Question about Electic Motors

[The Natural Philosopher]

High strength magnets merely reduce the weight of the motor, You can get as high flux densities and efficiences using electromagnets

[Fredxx]

Probably higher, but these take power, unless you're talking superconductors?

In terms of energy, a magnet is 100% efficient at providing a magnetic field.

[Andy Burns]

I guess that's why they waited until the change of Model S and Model X, to Model 3 (lighter, lower capacity battery, cheaper) to change motor.

AFAIK "rare" earths aren't especially rare, just that nowadays only China seems to bother mining them, still they'll probably dig out lots of Thorium while they're at it ...

[The Natural Philosopher]

China dumped them on the market well below cost, driving out the other mines, then only exported refined materials and finished goods once the competition had gone.

Cunning really.

[Tim+]

I think it?s more a case of ?China being one of the very few countries prepared to accept the horrendous environmental cost of refining?.

Tim

[Vir Campestris]

If you're going to put a drive shaft for each wheel connected to an electric motor why not put the disc inboard next to it? That would really cut the unsprung weight (to just half the shaft)

Andy

[Tim+]

One problem with that is that it moves the disc (and pads) out of reach of easy inspection/service.

Tim

[Johnny B Good]

The real problem with that is the possibility of a high speed wheel lockup under emergency braking snapping the drive shaft as a result of the rotational energy stored in the wheel. You want to decouple the mass of the brake assembly from the unsprung mass but not at the risk of completely disconnecting it from the wheel in an emergency braking situation.

[Johnny B Good]

You only need to observe the small size of the motors used on the TT lapping E- race bikes to see the absurdity of that statement.

[Johnny B Good]

... which, if the urgently required development of MSR technology becomes realised, can only be "A Good Thing". :-)

[Tim+]

Compared to the kinetic energy of a car I would have thought that the rotational energy of the wheel is relatively trivial..

I would agree that it introduces a new ?weak point? in the braking system though. Off the top of my head, Jaguar E-type, Rover 2000/3,500 and Citroen ZX all had inboard discs,.

Did they have a reputation for snapping drive shafts?

Tim

[Johnny B Good]

Which could, assuming Molten Salt Reactor (MSR) technology becomes the de-facto standard for nuclear power station design to tide us over the next half century or longer that it'll take for the dream of "Cheap Pollution Free Nuclear Fusion Power" to finally be realised, end the current fossil fuelled environmental problems resulting from the use of coal fired power stations.

Whatever we do to power our high energy western lifestyle, there's always going to be a 'Pollution Problem'. Putting aside the issue of 'Nuclear Accidents', nuclear power by necessity, pollutes the environment far less than conventional coal fired plants do in regard of radioactive isotope emissions.

The issue of 'Nuclear Waste' remains addressable even if it means simply locking it up and throwing away the key. Inherently safer LFTR based MSR technology can even help burn up the most lethally radioactive nuclear waste products to mitigate the problem of nuclear waste storage/disposal. On balance, China's interest in mining for rare earth elements (and the Thorium tailings) could well prove to be the lesser of two evils regarding the issue of planetwide pollution.

[Johnny B Good]

The fast startup times were more probably the result of employing solenoid operated pinch wheels. I'm sure the capstan motors were more likely kept spinning all the time the machine was powered up and ready to go.

[Johnny B Good]

It's the peak shock loading going via a couple of UJs that's the problem.

That may be but I doubt they were attached to the wheel via a UJ. Even though they might have been described as "inboard" brakes, the section of drive shaft involved would not only have been quite short but also beefed up to take the strain.

No Idea, gov.

The torque loading from translating the kinetic energy of the vehicle into heat energy in the brake disks at maximum braking force just shy of locking up the wheels is limited by the tyre grip to the road surface, circa 1.5 G. Locking a wheel through overenthusiastic application of the brakes can generate a very high shock loading on the UJs in a system that places the brake assembly at the sprung end of the suspension system rather than more directly at the unsprung wheel side of the UJs.

