Can you drive a 3.6V motor using PWM motor driver?

I'm trying to drive a 3.6v motor from an electric screwdriver using the standard PWM DC driver circuit. I know that it's 12v PWM, but I was able to run a small (3v) motor during testing the board with no problem.

Running from the original 3.6v or even 2.4v batteries, it runs fine. However, hooked up to the RepRap, it just buzzes (aprox 100hz?).

My initial guess was the 12v PWM was not enough - it needed constant current?

I added a 4500nF smoothing capacator - seemed to help a little, initially a half-turn of drive before stalling.

I don't know too much about electronics - has anyone any ideas? will this even work?

Try this thread for some hints.
[forums.reprap.org]

First guess. Lower the PWM switching frequency.

Voltage isn't a particularly good measure of a motor's rating. Because, for our purposes, a motor is essentially a piece of wire, its rating should be the same: current. There are many reasons for this discrepancy, chief among them that it takes much more effort for us to regulate current than to regulate voltage.

The voltage rating of a motor is the voltage at which the limiting current appears in the motor at its designed rpm and torque. In the case of a drill motor, that may be fairly close to locked-rotor current (where the motor isn't turning and is exhibiting its maximum torque). Regardless, the current rating of any motor is the current rating of the wire that makes up its windings.

Based on this, a current control technique is virtually always better for motors than a voltage control technique. By using a current control technique with a voltage greater than the maximum for the motor, you guarantee that the motor will always run at the highest torque possible while not overloading its drive coils.

My guess that the cause of your problem is that you're running the PWM too slowly. IF you increase the frequency of the PWM, the problem may go away.

--Annirak

The L293 on the DC motor board can only provide 0.6A continuously and 1.2A peaks. I expect a low voltage screwdriver motor takes more current than that, especially if driven from 12V.

Connecting a big capacitor to it is a bad idea unless you include a series resistor or inductor. A capacitor on its own will draw a big surge current every time the driver turns on and could well damage it, as could a motor with a stall current of more than 1.2A.

Edit:
Something like an L6203 would be more suitable.

Edited 1 time(s). Last edit at 03/25/2009 05:24PM by nophead.

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[www.hydraraptor.blogspot.com]

ahhh not this board
[www.reprap.org]

but this one
[www.reprap.org]

I tried using a screwdriver motor head for a extruder design I was working on and fried the power transistor when I went above 180 analog value for the speed.

You can do it with the PWM driver board if you gang two channels together and put a EMF kickback diode across the outputs(works). You will then have a single rotation direction and vast difference between the load and no load speeds.

Reporting back:

Nophead, you seem to be right (again).

I tried altering the arduino PWM frequency (hacking the timer division speed) both up and down and neither produced a significant improvement.

I removed the gearbox and replaced the motor with a 6V smaller one, ripped from an old toy. This worked better, but the motor seemed to stall easily. It still was able to drive the gearbox from the electric screwdriver reasonably well.

I might have a look at something like a scalextric motor (from memory they run on 12v and seem to cope with variable speed well).

To solve my immediate problem, I took the gearbox/motor from my BfB extruder and temporarily rigged a clamp so it drove my screwdriver bit. It seems to have much more torque and controllability than the original gearbox/motor, so I'll use that for now!

Thanks for the advice. I know much more about PWM and electric motors now.

Renoir