RE: Backup controllers

["Eric Chang" <ericchang@my-deja.com>]

>From: "David Roden (Akron OH USA)" <roden@ald.net>

>> They may be tempted to drive home under
>> "contactor control", placing their motor at great risk.

>I'm curious to know what the great risk is.  

There is a slight risk of subjecting the motor to currents
that could damage it.  The armature consists of two layers
of flat wire (on a GE motor).  If the outer layer burns
through, it can be repaired with a resistance soldering
tool and silver solder.  But, there is a layer of insulation
between the two layers and also the armature that can be
carbonized.  If this happens, it will be impossible to
clear the short to the shaft, causing a hazardous
connection between pack and chassis ground.  Also, 
any heat damage to the bonding wire can cause it to come
lose, and do damage to the brush rigging and field coils.
Using a flaky bipolar transistor controller which failed
and "contactor control" to drive home completed the event.
This is not hypothetical.  It can and has happened.  By
the way, I was not the one who drove home under contactor
control.  It was a previous owner.  Although I cannot say
that I would not have been tempted to give it a try.  The
inconvenience of waiting two hours for a tow is sometimes
more unbearable than losing $1000.  In retrospect, a
simple contactor controller that was a bit smarter may
have been a good idea.  Possibly, run the motor at half
voltage from half the pack through thin (steel?) current
limiting wires.  It seems like a good idea.

It may sound unbelievable, but you can overcurrent a motor
this way, because the shaft speed is so low when you are
starting.

Another unlikely, but possible thing that can happen is 
that when you take your motor in to be repaired, the
service manager might say, as she is running her fingers
over the commutator, "It's so big, and so thick; I don't
think I can handle it."  It can and has happened, however
unbelievable it may sound.




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