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PWM Drives The development of large power transistors in the 1980s
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spawned a new type of variable-frequency drive (Fig 75) 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 determines the voltage magnitude Figure 75a illustrates this process in generating a 320-V rms, 40-Hz sine-wave approximation from a
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Pump Drivers and Variable-Speed Drives Pump Drivers and Variable-Speed Drives 189
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Figure 74 Current-source and voltage source drives (From Keith H Sneker, private communication, Halmar Robicon Group, Pittsburgh, Pa, 1994)
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675-V constant-potential bus This is an actual requirement for a PWM drive on a 480-V, 60-Hz motor The switching frequency is about 1 kHz in this example At the beginning and end of the 40-Hz sine wave, the switching is mostly off, but the duty cycle rises to two-thirds at the peak to generate
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Pump Drivers and Variable-Speed Drives 190 HVAC Pumps and Their Performance
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Pulse-width-modulating (PWM) drives (From Keith H Sneker, private communication, Halmar Robicon Group, Pittsburgh, Pa, 1994)
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the required 453-V peak for 320-V rms In this fashion, the output voltage frequency and magnitude are controlled for the motor More sophisticated switching techniques are used in practice, but this example illustrates the basic approach Motor current is nearly sinusoidal A PWM drive consists of a diode rectifier, a filter capacitor bank, and a set of six switching transistors A simplified schematic is shown in Fig 75b The rectifier diodes charge the filter capacitors to a constant potential for approximately 675 V dc The transistors are controlled so that one transistor in each phase is always conducting, and a path for current exists through a transistor or its inverse diode
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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 lowfrequency 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 systems are shown in Fig 76 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 complexity 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 multipulse 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 algorithm 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 deceleration of an overhauling load These clean power drives are currently available through 1000 hp
Figure 76 Twelve- and eighteen-
pulse drives (From Keith H Sneker, private communication, Halmar Robicon Group, Pittsburgh, Pa, 1994)
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