DIY Electric Car Forums banner
1 - 19 of 19 Posts

·
Registered
Joined
·
5 Posts
Discussion Starter · #1 ·
Hey guys!

To start I must inform you that I don't have much knowledge about electric cars, I'm just an enthusiast who worries too much about the changes that are happening in the world and I don't want to be left behind, and I am an Industrial Engineer and mechanic...

I've been watching videos about electric cars do some researches, I can see that all electric cars have the same common problems which are the batteries of course, and why most e-cars have only one gear or two?

With some more research, I found out why which is logical! but what I can't wrap my head around is why using so powerful motor with high torques that uses more electricity while the technology behind the batteries is not so efficient.

My question is can I use a less powerful motor with low torque with a 5-speed transmission where the trans will help provide low-end torque to compensate for the less powerful motor and use less energy and gain more kwh. Am I making sense?
Since you guys have more knowledge please educate me!

My regards.
 

·
Registered
Joined
·
187 Posts
Using a transmission can make your vehicle work better with a lower power motor and will help the performance if your motor is undersized.
It's only a few percent more efficient though. Generally a motor takes the same amount of power to produce the same horsepower no matter the torque or speed, it's not like a gas engine where there is a 'power band' or more gas consumed at higher revs.
 

·
Registered
Joined
·
6,239 Posts
but what I can't wrap my head around is why using so powerful motor with high torques that uses more electricity while the technology behind the batteries is not so efficient.
You are incorrectly assuming that using a large motor to produce a lot of torque at low speed uses significantly more power compared to a smaller motor at higher speed (using a shorter - more reduction - transmission gear ratio). There is some efficiency loss, but not enough to make up for the complication, weight, and cost of a multi-speed transmission for most cars.

My question is can I use a less powerful motor with low torque with a 5-speed transmission where the trans will help provide low-end torque to compensate for the less powerful motor and use less energy and gain more kwh. Am I making sense?
It make some sense, and you can do that. Unless they need an extreme speed range (such as for the Rimac Concepts or the Porsche Taycan), production EVs use a single transmission ratio, and a larger motor to compensate, because the overall result is lower cost and higher efficiency. Even the Rimac and Taycan use only a two-speed, and only at the rear (although both have front motors as well, they use a single-ratio in the front).
 

·
Registered
Joined
·
5 Posts
Discussion Starter · #4 ·
Using a transmission can make your vehicle work better with a lower power motor and will help the performance if your motor is undersized.
It's only a few percent more efficient though. Generally a motor takes the same amount of power to produce the same horsepower no matter the torque or speed, it's not like a gas engine where there is a 'power band' or more gas consumed at higher revs.
So it's basically pointless to use a 5-speed trans with an electric motor, then we will lose all the fun in banging gears.
 

·
Registered
Joined
·
6,239 Posts
It's only a few percent more efficient though. Generally a motor takes the same amount of power to produce the same horsepower no matter the torque or speed, it's not like a gas engine where there is a 'power band' or more gas consumed at higher revs.
It's not really the same efficiency at all power levels, or even at all speed/torque combinations for the same power level but the efficiently usable range of speeds is wider than for an engine. Most importantly, performance and efficiency are acceptable down to zero speed, so most of the reasons that a multi-speed transmission and some sort of clutch are absolutely needed with a gasoline or diesel engine don't apply with an electric motor.

A gas engine doesn't just consume more gas at higher speed. The fuel consumed per unit of energy produced is optimal (minimized) in a speed range depending the power level; higher output power is most efficiently produced at higher speed (usually well over the midpoint of the engine's speed range).

Search the web for charts showing the efficiency map of the Nissan Leaf motor; this motor (with controller) has been around for a long time, and Nissan is not insanely secretive about it, so it is well-studied and the data is widely available from multiple sources. For the vast majority of driving conditions motor (including controller) efficiency is over 90%, compared to a peak in much smaller region (that is rarely experienced in driving) of 97%, but in extreme conditions (very high load and near zero speed, or the top half of the speed range at very low load) it can drop to 70%.
 

·
Registered
Joined
·
6,239 Posts
So it's basically pointless to use a 5-speed trans with an electric motor, then we will lose all the fun in banging gears.
Essentially true, for most situations. You can still use a multi-speed transmission (although more than 3 ratios is probably pointless in almost any case) and enjoy trying to get optimal performance... you just can't really justify the mechanical complexity, space consumption, weight, or cost in a purpose-built EV. In some conversions, limited by various factors, it could make sense.

I joined this forum when considering conversion of our Triumph Spitfire, then realized it didn't make sense. If I ever do it, I would like to keep at least a two-speed transmission... mostly for fun.
 

·
Registered
Joined
·
5 Posts
Discussion Starter · #7 ·
It's not really the same efficiency at all power levels, or even at all speed/torque combinations for the same power level but the efficiently usable range of speeds is wider than for an engine. Most importantly, performance and efficiency are acceptable down to zero speed, so most of the reasons that a multi-speed transmission and some sort of clutch are absolutely needed with a gasoline or diesel engine don't apply with an electric motor.

