Re: [CAD_CAM_EDM_DRO] Re: STEPPER MTR VOLTAGE SUPPLY HELP!!!

cnc6805 wrote:

> Is there a way to use this L/R stepper drive that I have to run
>at two seperate voltages? at idle it holds the position
>satisfactorily but, as soon as the axis begins to move the motor has
>no power and is of little use.
>

Yes, this is essentially what Bridgeport did with the BOSS series of
machines. They used a magnetic amplifier on the AC supply to each
driver, and saturated the mag amp when the motor was moving, applying
a much higher voltage to the driver. It is a pretty crude hack, and part of
the reason that they occasionally burned up motors, and often burned up
the drive.

> Is there a way to run the board from a
>small transformer with the rated motor output then when the motor
>starts to turn switch to another transformer of higher voltage? and
>then back to the other transformer for idle current. The maker of the
>board suggested that I use one of the on board relays to switch the
>transformers via an M command in the program. Of course this would
>only be successful until someone failed to put in the command when
>the motor stopped and the board would be toast.
>

Exactly, a very bad idea to have this under program control. A one shot
(timer) could switch on high power whenever a step pulse was received,
and go to low power when no step pulses were received for 1/2 second
or so.

> Is there some simple
>electronic device that would do this very same thing? I'm thinking
>like something that senses the higher current at the board and
>automatically switches it to the lower holding current.
>

No, the driver doesn't really draw higher current when stepping slowly.
You can sense the step pulses themselves, though, to control the power.

> There has to
>be a way to do it without getting too complicated. It works as is, I
>just need a way to supply the motors with a higher current
>

No, not current, voltage!

> when
>moving. I paid quite a bit for this board
>

That is truly unfortunate, that you paid a lot of money for 40-year old
technology!

> and the maker is not
>wanting to give me a refund so, I'm ready to try anything. It's
>useless to me as it is.
>
>

Well, the problem is that even if you fix this, it is still a two-step
control.
A modern chopper-type drive always provides the right current, from
stopped up to some maximum speed where the supply voltage is no longer
enough to ramp up the phase current before another step comes in. And,
you are stuck with full, or at best, half-steps. This leads to resonances,
vibration and a host of problems typical of the "old way" of running
steppers. Once you've seen what a good microstepping drive, like the
Gecko 201A can do, you'll never want to see another full-step drive.

There is really no good solution with the two-step approach. You need
a lot of voltage for high speed, and need a large series resistor to
accomodate speed variations from slow to fast (maybe a 100:1 range),
without causing excessive current at low speed or loss of torque at
high speeds.

Jon