Re: [CAD_CAM_EDM_DRO] What ratio to use for stepper motor drive?

JanRwl@... wrote:

> In a message dated 25-April-00 21:52:30 Central Daylight Time,
> bryan@... writes:
>
> << I find that the motors lack power at higher feed rates and have
> begun
> to think that, if I put bigger pulleys on the motors, I will be able
> to travel the same distance with lower motor RPM and greater torque.
> I don't need that much resolution. >>
>
> Bryan: It may still be April, but the first is long-gone! You seem
> to have
> things backwards in your mind! You would put a SMALLER pulley on the
> motor,
> and this would give you more torque. It would ALSO give you MORE
> (unneeded!)
> resolution!

Well, not necessarily, with steppers. If he in fact has more than
enough torque at LOW speeds,
then the speed-up gearing might actually allow him to go to higher feed
rates before losing steps.
The problem with most stepper drives is when the speed increases until
full current is no longer
developed in the windings before the drive is advanced to the next step,
the torque suddenly
falls off VERY rapidly. It is pretty common near that point to see
torque falling 50% with a
10 to 20% speed increase.

Guven the above considerations, it MIGHT help to put a bigger pulley on
the motor.
Since his problem is only at high speeds, it would NOT help to put a
smaller pulley
on the motor, because that would require the motor to turn FASTER at the
same
feed rate. Now, this works only if you have a lot of excess torque at
low speeds.
Another way to solve the problem is to increase the power supply voltage
to the
stepper driver, if it can accept a higher voltage. This will extend the
torque curve
so that the torque falls off less rapidly at higher speeds.

Jon