"Peak Efficiency" Control Mode?

General topics and discussions about the VESC and its development.
markorman
Posts: 14
Joined: 25 May 2016, 11:35

Re: "Peak Efficiency" Control Mode?

Postby markorman » 05 Sep 2017, 06:55

First of all stop posting this graph, which has nothing to do with 3 phase BLDC or PMSM motors.

rew
Posts: 943
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: "Peak Efficiency" Control Mode?

Postby rew » 05 Sep 2017, 07:34

devin wrote:
rew wrote:Bullshit


@rew -can you be more specific i.e. where is the math wrong?

Mark is right.

pf26
Posts: 309
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: "Peak Efficiency" Control Mode?

Postby pf26 » 05 Sep 2017, 09:01

@devin: your graph assumes the motor torque/current decreases linearly with RPM, which is obviously a wrong assumption in all practical applications. RPM and torque/amps can both be varied depending upon the motor load and ESC control parameters. For a given motor, the efficiency varies with both RPM and current. Highest efficiencies are obtained at high RPM and moderate currents, and can exceed 90% for good quality motors. At higher RPM, the iron losses become predominant though but motor datasheets generally don't specify this clearly.

devin
Posts: 255
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: "Peak Efficiency" Control Mode?

Postby devin » 05 Sep 2017, 12:24

rew wrote:Bullshit

devin wrote:@rew -can you be more specific i.e. where is the math wrong?

markorman wrote:First of all stop posting this graph, which has nothing to do with 3 phase BLDC or PMSM motors.

rew wrote:Mark is right.

pf26 wrote:@devin: your graph assumes the motor torque/current decreases linearly with RPM, which is obviously a wrong assumption in all practical applications. RPM and torque/amps can both be varied depending upon the motor load and ESC control parameters. For a given motor, the efficiency varies with both RPM and current. Highest efficiencies are obtained at high RPM and moderate currents, and can exceed 90% for good quality motors. At higher RPM, the iron losses become predominant though but motor datasheets generally don't specify this clearly.


@rew:

-earlier you proposed a scenario of an e-bicyclist at 25kmh with 100kv motor, 0.1ohm resistance, 50V and no load = 50kmh (@25kmh, the present speed is half of no load speed which on the objectionable graph shows this rpm point should supply peak power not peak theoretical efficiency, and yet as I describe below, you can achieve peak theoretical efficiency, even at 50% of no load rpm):

-the algorithm recommends %58.82 duty cycle for peak 85% electrical to mechanical conversion efficiency at 50% of no load rpm in the scenario

rew wrote:You are suggesting that we find a control method that is more efficient at driving a motor than "standard".

Let's try this with a theoretical motor first. Let's try this first "by hand".

Easy numbers. 50V max, about 100KV: 10 rad/sec/V, 0.1 Ohm resistance. Max 50A.

The 10 rad/sec/V means that this motor produces 0.1 Nm/A . So at 50A, a maximum of 5Nm.

So I've mounted this on my bike (or skateboard if you like) and the gearing is such that at no load and 50V on the motor, the board/bike goes 50km/h. So now I'm cruising along at 25km/h


1297.275W Electrical = 25.9455 battery amps * 50V <-------- 1297.275W Electrical In

1102.756w = 4.212Nm*2500rpm/9.5488 <------ 1102.756W Mechanical Output @ 2500rpm @ 29.411V Effective Volts @ 100kv @ 0.1ohm @ %58.82 duty cycle

(1102.756W/1297.275W)*100=85.00% <----------- 85.00% conversion efficiency of electrical to mechanical watts.


^ @rew the math i posted prior suggests 58.82% duty cycle provides peak theoretical 85% electrical to mechanical conversion efficiency at half of no load rpm in your e-bicyclist scenario (in contrast to the objectionable graph which indicates peak 85% efficiency isn't available at 50% of no load rpm)... as proof the algorithm works as described, in my next post I will analyze how efficiency would be effected if the controller supplied the motor with either 55.82% duty cycle (-3% from algorithm suggested duty cycle) or 61.82% duty cycle (+3% from algorithm suggested duty cycle). I will also analyze what happens at 12.5kmh instead of 25kmh, with the algorithm recommended duty cycle, and then compare what would happen if +3 or -3 duty cycle were used compared to the algorithm.

markorman
Posts: 14
Joined: 25 May 2016, 11:35

Re: "Peak Efficiency" Control Mode?

Postby markorman » 05 Sep 2017, 13:07

Again your graphs are not for 3-phase BLDC or PMSM motors, which can have efficiency around 90-95%. Don't know where you got your 85% theoretical limit.

If you want to maintain desired speed you need to apply proper torque to counteract all the forces braking you. This torque will be dependent on the load. Depending on the speed and desired torque regulator will set proper duty cycle to achieve this required torque. If your "algorithm" calculates different duty cycle, you will either accelerate (if duty cycle is higher than needed) or decelerate (if duty is lower than needed) since you will apply different torque than needed. In almost all applications, especially in EV's you want to control the speed or the torque.

devin
Posts: 255
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: "Peak Efficiency" Control Mode?

Postby devin » 05 Sep 2017, 14:15

markorman wrote:Again your graphs are not for 3-phase BLDC or PMSM motors, which can have efficiency around 90-95%. Don't know where you got your 85% theoretical limit.

If you want to maintain desired speed you need to apply proper torque to counteract all the forces braking you. This torque will be dependent on the load. Depending on the speed and desired torque regulator will set proper duty cycle to achieve this required torque. If your "algorithm" calculates different duty cycle, you will either accelerate (if duty cycle is higher than needed) or decelerate (if duty is lower than needed) since you will apply different torque than needed. In almost all applications, especially in EV's you want to control the speed or the torque.


