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"Peak Efficiency" Control Mode?

Posted: 28 Aug 2017, 17:17
by devin
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.

Re: "Peak Efficiency" Control Mode?

Posted: 28 Aug 2017, 17:42
by markorman
So you want speed control?

Re: "Peak Efficiency" Control Mode?

Posted: 28 Aug 2017, 18:16
by devin
markorman wrote:So you want speed control?

I don't believe the algorithm as described would result in constant speed if the load was changing. However, at low rpm, as outlined there would be negligible watts available, so the algorithm would need a separate, possibly wattage based algorithm to get the motor smoothly up to desired minimum operating speed before switching to the efficiency-preserving mode.

Re: "Peak Efficiency" Control Mode?

Posted: 28 Aug 2017, 18:41
by rew
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.

Re: "Peak Efficiency" Control Mode?

Posted: 28 Aug 2017, 18:59
by devin
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.


@rew If we take a 100kv motor, and apply 50% duty cycle at 50V, unloaded, would you agree it will spin at max 2500rpm? (and @ 100% duty it will spin 5000rpm?) (and on a VESC @ maximum duty 95% it will spin 4750rpm?)

Re: "Peak Efficiency" Control Mode?

Posted: 29 Aug 2017, 00:57
by elux
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.

Re: "Peak Efficiency" Control Mode?

Posted: 29 Aug 2017, 01:22
by devin
elux wrote: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.



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.


Assuming 100% duty cycle, the motor physics are the same, & I specified 100% duty cycle. (yes the vesc isn't capable of this at stall @ 50V, but the physics ratios remain). If one substitutes 10V for 50V, the math is the same, and the physical description is also possible and safe. Yes the VESC enables safe high-voltage operation but this doesn't change the motor ratios. Also, strictly considering the formula in the first post it would be safe as well, but as mentioned it would require a different algorithm for low speed operation due to the limited watts.

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).


@elux... and yet the correlation remains... I think you will see the following formula agrees with what you are saying regarding the number of turns without specifying the number of turns or termination.

D=sqrt(E/(V*N))
E=N*V*D^2
V=E/(N*D^2)
N=E/(V*D^2)

D = Change Factor of KV (rpm/v)
E = Change Factor of Conductor Resistivity (ohm-meters)
V = Change Factor of Conductor Volume (meters^3)
N = Change Factor of Conductor Resistance (ohm)

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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.


@elux... I agree copper vs aluminum with same geometry winding should be the same KV in theory, which agrees with your position and the aforementioned formula, however despite searching I haven't yet seen hard fully convincing proof for this, and I intend to do the copper vs aluminum experiment and post the results. I've also observed that in a copper winding wye motor, changes in no load rpm correlate linearly & proportionally with changes in electron drift velocity at stall (100% duty), and copper and aluminum have different drift velocities, so I personally want to use a VESC as an experimental tool to observe the discrepancy.

devin wrote:I've also observed that in a copper winding wye motor, changes in no load rpm correlate linearly & proportionally with changes in electron drift velocity at stall (100% duty), and copper and aluminum have different drift velocities, so I personally want to use a VESC as an experimental tool to observe the discrepancy.


Examples:
- doubling battery voltage doubles electron drift velocity at stall and doubles no load rpm.
- halving the number of turns doubles electron drift velocity at stall and doubles no load rpm.
- doubling the wire thickness doesn't change the drift velocity and doesn't change the KV.

If copper no load rpm changes are proportional to changes in electron drift velocity at stall, and copper and aluminum have different drift velocities at the same number of turns, what would then cause the copper and aluminum to have the same KV at the same number of turns?

Re: "Peak Efficiency" Control Mode?

Posted: 29 Aug 2017, 03:43
by rew
Bullshit

Re: "Peak Efficiency" Control Mode?

Posted: 29 Aug 2017, 04:29
by devin
rew wrote:Bullshit


Source: https://en.wikipedia.org/wiki/Drift_velocity
Image

^We can see halving the conductor length doubles the drift velocity... & doubling the voltage doubles the drift velocity... & and doubling the wire cross section has no effect on drift velocity... behaviors also describing changes to the rotor rpm at no load rpm (halving conductor length doubles no load rpm, and doubling voltage doubles no load rpm, and changing the thickness of the conductor has no effect on no load rpm).

Re: "Peak Efficiency" Control Mode?

Posted: 29 Aug 2017, 06:16
by rew
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?