"Peak Efficiency" Control Mode?

General topics and discussions about the VESC and its development.
devin
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Re: "Peak Efficiency" Control Mode?

Postby devin » 02 Oct 2017, 14:16


rew
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Re: "Peak Efficiency" Control Mode?

Postby rew » 03 Oct 2017, 07:37

Just a quick question. Say I have a 50V motor, currently running at 25V BEMF. Suppose the motor can do 50A, but the torque required is only 5A. The motor has 20mOhm resistance. What is the effective voltage that you propose and what is the efficiency of the motor? (the last example that I read, and that was a while back, had the "normal" and your proposed settings very close together.).

devin
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Re: "Peak Efficiency" Control Mode?

Postby devin » 03 Oct 2017, 13:05


devin
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Re: "Peak Efficiency" Control Mode?

Postby devin » 05 Oct 2017, 23:54

@rew Suppose I have a (4) 140kv 0.02ohm hub motors on an electric skateboard with 84mm tires which can do 80A and a 10ah 30C 50V battery. During a lighthearted ride through the park I'm willing to draw up to 300a battery amps for a short time, & on the VESCs I use a 75a battery amp limit and an 80a motor amp limit... I might experience something like this:

Image

----------------------------

Now suppose I engage efficiency control with the following settings:

Desired Efficiency Setting: 96%
Min Desired Watts Setting: 100w (per motor)
Max Desired Watts Setting: 3700w (per motor)
Max Motor Amps Setting: 80a (per motor)
Max Duty Cycle Setting: 95%

Now I might experience something like this:

Image

Image

rew
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Re: "Peak Efficiency" Control Mode?

Postby rew » 08 Oct 2017, 09:10

So, I managed to find a situation, a not so theoretical example, where you fail to reach the "desired efficiency" by getting a much higher efficiency. Now I'm going come up with an example where you fail to reach the desired efficiency by coming short a long ways....

Same motor, but now we're moving at 1.0km/h, and want close to full throttle to accelerate to that 25km/h as quickly as possible. My guess is that you can't get more than about 50% efficiency. (*)

Sure, most of the mechanical energy will go towards increasing the kinetic energy of the vehicle that will quickly accelerate, so this situation won't last long.

In slightly other applications, say when winding a string onto a coil, where the force on the string and the rotation speed of the coil can vary by a lot, then this situation with low efficiency may persist for quite a while.

A quick glance at your latest plot shows that the most important difference between "classical" and your method is that you reduce the throttle by as much as 75% at low speeds (15% of max). So... At low speeds, high throttle settings will result in low efficiency. Low throttle settings (especially when combined with a high speed) result in high efficiency. Of course, you can reduce the throttle to maintain a desired instantaneous efficiency. The user of your algorithm will feel the board accelerate, hold back and then pick up speed quicker and quicker.

(*) This situation will persist if say you're pulling a load up a hill where things just won't start moving quicker.

devin
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Re: "Peak Efficiency" Control Mode?

Postby devin » 09 Oct 2017, 00:35


rew
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Re: "Peak Efficiency" Control Mode?

Postby rew » 09 Oct 2017, 07:26

You suggest that while I command 50A of motor current through the throttle, I would accept getting only 0.8A. Well, I don't. I think most people would agree that such a big difference is unacceptable.

The general idea is that the more throttle you use, the worse the efficiency becomes. At low speeds you'll have to accept that the instantaneous efficiency is low.

Again the thing is: WHAT are you trying to optimize. If you're designing the solar car to race across australia, then the steady state efficiency matters a lot because the stretches where you're already at speed are so long. The energy use during starting and stopping can (at first) be neglected.

But in general, you're riding an electric skateboard. You've charged the battery at home, and you could ride 5km/h to arrive at your destination in one hour. This is way more energy efficient than riding at 25km/h and arriving at your destination in 12 minutes. I would hazard a guess that even you would prefer to ride for 12 minutes at 25km/h than an hour at 5km/h. You are willing to spend some energy to get to your destination quicker.

So.... to travel, the energy-per-meter-traveled goes up the faster you go.

devin
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Re: "Peak Efficiency" Control Mode?

Postby devin » 09 Oct 2017, 13:20


rew
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Re: "Peak Efficiency" Control Mode?

Postby rew » 13 Oct 2017, 07:50


devin
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Re: "Peak Efficiency" Control Mode?

Postby devin » 13 Oct 2017, 12:22



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