Electric motors in series with unconnected shafts

Hi - I'm looking to replace the standard differential in my car with 2 opposing electric motors (separate shafts) but connected in series. The theory is that as I turn a corner (and the outer wheel turns faster than the inner), the two motors will accept the different torque required, and behave nicely.

Is this so? What behaviour should/would I expect from this setup?

And also, is a DC electric motor suitable to regenerate power (when slowing down or going downhill)?

It should work, but possibly emulate a mechanical one too well!
If a wheel spins free, the motor voltage will increase without limit to the point it could be running on near twice the voltage it's supposed to be, with the other wheel stalled.

Some electric vehicles do use separate motors for each wheel, but I suspect with those the motors have individual drives and they in some way share or average the speed feedback and use that to modulate the current loop (torque control) output stage of the drive, to emulate a limited-slip type setup and avoid motor overspeed.

A DC (or brushless DC, BLDC) motor will regenerate, but on it's own the output voltage and braking effect is proportional to speed.

Either type is normally controlled with a PWM drive system, and a fairly sophisticated one in applications with variable loads such as industrial machine tools or vehicles.

They are "four quadrant" drives, meaning they can deliver full torque throughout accelerating or braking in either direction.

Braking is done by commanding the motor to run in the same direction but at half (if I remember right) its present speed. That causes the excess energy to be returned to the DC supply side of the system, where its either returned to the supply source or dumped in large resistors to prevent the DC side going overvoltage.

Drives such as that are also "double loop"; the first stage compares the commanded speed to actual speed and produces a torque (current) command signal. The second stage compares the current command to actual current and modulates the PWM duty cycle to achieve the commanded level.

That means the motor will run at the set speed regardless of it running free or at maximum load, unlike a simple PWM that directly sets a duty cycle - with that, the motor slows down load and speeds up when the load is removed.

This will work, but as rienkinsgb said, it might work too well.
If you monitor the speeds of both motors and look at the delta between them, you may be able to implement an anti-lock braking system by dynamically "pulsing-braking" the faster of the two motors proportional to the amount of speed delta.

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A quick answer is that DC motors produce a torque that is proportional to the current. If two motors are in series, they will produce the same torque, and they will in that way behave like a differential.

A differential is needed in a car because a solid shaft makes slow speed cornering virtually impossible. However, putting two DC motors in parallel could be better. Two DC motors in parallel will tend to turn at the same speed, whatever torque is applied, but they won't be strongly held at the same speed. A small speed difference will not result in a large amount of torque trying to get them to the same speed. There is never a larger speed difference between the wheels in a car, unless one side is slipping. There can be a large percentage difference while manoeuvring, but the absolute speed difference is small. At high speeds, turns have larger radius so the percentage difference is small.

I think that DC motors in parallel would be more like a good limited-slip differential.

You should be aware that many cars don't have limited-slip differentials, and so their suspension systems never have to handle different torques on the driving wheels. On a front-wheel drive car, different torques on the driving wheels can lead to torque-steer. (https://en.wikipedia.org/wiki/Torque_steer).

Even if you have two motors in series, the inertia of the motor will be much more than the inertia of a single wheel, because of the gearbox reduction ratio, which I assume will be needed. The large inertia of the motor will mean that a wheel slips, it will take time for the motor speed to increase, and while it is increasing, there will still be a lot of torque available the other wheel. That could lead to torque-steer happening for a lot longer than on a car with conventional differential that isn't a limited-slip one.

Also, DC motors may not be very efficient, so you might want to look at figures before you commit to a design. None of the modern electric vehicles use DC traction motors.

Diver300 said:

A quick answer is that DC motors produce a torque that is proportional to the current. If two motors are in series, they will produce the same torque, and they will in that way behave like a differential.

A differential is needed in a car because a solid shaft makes slow speed cornering virtually impossible. However, putting two DC motors in parallel could be better. Two DC motors in parallel will tend to turn at the same speed, whatever torque is applied, but they won't be strongly held at the same speed. A small speed difference will not result in a large amount of torque trying to get them to the same speed. There is never a larger speed difference between the wheels in a car, unless one side is slipping. There can be a large percentage difference while manoeuvring, but the absolute speed difference is small. At high speeds, turns have larger radius so the percentage difference is small.

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I've had a couple of Suzuki Jimny's, small 'Jeep' type vehicles - they are normally rear wheel drive, but can be switched to four wheel drive, or low four wheel drive - and they are excellent in snow, or off road.

However, you can't use them in four wheel drive under normal road conditions - while they have a front diff, and a rear diff, they don't have a middle diff. In the picture below I was coming up from the left (where the blue car is), and attempting to turn sharp left down the hill where the red car is going. The turn is sharper than it looks, and the hill is steeper than it looks - it was winter, and the roads were covered in snow (the road up from the left is pretty steep as well, both go to the same road below), I was having no problems as I was in 4WD

However, while the road was covered in snow, the corner itself had been cleared and as I tried to go round the corner all the wheels locked up due to the lack of a centre differential. Even worse, I couldn't switch back to 2WD as the front wheels weren't straight - the only solution was lot's of throttle and make the tyres 'scrub' round the corner.

