(ET) Motor overvoltage

[Rhett George <rtg ee duke edu>]

 - Greetings -

Here's a couple of thoughts on what happens with higher voltage on the
drive motor.

If the motor is a series motor, running at near a constant torque (and
consequently a constant current) it will run faster and generate the 
needed increase in EMF (back EMF).

If the motor is a shunt motor with the magnetic field below saturation 
with the increased applied voltage, the speed stays almost constant.
The needed increase in EMF comes from the increased field strength and
an increase in motor speed is not needed.  Historically, in the days 
of autos having generators instead of alternators, heater blowers and 
windshield wiper drives used shunt motors which yielded drive speeds 
mostly independent of the vehicle engine speed.

If the motor is a shunt motor in which the field magnetization current
has already saturated the iron, applying a higher voltage will make little
if any difference in field strength and the motor will run faster to 
generate the needed EMF.

Cheers,

                                        Rhett