Phase current VS Battery current...

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esoria
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Phase current VS Battery current...

Post by esoria » Jan 07, 2012 1:29 pm

Good evening guys, I have a doubt that I could never quite understand the difference between the phase current and the battery. For example, I usually set up when the battery 100A, on my lyen
, Set the current phase to 260A (2.6 X). Now I have a 200A Kelly (phase), but why if i have max output of Phase 200A, 200A i can read on AC, battery side? I think I understood that the phase current is always larger than that of the battery, right? Someone I can 'explain the difference, or trackback some discussion? I could not find a discussion about it.

Thanks to all

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Re: Phase current VS Battery current...

Post by Lebowski » Jan 07, 2012 1:48 pm

I_bat = y * I_phase

I_bat = DC battery current
I_phase = amplitude of sinusoidal current in 1 phase
x = amplitude of sine wave in PWM with 1 being 100% fill and > 1 a clipped sine wave

then, for x in 0 to 2

y = 0.012 + 0.495*x + 0.825 * x^2 - 0.774 * x^3 + 0.175 * x^4

capisci ?


boewhahahaha :mrgreen:

The above equation is actually serious and is what i use to derive battery current from measured phase currents in
my controller build.

Assuming a simple china controller, let's say you draw 50 amps phase current and you have a PWM of 20%.
This means that 20% of the time the current is drawn from the battery via the high side FET and 80% of the time
the current runs around in circles via the motor inductance and the low-side FET. So the current you see in the
battery wire is 50Amps during 20% of the time and 0Amps during 80% of the time. So the average current is 10 Amps
(and you need big capacitors to deal with this spiky current).

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Re: Phase current VS Battery current...

Post by esoria » Jan 07, 2012 2:28 pm

Thanks for answer :), now i can understand better...
But for example, now i have a 200A phase Kelly, i try to set max motor current at 100%, battery at 20%, i see this:
Torque is good at start, and max battery output i read on CA is 34A....the motor current is 34*2=70A phase?
If i set battery current to 100%, and phase current to 20% i see this:
Torque is toooooo weak, no torque, and battery output is 16A....

Now, when i set kelly 200A at motor 100% and battery 100%, i read on the CA 211A peak, 211 battery side=422 motor current???
How is possyble to have 422A motor if the controller is max 200A phase?

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Re: Phase current VS Battery current...

Post by Doctorbass » Jan 19, 2015 9:54 pm

Let's wake up this thread again!

A little discover with many test on a dyno:

Here is some conclusion from the graph i got:


About torque:

With WOT here is what happened:

If Battery and Phase current is identical the torque should be constant for all the speed (the torque curve will be flat and no slope ( horizontal))

If Phase current is 2x Battery current the torque should peak and drop as the speed increase (the torque curve will be negative slope)

By deduction , If Phase current is half the Battery current the torque should increase as the speed increase and peak at the end (the torque curve will be positive slope) ? (edited Jan 20 2015)

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Re: Phase current VS Battery current...

Post by major » Jan 19, 2015 11:50 pm

Hi Doc,

Another topic that is of interest to me :) A while ago I attempted to show the relationship for sinewave 3 phase from battery. Here it is:
P = √3 * U * Irms * cosΘ

That is power in a 3-phase sinusoidal system. P in Watts, U is line to line RMS Voltage, Irms is RMS phase current and cosΘ is power factor.

Of course the battery power is Vb * Ib.

So, Vb * Ib = √3 * U * Irms * cosΘ

Or, Ib = √3 * (U/Vb) * Irms * cosΘ

With an inverter synthesized sinewave, the peak voltage cannot be greater than Vb, and the maximum RMS value of U = Vb/√2. Substitute that into the equation:

Ib = √3 * ((Vb/√2)/Vb) * Irms * cosΘ

Or, Ib = (√3/√2) * Irms * cosΘ = 1.24 * Irms * cosΘ

So it would appear that battery current can exceed RMS phase current at full voltage to the motor and when the power factor is better than 0.81. Of course this assumes no inverter losses for simplicity sake.


