Page 178 - HVAC Pump Handbook

Rishel_07.qxd 20/4/06 6:29 PM Page 175

Pump Drivers and Variable-Speed Drives

Pump Drivers and Variable-Speed Drives 175

7.2.2 Motor output ratings
The following discussion pertains to induction-type electric motors,
since most of the pump motors in the HVAC industry are of this design.
These ratings are for 104°F ambient air and for motors installed
between sea level and 3300 ft of elevation. For motors installed at
higher elevations:

1. A motor with a service factor of 1.15 can be operated at higher ele-
vations with a service factor of 1.0.
2. A motor can be operated at its normal service factor at higher ele-
vations at lower ambient temperatures. Refer to the manufacturer’s
nameplate information.
3. Medium- and high-voltage motors should be operated at higher
elevations with caution due to a corona effect.
All high-altitude motor operations should be referred to the motor
manufacturer for approval.

7.2.3 Motor speed
The speed at which an induction motor will operate depends on the
input power frequency and the number of electrical magnetic poles for
which the motor is wound. The speed of a pump varies linearly with
the frequency; the higher the frequency, the faster the motor runs.
This is how the present electronic-type variable-speed drives are
designed and why they are therefore called variable-frequency drives.
Conversely, the more magnetic poles in a motor, the slower it runs.
For an induction-type motor, the speed of the rotating magnetic field
in the stator is called the synchronous speed, and it is determined by
the following equation:
60 2 frequency
Synchronous speed, rev/min (7.1)
number of poles

For example, a motor operating at 60 Hz with four poles would have
a synchronous speed of

60 2 60
1800 rev/min
4
In the pump industry, a 1200 rev/min motor is called a six-pole machine,
an 1800 rev/min motor, a four-pole machine; and a 3600 rev/min
motor, a two-pole machine.

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.