Page 190 - HVAC Pump Handbook

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Pump Drivers and Variable-Speed Drives

Pump Drivers and Variable-Speed Drives 187

that the pump should not overload the motor beyond its nameplate
rating, a 40-hp motor would be required with a drive equaling the
nameplate rating of the motor in amperage.
With the running limit of the drive, it is possible to furnish a 25-hp
motor and drive since that is the motor horsepower required at the
design condition of 700 gal/min at 116 ft. The drive prevents the pump
from overloading the motor beyond 25 hp. The pump follows the motor
horsepower curve, not the pump curve. This does not guarantee higher
pump performance, i.e., efficiency, since that is determined by the
speed control and sequencing described in Chap. 10, “Basics of Pump
Application.”

7.3 Variable-Speed Drives
for Electric Motors

7.3.1 History
Up to about 1970, variable-speed drives in the HVAC industry usually
meant eddy-current or fluid couplings between a fixed-speed induc-
tion motor and a variable-speed pump. The coupling was controlled
electrically or hydraulically to allow a variable slip between the motor
speed and that of the coupled load. These drives established a high
order of reliability and were quite satisfactory for pump loads in
which the torque requirements dropped off rapidly as the speed was
reduced. In such applications, the losses inherent in the coupling
between the motor and the pump were offset by the reduction in
losses due to overpressuring caused by a constant-speed pump. For
new installations, however, this arrangement has largely been super-
seded by variable-frequency drives. On applications that have environ-
ments hostile to variable-frequency drives, mechanical variable-speed
drives should still be used. This includes dusty or corrosive atmos-
pheres and high ambient temperatures where it is impossible to pro-
vide adequate cooling for the variable-frequency drive.
The advantages of variable-frequency drives (VFDs) for fans,
pumps, and chillers have been known for many years. They permit
the use of simple, reliable, and inexpensive induction motors yet pro-
vide the operating economies of variable speed. Unfortunately, motor
generator sets, thyratrons, and ignitrons, the only methods of obtain-
ing a variable-frequency source, were too expensive for all but the
most critical applications.
The invention of the thyristor (SCR) in the mid-1960s changed the
picture dramatically. Here was a device that could control power at
the megawatt level yet was both economical and reliable.
Variable-speed drives soon appeared for direct-current (dc) motors
and shortly thereafter for alternating-current (ac) induction motors.

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