Even assuming the UJs can cope with a limited number of such shock loads, the further away the brake assembly is mounted along a relatively spindly shaft from the wheel, the greater the risk of damage from the sudden torsional forces being applied.

A conventional brake is still required on an all electric vehicle that uses regenerative braking just to cover the final 15MPH or so to 0MPH end of the braking phase where the regenerative braking effect fades to nothing.

In a direct drive design using wheel hub motors where unsprung mass is an issue, the temptation is there to reduce the mass of the conventional disk brake assembly to a minimum which will reduce the maximum speed rating to just above the tail end of the effective minimum speed range of the regenerative braking system.

However, in practice, rather than qualify them for say 20MPH, they're more likely to be qualified for 50MPH to give some margin for emergency braking on a long downhill gradient (provided the speed is held to no higher than 50MPH in this case).

Whilst this will add a little more unsprung mass than strictly necessary when assuming the regenerative braking system is never ever going to fail, being mindful that even the best designed systems can suffer catastrophic failure, they'll no doubt hedge their bets on this and add an independant secondary emergency dissipative braking circuit[1] to the hub motor circuit which can, along with the transmission power management control and monitoring logic, log any problems to reduce the likelihood of two seperate, but extremely unlikely (it is hoped) faults occurring in both electrodynamic braking systems simultaneously by alerting the user and the service engineer to any symptoms of impending problems in either system (regenerative or dissipative) which need to be immediately addressed. The 'weedy' lightweight disk brakes can act as a last chance saloon backup in the event of such a double failure (hence the likelihood of them being rated for 50 rather 20MPH).

The point I was trying to make was that, given sufficient development, the all electric transmission direct drive system offers far more benefit than deficit in a 'normal' electric road car. I think the issue of 'unsprung mass' is perhaps being a little over stated in this case thanks to rare earth permanent magnet DC brushless motor technology.

[1] Such a 'secondary' dissipative electrodynamic braking system will be needed anyway just to cover the worst case scenario of a vehicle setting off with a fully charged battery from the top end of a long downhill stretch of road. Apart from the initial burst of acceleration to reach a sane cruising speed, the battery will be in no condition to accept a prolonged recharge from the regenerative braking system which will then have to call on the dissipative electrodynamic system to keep the kinetic energy build up in check by converting it into waste heat as the vehicle converts its potential energy into kinetic energy during its prolonged descent.

This waste heat would simply be dissipated to the environment in mild to warm weather conditions but could be put to good use as cabin heating on early frosty winter morning runs. In any case, an element of electrodynamic dissipative braking will probably still be required to limit the peak charging rate of even a discharged battery under extreme high speed braking conditions to avoid exceeding the battery's maximum charging limit.

The control logic to manage the power flows will not only need to be very sophisticated but also very robust. I'm sure the car manufacturers will be able to come up with a safe and reliable solution, after all we've been putting our lives into the care of such 'Fly by Wire' systems for well over a decade now with commercial aviation.

[The Natural Philosopher]

Jagura XJS too

Mostly when a drive shaft goes iys where the splines enter the diff

GF did that years agop - MG Midget, Revved it and dropped the clutch with a bang..

[harry]

weight (which

aren?t actually rare, just

e, and that means a lot more weight than the wheels. The resulting heavines s is unsuitable for ordinary road holding performance.

The "Tweel" offers good possibilities for low unsprung weight.

[harry]

Not at all. The old Citroen had this arrangement. The brake pads were changed very easily under the bonnet. Not even neccessary to jack the car up. A ten minute job.

[harry]

But nowhere near as compact.

[Vir Campestris]

The limiting torque on the shaft will be the braking effort available, which is unlikely to be much beyond the traction limit. It's not beyond the wit of man to overspec the thing enough so it doesn't happen.

Quite a few manufacturers.

Andy

[Dave Plowman (News)]

No. As I said, the motors stopped. Constant run capstan was the far more common method.