Page 183 - HVAC Pump Handbook

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Pump Drivers and Variable-Speed Drives

180 HVAC Pumps and Their Performance

group D) motors may have a service factor of 1.0 and no overload or
1.15. The manufacturer or supplier of a motor should verify the actual
service factor for a motor. Also, the nameplate for existing motors
should be inspected to confirm the allowable service factor.

7.2.8 Power factor
The power factor for a three-phase motor recognizes the magnetizing
current of the motor. Its equation is

watts applied
Power factor (7.4)

3 volts amps
Power factor is a characteristic of a polyphase electric motor’s opera-
tion. Electric utilities may charge penalties for low power factor. Like-
wise, some governing bodies may establish a minimum power factor.
Power factor can be improved by purchasing motors with high
power factor ratings, providing no loss in efficiency results. A more
satisfactory method may be the installation of power factor correction
capacitors. Total power factor correction programs are within the
province of the consulting electrical engineer, but the HVAC designer
should have a working knowledge of power factor and its correction.
One advantage for some variable-speed drives, particularly pulse
width modulation type, is that the input power factor of the variable-
speed drive and the motor as a combination is equal to the power fac-
tor of the drive itself. Most of these drives have a power factor close to
95 percent. More on power factor will be discussed later in Sec. 7.3 of
this chapter.

7.2.9 Motor efficiency
Motor efficiency is an important design consideration and has been
the object of much redesign and rating in the motor industry. The
equation is
hp output 746
Motor efficiency (7.5)
watts input
IEEE has established and NEMA has adopted Standard 112, Method
B for the Testing of Electrical Motors. This test establishes uniform
methods of testing and rating electric motors and is the basis for rating
motors for compliance with the latest government requirements.
The Energy Policy Act of 1992 establishes nominal full-load effi-
ciencies for both open and closed motors. Table 7.3 lists these efficien-
cies for up to 200-hp motor sizes. These efficiencies apply to all motors
manufactured after October 1997.

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