RE: [CAD_CAM_EDM_DRO] Big servo motors

I agree Brian.

Presently we have the cheap DIY drives which seem OK for small motors up to
say 100V ish sizes. These motors don't really need true torque command and
can get away with simple PWM, the PID able to compensate for it's
inadequacies.

We then have a limited number of servo drives for HV drives up to 200V and
very low current, mostly about 10 to 15 amps, and Rutex's higher current
rating being unrealistic. Most of these are overgrown high voltage version
of the LV drives above, they work for a few motors but as we go bigger
inductance and pwm control rather than torque/current start to limit the
dynamic range until the motor gets big enough it just won't work.

Then there are your big Getty motors, wow have they some power needs. But
when you look at that sort of scale it's is pretty common sense that at some
stage you are going to run out of dynamic range.



But if we can start improving the HV drives, either adding digital speed
compensation to the pwm signal, or true current torque command with a bit of
analogue circuitry we have the second stage of drives we can build upon.
From then on higher voltage and current is just down to driver stages and
selecting enough fet's or IGBTs to correctly parallel up to do the job. The
task isn't that difficult really.

I do wonder how many digital servo guy's are quietly watching this, come on
guy's it's laid out now, it's pretty easy from now on.

Otherwise we are back to analogue cards and something like Pixie drivers.
This may not be that bad though, it allows us to keep encoder signals well
clear of the motor power signals.







Dr. Mark Vaughan Ph'D. B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd, reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
Phone/Fax 44 1872 561288

RSGB DRM111(Cornwall)

_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of BRIAN FOLEY
Sent: 27 May 2007 18:24
To: Cad_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Big servo motors



Hi, been reading all the stuff comming across on big servo motors, here is
my go at it, take a typical Getty's servo motor, or a Peerless Porter servo
motor. BIG, heavy but very powerfull...current levels high enough to weld
steel with. you have in your hand a servo drive that weights 6 or 8 oz's is
not gonna do it.
add 500 or 1,000 lbs of table and the drive needs to handle 150 to 350 amps
capacity for seconds at a time. the last 2 Gettys servo boxes i junked
weighed 145 lbs and had (3) 300 amp SCR's to control the motor with.
I do think the electronics inside the late model servo drives have good
stuff, but time for someone to design a servo drive with say 100 amps of
real power handling, would make these motors very usefull again. IMHO cul
brian f.

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