@markorman... Thanks for having patience with me, I have now obtained & seen mathematical proof that your assertion is correct that BLDC motors can surpass 85% conversion efficiency. On that basis I think "Peak Efficiency" Control Mode should be renamed henceforth to "Desired Efficiency" Control mode, because if we rename the algorithm to "desired efficiency" control mode, then basically the algorithm can ensure efficiency does not drop below a minimum desired conversion efficiency -- 85% conversion efficiency in the examples I've used so far.

devin wrote:The cold logic of the algorithm is "whenever I am pushing full throttle, my motor is at peak electrical to mechanical conversion efficiency, whatever amount of power that happens to be at the time*"

*except at very low rpms-- full throttle equates to a rider-specified minimum wattage-- otherwise the formula will produce insufficient electrical and mechanical wattage to propel a human rider. (less than full throttle, throttle % equates to % max available motor amps as defined by the algorithm)

@ full throttle:
if according to the algorithm at full throttle G < E, then full throttle = E or max duty cycle, whichever is lower wattage, & %throttle = %max motor amps as defined by E or max duty cycle, whichever supersedes based on lower wattage. (E supersedes & is lower wattage when rpms are below the useable max efficiency power threshold,and max duty cycle supersedes & is lower wattage when Back EMF at high rpms prevents obtaining E).

if according to the algorithm G > F, then full throttle = F & %throttle = %max motor amps as defined by F

B = (D)(AC)

where:

A= present effective voltage
B= present rpm
C= motor kv or rpm per volt
D= 85/100
E= minimum full throttle wattage setting
F= maximum wattage setting
G= present wattage

^by variably changing D the (85/100) ratio, the rider could change the power curve, for example changing this ratio to (90/100) would give lower acceleration, or changing the ratio to (80/100) would give greater acceleration.


^in a couple mathematical examples I've done, I've found that changing variable D to a fraction higher than 85/100 (such as 90/100) can provide even higher conversion efficiencies than 85% at the sacrifice of acceleration. On that basis, as mentioned, I propose calling the control mode "Desired Efficiency Control Mode" rather than "Peak Efficiency" control mode.

markorman wrote:If you want to maintain desired speed you need to apply proper torque to counteract all the forces braking you. This torque will be dependent on the load. Depending on the speed and desired torque regulator will set proper duty cycle to achieve this required torque. If your "algorithm" calculates different duty cycle, you will either accelerate (if duty cycle is higher than needed) or decelerate (if duty is lower than needed) since you will apply different torque than needed. In almost all applications, especially in EV's you want to control the speed or the torque.


^The plan here is to have E -- "minimum full throttle wattage setting" for low rpms when the algorithm provides less wattage than what would be desirable for the rider.

markorman
Posts: 14
Joined: 25 May 2016, 11:35

Re: "Peak Efficiency" Control Mode?

Postby markorman » 05 Sep 2017, 14:49

Stop thinking about controlling the motor with watts. Nobody nowhere is controlling the motor with watts. The only time you care about watts consumed by motor is when you have to limit the power drained from battery.
Almost are motors are in torque control mode (at least in automotive world) since the torque is the force the motor produces and force you feel when accelerating/decelerating. In BLDC and PMSM motor you have to limit the torque because the mechanical limits, and limits due to current in copper windings.

This is typical graph for PMSM motors. You have to limit the torque (motor current) and then limit the power.

Image

At this is graph for Tesla's model S

Image

Both graphs shows maximum Output the motor can achieve, without destroying it. Torque and current (motor current) have the same shape.

devin
Posts: 255
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: "Peak Efficiency" Control Mode?

Postby devin » 05 Sep 2017, 19:00

markorman wrote:Stop thinking about controlling the motor with watts. Nobody nowhere is controlling the motor with watts. The only time you care about watts consumed by motor is when you have to limit the power drained from battery.
Almost are motors are in torque control mode (at least in automotive world) since the torque is the force the motor produces and force you feel when accelerating/decelerating. In BLDC and PMSM motor you have to limit the torque because the mechanical limits, and limits due to current in copper windings.

This is typical graph for PMSM motors. You have to limit the torque (motor current) and then limit the power.


@markorman... i think the user should be able to specify variable E either in watts mechanical or watts electrical... either of these can be converted to motor amps or torque via simple existing formulae.

markorman
Posts: 14
Joined: 25 May 2016, 11:35

Re: "Peak Efficiency" Control Mode?

Postby markorman » 05 Sep 2017, 19:31

Well if you think this is better than what all of the industry and automotive field is using go ahead and work with useless watts. But please read more about motor control techniques since i believe you don't understand it and it doesn't look anybody on this forum can explain it to you.

Over and out.

devin
Posts: 255
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: "Peak Efficiency" Control Mode?

Postby devin » 05 Sep 2017, 19:42

markorman wrote:Well if you think this is better than what all of the industry and automotive field is using go ahead and work with useless watts. But please read more about motor control techniques since i believe you don't understand it and it doesn't look anybody on this forum can explain it to you.

Over and out.


@markorman... i didn't say better i said effectively the same via simple formulae... ie watts can be converted to torque or motor amps value, etc,...

markorman wrote:The only time you care about watts consumed by motor is when you have to limit the power drained from battery.


in an electric vehicle for human propulsion, regardless of industry practice, power drained from battery is a large concern, hence my primary focus on efficiency control.


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