A gas engine doesn't just consume more gas at higher speed. The fuel consumed per unit of energy produced is optimal (minimized) in a speed range depending the power level; higher output power is most efficiently produced at higher speed (usually well over the midpoint of the engine's speed range).

Search the web for charts showing the efficiency map of the Nissan Leaf motor; this motor (with controller) has been around for a long time, and Nissan is not insanely secretive about it, so it is well-studied and the data is widely available from multiple sources. For the vast majority of driving conditions motor (including controller) efficiency is over 90%, compared to a peak in much smaller region (that is rarely experienced in driving) of 97%, but in extreme conditions (very high load and near zero speed, or the top half of the speed range at very low load) it can drop to 70%.
Interesting I will take a look at this article later! I appreciate it!
 

·
Registered
Joined
·
5 Posts
Discussion Starter · #8 ·
Essentially true, for most situations. You can still use a multi-speed transmission (although more than 3 ratios is probably pointless in almost any case) and enjoy trying to get optimal performance... you just can't really justify the mechanical complexity, space consumption, weight, or cost in a purpose-built EV. In some conversions, limited by various factors, it could make sense.

I joined this forum when considering conversion of our Triumph Spitfire, then realized it didn't make sense. If I ever do it, I would like to keep at least a two-speed transmission... mostly for fun.
Essentially true, for most situations. You can still use a multi-speed transmission (although more than 3 ratios is probably pointless in almost any case) and enjoy trying to get optimal performance... you just can't really justify the mechanical complexity, space consumption, weight, or cost in a purpose-built EV. In some conversions, limited by various factors, it could make sense.

I joined this forum when considering conversion of our Triumph Spitfire, then realized it didn't make sense. If I ever do it, I would like to keep at least a two-speed transmission... mostly for fun.
The electric vehicle is not a thing yet in my country but I was looking at it as a future build because I go to the track frequently and gasoline is getting quite expensive here, and we use GLP conversion to our vehicle but when it comes to force induction it's pretty delicate, me and my team were looking for something else that we can bring to the motorsports here.
 

·
Registered
1996 Toyota Land Cruiser
Joined
·
144 Posts
I joined this forum when considering conversion of our Triumph Spitfire, then realized it didn't make sense. If I ever do it, I would like to keep at least a two-speed transmission... mostly for fun.
What do you mean it doesn't make sense?? It's an old sportscar with a junky tractor motor and beautiful Italian styling. It's lightweight and full of character. If any car is worthy of an EV conversion it's got to be a vintage roadster? Sure maybe it doesn't make sense economically, but as soon as you slide behind the wheel and take it out on that maiden voyage everything comes into focus.
 

·
Registered
Joined
·
6,239 Posts
What do you mean it doesn't make sense?? It's an old sportscar with a junky tractor motor and beautiful Italian styling. It's lightweight and full of character. If any car is worthy of an EV conversion it's got to be a vintage roadster? Sure maybe it doesn't make sense economically, but as soon as you slide behind the wheel and take it out on that maiden voyage everything comes into focus.
:D
A Triumph Spitfire certainly has character - that's why we bought it.

The conversion doesn't make sense for me. Due to the location of my home, the range that could be readily achieved (given the space for battery) is not long enough for the car to be practical - it would make more sense in other locations. The usual battery space fixes of putting modules beyond the axles would result in handling degradation which would be unacceptable to me. A conversion that I would be happy with would be too expensive for the use that I would get out of it.

Spitfire conversions certainly make sense for some people, and many have been done... with the original transmission, with alternate manual transmissions, and with the transmission removed (using only the final drive for reduction gearing). There's probably one out there with a fixed-ratio reduction gearbox on the motor.
 

·
Administrator
Joined
·
6,296 Posts
The electric vehicle is not a thing yet in my country but I was looking at it as a future build because I go to the track frequently and gasoline is getting quite expensive here, and we use GLP conversion to our vehicle but when it comes to force induction it's pretty delicate, me and my team were looking for something else that we can bring to the motorsports here.
Hi
If you fancy hooning around and a bit of motorsport have a look at my "Device"

I'm using a DC forklift motor - cheap and cheerful
I initially bought a four speed gearbox - but I calculated that I would be able to spin the rear tyres in top gear - so I went direct drive

With the forklift motor my revs are limited

Modern AC motors fitted to production EVs can handle lots of rpm

The basic equation is that Torque is proportional to motor weight - so Tesla use a 35 kg motor and a 10:1 reduction gearbox to get the same torque as a 350 kg motor

You NEED the reduction gear to get the wheel torque
But the motor can keep on giving you that torque right up to about 14,000 rpm

The result is that for zero to 150 mph a single gear will work - a 35 kg motor will break traction at any speed from zero to 150 mph

As far as the "fun factor" is concerned I let one of the faster guys drive my car on a recent event
He spun twice and came back saying that
"I was worried about electric cars taking the fun out of motorsport - now I'm NOT worried about that!"
 