So I don't think it is possible to have phase current = half of battery current as in your third scenario.

But I am here to learn, so if you did see that, I'd like to know more about the experiment.

Regards,

major

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Re: Phase current VS Battery current...

Post by gwhy! » Jan 20, 2015 3:26 am

esoria wrote:I think I understood that the phase current is always larger than that of the battery, right?
Only if the controller will allow it.. So im assuming the kelly has a max phase current setting (limit) of 200A and this also happens to be the max setting for battery current.
esoria wrote:If i set battery current to 100%, and phase current to 20% i see this:
Torque is toooooo weak, no torque, and battery output is 16A....
the battery current will hit the 100% (200A) in the right circumstances ( maybe not in this extreme case of phase 20% and battery 100%) , its just that due to the very low phase current setting ( limiting ) its also holding back the battery current as the controller will not have the torque to get you up to your maximum speed.

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Re: Phase current VS Battery current...

Post by smeagol222 » Nov 24, 2016 8:48 am

Is there a simpler explanation to this? I'm new to this and cannot understand those complex formulas. I thought battery current is about half phase current? If its not exactly half, when what is the precise reduction?
I have this kelly http://www.qs-motor.com/product/4kw-4-5 ... rkeb72601/

it says
•Peak Phase Current, 10 seconds: 280A. ------> battery peak current 140a?
•Continuous Phase Current Limit: 110A. ------> battery cont. current 55a?

If my battery is 72v with discharge BMS 60a cont. / 120a peak

my motor is QS 10inch v3 4000w rated / 7000w peak (61a cont. 90a peak - surely this is 'battery current')
https://goo.gl/wC3NmK

72v x 97.22a = 6999.84w --> set kelly to 69.44% for peak current? (69.44% of 140a[batt current] is 97.21amps)?
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Re: Phase current VS Battery current...

Post by ferret » Nov 24, 2016 11:17 am

The term battery current can refer to several different things:

Actual battery current - the current you will see in an ampermeter connected to the battery at a given moment when riding.
Rated peak battery current - the maximal current the manufacturer claims that will not instantly fry the hardware.
Rated continuous battery current - the maximal current the manufacturer claims that will not fry the hardware when applied for an extended period.
Battery current limit - the maximal current the controller is allowed to draw from the battery.

The same terms apply for phase current (simply replace "battery" with "motor").

The actual battery current and actual phase current are constantly changing according to riding conditions.
The relationship between actual battery current and actual phase current is not fixed, it is also constantly changing according to riding conditions.

A simplified explanation for the relationship is: Actual battery current = actual phase current X %PWM .

The PWM is effected by motor speed, throttle position and load on the motor.
When at low speeds/high loads/a lot of throttle, the phase current will be much higher than the battery current and when cruising at high speed they will be very close.
If either battery or phase currents exceeds the maximum set in the controller it will reduce current draw accordingly.

For example:

Suppose the controller is set to limit the phase current to to 50 Amps and the battery current to 25 Amps.

If you are running at 25% PWM and the motor is drawing 40 Amps, the actual battery current will be 40 X 25%=10 Amps. No limiting is in effect.
Then a you start climbing a hill. Still running at 25% PWM, the motor now needs to draw 60 Amps. The phase current will be limited 50 amps and the battery current will be 50 X 25% =12.5 Amps.
After the hill you ride at high speed, say %90 PWM. The motor needs 45 Amps which means that the controller will need to draw 45 X %90 = 40.5 Amps from the battery. Since it is limited to draw only 25 battery Amps, the actual phase current will be 28 Amps (25 X %90 = 28).

I hope this helps to clarify things.

Avner.
Last edited by ferret on Nov 29, 2016 3:18 am, edited 1 time in total.
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Re: Phase current VS Battery current...

Post by smeagol222 » Nov 28, 2016 6:41 am

I understand that using the throttle on flat vs downhill vs uphill is going to draw different amounts of current from the battery/controller/wheel

But what is PWM? Are you able to modify the example to use the numbers for my hardware so I can better understand? (above post)
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Re: Phase current VS Battery current...

