PulseWidthModulation comments

[rauscher at icst.com]

Ludis wrote:
>Years ago as a summer intern I worked on a motor controller which used
>PWM.  A brushless DC motor ran on up to 120 to 170 volts at up to 10 to
>15 amps.  A switching (PWM) power supply controlled the voltage applied
>to the motor.
<snip>
>An improved version used a microcontroller controlled digital PWM.  I
>think the PWM frequency was about 20 KHz and the PWM had 5 bits of
>range.  The same microcontroller monitored the motor position/speed to
>form a closed loop system.

Was this motor designed specifically for this freq? When I first developed
this PWM motor controller, I was trying to run it at 20KHz also. But the
motor was down on power. Motor was an AstroFlight cobalt, 9.6V at 35Amps,
such as those used in R/C model aircraft. Going to the lower freq brought
the power back up. I looked at possibly skin effect, but the wire used in the 
winds wasn't that large.

>Some comments applicable to fuel pump / coolant fan PWM control:
>
>The pulse width and frequency needs to be very stable, otherwise all the
>inductors in the circuit will squeal like stuck pigs.  The original
>analog system screamed constantly.  The digital system (running the same
>switcher!) was totally silent as long as the pulse width wasn't
>changing.  Software in the microcontroller updated the pulse width every
>millisecond.  When the pulse width needed to be changed often, this
>produced a 1 KHz tone from the electronics.  The amount of noise was
>directly proportional to the load on the motor.  You could actually hear
>the computer "straining" against a high load.

Sounds as thou the noise is from the P/S, and not the motor. I've only
heard a light whine from the motors, and only at lower speeds. So hopefully
the noise won't be enough to drive folks out of the car <g>.

>The main switching transistor was a TO-220 package mounted on a dinky
>little heat sink.  You don't need a massive TO-3 transistor.  In the
>above system, cooling the rectifier diodes was actually more of a problem.

Oh so true, in the writeup I showed that even at 15 Amps, the MOSFET only
needed to dissipate a little more than 5 watts. Only a small heat sink required.
The key to the proper MOSFET, is low voltage drop through it. This keeps
losses down and heat also.

>BTW, in one mode, this motor was reversed every several tenths of a
>second.  This applies to a thread from several weeks ago.

My brain is fading here, possibly relative to a motor driven FPR?

BobR.

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