Page 182 - HVAC Pump Handbook

Rishel_07.qxd 20/4/06 6:29 PM Page 179

Pump Drivers and Variable-Speed Drives

Pump Drivers and Variable-Speed Drives 179

With the preceding three taps of 80, 65, or 50 percent, the start-
ing current is limited to approximately 125 to 390 percent of full-
load current. This starter is large and expensive.
2. Primary resistor. Starting characteristics are fixed; starting cur-
rent is 390 percent of full-load current. This starter has an added
power loss in the resistors.
3. Part winding. Starting characteristics are fixed; starting current is
390 percent of full-load current. It requires special motor connec-
tions for 460-V service. It is the smallest and least expensive reduced-
voltage starter.
4. Wye delta. Starting current is approximately 200 percent of full-
load current; starting torque is low, only 33 percent of full-load
torque. This starter requires special motor design and more com-
plex control than the other reduced-voltage starters.
5. Solid state. Solid-state starters are being used on both constant-
speed and variable-speed pumps due to their adjustability and
size. A typical range of adjustable starting currents is from 100 to
400 percent of full-load current. They may or may not be less expen-
sive than other types of reduced-voltage starters.
6. Variable-frequency drive. The variable-frequency drive has proved
to be an excellent method for reducing the current inrush. Since
the starting torque on most HVAC pumps is limited to inertia
2
torque WR , the variable-frequency drive can be used on almost all
HVAC pump motors. It is usually more expensive at this date than
other reduced-voltage starters; its ability to be programmed into
the overall duty cycle of HVAC pumps often results in its selection
due to the energy savings achieved through variable speed.

Due to special motor windings, the part winding and wye delta
types of reduced-voltage starters cannot be used as standby starters
for variable-speed drives at 460-V power supply.

7.2.7 Output horsepower
A motor is designed to produce its rated horsepower with nameplate
voltages and frequency applied to its power terminals. Most HVAC
pump motors have a service factor that has been developed by NEMA
for standard polyphase motors. These service factors are defined as
the permissible overload beyond nameplate rating with standard
voltage and frequency. Most NEMA design B, open-frame, drip-proof
motors have a service factor of 1.15 up to 200 hp and 1.0 for over 200
hp. Totally enclosed, fan-cooled (TEFC) and explosion-proof (class 1,

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