# Transmissions



## TKosinski02 (Dec 28, 2010)

So I have a conceptual question. I understand that with an internal combustion engine we use the transmission to gain a mechanical advantage to move the vehicle and keep the Rpms in the power band so that we always have peak performance and economy. I've looked a little into the volt and leaf and noticed they both have a single speed. I understand that electric motors making peak power straight off but wouldn't it be advantages to use a transmission even if it was a simple 2 speed (city/highway) one? Couldn't use end up using less electricity because it's not having to work as hard? I don't understand completely how electric motors work but I am studying(early stages) to become and electrical engineer so eventually I will understand. 
Or even using a transmission to help with towing? That is if you were to make a truck.


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## TigerNut (Dec 18, 2009)

A transmission can help you to match the motor's RPM and torque capabilities to the operating regime of the vehicle. Many EV conversions use a transmission because the final drive ratio is not high enough to support direct drive with the torque available from the motor. A second ratio is then helpful for achieving higher speed, but the low and high ratios that are most effective might be second and third, or second and fourth gear. 

An electric motor's torque capability is limited by the current you can source from the batteries through the controller (and the amount of heat you can dissipate due to resistive loss in the motor), while its ultimate RPM is limited by the lesser of the rotor burst speed, and its kV (speed-voltage) constant, which is fixed in series-excited DC motors and AC induction motors but it can be controlled in DC sepex and AC wound-field motors. 

There are purpose-built electric motor drives available that have an integrated final drive such that effective operation can be had without changing gear ratios, but they're mostly designed around AC motors that have higher maximum speed ratings than most EV oriented DC motors.


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## rwaudio (May 22, 2008)

TKosinski02 said:


> I understand that electric motors making peak power straight off


Be careful of your terminology as it can confuse how you see an electric motor. A series dc motor for example can make peak torque (not power) right away. Power is still a function of torque and RPM so it behaves just like a gas engine in that respect. Because of the increased torque and ability to start from 0 rpm the low gear(s) aren't needed this means 1st and maybe 2nd is gone. The ideal running rpm is in the mid rpm range 3-4k (based on DC) depending on motor size, so no need for 5th gear (maybe 4th as well) to get the rpms down to a fuel efficient range. So if you eliminated 1st, 2nd, 4th and 5th you just have 3rd gear. Why add a transmission? 
The OEM's are using AC, so they have a larger RPM range (even if the power band isn't really that much greater than DC they can still rev higher) Pick the right ratio for the cars torque/weight/desired performance and you're set. Is it ideal? Perhaps not, but it's simpler, cheaper and perhaps more reliable until the OEM's master electric motors. Look at the evolution of transmissions, 2/3 speeds were common, then it was the 4 speed auto and 5 speed stick, then a 5 speed auto and a 6 speed stick, some cars now have 7 and 8 speed transmissions?!?! I have a feeling that as EV's diversify so will the gearing needs, there will be trucks, suv's, etc. 

Good question though.


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## TKosinski02 (Dec 28, 2010)

So when people convert there Cars over why do they sometimes keep the transmissions? Could you technically get rid of all that and as long as the rear was independent bolt it straight to the rear end? Or do they use the transmission to gain a torque advantage to move the vehicle with a small electric motor? Sorry I'm trying to understand a little better. I had another post about a diesel electric truck(similar idea to a volt) that I did and the transmission just spures more questions. 

So why not have a simple city/hwy transmission? Just too much money?


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## TigerNut (Dec 18, 2009)

TKosinski02 said:


> ...
> So why not have a simple city/hwy transmission? Just too much money?


That is more or less what people do, when they keep the OEM transmission - except they will just start off in 2nd or 3rd gear and then only shift to 4th when they get onto the highway.

The main issue with making a custom transmission is that it's beyond the capabilities of most folks to make a driveline (everything between the motor and the wheels) that has anything near the performance, reliability, and cost of an OEM driveline. 

If you look at something like a Gear Vendors overdrive, which is the nearest thing to an easily adapted (for RWD) two-speed transmission you'll see that the cost is far higher for that unit than for a four or five speed manual gearbox - that you may already have. The only benefit of such a gearbox over a conventional manual transmission would be the planetary gearset allows clutchless operation. The time may come when someone markets a low-cost EV transmission that incorporates a simple planetary gearset for two-speed selection... you could do that using aftermarket Powerglide parts. However, such a gearbox would still cost you $2000 or some such number because the volume is low.

So... the really lightweight or limited-purpose high-power applications can go direct drive (still through a final drive of between about 3 to 1 and 8 to 1); the rest of us will be using some kind of intermediate gears and multiple ratios to spread out the available torque and RPM range.


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## rwaudio (May 22, 2008)

TKosinski02 said:


> So when people convert there Cars over why do they sometimes keep the transmissions? Could you technically get rid of all that and as long as the rear was independent bolt it straight to the rear end? Or do they use the transmission to gain a torque advantage to move the vehicle with a small electric motor? Sorry I'm trying to understand a little better. I had another post about a diesel electric truck(similar idea to a volt) that I did and the transmission just spures more questions.
> 
> So why not have a simple city/hwy transmission? Just too much money?


I'm keeping the transmission because I don't know what the best ratio would be. So I have a box with 5 to choose from. I would say that goes for many converters, the other reason would be like you said it lets some use a smaller motor and/or controller and still have decent performance. I tend to agree with your city/hwy idea, on paper that would be 2nd and 3rd gear for me, or perhaps 2nd and 4th. I'm basing this on what I've learned about DC performance though, I don't know if everything is the same in a higher power AC system. (by higher power I mean higher than the low voltage curtis systems) I'm also keeping the transmission for easy reverse even though it could be done with a reversing contactor.


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## DJBecker (Nov 3, 2010)

That last point is important: you usually need a transmission for reverse.

Series DC motors for EV usually have "advanced timing". The brush assembly is mounted a few degrees rotated from its optimal low-speed point. This allows higher forward speeds with reduced brush heating and sparking, but increases sparking with reverse operation and regeneration. So you generally don't want to operate them in reverse.

Your controller also needs to be able to reverse the voltage. This requires 2x or 4x the number of transistors of a basic DC controller, and it will double the heat loss for forward driving. Or you can use a reversing relay/contactor. Either of those solutions is very expensive once you go above golf cart voltages and current.


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## normalmitra (May 11, 2011)

I'm also interested in this component. It seems the last piece of the EV puzzle that has yet to be fully addressed (besides high-capacity/low-weight/low-volume batteries).

Two questions:
1. why not add a low stall speed torque converter (like those found in diesel trucks) in place of a clutch? This would smooth power delivery and allow for clutchless shifts.

2. what about using a lightweight & strong racing transmission? Like a powerglide - or better a Brinn Modified Open (http://www.brinninc.com/)?


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## TKosinski02 (Dec 28, 2010)

So I guess another question would be why didn't GM or Nissan develope a simple two speed transmission for the volt or leaf to take advantage of better consumption of electricity or utilizing a smaller electric motor because of the mechanical advantage?


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## Coulomb (Apr 22, 2009)

TKosinski02 said:


> So I guess another question would be why didn't GM or Nissan develop a simple two speed transmission for the volt or leaf to take advantage of better consumption of electricity or utilizing a smaller electric motor because of the mechanical advantage?


Because they use brushless DC (Permanent Magnet AC) motors with flux weakening, or high speed induction motors with field weakening, which have a wide constant power region. If you have a wide constant power region, then a transmission is basically pointless, so why not save the space, weight, and expense. Reverse is easy with any AC motor, so no need for mechanical reverse either.


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## Coulomb (Apr 22, 2009)

normalmitra said:


> 2. what about using a lightweight & strong racing transmission? Like a powerglide ...


It's been done. See Powerglide Secrets EVers Should Know.


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## Coley (Jul 26, 2007)

Using the original transmission also makes mounting the electric motor much easier.
Once you have an EV, even a light weight one, you will appreciate the gears in a transmission.
Getting home on an unexpected low pack is one.


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## Anaerin (Feb 4, 2009)

normalmitra said:


> 2. what about using a lightweight & strong racing transmission? Like a powerglide - or better a Brinn Modified Open (http://www.brinninc.com/)?


You mean something like the EVGlide? http://www.kansasev.com/evglide-powertrain.html


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