Electric Vehicle Drivetrain

[Goyds]

G'day Everyone

I have a bit of an unusual one for you all. We are making an Electric SAE vehicle and have a single large (85kW at 4000RPM, 250Nm 0-3000RPM) AC induction motor. This is all very nice but the trouble we have is packaging it into our frame. It is very long and so to chain drive the back axle the motor needs to sit around 250mm off to one side and as it weighs 68kg this upsets our weight disto rather badly. Oh and our motor runs just fine with a single speed transmission and we are pretty much stuck with it for now.

So on to my actual question... We are considering a north/south motor arrangement and a right angle drive into a spool or diff but we need a gear reduction of around 5.5:1 This is possible with some car diffs but they are far too large/heavy for us. Assuming we cannot use an actual car diff, does anyone have any experience with ATV diffs if they exist in this kind of ratio? Or can someone honestly recommend designing gears from scratch? This seems like a really good way to make expensive mistakes to me so i am a bit worried about committing to this at this stage.

Just to make it more difficult, the most convenient packaging arrangement would call for the pinion gear to directly mount onto the motors shaft. We aren't too fussed about machining the motors shaft (its not hardened) but if the pinion gears is already shaft mounted, we probably can't use it.

At this stage we are looking into basic packaging for our next vehicle so we don't feel too pushed for time as yet, but if the north/south idea is a no go we may need to look into a different motor option.

Cheers for the help

If anyone is interested i'll see about posting some pics of our 2011 vehicle at some stage.

[Yannick]

Hi Goyds

I'm not sure to understand your question 100% but here I go.

In FSE quite a lot of teams have manufactured their own (fixed ratio) gearbox. Delft and us did a spur gear, Zwickau and Karlsruhe made planetary gears and Stuttgart (and also Aachen I think) made bevel gears. Except for some delays I didn't hear of problems. All those team however used 2 (or 4 in the case of Zwickau) independent motor to get rid of a differential. As far as I know, all teams had the gears manufactured for them.

There is a good presentation on the FSE website of Stuttgart explaining their gearbox (with a reduction of 5.5:1):
http://www.formulastudent.de/a...-euromold-frankfurt/ (http://www.formulastudent.de/academy/workshops/20111202-euromold-frankfurt/)

And if I understand correctly on slide 8 they show a possible concept that would be what you want to do.
You need an account of the FSE page to see the presentations I think.

On another topic I am wondering why you choose to run only one motor, which also seems not to be well suited to this application. It has a low power density (1.25 kW/kg) which will penalyze the weight of the car. Furthermore you eliminate all possibilities of individual rear wheel drive, which is one of the reasons to use electric motors in race cars in my opinion.

Bye
Yannick

[Goyds]

G'day Everyone

Thanks for the replies. At this stage we are a little stuck with our motor as the uni does significant research into AC electric vehicles so a DC permanent magnet motor would be frond upon. That said the motor is 68kg stock and we plan on lightening it fairly soon. The motor is actually capable of 124kw, which we did by accident on the dyno one night. Scared the hell out of us. If only we could make it do that a bit more reliably. Youtube vid: http://www.youtube.com/watch?v=PuhiSfeETfw

The vehicle we are building in 2012 has a target weight of 240kg and from looking at the layout of the vehicle that seems pretty reasonable. I know that's on the upper end of the petrol vehicles but it think it's something we'll have to live with.

I think based on the responses we will look into mounting the motor north/south with a bevel drive into a spool for 2012.

Oh and cheers for the perm 132 suggestion. I looked into those but we are concerned they are easily damaged from overheating so getting more than the rated 7.7kW out of them is risky, still other teams seem to do it...

Thanks for the help

[Goyds]

Oh and we have a vid of the 2011 car doing its first run. It hadn't had an alignment before the vid so don't be too harsh on its handling...

Without a petrol engine you can hear the spool scrabbling in the loose tarmac.

http://www.youtube.com/watch?v...76I&feature=youtu.be (http://www.youtube.com/watch?v=6KV38H_a76I&feature=youtu.be)

[Goyds]

G'day Cole

Out of curiosity how are you acheiving that kind of peak power. The obvious ones are very high RPM/high voltage and high current. but i'm curious how high?

