Phil Allison wrote:

"Robbie Hatley"

Greetings, group. Where I work, we repair several different types of
variable-speed
3-phase motor drives. All of them I've seen so far have very similar
circuitry.
The main power-control module generally contains:
6 hi-current hi-voltage silicon diodes in 3-phase bridge (to generate
+320VDC)
6 hi-current hi-voltage IGBTs in 3-phase bridge (to power the motor)
thermistor (for temp sense)
braking IGBT (in some units)

The IGBTs are always wired as 3 push-pull pairs, with the 3 center points
connected to the 3 phases of the motor. (The down side of this technology
being that if both transistors on the same phase turn on at same time, the
IGBTs explode with a deafening "BANG!!!" and you're out $65 in 65ns.)

I had assumed that the IGBTs would be switched so as to generate 3 AC
waveforms,
120deg out-of-phase with each other, and that the frequency would be
variable
from about 2Hz to about 60Hz, with motor speed proportional to frequency.

But no such thing is true!!! When I actually looked at the waveforms at
the gates of
the IGBTs, with the motor at lowest speed (about 0.5 RPM), the frequencies
of
all 6 IGBTs are all about 400 Hz, about 50% duty cycle. When I turn
increase
commanded speed, the frequencies all stay at 400 Hz, but the duty cycles
begin to fluctuate, with rate of fluctuation of duty cycle being equal to
motor
speed in revs/second, and amplitude of fluctuation also increasing with
speed.
(Eg, if motor is turning at about 10 revs per second, the duty cycles of
the signals
are fluctuating from about 40% to about 60% at about 10 fluctuation per
second.)

So how the hell does this bizarre technology work? If the frequency is
always
stuck at 400Hz, why isn't the motor always turning at exactly 24000 RPM?

(My guess: at minimum speed, the three signals are almost perfectly
in-phase;
but at higher speeds, the phases fluctuate slightly away from 0deg, with
the
fluctuations "cycling" around the 3 phases at a rate equal to motor
revs/sec,
but the "carrier" frequency still staying at 400Hz all the while. But I'm
just guessing.)

So, does anyone here know what this bizarre motor-control technique is
called? (Hard to google something one doesn't know the name of.)
And, roughly, how it works?



** The technique is called PWM:

http://en.wikipedia.org/wiki/Pulse-width_modulation

The duty cycle of the 400Hz rectangular wave is slowly modulated to create
low frequencies in the 3 phases required.

A steady 50% duty cycle wave at 400Hz creates no torque in the motor as the
average value is zero and the motor's inductance at that frequency causes
little current to flow.

Shallow modulation depth creates the low voltages needed for slow running.



... Phil

Yeah, what ever you say Phil.,.

Talk about something you know more of..

From what you just spit out, I can tell this isn't your line of work.

Jamie