Power Limiting instead of Current Limiting

General topics and discussions about the VESC and its development.
BlakeCarmichael
Posts: 16
Joined: 06 Sep 2017, 12:12
Location: Florida

Power Limiting instead of Current Limiting

Postby BlakeCarmichael » 06 Sep 2017, 12:18

Hi,

I am using a VESC from diyelectricskateboard on a 24 V BLDC motor that has a 177 Watt maximum power rating.
The battery I am using is a 24 V lithium ion which ranges from 21-29.75 V throughout its cycle.

Using the VESC and BLDC tool, I would like to limit the power consumption of the motor such that it never exceeds the 177 W rating.
The only option for this I have seen is to limit the current, but this causes problems as the battery voltage drops because a fixed current limit will reduce the maximum power output to the motor at lower voltages.

Is there a solution to this that I am overlooking?

Thanks for any help.
Blake

devin
Posts: 253
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: Power Limiting instead of Current Limiting

Postby devin » 06 Sep 2017, 13:12

BlakeCarmichael wrote:Hi,

I am using a VESC from diyelectricskateboard on a 24 V BLDC motor that has a 177 Watt maximum power rating.
The battery I am using is a 24 V lithium ion which ranges from 21-29.75 V throughout its cycle.

Using the VESC and BLDC tool, I would like to limit the power consumption of the motor such that it never exceeds the 177 W rating.
The only option for this I have seen is to limit the current, but this causes problems as the battery voltage drops because a fixed current limit will reduce the maximum power output to the motor at lower voltages.


Is there a solution to this that I am overlooking?

Thanks for any help.
Blake


do an foc winding resistance detection... the true value will be double since it calculates to virtual ground point.

then (assuming, for example 0.1ohm detection, 24v pack)

0.1ohm*2=0.2 ohm lead to lead

177w=29.74^2 motor amps * 0.2ohm=7.37battery amps * 24V pack v

^simply assuming 177w max, 24v pack, and 0.2ohm lead to lead, the battery amp limit should be 7.37amps, the motor amp limit should be 29.74 amps, and the absolute max amp setting should be a hair higher... say 35a

BlakeCarmichael
Posts: 16
Joined: 06 Sep 2017, 12:12
Location: Florida

Re: Power Limiting instead of Current Limiting

Postby BlakeCarmichael » 06 Sep 2017, 13:27

Ok, I need to do some reading on FOC.

But, I haven't had any success with running motor detection on this motor. The motor is permanently attached to a compressor and I think that may have something to do with why the motor detection is failing. Is this correct? I have tried adjusting the current, min erpm, and low duty values in the motor detection field.
Do I need to resolve the motor detection issue before I can make headway with FOC?

BlakeCarmichael
Posts: 16
Joined: 06 Sep 2017, 12:12
Location: Florida

Re: Power Limiting instead of Current Limiting

Postby BlakeCarmichael » 06 Sep 2017, 14:01

devin wrote:^simply assuming 177w max, 24v pack, and 0.2ohm lead to lead, the battery amp limit should be 7.37amps, the motor amp limit should be 29.74 amps, and the absolute max amp setting should be a hair higher... say 35a


If I set the battery amp limit to 7.37 A, then when the battery is charged to 28+ V, the maximum power output will exceed the rated power won't it? Or am I missing something here?

devin
Posts: 253
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: Power Limiting instead of Current Limiting

Postby devin » 06 Sep 2017, 14:42

If I set the battery amp limit to 7.37 A, then when the battery is charged to 28+ V, the maximum power output will exceed the rated power won't it? Or am I missing something here?


it wouldn't exceed the 177w rated power because with 29.74A motor amp limit:

29.74^2A limit*0.2ohm=176.89w

&

176.89w=6.3175A battery amps * 28v

^simply when the battery is charged to 28v, with those settings, due to the 29.74A motor amp limit setting, the "effective" battery amp limit will be 6.3175A

devin
Posts: 253
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: Power Limiting instead of Current Limiting

Postby devin » 06 Sep 2017, 14:52

Blake, on second thought, yes you are correct, the battery amp limit should be 6.31A based on the 28v peak voltage because above stall speed, the 29.74 motor amps and 7.37A battery amps @ 28v will be equivalent to more than 177w
Last edited by devin on 06 Sep 2017, 14:56, edited 1 time in total.

BlakeCarmichael
Posts: 16
Joined: 06 Sep 2017, 12:12
Location: Florida

Re: Power Limiting instead of Current Limiting

Postby BlakeCarmichael » 06 Sep 2017, 14:55

devin wrote:on second thought, yes the battery amp limit should be 6.31A based on the 28v because above stall speed, the 29.74 motor amps will be equivalent to more than 177w


Well if I limit the battery amp limit to 6.31 A, then say the battery drops to 23 V, I will only be putting out 145 W to the motor. That's an 18% loss in power, although I think I should still be able to use the battery at full power at 23 V.

devin
Posts: 253
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: Power Limiting instead of Current Limiting

Postby devin » 06 Sep 2017, 14:57

Well if I limit the battery amp limit to 6.31 A, then say the battery drops to 23 V, I will only be putting out 145 W to the motor. That's an 18% loss in power, although I think I should still be able to use the battery at full power at 23 V.


this is why i previously suggested in a different thread the battery amp limit should be dynamically variable with fluctuating voltage based on user-desired wattage.

devin wrote:http://vedder.se/forums/viewtopic.php?f=6&t=18&start=50
In the future I would love to see a watt control mode implemented as follows:

1- user selects electrical wattage, for example "500w," equivalent to 500w electrical full throttle at all physically possible rpms

2- based on chosen wattage, battery amp limit and motor amp limit are chosen automatically based on detected winding resistance and pack voltage. the battery limit would have to change dynamically with fluctuating pack voltage to achieve this while the motor amp limit setting would remain constant.

3. user experiences full throttle = 500w electrical equivalence at all physically possible rpms, even during voltage sag

BlakeCarmichael
Posts: 16
Joined: 06 Sep 2017, 12:12
Location: Florida

Re: Power Limiting instead of Current Limiting

Postby BlakeCarmichael » 06 Sep 2017, 15:02

devin wrote:this is why i previously suggested in a different thread the battery amp limit should be dynamically variable with fluctuating voltage based on user-desired wattage.

devin wrote:In the future I would love to see a watt control mode implemented as follows:

1- user selects electrical wattage, for example "500w," equivalent to 500w electrical full throttle at all physically possible rpms

2- based on chosen wattage, battery amp limit and motor amp limit are chosen automatically based on detected winding resistance and pack voltage. the battery limit would have to change dynamically with fluctuating pack voltage to achieve this while the motor amp limit setting would remain constant.

3. user experiences full throttle = 500w electrical equivalence at all physically possible rpms, even during voltage sag


Yes, that it what I need. So there is no way to accomplish this yet? Any news on plans to implement?

Addy
Posts: 7
Joined: 10 Aug 2017, 17:56
Location: BC, Canada

Re: Power Limiting instead of Current Limiting

Postby Addy » 06 Sep 2017, 16:19

Why not limit the motor current instead of battery current? Then it doesn't really matter what the battery voltage is.


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