# Direct drive motor and gear ratio to use



## lendellmm3 (Jan 19, 2012)

If I were going to use a direct drive DC motor hooked directly to my differential would I be limited to a certain speed such as the optimal gear ratio for 40 mph? or can I use the electronic controller to adjust the RPM on the motor to compensate for having a single fixed gear ratio


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## major (Apr 4, 2008)

lendellmm3 said:


> If I were going to use a direct drive DC motor hooked directly to my differential would I be limited to a certain speed such as the optimal gear ratio for 40 mph? or can I use the electronic controller to adjust the RPM on the motor to compensate for having a single fixed gear ratio


Hi lend,

You need the motor controller regardless of whether you use direct drive (a single fixed ratio) or a transmission (multiple ratios).

Regards,

major


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## lendellmm3 (Jan 19, 2012)

Yeah I understand that. I dont want to use my manual transmission and just want the DC motor directly to my differential. I was thinking that if I used the best gear ratio for going 65 mph (The max i'd go on freeways), I would still have enough torque to get around in lower speeds. 
I know you can control the voltage going into the DC motors so that could control the RPM of the motor. So would be using the controller to change voltage going to the motor act as a "virtual" gearbox to replace a mechanical transmission?


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## major (Apr 4, 2008)

lendellmm3 said:


> Yeah I understand that. I dont want to use my manual transmission and just want the DC motor directly to my differential. I was thinking that if I used the best gear ratio for going 65 mph (The max i'd go on freeways), I would still have enough torque to get around in lower speeds.
> I know you can control the voltage going into the DC motors so that could control the RPM of the motor. So would be using the controller to change voltage going to the motor act as a "virtual" gearbox to replace a mechanical transmission?


You have to size the motor & controller to have enough torque without the benefit of a "low gear" to multiply the motor torque. So if you're geared for 65 with a direct drive you need more motor torque for acceptable low speed acceleration compared to using a shifting transmission.

A physically larger motor is often used for direct drive. Motor size relates to torque. The other method is to crank up the current to the motor. More motor current means more motor torque. Depending on your duty cycle (driving habits), higher motor current may require additional motor cooling and larger (more expensive) controller.

The gear ratio multiplies torque. It transforms the RPM/torque ratio but maintains the power constant input to output. The controller is an electric analogy to the variable ratio gearbox. The controller transforms the V/I ratio from the battery to the motor while holding power constant input to output. So in that respect, the controller is similar in function to a shifting transmission. However, it cannot make greater torque than the maximum from the motor, where the gearbox can.

Hope that explains it,

major


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## dougingraham (Jul 26, 2011)

lendellmm3 said:


> Yeah I understand that. I dont want to use my manual transmission and just want the DC motor directly to my differential. I was thinking that if I used the best gear ratio for going 65 mph (The max i'd go on freeways), I would still have enough torque to get around in lower speeds.
> I know you can control the voltage going into the DC motors so that could control the RPM of the motor. So would be using the controller to change voltage going to the motor act as a "virtual" gearbox to replace a mechanical transmission?


The motor controller is not a virtual gearbox. It can only control the voltage the motor sees. The voltage at the motor is proportional to the desired RPM. The motor will draw current from the controller to try to reach this desired RPM. Controllers also typically will protect themselves from excessive current and they do this by lowering the voltage the motor sees so it tries to pull less current. The battery will try to produce the current requested by the controller.

The Tesla Roadster has a single ratio final drive. But the AC motor they use has a higher RPM limit (14krpm) vs the approx 5 to 6 krpm of the typical DC motors. The wider RPM band allows this to work reasonably. I believe they use about an 8:1 ratio. You may not be able to get a final drive ratio that will be suitable with your selected motor and desired top speed. You need to select a ratio such that you can achieve your desired top speed (in a reasonable amount of time). The trade off is going to be excessively high current's at start and when traveling slowly.

Unless your motor has a much higher RPM limit than the typical DC motors it makes sense to keep the transmission. You might only use 2nd gear around town and 3rd on the highway. A two speed transmission with reverse would be ideal for typical EV conversions.

Another downside is that you wont have reverse without a transmission. With a series wound DC motor you need to flop the ends on the field windings to reverse the motor rotation. This would require two high power relays to accomplish.

I am guessing that you would want somewhere between a 5:1 and 6:1 final drive ratio. There are a lot of things that would determine this including the drive wheel diameter, maximum motor RPM, traction pack voltage, desired maximum speed. Finding a final drive ratio that high might be tough. I know I could get a differential ratio for my RX-7 of 5.125 at a cost of $950. And this wasn't high enough for what I was looking to do so I kept the transmission and the original rear end.

Good luck on your project. I hope this helps.


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## Yabert (Feb 7, 2010)

Hi Lend

You can do 0-65 mph with a single ratio. But performance depend of vehicule weight and motor controller set-up. 

For example, I can do 0-65 mph with my car in third gear (4.95 : 1) with good acceleration.
I can also do 0-65 mph in fourth gear (3.58 : 1) with slower acceleration.

But my car weight is less than 850 Kg (1870 lbs) and I use a 1000A controller.


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## EVfun (Mar 14, 2010)

dougingraham said:


> The motor controller is not a virtual gearbox. It can only control the voltage the motor sees. The voltage at the motor is proportional to the desired RPM. The motor will draw current from the controller to try to reach this desired RPM. Controllers also typically will protect themselves from excessive current and they do this by lowering the voltage the motor sees so it tries to pull less current. The battery will try to produce the current requested by the controller.


It can be used as a transmission, in a manner of speaking. That is largely the effect of setting the controller motor current limit higher than the controller battery current limit. 

I have been running the motor current limit as 840 amps while I limit the battery current to 420 amps (7C for my Li cells.) This gives me a flat torque band of about 136 ft-lb or torque up to about 1500 rpm. From 1500 rpm to 3100 rpm I have a flat horsepower band, meaning the torque declines as the rpm rises until I get to about 68 ft-lb of torque at 3100 rpm (40 horsepower.) Above 3100 rpm the motor can see full pack voltage and draw less than 420 amps (declining torque and horsepower as the rpm increases.)

Compare this to a controller than only limits motor current, like a Curtis 1221 or 1231. With a single current limit of 420 motor amps (and battery amps be default, because battery amps will never exceed motor amps in a buck converter) I would have a flat 68 ft-lb of torque from 0 to 3100 rpm. Horsepower would climb linearly from 0 at 0 rpm to 40 at 3100 rpm. 

The first case pretty much eliminates the need for using second gear. 136 ft-lb times 1.32 third gear gives me 180 ft-lb of torque into the differential. With the Curtis type controller I only had 140 ft-lb of torque into the differential in second gear (2.06 second, 68 ft-lb.)

Of course this isn't literally a virtual gearbox. The motor will be operating at a lower rpm and higher motor current at the speeds where you would have used second gear. It will need to be large enough to take the extra current. The controller also needs to be large enough and well cooled so it can dish out the extra current at a rather low motor voltage without overheating.


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