I must admit, I was absolutely amazed the staggering difference it made, I was aware of the issue, but thought it was far less of a problem than it was.

The lack of a centre diff means that at least two wheels are been driven at all times, so avoids the need for diff locks.

Diver300 said:

I think that DC motors in parallel would be more like a good limited-slip differential.

You should be aware that many cars don't have limited-slip differentials, and so their suspension systems never have to handle different torques on the driving wheels. On a front-wheel drive car, different torques on the driving wheels can lead to torque-steer. (https://en.wikipedia.org/wiki/Torque_steer).

Even if you have two motors in series, the inertia of the motor will be much more than the inertia of a single wheel, because of the gearbox reduction ratio, which I assume will be needed. The large inertia of the motor will mean that a wheel slips, it will take time for the motor speed to increase, and while it is increasing, there will still be a lot of torque available the other wheel. That could lead to torque-steer happening for a lot longer than on a car with conventional differential that isn't a limited-slip one.

Also, DC motors may not be very efficient, so you might want to look at figures before you commit to a design. None of the modern electric vehicles use DC traction motors.

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To be fair, VERY few cars have limited slip differentials, it's more a racing car or rally car innovation, and of little use for normal road use. It's a popular addition for those trying to increase performance of a normal road car, particularly for rallying.

Nigel Goodwin said:

However, while the road was covered in snow, the corner itself had been cleared and as I tried to go round the corner all the wheels locked up due to the lack of a centre differential. Even worse, I couldn't switch back to 2WD as the front wheels weren't straight - the only solution was lot's of throttle and make the tyres 'scrub' round the corner.

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It was possible that the big pre-load on the transmission, which is what stopped you, also made it impossible to move the lever that disengaged the front wheels.

Diver300 said:

It was possible that the big pre-load on the transmission, which is what stopped you, also made it impossible to move the lever that disengaged the front wheels.

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It's not a lever, it's a button (well three buttons) - and the front wheels are engaged or disengaged by air pressure - but it specifically says 'only with the steering straight, and at no more than 50 mph. It works by sliding a splined collar over splined shafts joining them together. I suspect there's not enough power in the system to disengage it while it's under anything more than a light load.

My current car, also a Suzuki, is AWD (All Wheel Drive), which basically means there's no selecting of the drive system - it's front wheel drive normally, but if a front wheel starts spinning it transfers power automatically to the rear wheels as well. There's also clever 'tricks' like 'hill descent control' and a system for going uphill that will apply braking to any wheel that starts spinning.

Nigel Goodwin said:

However, you can't use them in four wheel drive under normal road conditions - while they have a front diff, and a rear diff, they don't have a middle diff.

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How do you explain why it is that a four wheel drive truck or one of the old Jeeps, army or civilian, can drive and operate in 4WD on the road? They have no center differential, just a transfer case, to send the power to both ends of the vehicle.

The only vehicles I'm aware of that use a center differential are those that are called "all wheel drive", not 4WD.

As far as using two electric motors on one axle, especially a DC motor, how are you going to gear it down to normal road RPM's/wheel speed? That is the reason electric cars aren't direct drive, the motors turn too fast. And slowing them down to normal driving RPM is bad because they will overheat. A simple calculation will show what I mean, wheel RPM is much lower than people think. So that is why they use some sort of differential gear in an electric car. Though not a transmission like an IC vehicle.

shortbus= said:

How do you explain why it is that a four wheel drive truck or one of the old Jeeps, army or civilian, can drive and operate in 4WD on the road? They have no center differential, just a transfer case, to send the power to both ends of the vehicle.

The only vehicles I'm aware of that use a center differential are those that are called "all wheel drive", not 4WD.

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All the old cars with 4WD will get some wind-up of the transmission. Tyres don't grip perfectly, so in 4WD, if the rear wheels are trying to go faster, they will take a bit more torque, and slip a bit more, which takes up the speed difference. There may be slightly more overall drag, and tyre wear may be a bit more

It's a matter of degree, as Nigel found out when his Suzuki got stuck on a corner, but with a big enough force, the tyres slipped.

Any lorry or van with twin rear wheels will have some slip between the inner and outer wheels on a corner. There even has to be some slip when a single tyre goes round a corner, but it is too little to worry about. At the other end of the scale, vehicles with no differential and widely spaced wheels, like some quad bikes, or an Alvis Stalwart that has no front-rear differentials, can suffer badly from wind-up if driven on the roads.

shortbus= said:

How do you explain why it is that a four wheel drive truck or one of the old Jeeps, army or civilian, can drive and operate in 4WD on the road?

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My first 4x4, a Suzuki Vitara, did not have a centre differential.
If I was driving on slippery roads (snow/ice) with 4WD selected, it was fine.
But if I got on to a clear patch of tarmac, the tyres would judder and jump around in a most excited manner.

