Clutch - leave in or not?

Have you made or bought a converted vehicle if so this is for you
User avatar
geekygrilli
Posts: 325
Joined: Sun Jun 24, 2007 6:56 pm
Location: London
Contact:

Postby geekygrilli » Mon Aug 18, 2008 9:52 pm

I can't comment on the FET discussion, but...

One thing I do I have to do is change is the progression of my throttle. Its sort of round the wrong way as it is: For a given pedal displacement I have a lot of movement in the potentiometer at initial pick up, and a little at full pedal travel.

So for people who haven't driven the car it can be quite jerky on intial acceleration - esp in reverse. I'm used to it, but it still needs changing.

User avatar
EVguru
Posts: 305
Joined: Fri May 25, 2007 5:17 pm
Location: Luton
Contact:

Postby EVguru » Mon Aug 18, 2008 10:13 pm

You can't do pulse by pulse current limiting or desat detect if you're barely reaching saturation.

EMC and turn off spikes are a real consideration. People that design controllers for a living have found frequency shifting or pulse skipping to be necessary. A shift to a resonant technology would help, but brings in a whole new set of design problems.

A reverse progression is often used on small engines to make them feel more powerful. Likewise initial servo valving is often quite agressive to make the brakes seem more powerful.

Current control can work very well, Cedric is a big fan. The only downside is a situation like climbing a kerb. You demand enough torque (current) to climb the kerb, then suddenly accelerate as the torque demand drops. There is an add on device to give current control to the alltrax. I did once ask Damon if a PD (proportional Difference) throttle response could be done.
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
http://www.compton.vispa.com/the_named

User avatar
Jeremy
Posts: 472
Joined: Thu Jul 31, 2008 6:35 pm
Location: Salisbury

Postby Jeremy » Tue Aug 19, 2008 6:28 am

There are now well over a million vehicles driving around the highways of the world, with no clutch and a directly coupled electric motor to final drive. Their controllers use direct motor current measurement and pass the strictest EMC regs. What's more, they run at high voltage too, using IGBT switches to control both motor generators. I owned one for three years and can confirm that it has superb low speed control; much, much better than a car with a clutch and gearbox, and significantly better than any automatic transmission car I've driven.

Paul, I appreciate what you're trying to convey with regard to controllers derived from crude golf buggy ones, as many commercially available "hobby" EV controllers are, but I'm afraid it's perfectly feasible to design and build a controller that will operate down to very low PWM duty cycles without adopting antiquated techniques such as slowing the PWM frequency or adopting a pulse skipping approach.

As someone who was the head of UK Type Approval for EMC (and other forms of) approval of certified very high power electronic safety equipment back in the nineties, I can assure you that I don't need a lecture on the challenges of EMC (no offence intended, but you seem to assume I'm ignorant on the subject and need educating).

The "motor current control" approach needs an additional limiting control loop to prevent speed runaway, plus some time-bound filtering probably, to give a smooth feel, but that should be possible easily enough with a simple microcontroller. I've been looking at pulse by pulse current measurement for a 15kHz PWM brush motor controller and it seems quite straightforward, although it's complicated by needing to account for the actual installed motor/wiring inductance, so would be more difficult to incorporate into a universal product. If designing a universal controller along these lines then using a simple shaft torque sensor might be a better approach, especially as the same sensor would automatically give motor shaft rpm.

Jeremy

hyve
Posts: 60
Joined: Sat Oct 06, 2007 1:35 pm
Location: Chesterfield, Derbys.

Postby hyve » Tue Aug 19, 2008 8:48 am

Jeremy, getting back to the question of clutch or not: this million or so vehicles with no clutch or gearbox; what would be a typical motor kw output to vehicle all up weight, to permit one-speed transmission ?
All along in my study of requirements for a sensible EV I keep running up against this question. Remember that after a century of motor vehicles the public is rather used to a decent level of performance, so we're not talking about what you can "get away with", but something reasonable.

Getting rid of as much as possible of the junk associated with ICE motor limitations seems to me an important feature of EV's, especially since by the above posts it seems we're having to add in their place some very seriously complex electronics ! One stage final drive gearing has to improve transmission efficiency, but the above debate about very slow speed control has revealed yet another difficulty for an EV converter to deal with. Can it be done without a clutch or vast sums of money for controllers which make Curtiss look primitive ?
Peter Ph

User avatar
EVguru
Posts: 305
Joined: Fri May 25, 2007 5:17 pm
Location: Luton
Contact:

Postby EVguru » Tue Aug 19, 2008 10:00 am

There are now well over a million vehicles driving around the highways of the world, with no clutch and a directly coupled electric motor to final drive.


Care to tell us what this vehicle is?
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
http://www.compton.vispa.com/the_named

User avatar
qdos
Posts: 2089
Joined: Fri Apr 27, 2007 10:26 pm
Location: Dorset
Contact:

Postby qdos » Tue Aug 19, 2008 10:57 am

Care to tell us what this vehicle is?


