[email protected] wrote:
> Why do some controllers use 1.5KHz for low speeds and 15KHz for high
> speed? Why specifically is a lower frequency better for low speeds?
The switching frequency isn't associated with vehicle speed. In general,
lower switching frequencies allow slower transistors and diodes to be
used, which are cheaper for a given power level. Lower frequencies also
reduce switching losses in the transistors and diodes, which makes them
run cooler and raises efficiency.
But... frequencies in the audible range of human hearing can be heard.
The motor acts as a loudspeaker, and people complain. So, most modern
controllers accept the higher price and lower efficiency to get the
switching frequency above the audible range.
Curtis originally designed their controllers to switch at 15 KHz, which
is high enough that most people can't hear it. However, they also
skimped on the parts, so they are barely strong enough. When people put
these controllers in a heavy road-going EV with a huge motor, the
controllers were over-stressed and failed. Rather than redesign it with
bigger parts, they chose to make it drop down to 1.5 KHz under heavy
stress conditions. So, you'll hear them "whistle" when hot or trying to
start a large motor.
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
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