Re: [CAD_CAM_EDM_DRO] Re: LARGE DC MOTOR speed control question

STAN MCDONALD wrote:

> When i rectify 240 I get 280 DC when i rectify 120 I get 130 DC why do
> I gain 40 volts on 240 and 10 volts on 120? Since I get 130
> from 120 what besides a transformer can I do to get the correct
> voltage? The motor seems to hum on 130 and wine on 280 but it does
> work on both.. Since I get 130 from 120 what can be done to operate a
> 90 volt dc motor?
>
> Do you have an idea on how to make a speed control besides a varac?

Yes. But, it is not for the amateur! A 3 Hp PWM variable speed DC
drive would normally
be done today with a big IGBT (Insulated Gate Bipolar Transistor).
You'd take the DC
supply we have been discussing, and feed the + output to an IGBT. The
output of the
IGBT would go to a smoothing inductor, then a good size capacitor to
minus, and finally
to the motor. A "freewheeling" diode would be connected to the junction
of the transistor
and inductor, with the other end to minus. The motor minus terminal
connects to the
bridge rect. minus.

Then, you need a variable width pulse, which can be provided by a 555
timer, or a 556
can be set up as a frequency source and a pulse width modulator. But,
the IGBT needs
to have a very powerful driver, as it needs to be turned on and off very
quickly or it
will self destruct. There are commercial chips designed specifically
for doing this.
International Rectifier would be my first choice.

If you are not fairly confident of your abilities as an electronic
circuit designer and
power systems type, I would not attempt this! If you don't have an
oscilloscope,
forget it!

Jon