Re: [CAD_CAM_EDM_DRO] 90v DC motor as servo?

Les,

Thanks for the info. Obviously, one is not going to build a high
speed servo system with 90v DC motors. I do wonder however if a
surplus motor of this type plus a G320/340 could be used to build a
cheap system with stepper-level performance.

I believe the SCR motor develops its rated power at 1750 RPM as
opposed to maybe 6000 for the servo. This would mean a lower
reduction ratio - possibly direct drive, which would reduce the
effective rotor inertia, right? (well really, the rotor wouldn't have
to be accelerated to as high a speed)

I'm dealing with a lathe application that could be characterized as a
'smart power feed'. What I really need to do is inexpensively get
lots of torque for low speed cutting, and the ability to rapid back to
the starting position at maybe 60 IPM. Perhaps I should just buy big
steppers, but I'm afraid it is going to take a fairly large, expensive
motor to drive the carriage if the rack & pinion system is left in
place for manual operation.

(I'm halfway tempted to hang a surplus gear motor on the thing, pwm it
with a single N fet and reverse with a relay, since I'll probably be
using the existing leadscrew with its substantial backlash anyway.)

Chris

> The biggest difference I see is in polar moment of inertia.
> The dc motors you mention often accelerate slowly compared
> to a servo. They are designed for steady state operation.
> For example a typical 1/2 hp pmdc scr rated motor has
> .2 - .4 oz-in/sec^2 while a dc servo of the same torque
> might be .05 or less.
>
> The biggest give away with conventional rotors is simply diameter. Moment of
> inertia grows rapidly with diameter,
> so servos are often long and low diameter. Special rotors need not be.
>
> Motor moment of inertia needs to be a specific value
> for a given load for optimum performance. If it is too high
> very large currents ($ for servo amps) will be required to
> start and stop it rapidly.
>
> I have used pmdc motors for servos in some apps and never had too much
> problem with cogging. I test them by seeing if they will run smoothly at
> some tens of rpm at a low voltage
> with no load. It just depends on the application.
>
> Leslie Watts
> L M Watts Furniture
> Tiger, Georgia USA
> http://www.rabun.net/~leswatts/wattsfurniturewp.html

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Christopher C. Stratton, stratton@...
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