# back emf and a transmission



## appleseed (Sep 12, 2010)

hey guys,

i have a question,
i have searched many forums and no one has said it in a way i could understand well.

is back emf from a dc motor a cause from the RPM of the motor or load?
if back emf is caused by increasing RPM, then if i use a transmission is the top speed of my ev based on the amount of torque i can provide to keep accelerating? or is back emf goin to limit my top speed even thou i am using a lower RPM?

thanks to anyone that helps out


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## DavidDymaxion (Dec 1, 2008)

Your motor is both a motor and a generator at the same time -- the generator part is called "back EMF."

The amount of back EMF is a function of field current and rpm -- more of either gives more back EMF.

Suppose your motor won't rev above some rpm due to back EMF. You can often go faster by shifting up a gear, so you can operate in the RPM region that can deliver power.


appleseed said:


> hey guys,
> 
> i have a question,
> i have searched many forums and no one has said it in a way i could understand well.
> ...


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## appleseed (Sep 12, 2010)

so back emf is caused by rpm of the motor and the current through the motor? so by changing gears you are only changing the rpm of the motor into a higher "powerband" (of course if possible)

another question

buck converters vs pulse width modulation controllers? pros and cons of each
why would someone pick one over the other

i heard the solition 1 (something close to tht) controller is a buck converter
while other controllers use pwm


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## Coulomb (Apr 22, 2009)

appleseed said:


> buck converters vs pulse width modulation controllers?


All modern controllers are both. They use PWM to buck the pack voltage to a little more than the back EMF of the motor.


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## appleseed (Sep 12, 2010)

thank you 
it seems i misunderstood exactly how buck converters work but now i know 

i did find this about emf thou

"If a load is applied that slows the motor, the back-EMF is reduced and more current flows, increasing the torque to better handle the load. "
so the website says by going to a higher gear (load) back emf is reduced, so my top speed can only be limited by the amount of current i can apply to the motor and the number of gears in my transmission, right?

so motor current shouldnt have any effect on back emf, i think so anyway


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## DavidDymaxion (Dec 1, 2008)

You got it. It's like a gas or diesel engine -- eventually you run out of power (or safe rpm) and you have to upshift to go faster. This just tends to happen at a lower rpm in a low voltage conversion.


appleseed said:


> so back emf is caused by rpm of the motor and the current through the motor? so by changing gears you are only changing the rpm of the motor into a higher "powerband" (of course if possible)


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## Coulomb (Apr 22, 2009)

appleseed said:


> "If a load is applied that slows the motor, the back-EMF is reduced and more current flows, increasing the torque to better handle the load. "
> so the website says by going to a higher gear (load) back emf is reduced, so my top speed can only be limited by the amount of current i can apply to the motor and the number of gears in my transmission, right?


Sort of.

The fundamental principle is that back EMF is proportional to the product of field strength and motor speed.

Top speed in a vehicle with a series DC motor is set by two things. First of all, you need enough speed from the motor. With a series DC motor, it turns out that they are capable of almost infinite speed, if the field current is low enough; they can be seriously damaged by overspeeding at low loads (e.g. in neutral). But when determining the top vehicle speed, air resistance will provide a considerable load (say 20 kW), so you will need a considerable field strength to get that sort of power. If you look at the performance graph for a DC series motor, you will see that high power comes at low motor RPM (the opposite is true for most other motor types), so to get the maximum speed, you need to carefully choose the gear to get enough load on the motor, yet enough speed to the wheels. [Edit: this is wrong; see my next post.]



> So motor current shouldn't have any effect on back emf, i think so anyway


I don't see how that follows from what was said before. Back EMF is set by two things; speed and field strength. These two quantities are multiplied together, then multiplied by a constant, to determine back EMF. So field strength is just as important as speed in determining the back EMF of the motor.

Field strength is closely related to field current, and in a series DC motor, field current is equal to motor current. For most purposes, we can say that field strength is proportional to motor current. So motor current directly influences back EMF.

So load has a double effect on back EMF. Load affects RPM: increasing the load reduces the RPM, which _reduces_ the back EMF. This increases the current, which increases the field strength, which _increases_ the back EMF. So there is a sort of balance that gets struck as load is increased. The controller and pack may interact with this; the increased current load may cause the controller to limit its current, and the batteries may sag more which could decrease the current to the motor.

Overall, I think that increased load will always result in decreased back EMF, though people more familiar with series DC motors might have more to say on that.

Finally, the increased torque from the increased field strength will eventually increase the motor's RPM. But that's not an instantaneous effect; the other effects all happen more or less instantaneously.


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## Coulomb (Apr 22, 2009)

Coulomb said:


> ... so to get the maximum speed, you need to carefully choose the gear to get enough load on the motor, yet enough speed to the wheels.


Sigh. I got that wrong. The higher the gear (the lower the ratio, less tall, more (towards) overdrive), the higher the load on the motor, and therefore the higher the power, and of course the higher the gear the higher the higher the speed. So generally, higher gears will give you a higher top speed for both reasons.

There may be some point where a really high gear (say 6th gear if there is one) might put such a load on the motor that the controller or pack can't supply as much power as a lower gear, so you might actually lose top speed with very high (low ratio) gears.

Sorry for the confusion.


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