Re: Anyone ever built a voltage-modulated controller?

["Peter VanDerWal" <peterv@peoplepc.com>]

> Instead of modulating the current by chopping it up, what would
> happen if the voltage across the motor was monitored in a simple
> feedback loop.  A large capacitor in parallel with the motor would
> store enough electricity to smooth the voltage a little.  When the
> motor voltage is less than the pedal voltage, a relay closes, then
> opens after the motor voltage has risen to the pedal voltage.  A
> delay would be effected with resistors in the control circuits.
>
> Would this work?  Or would having a low voltage instead of pulses
> of high voltage be bad for the motor?

Sure it's called a Pulse Width Modulator.
I.e. it's doing the exact same thing they are doing with MOSFET controllers.
A MOSFET is essentialy a silicon switch that they turn on and off very
quickly.  Only difference is that MOSFET controllers use the motors
inductance to smooth out the DC instead of a huge (expensive) capacitor like
your idea would require.
FWIW The MOSFET controllers usually have a large capacitor bank in them, but
it's designed to smooth out the current pulses on the input to the
controller (helps the batteries).


>
> Another aspect is, the same circuit could be used to do regenerative
> braking by switching the armature out of the powered circuit.  Having
> your accelerator double as a brake pedal when you flip the regen
> switch is obviously unsafe for any circumstance other than a roll
> down a long hill.
>

Curtis REGEN controllers do essentially the same thing.

> I am not an EE.  In general, does this kind of switched continuously
> veriable voltage power supply have problems?  Where are the
> big losses?  For this application we obviously don't care about
> if the power is smooth or not :)
>

With mechanical components?  Yes..contactors don't last long when they have
to continuously switch high currents on and off.