# Confused: Battery Voltage versus HP/torque/current/efficiency



## Clipper (Jan 30, 2011)

This is from a thread long ago:



major said:


> Undoubtedly, you will be current limited by the controller. For example, 500 amps. So peak power will occur at 500 amps when the motor controller reaches 100 % duty cycle, or passes full battery voltage to the motor. At RPM higher than that, the torque and HP decrease, as well as current.
> 
> So, peak power for you is at 500 amps and battery voltage. If you increase battery voltage, you increase motor RPM at 500 amps and also increase peak power.
> 
> ...


So I'm still a little foggy on the concept.

If I take the 72V curves from Netgain's website (attached below):
http://www.go-ev.com/PDFs/003_09_02_WarP_9_96VSpreadSheet.pdf

What do I do to these curves if I operate the motor at 144V? 
From the reading and studying I've done, It looks like I'm supposed to double the RPM.

Eg: The NetGain numbers roughly show the torque at 40 ft-lbs at 2600 RPM and 240 amps.

Does that mean that at 144V the motor is developing 40 ft-lbs at 5200 rpm and 240 amps?

Or that at 2600 RPM, the 144V motor is developing 40 ft-lbs, at 120 amps? 

Conversely, am I able to develop more torque at a lower RPM at the same current with a higher voltage?

I'm confused. Too many variables.

I guess the REAL question is: 
what is the advantage, if any, of running the Warp 9 (or any series motor) at higher voltage?

_*Theoretical question:*_
If I have a certain vehicle, say, my 1973 VW squareback (photo attached)

and I have a 72V LiFe battery pack,

and run the vehicle at exactly 60 mph down a straight flat road,
until the pack voltage dropped to the minimum voltage (say 3.1V per cell), assuming the cells are perfectly balanced, of course,

under good weather conditions, say, no wind, dry and 72 degrees,

and I replace that 72V 160Ah LiFe battery pack 
with a 144V 160Ah LiFe battery pack in THAT SAME vehicle,

remove some weight so the vehicle weight is the same 
as with the 72V pack,

using the same controller, say, a Soliton Jr,

And make the same run down the road, till the pack voltage drops to 3.1V per cell, (perfectly balanced, of course)

At the higher voltage, will I be drawing less BATTERY amps to push the exact same vehicle down the road under the exact same conditions?
Thereby using half the amp-hours from the pack to go the same distance down my theoretical road?

Conversely, will that vehicle theoretically go twice as far with the 144V pack?

I am thinking that I must be using the same motor amps to push the car down the same road...and the controller is changing the duty cycle to so that the apparent MOTOR voltage is the same as it was with the 72V pack.

So by changing packs, I am not changing the MOTOR voltage or amperage to make the same run. 

I think that the confusing part is that I am changing the PACK voltage, but the MOTOR voltage doesn't change to produce the same HP; the controller reduces the apparent MOTOR voltage to the same as it was with the 72V pack, to do the same work. Since I am using twice the BATTERY voltage to give the controller and motor the power they need (develop the same motor voltage and horsepower, or KW,) ohms law says I am only drawing half the amps from the battery pack, and thus I'll get twice the range from 160Ah of the higher voltage pack.

What the higher voltage does is to give the controller more voltage to feed the motor *if I demand it with my pedal*. I can get more horsepower, more torque, more power at a lower RPM. At 72V and 5200 RPM, I max out at about 6 ft-lbs of torque. At 144V and 5200 RPM, I can get up to about 40 ft-lbs. Am I reading that right?

Theoretically, of course. Ignoring minor losses in the controller and motor.

And since force = mass x acceleration, and the mass of the car doesn't change, if I floor the pedal, I can get more acceleration out of the same car with higher voltage. Great if I'm drag racing. Just because I have more power to (potentially) use, doesn't mean I have to use it when tooling around town. Nice for freeway on-ramps, tho.


Is that right?

Forgive me for the long post; I'm confused, and trying to think it through.


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## major (Apr 4, 2008)

Clipper said:


> This is from a thread long ago:
> 
> 
> major said:
> ...


Yes, that would be correct.



Clipper said:


> Eg: The NetGain numbers roughly show the torque at 40 ft-lbs at 2600 RPM and 240 amps.
> 
> Does that mean that at 144V the motor is developing 40 ft-lbs at 5200 rpm and 240 amps?


Yes.


Clipper said:


> Or that at 2600 RPM, the 144V motor is developing 40 ft-lbs, at 120 amps?


No.


Clipper said:


> Conversely, am I able to develop more torque at a lower RPM at the same current with a higher voltage?


No, the torque per amp remains the same regardless of voltage.


Clipper said:


> I'm confused. Too many variables.
> 
> I guess the REAL question is:
> what is the advantage, if any, of running the Warp 9 (or any series motor) at higher voltage?


Higher speed at load and therefore higher power.


Clipper said:


> _*Theoretical question:*_
> If I have a certain vehicle, say, my 1973 VW squareback (photo attached)
> 
> and I have a 72V LiFe battery pack,
> ...


Yes, that is correct.


Clipper said:


> Conversely, will that vehicle theoretically go twice as far with the 144V pack?


Yes, you have doubled the energy (kWh) of the battery so for identical conditions, you double the range.


Clipper said:


> I am thinking that I must be using the same motor amps to push the car down the same road...and the controller is changing the duty cycle to so that the apparent MOTOR voltage is the same as it was with the 72V pack.


Correct.


Clipper said:


> So by changing packs, I am not changing the MOTOR voltage or amperage to make the same run.


Correct.


