Drives 185

relatively clean indoor applications, whereas weather-protected
motors can be used outdoors and are so constructed to minimize the
entrance of rain, snow, and airborne particles. Drip-proof motors are
adequate for most HVAC equipment room installations.
Totally enclosed, fan-cooled motors are cooled by an external fan
mounted on the motor shaft. This enclosure is available for applica-
tions where the motor is wetted periodically. It should not be used for
routine motor applications in this industry. Usually, it does not have
any higher efficiency than an open-frame drip-proof motor.
Explosion-proof class 1, group D construction is for applications
where the ambient air may contain combustibles such as gasoline,
petroleum, naphtha, or natural gas. There are other types of explosion-
proof constructions for other hazardous atmospheres. Most HVAC
atmospheres do not contain any combustibles. If a specific hazardous
material is encountered, the insuring agency should verify the correct
motor enclosure. These motors should not be used with variable-
frequency drives without approval of the motor manufacturer. They
may not be certified for this use.
Vertical motors are available in two physical constructions, vertical
hollow shaft (VHS) and vertical solid shaft (VSS). The hollow-shaft
construction is usually applied to vertical turbine pumps due to the
ability to adjust the lateral setting on these pumps without disassem-
bling the motor. This motor has a top drive coupling with an adjust-
ment nut that provides this adjustment.
Most motors for HVAC pumps are single speed. Although two-speed
motors are available for these applications, the variable-speed drive
has all but eliminated the use of multispeed motors for these pumps.
The added cost of the special two-speed motor and two-speed starter
is usually less than the cost of a variable-speed drive. However, the
complex control and the energy loss due to only two speeds, i.e., two
pump head-flow curves, usually result in the selection of a variable-
speed drive instead of a two-speed motor. Another disadvantage for
two-speed motors on large HVAC water systems is the hydraulic
shock that is produced when the motor changes from one speed to the
other.

7.2.12 Motor sizing for constant and
variable speed HVAC pumps
The important factors on most HVAC installations in the proper sizing
of an electric motor are long life and efficient operation. These factors
dictate that the motor should not operate beyond its nameplate rating.
For example, assume that a pump is selected for 2000 gal/min at 125 ft

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