Re: [CAD_CAM_EDM_DRO] Steppers run with Servo Drivers

datac@... wrote:

> I always have a pretty good laugh at the "battle lines" drawn when the
> "which is better" Step/Servo discussion erupts.
>
> A long time ago I decided to NOT CARE which was better !
>
> Anyhow, I am curious if any of the group have run Stepper motors with
> Servo Drivers. I am told this works very well because of the number of
> poles in the Stepper motor. It seems if it is true that it does work
> well, then, why don't we hear more about it ??

I don't know of a way to do this. Most stepper motors are 2 phase.
The other variant is 5-phase. there have been some odd 3-phase
steppers, but they are quite rare.

The servo drives I suspect you are talking about are for 'DC brushless'
motors, with are really permanent magnet AC synchronous motors.
These are generally 3 phase, and designed for fairly high voltages,
from 120 to 240 V line-line. If you could find a 3-phase stepper motor
capable of working at those voltages, you'd still have a pole mismatch.
The 'DC brushless' type of servo is generally 2 or 4 pole, and so
the drive is designed to operate these motors up to 5000 RPM or so,
or about 83 Hz or so, and deliver the rated motor voltage proportional
to that frequency. Unless the volts/Hz parameter was programmable,
you'd need a stepper motor to match it. Finally, practically all these
drives need an encoder to tell it which phase sequence is to be driven
now, and so you'd need an encoder specially made up to give the
right phase signals for driving the stepper. If it was a 150 step/rev
stepper, it would need 50 cycles of the phase drive pattern on the
encoder disc. (I came up with the 150 number because the standard
2-phase stepper has 50 poles, giving 200 steps when you go through
the 4 phase sequence 50 times.)

To get reasonable speeds out of the servo drive, it would need to
be capable of generating output frequencies WAY beyond what the
standard motors take. (Like, about 50 times higher!)

So, you can't just hook up a standard stepper to a standard brushless
servo drive and expect anything but a fault light.

Jon