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