Re: Motor controllers, RFI & stuff (esoteric question).

[Lee Hart <leeahart@earthlink.net>]

jcmassey@netspace.net.au wrote:
> 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?

A normal switching power supply, like the ones used in your PC and
virtually everything else, has an output LC filter to deliver
essentially pure DC. This LC filter is left out of EV controllers to
save money. EV controllers depend on the motor's inductance as the
filter.

This works, but the motor isn't a very good filter. It generates extra
heat in the motor, and the motor "sings" audibly at the frequencies it
is filtering.

Running the motor on filtered DC would have no effect on mechanical
performance (same torque, speed, and horsepower). But the motor would
run a little cooler, and would not "sing".

The filter losses would be shifted into the filter components in the
controller. Most likely, the controller's switching frequency would be
increased to 50 KHz or so (roughly the state of the art in high power
switchers) to reduce the size and cost of the filter components. Also,
it is likely that some form of "soft switching" would be used to reduce
switching losses. Overall, the system efficiency would probably rise a
bit, but the cost would go up.
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen