# If an electric motor produces max torque at 0 RPM, do I still need a transmission?



## electricdominic (Apr 3, 2012)

If an electric motor produces max torque at 0 RPM, do I still need a transmission? 

In a manual tranmissioned car with an ICE, I have to rev up the engine to 2500 RPM (top of it's torque band), slowly let out the clutch and drive away. With an electric motor (supposedly they make max torque at lowest RPMs), do I really still need to have that transmission?


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## lowcrawler (Jun 27, 2011)

I put my VW Beetle in 3rd gear and never move it. The only reason I kept the clutch was for safety reasons and the occasional desire to 'chirp the wheels'. 

Many have done direct-drive... perhaps someone with more info on that will respond.


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## Coulomb (Apr 22, 2009)

electricdominic said:


> If an electric motor produces max torque at 0 RPM, do I still need a transmission?


It depends on the motor. With an AC motor that can do well over 10,000 RPM and you get reasonable power at lower RPMs (not all of them can), you don't need the transmission. Typical conversions use a series DC motor, which are difficult to get spinning at high speed. So without a transmission, you need to use a high ratio (short) diff (or fixed reduction gearbox and diff) and use a motor with more power than you would need if you used a transmission, or put up with quite limited top speed, or of course some compromise in between.

Most conversions have a suitable gearbox in them already, so it's usually not hard to make use of the transmission. So many conversions retain at least the gearbox (some keep the clutch, others don't). For a designed from scratch EV, adding a gearbox costs money (as well as weight and space), and you can choose a motor (usually AC) with a wide constant-power region, so most commercial EVs don't have a transmission. The Tesla Roadster came out with a 2-speed gearbox, but it was so much trouble that they retrofitted them all with a better motor and a single ratio drivetrain.

I'm surprised that there aren't more separately excited DC motors used for direct-drive conversions, because presumably by controlling the field current, you can effectively "change gears" in the motor itself. It's probably due to the scarcity of available motor and controller options.


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## electricdominic (Apr 3, 2012)

Just want to update the thread with some specifics of what I'd like to accomplish (if possible).

I'd like to remove the transmission and direct drive the motor to my transfer case (yes a 4wd vehicle with a 2 high, 4 high and 4 low gearbox). The transmission has the following gear ratios. 

1st 3.652:1 --- here's what concerns me... this means without the transmission I need 3.652x50ft-lbs (183 ft-lbs) to "simulate" the motor with the gear-reduction of the transmission, right?
2nd 1.937:1
3rd 1.423:1
4th 1.000:1 
5th 0.865:1

When in 5th gear, I need to maintain about 4800 RPM to be at freeway speed (65). 

I know my 1.3l ICE produces about 50ft-lbs of torque at 3k and only 30ft-lbs at 5k.


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## Yabert (Feb 7, 2010)

electricdominic said:


> 1st 3.652:1 --- here's what concerns me... this means without the transmission I need 3.652x50ft-lbs (183 ft-lbs) to "simulate" the motor with the gear-reduction of the transmission, right?


That seem right.
Most typical 9'' and 11'' DC motor used in an EV can output over 200 lbs-ft of torque with 1000A controller.... from 0 rpm.


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## subcooledheatpump (Mar 5, 2012)

If you really want no transmission, you should use an AC motor. 

using a DC motor without a gearbox, like Coulomb explained, will require a higher gear ratio. That will give good torque at the wheels but means the DC motor will have to spin quite fast to make the vehicle go at higher speeds. DC motors don't like higher speeds

On the other hand you could use a lower gear ratio, but that means you'd need to put out more stall torque. You can't stall a DC motor at full current for very long. 

So AC motors work well in both cases, high speeds and full torque at stall.

This is why people usually stick with their transmissions, but there are a few exceptions.

Also with no transmission, you'd need to reverse the direction of the motor to reverse the vehicle, DC motors with "advanced timing" don't like to be reversed


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