Re: [CAD_CAM_EDM_DRO] how to safely stall a DC motor

On 18 May 2005, at 5:51 pm, Lance Hopper wrote:

> I've found somebody that has my same machine, but in working
> order, and has verified that the motor sees 24VDC while in forward
> operation, and 2VDC while in reverse/stall. From the manual they
> show a basic H-bridge circuit that drives the motor but virtually no
> details. At this point we are guessing that the motor is driven with
> PWM and the 2V we see is about a %10 duty cycle average voltage etc...

There could be current limiting in the drive circuit which lowers its
output voltage if the motor tries to draw too much current, or there
could be a power resistor in the leg of the H bridge which conducts
when it is driving the motor backwards. With the former the stall
torque will be the same in both directions (and significantly less
than you would get if you connected the motor directly to a 24V
supply); with the latter it will be less in reverse.

> Would it be feasible to simply control the motor with a DPDT relay
> that supplies 24VDC for forward operation, and then supply 5VDC and a
> resistor of some value for reverse/stall operation?

Yes, but I would forget the dual supplies and just choose a suitably
sized power resistor to limit the motor power from the 24V supply.
The simplest way would be to put the resistor in one of the motor
supply leads and put a high power diode in parallel with it, such
that it is conducting (and bypassing the resistor) when the motor is
being driven forwards.

--
------------ Alex Holden - http://www.alexholden.net/ ------------
If it doesn't work, you're not hitting it with a big enough hammer