Page 201 - HVAC Pump Handbook

Rishel_07.qxd 20/4/06 6:29 PM Page 198

Pump Drivers and Variable-Speed Drives

198 HVAC Pumps and Their Performance

1. Size in kilovoltamperes and percentage impedance of the distribu-
tion transformer
2. For current source and voltage source drives, the wire sizes and
lengths between any sensitive equipment and the distribution
transformer as well as wire sizes and lengths between any sensi-
tive equipment and the variable-speed drive installation (Wire
sizes and distances between variable-speed drives should be
secured where more than one is installed.)

Most public utilities or electrical designers will specify a maximum
allowable harmonic voltage distortion in percent for a specific instal-
lation. Typically, this can be 3 or 5 percent.

7.3.4 Advantages of variable-frequency drives
Variable-frequency drives are available for nearly any HVAC applica-
tion and have been the preferred means of varying the speed of a
pump. They have become the drive of choice for new applications for
many reasons, such as

1. First cost is lower in most sizes.
2. All are air-cooled. Larger slip-type drives require water cooling.
3. Wire-to-shaft efficiency is much higher than any slip-type drive.
For typical wire-to-shaft efficiencies for variable-frequency drives,
see Table 10.2.
4. It is very easy to integrate drive-control software into the software
of pumping systems or building management systems.

7.3.5 Application of variable-speed drives
The application of variable-frequency drives requires some care to
ensure proper operation and reasonably useful life. The manufac-
turer’s installation instructions should be reviewed carefully.
Following are some of the more pertinent concerns for installation.
Contemporary variable-speed drives are very reliable and will provide
years of uninterrupted service if installed and operated properly.
1. Ventilation. On variable-torque applications such as centrifugal
pumps, the heat expended by the variable-speed drive can be computed
easily.

Btu/h max. kW of drive 3412 (1 E ) (7.6)

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