Difference in acceleration with different voltage?

Get all your technical information about electric bikes here.
User avatar
teklektik   10 GW

10 GW
Posts: 4020
Joined: Mar 26 2011 1:15pm
Location: CT, USA

Re: Difference in acceleration with different voltage?

Post by teklektik » May 16 2018 11:50am

The thread was about answering the OP's very specific questions about some specific battery packs which were answered using a standard and respected tool. The Op posted back and the matter was brought to conclusion. The topic then derailed into broad generalizations that were erroneous. The original topic of discussion was never revisited or even subsequently referenced as the new discussion took on a life of its own and the thread became about a different topic with different participants. This is hijacking.

If you want to quibble about that particular term for changing the topic of discussion by new posters, then have at it.

I fully appreciate the desire to set the record straight and am not claiming evil intentions, but this thread went seriously sideways from what was a simple question and answer.
Visit Grin Technologies at www.ebikes.ca
Build Thread: 2WD Yuba Mundo V4

User avatar
parajared   10 kW

10 kW
Posts: 812
Joined: Apr 16 2012 11:33pm
Location: Northern Arizona

Re: Difference in acceleration with different voltage?

Post by parajared » May 16 2018 12:10pm

yeah,
an issue of perception I think. I see the subject very differently than everyone else it seems.

Fetcher said
You will have some increase in torque and acceleration with a higher pack voltage
the OP said
My gut feeling would have guessed the higher voltage would improve climbing ability/torque, but I had no idea on how big of a difference it would make.
Addy said this
Current is reduced by back-EMF as the motor speed increases. If your battery voltage is higher you can maintain higher currents before back-EMF starts reducing them
and you said this
The higher available input (battery) power allowed the controller to deliver higher phase amps and hence higher torque.
and that's what got me going. In my mind's eye all these statements are wrong and that's what got me triggered.
-Fetcher's comment a misunderstanding of watts per cell work, serial vs parallel doesn't matter, the wattage is still the same.
-OP's comment a misunderstanding of the way motors work
-Addy's comment a misunderstanding of how backfeed EMF works
-your comment a misunderstanding of phase amps and the role they play on torque.

Addy   100 W

100 W
Posts: 193
Joined: Aug 11 2017 11:15am
Location: BC, Canada

Re: Difference in acceleration with different voltage?

Post by Addy » May 16 2018 12:45pm

parajared wrote:
May 16 2018 12:10pm
Addy said this
Current is reduced by back-EMF as the motor speed increases. If your battery voltage is higher you can maintain higher currents before back-EMF starts reducing them
-Addy's comment a misunderstanding of how backfeed EMF works
I disagree that I am misunderstanding BEMF. I just think I could have explained it better.

User avatar
teklektik   10 GW

10 GW
Posts: 4020
Joined: Mar 26 2011 1:15pm
Location: CT, USA

Controller Battery Amps / Phase Amps / Limiting

Post by teklektik » May 16 2018 2:16pm

parajared wrote:
May 16 2018 12:10pm
and you said this
The higher available input (battery) power allowed the controller to deliver higher phase amps and hence higher torque.
and that's what got me going. In my mind's eye all these statements are wrong and that's what got me triggered.
...
-your comment a misunderstanding of phase amps and the role they play on torque.

AlanB gave a pretty good overview of how this all works, which should explain what is going on, but I'll give it another try:

The core difficulty is this statement which reveals a misunderstanding of motors and the individual roles that controllers and motors play.
parajared wrote:
May 15 2018 7:26pm
All the motor cares about is wattage.
Although the (unloaded) speed of a motor has a direct relationship to the phase voltage and the torque has a direct relationship with phase amps, there is no place in motor equations where the voltage and current terms appear together as a product (watts) which would in turn suggest that the product is the critical value not the individual voltage and current terms. So, volts and amps 'yes', but watts 'no'.

