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Old 05-28-2012, 12:55 PM   #17 (permalink)
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Originally Posted by fb_bf View Post

I found your reply interesting. In particular your claim about 25% loss in efficiency when not operating at WOT. My controller, and I've seen others claim 98% efficiency. Since they use PWM to get that, I'm interested in why you think it is only 75% , not the 98%.
I would agree that picking a gearing that allows for using one gear and eliminating transmission losses is a good idea. In my 4 speed transmission, the 4th speed is straight through, so It doesn't have losses due to gear friction, but I do have losses due to the transmission oil in any gear. I drive in 4th gear whenever I can.
Hmm, perhaps I am 15 years out of practice but...

You need to take motor and controller as a single drivetrain unit...
98% efficiency motor and controller only occurs at WOT, below that it must be another number, AKA switching losses affect both the controller loss, motor losses and battery/conduction losses not just one or another.

I was always taught to consider a DC ev drivetrain as being roughly 85% efficient overall, it goes up to 93-98% at wot (motor/controller/wires) But if it is 85% overall and it peaks at 93-98%, what happens if you are driving at low throttle below the average?

Anyway I deal mostly with lead and forklift style DC motors but many of the AC controllers suffer the same as a DC controller when your throttle is under 50%

Depending on switching speed you can easily loose 25% of power applied through the controller & motor to switching losses when throttle is "light", where that would occur would depend on voltage, rpm, and switching speed..

On my one EV (real world) 50amps at light throttle gets me to about 18mph and no faster without wind or hills, WOT I accellerate to 30mph and level out at about 65amps, the amount of energy difference is only around 30% (out of the battery) but the power generated is greatly different, the odd part is once I get up to speed, if I let off the gas slightly so I float around 50 amps my speed only decreases slightly and holds around 24mph, this is because field strength is higher so switching losses/motor losses end up being lower (higher throttle position to push 50amps whent he motor is spinning faster)

There were composite maps of efficiency on the diyelectriccar forums of one example controller that had a very fast (efficient) switching speed at various throttle positions, even on his controller things were not as pretty getting below 50% throttle. The thing is an AC controller does not need to PWM until RPMs and power are quite low since timing can just be adjusted, so AC tends to be more efficient at lower loads but usually, though not always less efficient (a percentage or two) at wot as compared to a DC controller and good motor combo.

All this said once you get to about 75% throttle most every controller & motor combo, even a terrible one gets over 90% efficiency, it is down on 50% or less that things get murky, especially at low RPMs.

So in other words if you aren't running WOT or close to it, it isn't 98% unless you have an extremely efficient controller & motor I haven't encountered. I have never understood why people ignore the bottom 50% of the throttle in terms of efficiency.

On my EV I only try to use that area when hypermiling because I can more or less drive WOT all day without speed worries. I have considered adding a large capacitor bank to improve my low speed efficiency but I am uncertain how much affect it would have, my controllers switch rather slowly Curtis 1204/1209 but aren't real bad, just not great.

Ah well, But then again mine are old, low tech, slow and reliable.
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