Motor controllers, RFI & stuff (esoteric question).

[jcmassey@netspace.net.au]

Hi all

I'm going to show another gap in my (our) experience here, we were having a
'how about' session, and the subject of RFI came up, leading to this
question: knowing how much RFI is generated and radiated by normal chopped
DC motor controllers, what would be the effect of running a motor off DC
provided from a controller that switched fast enough to provide "pure" DC
from 0V through to pack voltage? The fact of feeding the motor with
constant DC instead of chopped would eliminate the RFI, but what other
effects would happen? The freewheel diode would still be needed, but would
only go into conduction when the controller was sourcing less than the EMF
generated in the motor. There would probably be greater losses in
switching, but possibly gain in reduced losses from the motor?

Do the pulses from a conventional controller provide additional torque over
what you would have at steady DC? ie if a controller from 120V was at 20%
duty cycle giving an average 24 volts at (X) motor amps (and Y RPM),
achieving Z torque, would you get the same torque from 24VDC at (X) amps
(and Y RPM)? On the surface you would think you would get identical torque
wether under a pulsed DC average or a continuous DC of the same value, but
if this how it is in the real world?

While it is technically possible to build a high power switching regulator
(done all the time in welding equipment), it's not done in EVs, so for what
reason? Purely economics? or is it a case of "its always been done this
way" and the advent of fast enough powerful enough devices has been so
recent that no-one has done it yet?

This is another of these exotic questions that are irrelevant to what I'm
building (on a v. tight budget) but technically interesting, y'know? 

Interested in the answers,

James Massey