"Peak Efficiency" Control Mode?

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

Postby devin » 28 Aug 2017, 17:17

What if there was a control mode where full throttle always equates to peak electrical to mechanical power conversion efficiency from the motor?

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Summary:

Assuming 100% duty cycle, a motor gets maximum (about 85%) conversion of electrical to mechanical power at about 85% of no load rpm.

Image

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Logic:

full throttle applies an effective voltage (A) such that the present rpm (B) is 85% of (C) motor KV times (A) the present effective voltage.

B = (85/100)(AC)

therefore:

A = (20B)/(17C)

assuming the motor is 100kv, battery 50V, present rpm is 2500rpm:

A = XX.XXV = Full Throttle Effective Voltage
B = 2500rpm = Present RPM
C = 100kv = Motor KV
D = 50v = Battery Voltage

B = (85/100)(AC)

2500 = (85/100)(A*100)

therefore:

A = (20B)/(17C)

A = (20*2500)/(17*100)

29.41V = (20*2500)/(17*100)

therefore:

A = 29.41V = Full Throttle Effective Voltage
B = 2500rpm = Present RPM
C = 100kv = Motor KV
D = 50v = Battery Voltage
E = XX.XX% duty = Duty Cycle

&:

(A/D)*100 = E

(29.41/50)*100=58.82

therefore:

A = 29.41V = Full Throttle Effective Voltage
B = 2500rpm = Present RPM
C = 100kv = Motor KV
D = 50v = Battery Voltage
E = 58.82% duty = Duty Cycle
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Theory: If the motor KV is 100kv, the battery voltage is 50V, and the duty cycle is 58.82%, and the present RPM is 2500rpm, then the motor is running very close to peak electrical to mechanical conversion efficiency.

Simply if the duty cycle is always maintained such that the present rpm is always 85% of present effective voltage times the KV, then in theory in this control mode the motor should always be running at peak electrical to mechanical conversion efficiency.

markorman
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Re: "Peak Efficiency" Control Mode?

Postby markorman » 28 Aug 2017, 17:42

So you want speed control?

devin
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Re: "Peak Efficiency" Control Mode?

Postby devin » 28 Aug 2017, 18:16


rew
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Re: "Peak Efficiency" Control Mode?

Postby rew » 28 Aug 2017, 18:41

Devin, I think you should stop posting about stuff you know nothing about.

You are using formulas and graphs that DO NOT APPLY to the case at hand and drawing big conclusions from them.

devin
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Re: "Peak Efficiency" Control Mode?

Postby devin » 28 Aug 2017, 18:59


elux
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Re: "Peak Efficiency" Control Mode?

Postby elux » 29 Aug 2017, 00:57

Devin,

Please look at the title of the graph you posted. The torque-speed curve you posted is for a PMDC motor. That is a brush DC motor that has no controller. Apply DC Voltage and the commutator will change the DC into AC and the physics of the back EMF (Faraday's Law, google it) limits the current. It is a very different type of motor from the brushless DC motors that the VESC is designed to operate. FYI this type of motor was invented by Thomas Davenport in 1835 and it was awarded the first ever patent for an electrical device.

The VESC limits the current to the motor using pulse width modulation (PWM). It inverts the DC current into 3 phase AC in precisely the right frequency and phase angle for peak efficiency operation. So the mode you desire is already programmed in by Benjamin.

It is disheartening to read some of your posts. It's clear that you enjoy learning about motor theory and you have a great imagination and a spirit of inventiveness. Unfortunately, some of your posts contain misinformation that would be easily cleared up with a quick search on wikipedia.

For example, your misconception regarding the K_v rating of a motor and the winding resistance. The back EMF of an electric machine depends only on the rate at which the flux linking the windings is changing (Faraday's Law). This law is the fundamental basis for the connection between electricity and magnetism. If you change the material of the wire (to aluminum for example) and change with resistance without changing the number of turns or magnetic field strength, the back EMF and the K_v rating of the motor won't change.

devin
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Re: "Peak Efficiency" Control Mode?

Postby devin » 29 Aug 2017, 01:22


rew
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Re: "Peak Efficiency" Control Mode?

Postby rew » 29 Aug 2017, 03:43

Bullshit

devin
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Re: "Peak Efficiency" Control Mode?

Postby devin » 29 Aug 2017, 04:29


rew
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Re: "Peak Efficiency" Control Mode?

Postby rew » 29 Aug 2017, 06:16

A change from the pattern. Seems you're right.

If you keep the voltage constant, and double the wire (e.g. a second wire), you get twice as much current, giving the same "flow of electrons" in each wire. But I fail to see how this relates to rotor RPM and double the wire thickness. In that case, even though the rotor RPM ALMOST stays the same, the total current will rise "a little", and the drift speed will lower a lot.

There are lots of cases where you can find a "change this, and that other parameter doubles, change that and it still doubles, but change something else and it doesn't, but stays the same". I have a vat of water. Double the water height, and the pressure at the bottom doubles. Double the density of the fluid and the pressure doubles, but double the area of the vat, (keeping the water level the same) the pressure stays the same. So what?


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