Page 191 - HVAC Pump Handbook

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Pump Drivers and Variable-Speed Drives

188 HVAC Pumps and Their Performance

The early variable-frequency thyristor drives, which were mostly volt-
age source inverters, were sometimes less than ideal in their charac-
teristics; they opened up a huge new field of application in the HVAC
industry. Now, drive designs have matured, but there are still impor-
tant new developments that solve some of the major application prob-
lems on the electrical side.

7.3.2 Types of variable-frequency drives
For many years the variable-frequency drive field has been dominated
by six-pulse voltage-source and current-source thyristor inverters
(Fig. 7.4). Block diagrams of these drives are shown in Fig. 7.4a. Both
generate the output ac voltage by alternately switching between three
pairs of thyristors. Capacitor banks are used to force the load current
to switch from one set to the other. In the voltage-source inverter, a
set of six diodes in a rectifier is used to charge a filter capacitor bank
to a dc voltage equal to the peak output voltage of the load. The filter
capacitor bank serves to isolate the inverter from the ac line. This fil-
ter capacitor bank is, in addition to the capacitors, used for switching.
The current-source inverter replaces the filter capacitor bank with a
large inductor that serves the same purpose of isolation and filtering
but makes the drive more tolerant of line and load disturbances. It also
permits regenerating energy from the load to the line, an important
advantage when high-inertia loads must be brought to rest quickly.
Both these drives, in their basic form, generate a six-step output
waveform, voltage for the voltage source, and current for the current
source. Typical waveforms are shown in Fig. 7.4b. The magnitude of
the voltage is directly proportional to the load frequency so that the
volts-per-hertz ratio remains constant. The motor operates at a con-
stant flux level and, except for ventilation considerations, is able to
operate at constant torque. Harmonics in the output current cause
additional heating, however, and this generally results in a derating
of 10 to 15 percent in horsepower for standard motors.
PWM Drives. The development of large power transistors in the 1980s
spawned a new type of variable-frequency drive (Fig. 7.5). The
acronym PWM stands for pulse-width modulation, a totally different
technique from the six-step unit for obtaining voltage control and a
variable output frequency. Whereas the six-step voltage and current
source drives vary the amplitude of the switched voltage, PWM drives
vary the output voltage by repetitively connecting and disconnecting
a fixed voltage at rapid intervals. The ratio of “on” to “off” periods deter-
mines the voltage magnitude. Figure 7.5a illustrates this process in
generating a 320-V rms, 40-Hz sine-wave approximation from a

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