Post by ferret » Nov 28, 2016 8:23 am

PWM (Pulse Width Modulation) is the way the controller sends electricity from the battery to the motor. For example, when the controller is running %50 PWM, the battery is connected to the motor %50 of the time. This is roughly equivalent to giving the motor %50 of the voltage for %100 of the time, which will make the bike run at %50 max speed (assuming negligible aerodynamic resistance ).


"Your" example:

Controller Continuous Phase Current Limit=110A
Controller Peak Phase Current limit=280A

BMS Battery Current Limit = 60A cont. / 120A peak
Controller Battery Current Limit = Configurable

Let's assume that the Controller Battery Current Limit is configured higher than the BMS limit so the BMS limit will be activated before the controller limit is activated.

If you are running at 25% PWM and the motor phases are drawing 100A, the actual battery current will be 100 X 25%=25 Amps. No limiting is in effect.
Then a you start climbing a slow steep hill. Running at 20% PWM, the motor now needs to draw 300A. The phase current will be limited to 280 amps by the controller and the battery current will be 280 X 20% =56 Amps.
After the hill you ride at high speed, say %80 PWM. The motor needs 180 Amps which means that the controller will need to draw 180 X %80 = 144 Amps from the battery. Since it is limited to draw only 120 battery Amps, the actual phase current will be 150 Amps (150 X %80 = 120).

Earlier I tried to give a generic example to explain the principles without resorting to complex formulas and now another example using the data from your hardware. I think you better ask more specific questions, like what you want to know about your hardware or about principles you want to understand better.

Avner.
Last edited by ferret on Nov 29, 2016 3:10 am, edited 1 time in total.
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Re: Phase current VS Battery current...

Post by Punx0r » Nov 28, 2016 9:40 am

ferret wrote: After the hill you ride at high speed, say %80 PWM. The motor needs 180 Amps which means that the controller will need to draw 180 X %80 = 144 Amps from the battery. Since it is limited to draw only 120 battery Amps, the actual phase current will be 96 Amps (120 X %80 = 96).
Nice worked example but a minor mix-up at the end? 120 / 0.8 = 150A phase current ;)

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Re: Phase current VS Battery current...

Post by Alan B » Nov 28, 2016 10:52 am

Simple Motor Analysis

PWM controls effective voltage to the motor (not motor current). Motor current will vary with speed due to back EMF.

In a standard controller, throttle sets PWM unless overridden by a limit. In a torque throttle controller the PWM is adjusted by a feedback loop to control the motor current which is proportional to torque, and of course limits still apply.

Effective motor voltage is battery voltage times PWM (0-1).

Back EMF from the motor varies with speed, from zero when stopped to full battery voltage at no load maximum speed.

The voltage that drives current in the motor is the difference between Effective motor voltage and Back EMF. The motor current is this voltage difference divided by the system resistance, which is primarily the motor's resistance.

Sinewave and Trapezoidal are commutation techniques, varying in step size from small steps in Sine to big sixty degree jumps in Trapezoidal.

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Re: Phase current VS Battery current...

Post by footloose » Nov 28, 2016 11:29 am

Alan B wrote:Sinewave and Trapezoidal are commutation techniques, varying in step size from small steps in Sine to big sixty degree jumps in Trapezoidal.
Thank you for clarifying this.
I've wrestled with question of how "sinewave controllers" actually work. From my limited understanding, it seems like a true sinewave controller would necessarily be an analog system. If I understand what you say... then what actually happens is that a digital controller approximates an analog waveform by generating a large number of variable-length on/off pulses within each cycle of the waveform? If examined closely, the waveform would be "notchy", consisting of many tiny on/off time slices, but examined at a gross level the power delivery would appear as a smooth wave? (I'm probably not saying this clearly, but is that at least directionally correct?)

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Re: Phase current VS Battery current...

Post by Alan B » Nov 28, 2016 12:20 pm

The inductance in the motor essentially filters PWM into smooth waveforms.

A truly analog system would be terribly inefficient. When the steps are small enough the digital system is essentially equivalent.

