MIL-HDBK-1003/3
APPENDIX D (Continued)
D-1.04.1
PWM Design
a) The PWM drive has become the most commonly used
drive controller because it works well with motors ranging in
size from about 1/2 hp to 500 hp. A significant reason for its
popularity is that it's highly reliable, affordable, and reflects
the least amount of harmonics back into its power source. Most
units are rated either 230 volts or 460 volts, three-phase, and
provide output frequencies from about 2 Hz to 400 Hz. Nearly 100
manufacturers market the PWM controller.
b) In the PWM drive, an AC line supply voltage is
brought into the input section. From here, the AC voltage passes
into a converter section that uses a diode bridge converter and
large DC capacitors to create and maintain a stable, fixed DC bus
voltage. The DC voltage passes into the inverter section usually
furnished with insulated gate bipolar transistors (IGBT's), which
regulate both voltage and frequency to the motor to produce a
near sine wave like output.
c) The term "pulse width modulation" explains how each
transition of the alternating voltage output is actually a series
of short pulses of varying widths. By varying the width of the
pulses in each half cycle, the average power produced has a sine-
like output. The number of transitions from positive to negative
per second determines the actual frequency to the motor.
d) Switching speeds of the IGBT's in a PWM drive can
range from 2 kHz to 15 kHz. Today's newer PWM designs use power
IGBT's, which operate at these higher frequencies. By having
more pulses in every half cycle, the motor whine associated with
VFD applications is reduced because the motor windings are now
oscillating at a frequency beyond the spectrum of human hearing.
Also, the current wave shape to the motor is smoothed out as
current spikes are removed.
e)
PWM's have the following advantages:
(1)
Excellent input power factor due to fixed
DC bus voltage.
(2) No motor cogging normally found with
six-step inverters.
(3)
Highest efficiencies:
92 percent to
96 percent.
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