That said, when controllers are in limiting mode, they can be viewed as dealing with power. In particular, the phase power out of a controller is always equal to the battery power in, but a controller has the ability to reduce phase voltage and increase phase amps such that the resulting phase power remains unchanged. Additionally, since motor speed is directly proportional to voltage according to motor Kv, we can likewise say that phase voltage must be proportional to speed. This means that at any given speed, the phase voltage that the controller must deliver is fixed. So knowing the input power (batteryVolts x batteryAmps) and the phase voltage the resulting phase amps are known.

So looking at controller operation when limiting without all the details:
  • The motor behaves according to:

    Code: Select all

                                 Kv = rpm / volts
    So to achieve any given rpm the controller must provide:

    Code: Select all

                              volts = rpm / Kv
    The controller behaves according to:

    Code: Select all

                       (phasePower) = (batteryPower)
           (phaseVolts x phaseAmps) = (batteryVolts x batteryAmps)
                          phaseAmps = (batteryVolts x batteryAmps) / (phaseVolts)
    So at any given rpm the controller can supply this phase current:

    Code: Select all

                          phaseAmps = (batteryVolts x batteryAmps) / (phaseVolts)
                          phaseAmps = (batteryVolts x batteryAmps) / (rpm / Kv)            (from above)
                          phaseAmps = (batteryPower x Kv) / (rpm)
    The last relationship illustrates:
    1. Torque is dependent on the battery power not either of the battery volts and amps independently and at any given speed (rpm) different combinations of volts and amps can yield the same phase current and hence torque.
    2. The controller develops higher phase amps and hence more torque at lower rpm (when due to increased load) and the available phase amps (and hence torque) reduces as rpm increases (see blue lines in simulator plots).
Importantly, what we are looking at here is controller -not motor- operation. The motor role is to convert the phase amps directly into torque. It does not itself see the battery voltage or current but instead acts on the entirely different phase voltage and phase current. The controller does the magic.

For the discussion in this thread: the last equation shows that at any given speed the torque will be higher (acceleration higher) for the bike with the higher available input power (as long as the controller is limiting). Based on this I said:
The higher available input (battery) power allowed the controller to deliver higher phase amps and hence higher torque.
The twist in this case is that the OP's motor achieved higher acceleration with the higher voltage pack not because of the high voltage per se, but because the available power increased:

High voltage pack: 40Ax72V=2880W
Low voltage pack: 45Ax48V=2150W

If the higher voltage pack had been built with the same number of cells and just the S and P jiggled so the volts went up, the amps went down, and the available power remained the same, there would have been no increase in phase amps and no increase in torque.

This is why the specifics of the OP's question were important instead of dismissing the possible result out of hand as impossible because voltage doesn't control torque.

FWIW:
  • The last equation above explains the common experience of increasing battery voltage and getting better torque/acceleration. This typically happens because the controller limit is not changed (e.g. they are using a 40A controller). Since this discussion is based on the premise that the controller is limiting, the battery current remains fixed at the controller limit (e.g. 40A) but the pack voltage is higher and so the available battery power is increased. Just a variation of the OP's situation with similar confusion arising about the role of battery voltage.
Last edited by teklektik on May 17 2018 10:56am, edited 6 times in total.
Visit Grin Technologies at www.ebikes.ca
Build Thread: 2WD Yuba Mundo V4

User avatar
parajared   10 kW

10 kW
Posts: 812
Joined: Apr 16 2012 11:33pm
Location: Northern Arizona

Re: Difference in acceleration with different voltage?

Post by parajared » May 16 2018 6:30pm

Although the (unloaded) speed of a motor has a direct relationship to the phase voltage and the torque has a direct relationship with phase amps, there is no place in motor equations where the voltage and current terms appear together as a product (watts) which would in turn suggest that the product is the critical value not the individual voltage and current terms. So, volts and amps 'yes', but watts 'no'.
I think you inadvertently proved my point. I mean yeah, that's exactly right. The thing you are calling "phaseamps" is a combination of battamps and battvolts, you are dividing by rpm, or volts for simplification but in other words "watts". That's what wattage is.

I think the only thing we are balled up on is the way you are calculating KV into your equation. You are fooling yourself into thinking more voltage is more torque because you are forgetting about kv/rpm.

