Dave August wrote:
FWIW I've had no problems "plug reversing" (actually it was a classic DPDT
switch wired as reversing switch) PM DC motors, Both fractional gear motors
I've used for powerfeeds and a good sized 90VDC I've used for spindle
motors,
but my speed control was not electronic, but a big assed Variac and diodes
from hell. I did it all the time on the power feeds running at about half
speed and ocassionaly on the spindle motor usually running pretty slow when
threading, (made threading metric to a shoulder a snap. -) A big caviet
here is that doing this to PM DC motors WILL demag them over time... shrug
...


I suspect that the issue here is that reversed voltage blew the SCR's,
TRIACS or DIODES in the speed control.

Yes you should be able to drive the armature directly from some from of
rectifier, but remember if you use a full wave bridge, yer gonna be hitting
that thing with around 150VDC.. (the 120 is an RMS value, 120 x 1.4 is about
170 peek)

You might consider just a half wave... I know those, 50/100's are "growlers"
any way so....

Also if you are still going to "hot reverse" this motor you better get 400V
Diodes since you'll be adding your generated voltage to the 170 when you hit
the reverse switch.

--.- Dave

Yes! Thanks Dave!

I'm going to cut my losses on this and go for a set up like yours. I have a half dozen
90 volt pm dc drives and three 10 amp variacs and possibly one more if I could ever
find it. Got to find some "diodes from hell" and I'll be all set.

I don't understand the math involved in calculating ("if you use a full wave bridge, yer
gonna be hitting that thing with around 150VDC.. (the 120 is an RMS value, 120 x
1.4 is about
170 peek) )" but I think you are saying that if I pit 120 ACV across the Variac
and out through a full wave rectifier, at full rotation of the variac the output DC
voltage will be 150 DCV and peek at 170 V

This being the case,, I could limit the rotation of to variac and utilize the 20
volt tap on these variacs.

Better run, have to respond to a couple of other guys that were good enough to lend a
hand.

Thanks
T






"Mark Rand" wrote in message
...
On Tue, 11 Sep 2007 13:48:55 -0400, Ted Samuels
wrote:

Spent about 6 hours finding and wiring a reversing relay for my 8 Amp,
58/105 DC
volt, shunt wound GE motor. Worked like an absolute charm but I decided to
see if it
could stand being reversed without stopping. The smoke came out!

The field's still getting the full 58 volts but the armature voltage
(sadly) is now
only
working between about 19 and 36 volts. (no longer up to 105 V)

Probably asking the impossible but can anyone offer any suggestions as to
the
probable cause of failure. And might it be possible to strip out the speed
control
circuit (if it is screwed) and run the motor at its full rated speed using
current
directly from the full wave rectifier?

Had planned to to run this motor on a small lathe.

Thanks T

(Spam address)

You _cannot_ plug reverse a DC motor. When the motor is running it acts as
a
generator in opposition to the supply voltage. The current through the
armature is due to the difference between the supply voltage and the
generated
voltage, all divided by the armature resistance.

When you reversed the supply, that reverse voltage due to the rotation of
the
motor was added to the supply instead of being subtracted from it

So you probably went from about one volt, or less, driving the current
through
the armature to 115 volts... Exit magic smoke stage left.

If you need to reverse the motor rapidly, you need to think in terms of a
forward-stop-reverse switch. the stop position can have a biggish resistor
shorting the armature. The resistance can be sized to allow about twice
the
rated current at the rated voltage. The motor should be stopped before
reversing. Alternatively, It is possible to buy PTC based soft starters
that
might be used to limit the armature current with this sort of treatment,
but
those might slow down the stopping and starting process. YMMV

If the speed control circuit is designed smart enough, it can cope with
reversing the motor by absorbing the braking power itself and controlling
the
armature current so that you change speed at the fastest rate that the
motor
can handle (e.g. running at maximum torque decelerating and accelerating).
But
it sounds like your circuit isn't doing that :-(


HTH

Mark Rand
RTFM