I noted the s in Vehicles which implies more than one and I can think of a fair few myself for a start Milk Floats, Luggage tractors, Segways, Golf carts......... goes on and on really oh The Gwiz ...........

User avatar
EVguru
Posts: 305
Joined: Fri May 25, 2007 5:17 pm
Location: Luton
Contact:

Postby EVguru » Tue Aug 19, 2008 11:06 am

qdos wrote:
Care to tell us what this vehicle is?


I noted the s in Vehicles which implies more than one and I can think of a fair few myself for a start Milk Floats, Luggage tractors, Segways, Golf carts......... goes on and on really oh The Gwiz ...........


What's more, they run at high voltage too


Which eliminates most (all?) of the former.
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
http://www.compton.vispa.com/the_named

User avatar
qdos
Posts: 2089
Joined: Fri Apr 27, 2007 10:26 pm
Location: Dorset
Contact:

Postby qdos » Tue Aug 19, 2008 11:14 am

Ah yes sorry I did miss the "high voltage"

Out of curiosity what is considered high voltage?

User avatar
EVguru
Posts: 305
Joined: Fri May 25, 2007 5:17 pm
Location: Luton
Contact:

Postby EVguru » Tue Aug 19, 2008 11:25 am

what would be a typical motor kw output to vehicle all up weight, to permit one-speed transmission ?


It's not really the power, that just determines the top speed. It's the speed/torque range of the motor. Acceleration = force / mass, so the weight of the vehicle determines the torque you need at the drive axle to achieve a given acceleration. The drive ratio then gives you the required motor torque. You can either use a low torque motor and a large reduction ratio, or a high torque motor and a low reduction ratio. All other things being equal a high torque motor will be larger and heavier than a low torque motor. If you go with the large reduction ratio, then the motor has to spin faster for any given road speed. The question then is how high up the rev range can you maintain the torque? This is where a high voltage systems has an advantage, you can just keep increasing the motor voltage to counter the back EMF and keep the current (and hence torque) high enough for acceleration. Depending on the motor type you run into problems running at high rpm, so there is a limit to how far you can go.

The heavier and higher speed the vehicle the greater the speed/torque range required from the motor. Some time ago at a BVS Lunch we had a couple of talks giving the opposing veiws 'fixed ratio' vs 'multi ratio' (retained gearbox). At the end of the day the fixed ratio vehicle did not have sufficient torque to drive up the (fairly steep) ramp onto its trailer. Climbing a hill is functionally equivilent to accelerating, so needs to be taken into account.
Paul

http://www.compton.vispa.com/scirocco/
http://www.morini-mania.co.uk
http://www.compton.vispa.com/the_named

GregsGarage
Posts: 870
Joined: Tue Apr 01, 2008 5:27 pm
Location: Galashiels, Scottish Borders
Contact:

Postby GregsGarage » Tue Aug 19, 2008 11:27 am

hyve wrote:Jeremy, getting back to the question of clutch or not: this million or so vehicles with no clutch or gearbox; what would be a typical motor kw output to vehicle all up weight, to permit one-speed transmission?


You are actually asking 2 questions here, clutch or no clutch and multi speed gearbox or fixed speed. Regarding the question of gearboxes, on the Agni motors website FAQ section is a formula for figuring out if a fixed speed gearbox will be suitable for your requirements, click here. Find the section half way down called “Is the motor suitable for my vehicle?" This is of course for their motors but should give you somewhere to start.

Back to the clutch or no clutch discussion, after doing the Volvo conversion with no clutch when I started on the Fiat conversion, the clutch went straight into the bucket. Reverse gear in the Volvo is lively, but I am used to it. I will put some sort of simple half speed throttle on it for reverse at some point, but it's not much of a problem. so it's a little way down on the list of things to do. If someone else was driving it or I needed to do a lot of parallel parking it would move up the list. :shock:

It was suggested that a missed gear could potentially cause a problem with over reving the motor. In my case it has not been a problem. Gear changes take around 2 seconds for the gearbox syncros to match speeds. I could see that you could miss a shift if you need a quicker gear and tried to rush it. If this is your situation then keep the clutch. I find that on my 11 mile commute I only need 1 or 2 gear changes which offsets the slow gear change. 8) Sometimes the gearbox doesn't engage 2nd or reverse properly, we all have experienced that at some time. In my case the motor's own ineretia, even without the weight of a clutch and flywheel, and the throttle ramp built into the Curtis controller prevented the motor from "flying apart".

The question of clutch or no clutch will ultimately depend on what you require or expect out of your vehicle. Mine are availible for a test drive to anyone who finds themselves in the Scottish Borders, just PM me.

Oh and for a high voltage high speed fixed ratio vehicle with no clutch, how about the tesla roadster. O.K. so not exactly millions made, but its a start.
Greg Fordyce

Daewoo Matiz
http://www.evalbum.com/4191


Return to “Conversions”

Who is online

Users browsing this forum: No registered users and 9 guests