From: Jon Elson
Subject: Re: large (5HP) DC motor controller design issues
Date: Fri, 27 Sep 2002 16:53:39 -0500
Organization: Washington University
NNTP-Posting-Date: Fri, 27 Sep 2002 22:56:30 +0000 (UTC)
X-Mailer: Mozilla 4.78 [en] (X11; U; Linux 2.4.8-26mdk i686)
Richard Everett wrote:
> I would like to build a somewhat simple PWM controller for a largish DC
> brush motor (5HP). This is a shunt wound motor, with a 180V armature
> (guessing about 24 amps) and a 200V field (at 1 or 2 amps). I can live
> without field weakening if needed.
> One big problem is that I need to be able to control the motor position. It
> is the spindle motor in a cnc mill I picked up cheap because the spindle
> drive was shot ($7500 to replace)...the cnc mill has a tool changer and so
> the spindle has to stop in a certain position for a notch in the spindle
> taper to line up with a notch in the tool holder when changing tools.
> Ideally, I would like to build a solid state reversing drive (hbridge made
> up of IBGT devices, or mosfets) with quadrature encoder feedback. I have
> built discrete h-bridges for small motors and steppers, but have never
> tackled something of this power. I am worried about things like voltage
> spikes and inrush currents.
IGBTs are tricky devices to drive. They must never be allowed to operate in
the linear region, even for a few hundred nS, or the current will "hog" to
the highest gain part of the transistor, and that region will overheat by
thermal runaway. Only in deep saturation will the current distribute
across the die. I really don't think you want to build this yourself, unless
you're a die-hard power EE, or you don't know what you're getting into.
I've done a few power inverters, and a PWM servo amp of about 1 Hp.
If you can wait a while, you should watch eBay closely for large servo
in the Kollmorgen and Servo Dynamics brands. Also Yaskawa, Westamp,
Allen-Bradley, etc. Bit the first two are probably the best documented, and
most of their amps are DC brush. Note that some of these have integral
power supplies, and some need a separate supply.