PWM Inverters, both 1-Ф and 3-Ф were modeled and studied. The PWM signals were generated
by comparing either a triangular waveform with a sinusoidal waveform using relational
operators. These PWM signals were then applied to the gates of forced-commutation devices like
IGBT‟s so as trigger them in a specific sequence to be able to convert the DC supply voltage to
an AC output voltage. The DC supply could either be from a battery, a fuel cell, or from a
rectifier which receives AC supply from the mains.
A 3-Ф PWM Inverter was also fashioned using the Simulink Library blocks PWM Generator and
Universal Bridge. In all cases, successful Inverter action was obtained.
An Induction Motor was run with the help of a PWM Inverter without implementing any kind of
speed control mechanisms and the various characteristic curves were obtained. It was observed
that there were a lot of transient currents in the stator and rotor at the time of starting and they
took some time to settle down to their steady-state values. The lower the stator resistance, the
quicker the transients died down and hence, the stator resistance should be kept very low. In an
uncontrolled Induction Motor, torque was observed to rise to a maximum value and then settle at
the base value, while rotor speed was observed to rise to its rated value and remain constant
there.
Open-loop V/f Control was implemented using MATLAB and it was observed that by varying
the supply frequency and terminal voltage such that the V/f ratio remains the same, the flux
produced by the stator remained constant. As a result, the maximum torque of the motor
remained constant across the speed range.
Closed-loop V/f Control used a Proportional Controller to process the error between the actual
rotor speed and reference speed and used this to vary the supply frequency. The Voltage Source
Inverter varied the magnitude of the Terminal Voltage accordingly so that the V/f ratio remained
the same. It was observed that again the maximum torque remained constant across the speed
range. Hence, the motor was fully utilized and successful speed control was achieved