# Volts vs Amps in DC motors



## meanderingthemaze (Jan 25, 2010)

From what I understand, changing V increases max RPM. But how does changing V affect the torque curve over RPM band, generally speaking? 

Does it effectively allow for the same torque over a wider RPM band?

Does it change efficiency?

If you're using a transmission, is there much of an advantage to having a higher voltage pack?

Part of the reason I'm asking is that it seems easier on the batteries to use higher rated AH batteries at a lower voltage than lower AH at higher voltage if you're still going to be pulling the same amount of amps at any given RPM.


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## subcooledheatpump (Mar 5, 2012)

if you hook up a DC power supply without limits to a motor, then more volts does mean more speed and torque to a point (after which the motor is destroyed)

However, a motor controller regulates current regardless of the voltage. So unless the battery voltage is too low, then you will always get the same current at low speeds. As speeds increase, you do get a wider torque band if you put in more volts. Again the controller will regulate current and generally not voltage. At 100% throttle, the motor controller is essentially bypassed unless the current exceeds a present value and 100% of the battery is sent to the motor. hope that makes sense


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## meanderingthemaze (Jan 25, 2010)

What you said makes sense, thanks. 

What would be really helpful though would be to see a graph of torque over RPM of a specific motor (say a WarP 9) at various voltages. I'm curious to know the relative difference. Hard to understand the degree of change in the abstract. 

Also, any idea about how efficiency is affected?


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## Ziggythewiz (May 16, 2010)

It takes X watts to move Y lbs at speed Z. Volts * amps = watts, the relationship will be pretty linear. Generally you want the highest number of cells you can afford/fit that can power you with a reasonable C rate.


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## dougingraham (Jul 26, 2011)

meanderingthemaze said:


> Also, any idea about how efficiency is affected?


Generally the efficiency goes up with increased voltage. This is because the losses due to resistance of the windings and brushes are a smaller percentage of the whole when the voltage is higher.


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## meanderingthemaze (Jan 25, 2010)

So, as I understand, the resistive losses increase with A, right? And torque is directly related to A, not V, right?

So, if I need 100ft-lbs of torque to accelerate, I'm going to have to pull the same amounts of amps out of a 96V system than a 144V system, right? If I'm pulling the same amount of amps, isn't it the same amount of resistive losses regardless of V? 

I understand that P=A*V. That is not the question. The question is about the characteristics of DC motors, ie: their relative efficiency and torque characteristics at different voltages. Know what I mean?


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## somanywelps (Jan 25, 2012)

meanderingthemaze said:


> So, as I understand, the resistive losses increase with A, right? And torque is directly related to A, not V, right?
> 
> So, if I need 100ft-lbs of torque to accelerate, I'm going to have to pull the same amounts of amps out of a 96V system than a 144V system, right? If I'm pulling the same amount of amps, isn't it the same amount of resistive losses regardless of V?
> 
> I understand that P=A*V. That is not the question. The question is about the characteristics of DC motors, ie: their relative efficiency and torque characteristics at different voltages. Know what I mean?


Different voltage windings on similar motors give different torque per amp I've noticed.


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## dladd (Jun 1, 2011)

meanderingthemaze said:


> So, if I need 100ft-lbs of torque to accelerate, I'm going to have to pull the same amounts of amps out of a 96V system than a 144V system, right? If I'm pulling the same amount of amps, isn't it the same amount of resistive losses regardless of V?


no. Because the converter converts battery AxV to motor AxV. At low RPM my controller will put out 750a at 50v to the motor for a snapshot in time during acceleration. If I have a 96v pack, that translates into 750x50/96=390a. With a 144v system the it is 750x50/144=260a. With a 300v system I would only need 750x50/300=125a These are the required battery amps to supply the motor amps at that point in time.


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## EVfun (Mar 14, 2010)

Torque is set by the amps. Power is set by watts (volts times amps.) If you lack the voltage you will still have the low end torque, but you won't have the available rpm range. Voltage allows the motor to continue to take the peak amps to a higher rpm.


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## ruckus (Apr 15, 2009)

meanderingthemaze said:


> From what I understand, changing V increases max RPM. But how does changing V affect the torque curve over RPM band, generally speaking?
> 
> Does it effectively allow for the same torque over a wider RPM band?
> 
> ...


You are correct that voltage is directly proportional to torque.

However, it sounds like you assume Brushed DC are the only game in town? Google the number of failures that are happening... 

Increasing voltage is the ONLY way to get more from the same motor weight. Unfortunately, brushed DC motors have brushes. That means they turn into fire-balls above 288v!  or if you actually try to apply some decent amps.

If you want high power in a small package it requires a Brushless DC motor at 400-700VDC. Check out this drive system: http://www.greenstage.co.nz/PDFs/Scott_Drive_100.pdf
I think you will find it superior.

Cheers


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## DavidDymaxion (Dec 1, 2008)

That's true up to a certain speed in 1st gear.

The 96V system will run out of breath sooner, and have to upshift sooner, while the 144V system can stay in that gear longer. It takes more motor torque to accelerate or climb a hill in the higher gear, so more current is needed. The 144V system draws less current to climb the same hill by being able to stay in a lower gear longer.

As an extreme example, when my car was 48V acceleration and hill climbing was quite good... in 1st gear... up to about 10 mph... I was in 5th gear by 40 mph, and it took 100's of Amps to go highway speeds. Things will be much better when I finish going to 144V -- I anticipate highway speeds in 3rd gear, and around 100 Amps to cruise freeway speeds.


meanderingthemaze said:


> So, as I understand, the resistive losses increase with A, right? And torque is directly related to A, not V, right?
> 
> So, if I need 100ft-lbs of torque to accelerate, I'm going to have to pull the same amounts of amps out of a 96V system than a 144V system, right? If I'm pulling the same amount of amps, isn't it the same amount of resistive losses regardless of V?
> 
> I understand that P=A*V. That is not the question. The question is about the characteristics of DC motors, ie: their relative efficiency and torque characteristics at different voltages. Know what I mean?


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