I suggest that an old Jeep would do similar things resulting in rapid tyre wear compared to 2WD or a similar vehicle with a centre differential.

JimB

JimB said:

My first 4x4, a Suzuki Vitara, did not have a centre differential.
If I was driving on slippery roads (snow/ice) with 4WD selected, it was fine.
But if I got on to a clear patch of tarmac, the tyres would judder and jump around in a most excited manner.

I suggest that an old Jeep would do similar things resulting in rapid tyre wear compared to 2WD or a similar vehicle with a centre differential.

JimB

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If an older car were on cross-ply tyres, it might lead to less wind-up.

Cross ply tyres distort the tread more as the bottom of the tyre is pushed up by the road. That is why they are generally worse, and why they slip sideways more for a given side load. I would assume that they slip in the direction of travel more for a given rearward force. If so, any small difference in wheel diameter that leads to speed differences will result in much less force if cross-ply tyres are fitted.

Diver300 said:

If an older car were on cross-ply tyres, it might lead to less wind-up.

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A good point.
I had been thinking that a 1990s radial ply tyre would have more grip than a 1940/1950s cross ply tyre.

JimB

shortbus= said:

How do you explain why it is that a four wheel drive truck or one of the old Jeeps, army or civilian, can drive and operate in 4WD on the road? They have no center differential, just a transfer case, to send the power to both ends of the vehicle.

The only vehicles I'm aware of that use a center differential are those that are called "all wheel drive", not 4WD.

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As far as I was aware the difference between 4WD and AWD is essentially that 4WD has manual selection, and usually low ratio options as well. But anything without a centre diff (or some other kind of device) will have problems on the road in good conditions, simple physics - and will also scrub the tyres fairly quickly.

Certainly old Land Rovers had a bewildering array of levers and buttons, and even hand throttle.

shortbus= said:

As far as using two electric motors on one axle, especially a DC motor, how are you going to gear it down to normal road RPM's/wheel speed? That is the reason electric cars aren't direct drive, the motors turn too fast. And slowing them down to normal driving RPM is bad because they will overheat. A simple calculation will show what I mean, wheel RPM is much lower than people think. So that is why they use some sort of differential gear in an electric car. Though not a transmission like an IC vehicle.

Wheel And Tire Motion Calculator

Find information about tire motion based on speed, such as revolutions per minute (RPM) and revolutions per second (RPS).

www.csgnetwork.com

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There a UK TV show called 'Vintage Voltage' which converts classic cars to electric - and they essentially always seem to use Tesla motors - and sometimes have to strip and modify them (usually if it has be fitted backwards, so runs in reverse all the time). So I can confirm that there's a fair selection of gears in a Tesla drive train, and looks a pretty well made unit.

The backwards thing needs the oil pump altering to run the other way, as otherwise it's pumping oil down to the sump, rather than up from it via the pickup.

Nigel Goodwin said:

The backwards thing needs the oil pump altering to run the other way, as otherwise it's pumping oil down to the sump, rather than up from it via the pickup.

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Are you saying they use oil cooling for electric motors?

Mike.

Pommie said:

Are you saying they use oil cooling for electric motors?

Mike.

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I think the oil is for the gear box?, but has a pump - as far as I'm aware the drive units (as well as the batteries) are water cooled.

shortbus= said:

How do you explain why it is that a four wheel drive truck or one of the old Jeeps, army or civilian, can drive and operate in 4WD on the road? They have no center differential, just a transfer case, to send the power to both ends of the vehicle.

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A customer came in with a 4WD pickup that had larger tires on the back than front. He said the tires would chirp on the road and skid on gravel. I drove it strait a few hundred feet. When I stopped with the clutch disengaged, it backed up about 10 feet.

ClydeCrashKop said:

A customer came in with a 4WD pickup that had larger tires on the back than front. He said the tires would chirp on the road and skid on gravel. I drove it strait a few hundred feet. When I stopped with the clutch disengaged, it backed up about 10 feet.

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Nasty

I remember reading about a guy who had a Suzuki Jimny, similar to what I used to have, and he had a problem with the rear axle/differential or whatever.

Anyway, he was able to source one at reasonable cost from a scrap yard - only problem was manual and auto's have different ratio differentials, and he got the wrong type

The only 4X4 I ever had problems driving with 4WD engaged was my last Chevy S10. It had the front axle changed at some time in its life. The front had a different ratio in it than the rear, worked great off road but chattered badly on a paved surface with the front engaged.

I've had too many to count 4X4s in my life, starting with Jeeps and other than the last S10 have had no problems when driving at speeds under 55MPH in 4WD. The truck I'm driving now is a extended cab Chevy Silverado, and works just fine in 4WD on the road, did it just a couple of weeks ago during a bad snow fall here.

Stuff like you guys are talking about must not have the same ratio at both ends. I was in a 4WD club years ago and no one ever complained about what you guys are talking about, we drove Jeeps, Land Cruisers, Chevy trucks and Ford trucks All with no road complaints.