The real point of a Sinewave controller is to rotate the magnetic field more smoothly in the motor. So the current through the motor follows two paths through coils acting at different angles and the sum of the fields makes a net field in between the two that depends on how the current is divided. So the field can be rotated in between the fixed locations of Trapezoidal commutation.

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Re: Phase current VS Battery current...

Post by ferret » Nov 29, 2016 3:06 am

Punx0r wrote:
ferret wrote: After the hill you ride at high speed, say %80 PWM. The motor needs 180 Amps which means that the controller will need to draw 180 X %80 = 144 Amps from the battery. Since it is limited to draw only 120 battery Amps, the actual phase current will be 96 Amps (120 X %80 = 96).
Nice worked example but a minor mix-up at the end? 120 / 0.8 = 150A phase current ;)
Thanks,
I will edit the original posts to correct the mistake.

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Re: Phase current VS Battery current...

Post by smeagol222 » Nov 29, 2016 11:40 am

ferret wrote:PWM (Pulse Width Modulation) is the way the controller sends electricity from the battery to the motor. For example, when the controller is running %50 PWM, the battery is connected to the motor %50 of the time. This is roughly equivalent to giving the motor %50 of the voltage for %100 of the time, which will make the bike run at %50 max speed (assuming negligible aerodynamic resistance ).


"Your" example:

Controller Continuous Phase Current Limit=110A
Controller Peak Phase Current limit=280A

BMS Battery Current Limit = 60A cont. / 120A peak
Controller Battery Current Limit = Configurable

Let's assume that the Controller Battery Current Limit is configured higher than the BMS limit so the BMS limit will be activated before the controller limit is activated.

If you are running at 25% PWM and the motor phases are drawing 100A, the actual battery current will be 100 X 25%=25 Amps. No limiting is in effect.
Then a you start climbing a slow steep hill. Running at 20% PWM, the motor now needs to draw 300A. The phase current will be limited to 280 amps by the controller and the battery current will be 280 X 20% =56 Amps.
After the hill you ride at high speed, say %80 PWM. The motor needs 180 Amps which means that the controller will need to draw 180 X %80 = 144 Amps from the battery. Since it is limited to draw only 120 battery Amps, the actual phase current will be 150 Amps (150 X %80 = 120).

Earlier I tried to give a generic example to explain the principles without resorting to complex formulas and now another example using the data from your hardware. I think you better ask more specific questions, like what you want to know about your hardware or about principles you want to understand better.

Avner.
Thanks for the explanation. So the PWM is based on the kelly controller programming- more specifically the motor current and battery current limits? The controller just comes up with this %PWM based on hill/flat/downhill power requirements? I still dont know how that relates to battery current/phase current
Specifically I want to know the optimal kelly controller programming settings for my battery, controller, motor combination

I've had a read of the kelly programming reference
https://endless-sphere.com/forums/viewt ... 16&t=18916

So from what I can gather according to KEB type demo and my controller battery numbers, I just need to set max limits and downhill/flat will follow suit?
http://kellycontroller.com/KEBhelp.php

controller
•Peak Phase Current: 280A
•Continuous Phase: 110A

battery 60a/120a discharge

leave the max motor current to 100% and just change the max battery current to 70%?
70% of 280A = 196a phase?

motor accepts max of 90a (or there abouts 7000w peak at 72v)
leaving 100% motor current 280a phase from controller?

This feels like the wrong forum for me (too advanced) I'd really like to learn here, I threw myself in the deep and and paying the price now haha :oops: .
Is there any reference material / electronics course I should have read/done before posting & tackling this project? I suppose I will just email Kelly/QS motors for end user/consumer advice?
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Re: Phase current VS Battery current...

Post by ferret » Nov 30, 2016 8:37 am

I may have over complicated my explanation by introducing PWM into the discussion. Here is another way to look at it:
The controller acts like a transformer, it has an input: battery voltage and battery current and an output of phase current and "phase voltage". Because of the conservation of energy, Input Power= Output Power (neglecting the controller's efficiency losses). Power is voltage X current, therefore battery voltage X battery current = "phase voltage" X phase current.
Battery voltage and battery current are fairly easy to measure. "Phase voltage" is roughly proportional to the motor speed (and so is the controller's PWM).