If you calculate for an equal kv/rpm you will get the same numbers, here let me prove it to you using your own equation.

(batteryPower) = (phasePower)
check, check

(batteryVolts x batteryAmps) = (phaseVolts x phaseAmps)
got it, set

(batteryVolts x batteryAmps) / (phaseVolts) = phaseAmps
yep, we know that lower voltage = more amps to equal the same phase amps

(batteryVolts x batteryAmps) / (rpm / Kv ) = phaseAmps
ok, here's the meat of it.
example 1:
batterypower 36v x 10amps (360watts) / rpm/kv (10kv) = 36 phaseamps

example 2:
double the voltage x 5amps (360watts) / rpm half the kv (5kv) to make things equal = 36 phaseamps

This is where you are getting confused. You are forgetting to half the kv... or maybe forgetting to half the amperage to keep wattage the same. We aren't comparing what a 720 watt system can do compared to a 360 watt system. We are comparing two systems of similar rpm and wattage can do at different operating voltages.

User avatar
parajared   10 kW

10 kW
Posts: 812
Joined: Apr 16 2012 11:33pm
Location: Northern Arizona

Re: Difference in acceleration with different voltage?

Post by parajared » May 16 2018 6:37pm

We aren't comparing two systems of different wattage capabilities (or should I say phaseamp capabilities) because wattage is weight and size (more copper fill, more magnet). I don't think the idea in question is if a big beefy BLDC motor is capable of more wattage (fine okay phaseamps) than a little weenie motor are we?

User avatar
parajared   10 kW

10 kW
Posts: 812
Joined: Apr 16 2012 11:33pm
Location: Northern Arizona

Re: Difference in acceleration with different voltage?

Post by parajared » May 16 2018 6:59pm

Let me say it again for effect:
The magnets don't care what kind of repulsion they are getting. It can be volt heavy, it can be amp heavy. 10amps 20 volts of repulsion is the same as 20amps 10 volts of electromagnetic repulsion.

User avatar
teklektik   10 GW

10 GW
Posts: 4020
Joined: Mar 26 2011 1:15pm
Location: CT, USA

Re: Difference in acceleration with different voltage?

Post by teklektik » May 16 2018 7:47pm

parajared wrote:
May 16 2018 6:30pm
example 1:
batterypower 36v x 10amps (360watts) / rpm/kv (10kv) = 36 phaseamps

example 2:
double the voltage x 5amps (360watts) / rpm half the kv (5kv) to make things equal = 36 phaseamps
The math in these equations doesn't make sense - you use a fixed 360W, a fixed speed, and two different Kvs and get the same result (?). In any case, the Kv of the motor does not change because you changed the battery. It is a fixed motor characteristic at any voltage.

Both the speed and Kv are constants and identical in both examples. In your examples the battery capacities are the same so the phase amps are the same but not because of your messing around with Kv's. Regardless, no math example can make the motor see the raw battery volts and amps on the other side of the controller. The motor sees what comes out of the controller and responds to those amps and volts - not to the watts going through connections elsewhere in the system.

Phase amps and phase volts are not abstract calculations - they are actual quantities you can see on a scope. The fact that they are related in some calculable way to other currents and voltages does not make them the same thing. This is the point that AlanB and I are trying to explain - the controller actually changes the voltage and current that the motor sees to be different than the voltage and current that the battery supplies. This is not some trickery of algebra. This is all happening in the controller and not the motor as you suggested. The motor just responds to the amps it sees - which are different than the amps that are going into the controller from the battery.

The OP's question and the matter under discussion are the acceleration of the same bike at the same speed -- so the rpm and Kv are by definition identical. He just wants to know which battery will give him the best acceleration, etc. - a simple comparison by changing one element of the system (battery). Because in each case the input power is constant because the controller is limiting at max current, the Kv is fixed by the same motor, and we are looking at a specific speed, there is nothing to fiddle with and nothing to 'forget'.

I leave it to you to reconcile your view with the results the simulator produces. You can look at the tabular values there and see the phase amps (motor amps) are different and larger than the battery amps for cases where the controller is limiting.

