08-16-2017, 10:28 PM
Pulse Based Dead Time Compensator for PWM Voltage Inverters
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I. Introduction
The state of the art in motor control provides an adjustable
voltage and frequency to the terminals of the motor
through a pulse width modulated (PWM) voltage source inverter
drive. As the power devices change switching states,
a dead time exists. Although the dead time is short, it
causes deviations from the desired fundamental output voltage.
While each deviation does not appreciably affect the
fundamental voltage, the accumulated deviations result in
reduced fundamental output voltage, distorted machine currents,
and torque pulsations. To compensate for the dead
time in PWM signals, the industry has investigated this
problem [1-3], and has tried various methods of correction
B: Pulse Deviations
The effects of the dead time on the output voltage can
best be examined from one phase of the PWM inverter. The
basic configuration shown in Fig. 2 consists of upper and
lower power devices T1 and T2, and reverse recovery diodes
D1 and D2, connected between the positive and negative
rails of the power supply. Commutation of the power devices
comes from the PWM generator which creates the Swu
and Swunot base drive signals. Output terminal U is connected
to motor phase U and the current iu is positive with
respect to the motor.
II. Pulse Based Method of Correction
The Pulse Based Dead Time Compensator updates the
turn-on time of the power device at the beginning of a PWM
cycle and the turn-off time of the device at the midpoint of
the PWM cycle by software correction. Correction is based
on the polarity of the currents, independent of operating or
carrier frequencies. The method does not need current phase
detection, thus eliminating the a/d converters and software
overhead of other methods. Simple current polarity detectors
interfaced to the data bus make it compatible with microprocessor
or DSP I/O architecture's, which yields an
inexpensive dead time compensator.
IV. Comparison of Compensation Methods
Most dead time compensation methods are either voltage
compensation or current feedback types. Waveform correction
for the dead time effect is attained by returning the
lost volt-seconds to the applied PWM pulse pattern.
VI. Experimental Results
The TCR Pulse Based Dead Time Compensation
method applies the correct fundamental voltage to the motor
terminals. Fig. 11 shows the waveform correction that occurs
when the TCR Pulse Based Compensation method is
applied to the same motor used in Fig. 1. The motor current
is stable with the correct no load flux current for the 10 HP
460 vac high efficiency induction machine. The steady state
waveform is excellent and a frequency spectrum of the current
and line-line voltage showed a 12 fold reduction in subharmonics.