RE: [CAD_CAM_EDM_DRO] Stepper motor speeds

Marv,

Now sure what you are saying by "I haven't heard any discussions about
speed ranges and how they're controlled.". Speed is controlled by how
fast you feed your drive pulses. The higher the frequency the faster the
speed until you run out of torque on the motor/driver/power supply
combination. Stepper motors have a torque curve that is like a cliff (OK
more like a very steep face, not a vertical face). At low speeds they
are on the plateau and have a lot of torque. When you hit the knee where
inductance becomes the limit and not the current limit of the drive the
torque starts to fall off very rapidly with increasing RPM. So to go
fast you have to use a motor with a lot of excess torque at low speeds.
Gearing a stepper only makes reaching a high RPM harder as it required
more RPM. You will get higher feeds most often by reducing the gearing
and using a high torque motor.

The RPM you will get with any motor is quite variable depending upon a
lot of factors. The primary ones are the torque curve for the particular
motor, the voltage you are driving at, the smoothness of your pulse
stream, and how you connect the motors. Most motor manufacturers will
offer the same size and torque motor with different winds. Some will be
capable of higher RPM. The higher the drive voltage you can use the
faster you can go. As you get up towards the high end for a particular
motor the evenness of the pulses becomes a big factor. Basically a
bunched up pulse is really a rapid acceleration that you may not have
the torque to make. How you connect the motors will affect the
inductance and there by have an effect on the speed. If the manufacturer
has a torque curve for series and parallel bipolar you will see this
right off.

So after all this rambling, if you need 150 in/oz to retract and you
want to be fast then put on a 400 in/oz plus very low inductance NEMA 34
and run it direct drive at as high a voltage as you can.

Tim
[Denver CO]

> -----Original Message-----
>
> Guys,
> I have a pretty good handle on sizing stepper motors with respect
> to required torque, but I haven't heard any discussions about speed
> ranges and how they're controlled. I'm in the preliminary stages of
> designing a CNC retrofit for my Enco full size mill, and would like
> to use the smallest motor possible, to motorize the fine feed on the
> quill. My down feed requires almost no torque, but retracting
> requires about 120 to 150 oz/ins. I have enough room to gear the
> motor to feed at 1:1, 2:1, or 3:1, but I don't know the RPM range I
> can expect from the motor. Help please!!
>
> Marv Frankel
> Los Angeles
>
>
>
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