"Aidan Stanger" wrote in message
...

[2] Just to clarify, the devices - whether IGBTs or GTOs - switch to
form a
high-frequency square wave. The duty ratio of this square wave is ratio
of
the time it spends on to the total period, so if the duty ratio is 1/3,
it
spends 1/3 of the period on and 2/3 off. The duty ratio is varied over
many
switching cycles to follow a sinewave (in classic examples). However
the
motor is inductive, and this has the effect of filtering out the
switching
and producing a current proportional to the *average* of the square
wave.
This average is proportional to the duty ratio. Hence if the duty ratio
varies as a sinewave, the current will also be approx. sinusoidal. In
an
induction motor we need a variable-voltage, variable-frequency sinewave
on
each phase. Varying the duty ratio amplitude and frequency (=modulation
frequency) has this effect. The advantage of using pulse-width
modulation
(PWM) switching to achieve this is that the switching process is very
(90%)
efficient, since the devices only pass high currents at high voltages
(hence
burn lots of power) when switching. You could use a linear amplifier
(i.e.
a scaled-up audio amplifier) but its efficiency is rarely above 50%,
which
is clearly a no-brainer.

Why do you need a sinewave - what's wrong with a VVVF squarewave?

Square-wave excitation can be used, but produces a large torque ripple due
to large motor current harmonics. Sinewave excitation produces practically
constant torque as the current harmonics are much smaller. However as the
number of phases increases in an induction motor, any torque ripple produced
by square-wave excitation decreases and it becomes more attractive (mainly
because the inverter switching losses are almost non-existent). Someone in
my lab did a PhD on it a year or two ago, looking at total drive (inverter
and motor) losses.

Angus