(ET) Re: ET reliability Ref: Plug reversing DC permag motors and magnet damage

["The Nauglers" <snaugler earthlink net>]

> My only question is would tossing it from fwd to reverse would still
> cause an arc on the push-pull contactors. Also could that eventually
> damage the PM field magnet?

Chris and others,

Assume for the moment that my experience with industrial DC permag motors 
is
relevant.

There is a critical armature current, which is much higher than normal
nameplate currents, that will cause the permanent magnets in a permag DC
motor to become partially demagnetized.  When that happens the torque 
rating
of the motor drops and its rpm for any given armature voltage will 
increase.
We discovered than many of our industrial DC permag motors with an 1800 rpm
at 180 V armature voltage maximum rpm nameplate rating were actually 
running
at closer to 3000 rpm, and one that ran at over 4000 rpm.  That was under 
no
load.  Under low load they would appear to be OK, but stall when load was
increased.  We eventually replaced all of our permag DC motors with shunt
field DC motors, and now we only buy AC motors with variable frequency
drives.

Whenever you plug reverse your permag Elec-Trak traction motor you have a
momentary current spike which may well exceed the critical current
associated with field magnet damage.  (Plug reversing has nothing to do 
with
a plug and socket.  It is the attempt to instantly reverse direction on a
motor.)  I would avoid plug reversing our permag ETs under any
circumstances.

Steve Naugler
snaugler earthlink net