From: "Richard Everett"
Subject: Re: large (5HP) DC motor controller design issues
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Sat, 28 Sep 2002 01:44:04 GMT
NNTP-Posting-Date: Fri, 27 Sep 2002 21:44:04 EDT
"Jon Elson" wrote in message
> 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.
> > 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
> > 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
> > up of IBGT devices, or mosfets) with quadrature encoder feedback. I
> > 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
> 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,
> 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,
> most of their amps are DC brush. Note that some of these have integral
> power supplies, and some need a separate supply.
Actually, I did pick up a 6HP Westamp servo amplifier (on the label of the
amplifier, it says "Current output 30amp DC, 6HP") off of ebay a week or so
ago. It is somewhat older (the outside says 1990, but the circuit boards
have copyright 1980, so the technology is getting long in the tooth).
Besides the fact that it came with no documentation (motor armature voltage,
which pins are which on the encoder, if it even takes encoder inputs), it
takes a very weird 115V 3 phase 4KW input. This feeds into three bridge
rectifiers, in such a way that each phase connects to both AC inputs on its
bridge rectifier, and all three bridge rectifiers + and - terminals are
hooked together. The wires going from the + and - terminals of the bridge
rectifiers then go to a 7400uF 200V cap. What voltage do you get when you
rectify 115V 3 phase like this, and then feed it into a huge cap? I am
guessing 160V to 180V, since that seems about right for a 30 amp output (for
a 6HP DC motor). So anyway, I don't know what to feed it with (not having 3
phase 115V). Normally, I would guess you could just use 220VAC single phase
with a single bridge rectifier, but then I would have to take out that 200V
cap, and who knows what the ripple would do to the rest of the circuit.
Good thing it was only $26 :-)