Lets say your battery voltage is 72V and battery current 50A and the motor is spinning at %50 speed. 72V X 50A = 360 Watts going through the controller.
The "phase voltage" is 72 X %50 = 36V. The phase current is Output power/"phase voltage" = 360W/36V=100A.

The main points to keep in mind are:
The relationship between battery current and phase current is not fixed, it is constantly changing according to riding conditions.
Battery current is usually limited in order to protect the battery.
Phase current is usually limited in order to protect the motor.

The optimal settings are determined through trial and error and depend on the hardware and average riding conditions.
I suggest the following:
Since the battery is protected by the BMS' 120A current limiting, I would set the limit in the controller to 100% so that the controller won't do any battery current limiting.The BMS will protect the battery so there is no need for the controller to do it.

A common starting point for phase current limit is 2X the battery current limit, in this case 2 X 120 = 240.
Therefore, I would initially set the motor current limit to %85 (%85 X 280A=240A) and test.
If the motor is getting hot or the bike wheelies too much lower the limit and test again.
If the motor is struggling to get going from a standstill raise the limit and test again.

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Re: Phase current VS Battery current...

Post by Alan B » Nov 30, 2016 9:30 am

Using the BMS to protect the battery in normal operation is not advisable. The BMS can only "trip off" like a circuit breaker, removing power totally (and requiring something like unplugging the controller to reset it). The actual value the BMS trips at is generally not precise and is already too much for normal operation and long battery life. It is a safety mechanism, not a proper control. The controller can regulate at a limit, which is a proper control.

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Re: Phase current VS Battery current...

Post by Punx0r » Nov 30, 2016 10:36 am

I can't think how the current limit values for the QS motor have been derived? For one thing, 90A phase peak seems rather low for a motor of that size!

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Re: Phase current VS Battery current...

Post by smeagol222 » Nov 30, 2016 11:12 am

Punx0r wrote:I can't think how the current limit values for the QS motor have been derived? For one thing, 90A phase peak seems rather low for a motor of that size!
I pretty sure its 90a in battery current terms. QS say that it does 7000w peak. At 72v thats around 97.22a
Last edited by smeagol222 on Nov 30, 2016 11:25 am, edited 1 time in total.
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Re: Phase current VS Battery current...

Post by major » Nov 30, 2016 11:20 am

ferret wrote:I may have over complicated my explanation by introducing PWM into the discussion. Here is another way to look at it:
The controller acts like a transformer, it has an input: battery voltage and battery current and an output of phase current and "phase voltage". Because of the conservation of energy, Input Power= Output Power (neglecting the controller's efficiency losses). Power is voltage X current, therefore battery voltage X battery current = "phase voltage" X phase current.
Battery voltage and battery current are fairly easy to measure. "Phase voltage" is roughly proportional to the motor speed (and so is the controller's PWM).
...
Avner.
Hi Avner,

You appear to have neglected square root 3 and power factor in your 3-phase power calculation. Ref my previous post about 5 down from top.

major

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Re: Phase current VS Battery current...

Post by ferret » Nov 30, 2016 11:36 am

Hi Major.
I am aware that my description is a simplified one that doesn't include all the factors involved. My research into this subject was only deep enough to give me a enough understanding for my bike builds. I was trying to get an intuitive feel for what is going on in the controller rather than an accurate mathematical description.

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Re: Phase current VS Battery current...

Post by Alan B » Nov 30, 2016 5:58 pm

The motor vendor can only rate motor current and voltage limits. They have no idea what battery voltage is being used so cannot reference battery current limits.

Look at the current capacity of the motor wiring (what gauge is it?). Is it heavy enough to sustain more than 90 amps? The losses are predominantly I squared R.

The other factor is how they define of peak - does that mean for one millisecond or ten minutes? Big difference.

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