Clipper said:


> I think that the confusing part is that I am changing the PACK voltage, but the MOTOR voltage doesn't change to produce the same HP; the controller reduces the apparent MOTOR voltage to the same as it was with the 72V pack, to do the same work. Since I am using twice the BATTERY voltage to give the controller and motor the power they need (develop the same motor voltage and horsepower, or KW,) ohms law says I am only drawing half the amps from the battery pack, and thus I'll get twice the range from 160Ah of the higher voltage pack.


Yep.


Clipper said:


> What the higher voltage does is to give the controller more voltage to feed the motor *if I demand it with my pedal*. I can get more horsepower, more torque, more power at a lower RPM.


No. At the lower motor RPM you're at low motor voltage (below 72V on the motor), so the torque and power would be the same as if you still had the 72V battery.



Clipper said:


> At 72V and 5200 RPM, I max out at about 6 ft-lbs of torque. At 144V and 5200 RPM, I can get up to about 40 ft-lbs. Am I reading that right?


Yes


Clipper said:


> Theoretically, of course. Ignoring minor losses in the controller and motor.
> 
> And since force = mass x acceleration, and the mass of the car doesn't change, if I floor the pedal, I can get more acceleration out of the same car with higher voltage.


True only above the 72V base speed on the motor. Acceleration remains the same until you start putting more than 72V to the motor. 



Clipper said:


> Great if I'm drag racing. Just because I have more power to (potentially) use, doesn't mean I have to use it when tooling around town. Nice for freeway on-ramps, tho.
> 
> 
> Is that right?
> ...


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## Clipper (Jan 30, 2011)

Thanks, Major...you have come through for me once again...!
I think I've got it, but one more quick question... 



Clipper said:


> What the higher voltage does is to give the controller more voltage to feed the motor *if I demand it with my pedal*. I can get more horsepower, more torque, more power at a lower RPM.





major said:


> No. At the lower motor RPM you're at low motor voltage (below 72V on the motor), so the torque and power would be the same as if you still had the 72V battery.


However, if I mash the pedal to the floor at, say, 1000 RPM, I am telling the controller to go to full duty cycle, thus I am feeding the motor 144V, therefore I will get more torque/HP, and thus more acceleration, than I would have if I had mashed the pedal to the floor at 1000RPM with the 72V pack. Is that correct???
(assuming I have reprogrammed the controller for the higher maximum voltage)


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## major (Apr 4, 2008)

Clipper said:


> I think I've got it, but one more quick question...
> 
> However, if I mash the pedal to the floor at, say, 1000 RPM, I am telling the controller to go to full duty cycle, thus I am feeding the motor 144V, therefore I will get more torque/HP, and thus more acceleration, than I would have if I had mashed the pedal to the floor at 1000RPM with the 72V pack. Is that correct???
> (assuming I have reprogrammed the controller for the higher maximum voltage)


No, you hit current limit which has priority over the throttle position. So a 100% throttle will not result in 100% duty cycle (full voltage to the motor) until you're above base speed.


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## Clipper (Jan 30, 2011)

major said:


> No, you hit current limit which has priority over the throttle position. So a 100% throttle will not result in 100% duty cycle (full voltage to the motor) until you're above base speed.


Ah...that's right! Forgot about current limit! Thanks! LOL...


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## dougingraham (Jul 26, 2011)

The voltage you feed to a motor determines how fast it wants to spin.

The load on the shaft determines how much current the motor wants to draw.

The controller is there to limit the voltage because wide open is seldom desirable. Limiting the voltage will as a side effect lower the current the motor wants to draw and is how the current limits are imposed.

In a system where the motor current is limited by the motor controller if you plot the torque vs motor amps (not battery amps) you see a direct relationship. The torque will remain constant up to a particular RPM. What happens when that RPM is reached is the torque (and current) taper off as the RPM continues to climb. Increasing the pack voltage (up to the motor limits) will increase that particular RPM where the torque starts to taper off. Thus increasing the voltage will increase the width of the usable torque band. In my case I increased my pack voltage from a nominal 106 volts (33 cells) to 163 volts (51 cells) when I added my front battery box. This changed the torque band from 0-3000 rpm to 0-4500 rpm which delays when I want to shift during hard acceleration. This also increased my peak power from about 106kw to 163kw. This is a huge increase. Peak torque remains the same because the current is still limited to 1000 amps but you get to use it for a longer amount of time. Because the voltage limit of the WarP 9 with the Helwig brushes is 192 volts I would probably go with a battery that had that as its minimum sag voltage. Something like 80 cells but with 60AH so I could use the torque over the whole RPM band. Going to 80 cells would probably increase the point to near the rpm limit of the motor.


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## Clipper (Jan 30, 2011)

I see Hellwig brushes discussed on this forum...

H49, H60, H100, K254, split...which is the best one? And where can I buy some?
Do I need new brush holders?


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## major (Apr 4, 2008)

Clipper said:


> I see Hellwig brushes discussed on this forum...
> 
> H49, H60, H100, K254, split...which is the best one? And where can I buy some?
> Do I need new brush holders?


I assume you have a Warp9 from the discussion here. They've used a good brush when they made the motor. Why change


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## Clipper (Jan 30, 2011)

major said:


> I assume you have a Warp9 from the discussion here. They've used a good brush when they made the motor. Why change


It's 3 years old...not sure which brushes I have. It's at the motor balance shop right now and I may as well make sure they're the good ones.

I found out that EV West near me stocks them, so I can get them if I need them.


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## Clipper (Jan 30, 2011)

dougingraham said:


> The torque will remain constant up to a particular RPM. What happens when that RPM is reached is the torque (and current) taper off as the RPM continues to climb. Increasing the pack voltage (up to the motor limits) will increase that particular RPM where the torque starts to taper off. Thus increasing the voltage will increase the width of the usable torque band.


Wow, that's an interesting way to think about it. 
Increasing battery voltage (and thus available motor voltage) 
basically just moves the peak torque up the RPM curve.

Never seen that before.


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