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|Title: ||New optimal PWM strategies for a VSI induction motor drive|
|Authors: ||Abdulrahman, Shakir M.|
|Issue Date: ||1991|
|Publisher: ||© S.M. Abdulrahman|
|Abstract: ||The applications of robust squirrel-cage induction motors
in variable speed inverter drive systems have increased
considerably due to the availability of easily controlled
semiconductor switching devices. One problem encountered in
inverter drives is the non-sinusoidal nature of the supply
voltage, which results in increased motor losses and harmful
torque pulsations producing undesirable speed oscillations.
The latter effects are negligible at high frequency operation,
due to the damping effect of the rotor and load inertia.
However, torque pulsations and speed ripple may be appreciable
at low frequency, wore they may result in abnormal wear of
gear-teeth or torsional shaft failure. Hence, in applications
where constant or precise speed control is important, eg;
machine tool, antenna positioning, traction drives etc., it
is essential to establish a method for determining the
magnitudes of these torque pulsations and speed ripple, as a
first stage in minimizing or eliminating them.
When a voltage source inverter is used in such applications,
pulse width modulation (PWM) techniques are usually employed,
whereby the quasi square waveshape is modulated so as to
minimize or eliminate the low order harmonic voltage components
and thereby reduce the torque pulsations. Recent investigations
have shown that total elimination of low order components does
not produce optimal efficiency or torque pulsations and speed
ripple. minimization. This thesis describes new PWM strategies
which does not rely on complete elimination of low order
harmonics, but on controlling the magnitude and phase of these
components to achieve a smooth rotor motion.
Initially, a mathematical model for the inverter/induction
motor drive was developed, based on numerical integration of
the system differential equations. The changing topology of
the inverter bridge was simulated using tensor techniques.
Then an analytical method, based on harmonic equivalent circuit
analysis was proposed for calculating the induction motor pulsating torque components under steady-state operating
conditions, in terms of stator and rotor current harmonics.
The accuracy of this method was verified by comparing its
results with those obtained from the mathematical model
developed earlier. This provided an extremely rapid,
numerically stable and efficient means for evaluating harmonic
current and torque components with balanced non-sinusoidal
applied voltages. This method was then used to formulate the
torque performance function necessary to determine the new
optimal PWM switching strategies.
Throughout the work, the predicted performance was
extensively validated and supported by practical results
obtained from an experimental rig specifically designed to
drive the machine under different PWM techniques.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Mechanical, Electrical and Manufacturing Engineering)|
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