I know you are sincere in your view but I think at this point that we will just have to agree to disagree since you remain unconvinced by the simulator data or the explanations and I have done the best I can. Time to move on.
Visit Grin Technologies at www.ebikes.ca
Build Thread: 2WD Yuba Mundo V4

User avatar
parajared   10 kW

10 kW
Posts: 812
Joined: Apr 16 2012 11:33pm
Location: Northern Arizona

Re: Difference in acceleration with different voltage?

Post by parajared » May 16 2018 8:07pm

Let me start over:

Can we agree that larger magnets and more copper would make for a more torquey motor?
-because Gauss provides the magnetic repulsion
-because copper provides the electrical repulsion

If your answer is yes why do you think that skinnier copper wire wrapped more times for less kv and higher voltage creates more torque (aka repulsion) than fatter wire wrapped fewer times for higher kv more amps?

Lets assume copper fill is the same, so the same amount of wattage either way.

User avatar
parajared   10 kW

10 kW
Posts: 812
Joined: Apr 16 2012 11:33pm
Location: Northern Arizona

Re: Difference in acceleration with different voltage?

Post by parajared » May 18 2018 12:49pm

I think this is a good example of making sure you are swift and explicitly clear in your explanation. Since I didn’t really make myself clear over the span of a number of posts the matter ended up getting exhausted and ended in a deadlock instead.

I think I could have had this rabble sorted out about 10 posts ago had I said something to the effect of:

“so if given the chance would you re-wind a motor for more torque”

which would have brought this issue right to the heart of it all. Instead we danced all around the real issue with a bunch of malarkey about “phaseamps”, “buck switching” and accusations of “hijacking”. We never addressed the bottom line, the brass tacks of the matter.

Addy   100 W

100 W
Posts: 193
Joined: Aug 11 2017 11:15am
Location: BC, Canada

Re: Difference in acceleration with different voltage?

Post by Addy » May 18 2018 1:04pm

parajared wrote:
May 18 2018 12:49pm
I think I could have had this rabble sorted out about 10 posts ago had I said something to the effect of:

“so if given the chance would you re-wind a motor for more torque”

which would have brought this issue right to the heart of it all. Instead we danced all around the real issue with a bunch of malarkey about “phaseamps”, “buck switching” and accusations of “hijacking”. We never addressed the bottom line, the brass tacks of the matter.
That's the problem though, that isn't the bottom line. I don't know why you started talking about re-winding, that's not what the OP was asking about.

User avatar
parajared   10 kW

10 kW
Posts: 812
Joined: Apr 16 2012 11:33pm
Location: Northern Arizona

Re: Difference in acceleration with different voltage?

Post by parajared » May 18 2018 1:26pm

That's the problem though, that isn't the bottom line. I don't know why you started talking about re-winding, that's not what the OP was asking about.
Because the MAXIMUM torque a motor can do is a fixed thing. It cannot be magically increased by rewinding it, it can't be magically increased by adding voltage.

The title of this thread is "Difference in acceleration with different voltage?" and the assertion being made is that voltage increases torque, that's the bottom line.

User avatar
parajared   10 kW

10 kW
Posts: 812
Joined: Apr 16 2012 11:33pm
Location: Northern Arizona

Re: Difference in acceleration with different voltage?

Post by parajared » May 18 2018 1:31pm

The fact of the matter is that for a given stator and magnet combo, peak torque at reasonable efficiency is limited by the flux density in the magnetics or the heating effect in the copper.

If you want more torque, use a bigger stator and rotor. Period. Or a gearbox...or stronger magnets.

Addy   100 W

100 W
Posts: 193
Joined: Aug 11 2017 11:15am
Location: BC, Canada

Re: Difference in acceleration with different voltage?

Post by Addy » May 18 2018 1:43pm

I still think you're misunderstanding what this thread is about. No one is trying to redefine motor theory here.

The general question is that for a given motor, do different battery and ESC configurations result in different acceleration?

There are already enough attempts in this thread to explain the answer to that.

Post Reply