As it happens, Lucas actually has a separate motor connection for high
speed, which goes to a separate brush inside the motor. For high speed,
+12V is connected to this brush instead of the low speed brush.
There really is no 'speed governor' as such, except the inherent speed
limiting ability of a PM motor. Basically what happens is that, as the
motor turns faster, a counter voltage is developed across the armature,
which reduces current flow, thereby slowing down the motor. The balance
point depends on motor design (number of turns, strength of field) and
load. If you turn the motor fast enough (from an external power source),
it will actually reverse the current flow and act as a generator !
Because of the position of the 'high speed' brush, it 'sees' less of this
counter voltage (fewer effective turns) and so the motor has to turn faster
to generate the same balance.
Dan Masters has posted some nice diagrams for the TR6 wiper motor on the
VTR web site at
I've seen other cars that use a resistor as you describe, but it gets quite
warm, so they don't put it under the dash. The resistor is usually near,
or inside, the motor.
On Tuesday, April 20, 1999 5:03 PM, Ross Vincenti [SMTP:RVincenti@lbfc.com]
> Brad - Hmmm, help me here. Why would there need to be an additional
> wire? After all, 12 volts is 12 volts, whether fed to an electric
> motor by one wire or two. Wouldn't the speed govenor be built into
> the wiper motor itself? If you think about it, all you get is 12
> volts at the motor. The only way to create a variable speed motor
> a 2 speed motor is to have a reostat switch or a resistor of some
> that operates to allow a full 12 volts or a lesser amount to make
> motor run slower. If a reostate, then it is infinitely variable, if
> 2 speed switch, then a resistor would work (I think).
> Caveat - I am not an electrical engineer, nor am I even half certain
> that I know what the H_ll I am talking about. These are just random
> thoughts. Regards,