[Kobushi]

Slightly off-topic, but I figured this would be a good source of people to ask...

I've been thinking about the viability of using electri motors to directly drive the wheels of a car. No transmission, or perhaps a single gear reduction (say a 6500rpm max spin motor with a 3:1 gearing).

But my main curiousity is:

Will the amount of electricity required to keep the car at say, 40mph, be the same if the electric motor is at 3500, as it would be at 1500rpm(such as if it were going through a transmission)?

Or would it have to use more power since it is at a higher rpm, even though it would be under an identical load?

So I guess the question ismost easily condinsed as: Is the draw of an electric motor more dependant on load, or on rpm?


Thanks,
--Ash

 

[Foxpaw]

I believe that the draw of an electric motor is more or less constant - the exception being if the motor is not spinning. I believe that a higher RPM would cause the magnetic fields around the winding to be more frequent but smaller, so I think that it would balance out. If the motor isn't spinning at all then the magnetic field builds up fully and at that point current mostly just goes into heating up the wires.

Increased load should not really increase power consumption, but rather I would expect would simply decrease performance. (Which could be offset by increasing the voltage, and hence the power consumption, so.. err.. yeah. )

That sounds correct, but it has been a long time since I studied electronics, so it might not be correct. I am fairly certain that it's plausable to run a cars wheels directly with an electric motor, as I believe that the Alessandro Volta has a fixed gear ratio on it's rear wheels, which are connected to an electric motor.

However, with constant gearing, you will likely get poor performance at higher speeds, as the electric motor won't accelerate as fast at higher RPMs.

 

[b1shmu63]

Like a gasoline engine an electric motor is most efficient at one speed/load. The efficiency stays high over a much broader range, so you are less likely to need varying ratios. That is what makes the synergy system work. Advanced ironless rotor motors have the broadest range but are somewhatless robust than other types. As with everthig else designs are always compromises. Choosing the best compromise leads to the most viable design for mass marketing. In an ideal electric motor the power drain is directly and linearly related to the mechanical power output. That is, you don't get something for nothing, and the draw is mostly dependant on load.

 

[Aaron Cake]

At lower RPM, the motor will be required to produce more torque, and thus the current draw will go way up. I too have thought about driving the rear wheels with a motor, but the problem of gearing comes up. At highway speeds, the motor would be seriously oversped. The comm will probably just fly apart. If geared for highway speeds, the torque needed at low city speeds would produce current draws probably much larger then the battery pack could handle.

Now, you could use an electric clutch to disconnect the motor after you are up to speed, but then you're adding weight, complexity, etc. And I have not seen any affordable and small clutches up to the task...

 

[Holicow]

I have seen concept cars that have rear-wheel electric motors (Mitsubishi? Subaru?). I think the Honda/Acura DualNote concept may have been this arrangement also.

How did they handle the wide range in rpm?

 

[Foxpaw]

The Allessandro Volta uses that sort of system, and I don't think there is any transmission on the electric side, just fixed gearing. (I don't remember if it's gas on the front and electric on the rear, or vice versa.)

If I had to guess, I would say maybe it just has something like a centrifugal clutch in reverse, or perhaps a torque converter is used instead of the gearing. But that would be just a guess.

 

[Kobushi]

Well, with the Insight's tire size, and a 1:1 ratio with the electric motor.

That puts 64.4 mph at 1000rpms.

I don't think a clutch would be needed, since most of the motors operation would be under 1200 rpm. Depending how the motor is wound, this should be doable. Would just be dependant on where you do MOST of your driving, and you could even cut all the cuircuts to the motor and you wouldn't be turning that much more mass than a performance brake rotor.

(basing this roughly off the specs of eCycle's motors)

My thought was combining direct drive motors with a ceramic generator as the powerplant... cermanic engines are awesome, but brittle... having it not connected directly to the drive heels would allow for greater shock absorbtion for it (don't need stiff motor mounts, and it wouldn't get exposed to jerky shifts/downshifts/etc)

Also thought that since ceramics have sugh high exhaust temps (in 2000F and up) a thermoelectric generators could be placed in the path of the exhaust for even further efficiency ( even in Death Valley, the temp difference between 2000+ and ambient would be sufficient for power generation)

But I'm no engineer, just a tinkering thinkiner without enough money to actually try these crazy and $$$ ideas ) Thanks for humoring my post with replys.

Thanks,
--Ash

 

[Aaron Cake]

Even if the gearing is correct and at 64MPH you are only seeing 1000RPM on the motor, at, say 5MPH, you'll be asking the motor to produce huge amounts of torque, and probably drawing current greater then 500A.

Most direct drive EVs use an AC motor, which will spin past 10K RPM. The EV1 used this system quite well. DC direct drive EVs typically oversize the motor to handle the huge currents, and pull 1000A or more getting started because of the low motor speed.

 

[Mike Dabrowski 2000]

I looked at the rear wheels as the best place to try attaching an electric drive to the Insight. The concept that I had in mind was to have two DC motors,of ~5 HP ea , to kind of match the 13HP Insight motor, so the watts expected from the battery would be about the same as when driving the car. A 180V DC motor which I believe is a common industrial size, will crank at 3000+rpm. at rated voltage.
Lets consider that this motor with a 1:1 ratio to the wheel, would try to make the car go 193 MPH.
I was considering that my electric drive would only be used up to maybe 45MPH, since faster speeds start to have a big areodynamic load, and would need more that 10 HP to maintain.
It makes more sense to get some mechanical advantage to increase torque and lower amps by making the ratio so that the motor would be turning at ~ 2500-3000 rpm at the 45 mph.
The motor would drive the wheel through a sprag and chain, also called an over running clutch, which is the device that lets you coast on a bicycle. The 5hp sprag would decouple the electric motor when it was not powered up, so it would stop and let you run in the normal hybrid mode. For those small shots of electric boost, at under 45MPH, you could ramp up the electric motors, and when they began rotating fast enough they would start driving the rear wheels and push you over the hill, without the need for you to step down on the gas, therefore maximizing milage.
The logical way to accomplish the same thing would have been for Honda to provide a robust electric throttle and regen mode where the driver controlled the charging and assist.
You old timers may remember all of the discussions about this. I even sent an unanswered letter to honda asking for an electric control option.
No need to go there again..

The prius, partially due to the fun of driving on pure electric, is out selling Honda by a large margin (the last time I checked)
I would hope that Honda will soon figure out how to do a stelth mode on their hybrids. :roll: , so we don't have to have prious envy.

Mike

 

[b1shmu63]

Driving all electric puts a lot more strain on the battery pack. I don't sense that the benefits would outweigh the risk of an earlier battery replacement. An E-charger would give a lot more kick per ampere.