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Low Frequency Current Oscillation
Reduction for Six Step Operation of Threephase Inverters
Abstract:
This paper investigates a control method to significantly reduce the low frequency
current oscillation for six step operation of a three-phase inverter. The six step
operation of an AC electric machine drive system under high speeds has a few
advantages compared with pulse width modulation (PWM) operation including
reduced switching losses, a better utilization of the DC bus voltage, and an
enhanced speed capability. In digital implementation of the six step control, power
electronics may not be able to switch on and off exactly at the zero crossing points
of reference voltages, which will result in a DC offset in phase voltages and cause
a low frequency oscillation in phase currents. The low frequency oscillation will
create issues such as higher power losses, lower efficiency, and excessive heat that
may demagnetize magnets on the rotor for a permanent magnet synchronous
machine. This research presents a control algorithm using asymmetric PWM to
ensure that power electronics switch on and off exactly at the reference voltage
zero crossing points. Both simulation and experimental results demonstrate the
algorithm can reduce the low frequency oscillation in phase currents by more than
90%.
Existing system:
 A modern AC electric machine is usually driven by an inverter using PWM
control to ensure sinusoidal phase currents with a low total harmonic
distortion (THD). For an increasing fundamental frequency, fewer pulses are
created for each fundamental cycle and six step operation, another power
electronics switching technique, can often be applied to reduce the power
electronics switching losses and extend the machine speed capability as well.
 For the six step operation, it is desirable to have the power electronics switch
on and off exactly at the reference phase voltage zero crossing points.
However, the power electronics may not be able to achieve this using digital
implementation.
 This can create a DC offset in phase voltages over one fundamental cycle,
hence, a low frequency oscillation in phase currents. Low frequency is used
here as the oscillation frequency is lower than the fundamental frequency of
the AC voltage or current.
 This work prefers to use low frequency instead of subharmonic as the
subharmonic, in its pure sense, is defined as f1/n, where f1 is the
fundamental frequency and n is an integer.
Proposed system:
 This paper discusses how the low frequency current oscillation is produced
for the six step operation of a three-phase inverter. The contribution of this
work is to develop and implement a new control algorithm to eliminate the
low frequency current oscillation for the six step operation.
 The algorithm calculates the accurate duty ratio of the power switches if the
reference phase voltage changes polarity for the next sampling point to
ensure the switches can turn on and off exactly at the voltage zero crossing
points.
 Simulation and experimental results are presented to verify the control
algorithm using both a three-phase RL load and a three-phase interior
permanent magnet synchronous machine load.
Reference:
[1] P. Dallos, The Auditory Periphery Biophysics and Physiology, Chapter 6.
Nonlinear Distortion, pp. 448, Academic Press, 1973.
[2] J. Holtz, W. Lotzkat, and A. M. Khambadkone, “On continuous control of
PWM inverters in the overmodulation range including the six-step mode,” IEEE
Transactions on Power Electronics, vol. 8, no. 4, pp. 546- 553, Oct. 1993.
[3] R. J. Kerkman, D. Leggate, B. J. Seibel, and T. M. Rowan, “Operation of PWM
voltage source-inverters in the overmodulation region,” IEEE Transactions on
Industrial Electronics, vol. 43, no. 1, pp. 132-141, Feb. 1996.
[4] Z. Peroutka and T. Glasberger, “Comparison of methods for contrinuous
transition of space vector PWM into six-step mode,” EPE International Power
Electronics and Motion Control Conference, pp. 925-930, Portoroz, Slovenia, Aug.
2006.