·
Registered
Joined
·
5 Posts
Discussion Starter · #12 ·
Hi
If you fancy hooning around and a bit of motorsport have a look at my "Device"

I'm using a DC forklift motor - cheap and cheerful
I initially bought a four speed gearbox - but I calculated that I would be able to spin the rear tyres in top gear - so I went direct drive

With the forklift motor my revs are limited

Modern AC motors fitted to production EVs can handle lots of rpm

The basic equation is that Torque is proportional to motor weight - so Tesla use a 35 kg motor and a 10:1 reduction gearbox to get the same torque as a 350 kg motor

You NEED the reduction gear to get the wheel torque
But the motor can keep on giving you that torque right up to about 14,000 rpm

The result is that for zero to 150 mph a single gear will work - a 35 kg motor will break traction at any speed from zero to 150 mph

As far as the "fun factor" is concerned I let one of the faster guys drive my car on a recent event
He spun twice and came back saying that
"I was worried about electric cars taking the fun out of motorsport - now I'm NOT worried about that!"
Well, we will always find a way to have fun in a way or another, like I was just watching a report about Jay Leno laying down a 9.23-second pass with a trap speed of 152.16 MPH In the new Tesla model S plaid and a 9.50 with two passengers in it that's incredible it must be fun or scary hitting 0-60 in just 1.99 seconds!:eek:

But some stuff is nostalgics and it's kinda hard to let it go but what can we do other than embracing the future?
 

·
Registered
Joined
·
6,239 Posts
The basic equation is that Torque is proportional to motor weight - so Tesla use a 35 kg motor and a 10:1 reduction gearbox to get the same torque as a 350 kg motor

You NEED the reduction gear to get the wheel torque
But the motor can keep on giving you that torque right up to about 14,000 rpm

The result is that for zero to 150 mph a single gear will work - a 35 kg motor will break traction at any speed from zero to 150 mph
It can keep producing its rated power way up in the operating speed range, and good power all the way to 14,000 RPM, but the torque falls off in proportion to speed after about the first third of the speed range and even the power falls off after the second third. That's fine and works well, but people just shouldn't expect constant torque through the operating speed range... and it won't break traction at 150 MPH the way it would at zero.
 

·
Administrator
Joined
·
6,296 Posts
It can keep producing its rated power way up in the operating speed range, and good power all the way to 14,000 RPM, but the torque falls off in proportion to speed after about the first third of the speed range and even the power falls off after the second third. That's fine and works well, but people just shouldn't expect constant torque through the operating speed range... and it won't break traction at 150 MPH the way it would at zero.
That is when its voltage limited!!
If you feed it enough Volts to maintain the Current then the Torque remains the same - that does mean a lot of volts

My 48 volt motor when fed with 340 volts maintains the same torque until about 80 kph when the back EMF starts to reduce the current -
 

·
Registered
Joined
·
6,239 Posts
That is when its voltage limited!!
No, the voltage limitation in that Tesla curve doesn't hit until the upper third; the Nissan Leaf equivalent is nearly at the top of the operating range. In the middle portion, the motor power is limited deliberately to protect the battery and the inverter. Shifting gears to change motor speed from somewhere in this middle region to somewhere else in the same region makes no difference to the power available from the system, so it makes no difference to the torque delivered to the wheels.

Duncan, you have a massive controller and are willing to risk frying the battery (due to your very short high-power demand), so you can get away with high power in the middle speed range, not applying the power limitation that all production EVs use. That works for you; it will not apply with salvaged EV components unless the controller logic is replaced.

If you feed it enough Volts to maintain the Current then the Torque remains the same - that does mean a lot of volts

My 48 volt motor when fed with 340 volts maintains the same torque until about 80 kph when the back EMF starts to reduce the current -
It is true that current determines torque, so the constant-torque region is the constant-current region. In a production EV (and so in a drive unit using the production controller and protective logic) the current is limited to limit power, long before voltage becomes the limitation.
 

·
Registered
Joined
·
536 Posts
@Duncan: I don't want to hijack this thread but I've heard you "say" several times that you're feeding your motor at 340 volts. I'm assuming that's pack voltage (pre-sag): is your motor interpole?
 

·
Administrator
Joined
·
6,296 Posts
@Duncan: I don't want to hijack this thread but I've heard you "say" several times that you're feeding your motor at 340 volts. I'm assuming that's pack voltage (pre-sag): is your motor interpole?
I'm using a Hitachi Series motor - no interpoles
And you are correct that is before "sag" - and at "full battery"
About half of the way down the page I did some sag measurements during a 1/8th drag run
 

·
Registered
Joined
·
536 Posts
I remember that post now.... but surely the controller must have a motor voltage setting to control the max. voltage the motor sees?
 

·
Administrator
Joined
·
6,296 Posts
I remember that post now.... but surely the controller must have a motor voltage setting to control the max. voltage the motor sees?
Nope! - mine does not have that feature - it just keeps on changing the mark space ratio until its 100%
 
1 - 19 of 19 Posts
Top