Also you may be safe now i come to think of it. Our car uses an average power of about 13kW so in only few minutes i spose it'd be hard to heat a 7kW motor too much if you were only pulling 10-15kW average.

[TMichaels]

I would suggest to have a look at the webpages of the FSE teams and see which motors they use. There are teams using single AC motor setups and their motors are less bulky and have a higher power-to-weight-ratios than your current choice does.
GFR Electric for example used a single motor setup in 2011.

[Goyds]

G'day again everyone

Well it looks like we have a weight problem with our motor. Currently it weighs 68kgs and the afternoon's work tells me it will be possible to drop that to around 55 without hurting it too badly.

Also as a matter of pride, I feel i need to share that the motor is actually 55kW continuous and we have made over 120kW on our dyno. That said i doubt if we could manage that for more than a few secs before the motor would be out of action for several mins of cooling but still... Looking at the specs our motor is pretty similar to the REMY motors Tobias mentioned (but heavier).

Thanks for the input everyone. It looks like we will be ok'ish with what we have but we obviously need to put some serious thought into shaving weight out of it.

I suspose what i really need to consider is how much power is enough. The motor will easily do 85kW peak for actual track work but given a FSAE car, is that too much? I know that petrol teams run less than that and they are usualy traction limited anyway. If 85kW really is too much, maybe it is time to ditch the old motor and look into something lighter.

Lloyd Morrissey
Chief Engineer
Team Swinburne 2012

[TMichaels]

85kW is the limit given by the rules, that does not necessarily mean that you have to reach that limit.
Additionally it is measured at the battery, therefore you have to take into account losses by the inverter, the motor and the gearbox which takes you closer to the peak power of the combustion cars.
Do not forget the superior torque characteristics of electric motors.
Setting up a lap time simulation is probably not the worst idea to get a feel for the needed peak and continuos power.

The values that you have given with 55kW and 120kW, is that electrical or mechanical power?

Because if it is electrical power, it is more or less useless.

[Z]

Originally posted by Goyds:
... how much power is enough...
Lloyd, try looking here (http://fsae.com/eve/forums/a/tpc/f/125607348/m/824105905?r=86720419051#86720419051), for an "objective" way of looking at it.

(Note that your car will be heavier than the linked example.)

Z

[Goyds]

G'day Tobias

I'm interested in the 85kW limit. Last year at the Aussie comp we had a 100kW limit, i haven't had a chance to check the new rules but i presume that is new for this year.

Anyway, Yes it is mechanical. Electrical we are drawing around 62kW for 55kW mechanical and around 100kW for 85kW mechanical. The motor goes into saturation so the efficency drops above 100kW so 120kW mechanical draws around 160kW electrical.

The motor is a heavily modified Electro Vehicles Europe with a rebalanced rotor, rewound stator with class H windings and very high purity copper. We also added high temperature bearings. The motor is liquid cooled but now we have added all the other features we feel we don't need the water jacket anymore. Personaly i was pretty pleased with what we acheived as students but if 85kW is the limit for the 2012 comp you are right - the motor is too heavy and is really no use for us.

Lloyd Morrissey
Chief Engineer
Team Swinburne 2012

Oh and G'day Z, i'd forgoten about your post... I'll have to have a re-read.

[TMichaels]

The new limit for 2012 is 85kW.

The current FSE rules set can be found here:
http://www.formulastudent.de/u...ules_2012_v1.0.0.pdf (http://www.formulastudent.de/uploads/media/FSE_Rules_2012_v1.0.0.pdf)

At FSAE-A 2011 the FSE2011 rules set was used.
The FSE Rules are currently integrated in the FSAE2013 rules and a draft will be released in spring. Therefore my advice would be to look at the current FSE rules, if you are planning to build an electric car, as they will become global with the FSAE2013 rules set.

If you are interested in FSE cars in general I suggest you to have a look here:
http://media.formulastudent.de/

There is probably no better way to waste time http://fsae.com/groupee_common/emoticons/icon_smile.gif

After registering at the FSG website, you may also access the slides of our workshops, which also carry a lot of useful information regarding the design of electric and combustion cars:
http://www.formulastudent.de/a...-euromold-frankfurt/ (http://www.formulastudent.de/academy/workshops/20111202-euromold-frankfurt/)