Re: AC Servo Motors

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "camerashy44"
<camerashy44@y...> wrote:

> Thanks to those that responded to my question. I guess that I will
> keep looking for some good (inexpensive) DC Servos. I am not sure

that

> this is possible! Anyone got any sugestions? I am in Canada so
> shipping and duty from the US can make what is reasonable in the US
> out of the question here.
> Should one be looking for brush or brushless. I am looking for
> something in the 400 -600 in/oz size. Could be less depending on the
> speed and if I can use a reduction drive. I will be using 5:1 ball
> screws and am looking for around 100 in/min max speed. Thanks in

advance!!

> Wayne

I think you will find that for most hobby applications of servos that
400 in/oz motors are overkill. One exception is the larger knee
mills. You will find that brushed servo motors ratings are somewhat
different than steppers. The torque on steppers is measured at low
rpm where it is greatest. You have to derate your design based on the
max speed you want and upsize the stepper motors to compensate. With
servos you can plan on using the torque of the motor at higher RPM to
get multiplication through gearing. The 5 TPI leadscrew will give
you a mechanical advantage but reduce you speed by 1/5. Given that
you want 100IPM from your system you only need to spin the motors at
250 rpm to get there with a direct drive. Most Servos will easily do
1500 to 3000 RPM at full voltage so you are wasting a lot of torque
in that design. By using toothed belts and ratio pulleys say 3:1 now
you place the needed RPM at 750. and you gain mechanical advantage.

I have a 100lb 6' wide dual drive gantry that will zip along at
200IPM using two 120 in/oz 23 frame servo motors at 5TPI leadscrews
with 3:1 belt ratios. I have run it with a full sized router and it
will happily snap a 1/4" bit and never slow down! The mechanical
advantage of a screw mechanism is a function of the TPI and diameter
(actually the length of a thread if you were to stretch it out). The
downside is that with long screws of smaller diameter there are
centrifical forces that develop along the screw at higher RPM's cause
a "whipping" condition.

If you can find some PM DC brushed motors rated for PWM control then
you have a basic DC servo motor. You will need to retrofit encoders
but a lot of the motors have rear hole patterns for standard shaft
encoders.