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Author Topic: Anyone ever used a DC motor drive as a variable DC supply  (Read 1226 times)
N4AEQ
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Posts: 90




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« on: December 20, 2012, 04:00:16 PM »

 I have several surplus DC drive controllers, 120vac in and 0-90vdc output, used to control speed of DC motors.  I don't have the motors so I was thinking of adding volt / amp meters, pot to control voltage and use it as a bench supply. These have output up to 10a, controls for fwd/rev current limit, fwd/rev accell and decell, ir comp, min and max speed.
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W6EM
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Posts: 791




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« Reply #1 on: December 20, 2012, 04:30:36 PM »

Before you hook up any electronics, you should look at the DC output on an oscilliscope.  Chances are, it produces a pulse-width variable output.  And, in so doing, would be a rather heavy noise generator.  DC motor supplies often vary the pulse width to adjust speed and torque.

Another concern might be the effect of switching transients from such a pulse train.

If it just produces what amounts to a clean, pulse-free adjustable DC output, then it should be OK.  But, be careful.

I'd guess that the speed control would require some type of feedback input from a tachometer of some sort that looks at shaft speed, not just motor input current.
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KA4POL
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Posts: 1978




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« Reply #2 on: December 20, 2012, 09:53:57 PM »

These pulse width regulators are definitely not good for bench supply. They deliver mostly rectangular pulses. Rectangular means lots of harmonics. Also to take into account that you are using 120 V input. Probably you will be getting similar voltake output spikes.
I'd really go the good old way of transformer, rectifier and depending upon your needs a LM317 or the LM723 with pass transistor/s. Or even use a switch mode power supply. The latter may be critical though if you want to build it yourself for generating unwanted RF noise. Lots of those circuits can be found searching the web.
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KA1MDA
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« Reply #3 on: December 22, 2012, 12:00:16 PM »

Besides the reasons others have mentioned, there is a SERIOUS SAFETY ISSUE with using a variable DC drive as a poiwer supply. Most of these drives do not isolate the output from the AC line as a true P/S would. To make matters worse, in many variable DC drives the common return line for the 0-10 volt speed reference signal (used for the speed control pot) is actualy floating at -60 volts!
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W6EM
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« Reply #4 on: December 23, 2012, 05:48:03 AM »

Besides the reasons others have mentioned, there is a SERIOUS SAFETY ISSUE with using a variable DC drive as a poiwer supply. Most of these drives do not isolate the output from the AC line as a true P/S would. To make matters worse, in many variable DC drives the common return line for the 0-10 volt speed reference signal (used for the speed control pot) is actualy floating at -60 volts!
Very good points.  A neutral should never be grounded outside of the main panel, and it likely would be if the controller didn't use some type of transformer.

I built a small PWM motor controller for use in controlling a small Dremel-like 12V motor.  It worked well.  However, I added a capacitor and low ohmic value resistor to the output to smooth out the square wave pulses into at least strings of ramped pulses.  I don't like what Lenz's law taught me with respect to switching inductances on and off.  Even with a reasonable back-emf from the motor, transients can wipe semiconductors quite handily.  Although, the power MOSFET I used had a back-biased diode internally for protection.
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KE3WD
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« Reply #5 on: December 23, 2012, 07:46:38 AM »

Of course, when the shiny plastic rig presents on the poor factory service tech's bench, there will have been no mention of the attempt to run it by feeding 90V of pulsed DC into the poor thing...

Welcome to my world.  <g>


73
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