Page 194 - HVAC Pump Handbook

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Pump Drivers and Variable-Speed Drives

Pump Drivers and Variable-Speed Drives 191

The diode rectifier and filter capacitor act to greatly reduce the
harmonic effects on the power system. Although the low-frequency
harmonic currents remain, line notching is virtually eliminated.
The interference potential is much less, but elimination of the low-
frequency current harmonics still requires corrective measures at
increased cost. Power factor is also much better for the PWM drive
than for the six-step units. It is always 90 percent or better and is
nearly independent of motor speed.
Multipulse input circuits. An input transformer or autotransformer can
be used to power two or three input rectifiers to a drive. Phase-shifted
voltages cancel the low-frequency current harmonics and greatly reduce
the line-current distortion. Block diagrams of 12- and 18-pulse sys-
tems are shown in Fig. 7.6. The disadvantage of multipulse systems is
an increase in cost over the simpler systems. In drives over 500 hp,
however, multipulse operation is usually the arrangement of choice. In
general, a 12-pulse drive is sufficient, and the added cost and complex-
ity of an 18-pulse drive are seldom warranted.
Clean power variable-speed drives. A dramatic series of technical devel-
opments has recently yielded variable-frequency drives that will meet
the requirements of IEEE Standard 519-1992 with no filters or multi-
pulse circuitry. All these new PWM drives are based on fast-switching
power transistors. Most use an active filter that detects the current
distortion and injects corrective currents to cancel the harmonics.
Some manufacturers, however, employ an ingenious switching algo-
rithm that eliminates the harmonic currents in the first place. This
arrangement requires transistors in place of the rectifier diodes, but it
needs no active filter. It also permits regeneration for rapid decelera-
tion of an overhauling load. These clean power drives are currently
available through 1000 hp.

Figure 7.6 Twelve- and eighteen-
pulse drives. (From Keith
H. Sneker, private communi-
cation, Halmar Robicon Group,
Pittsburgh, Pa., 1994.)

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