Power/weight ratios

Have you made or bought a converted vehicle if so this is for you
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Postby qdos » Thu Aug 21, 2008 11:45 am

Whilst I agree there's less gear changes necessary it's easy to damage a gearbox quickly without a clutch especially when you don't have the noise of the ICE revs to judge the changes by

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Postby Peter Eggleston » Thu Aug 21, 2008 5:19 pm

If you have a copy of Bob Brants book it explains exactly how to work out what size motor you need for whatever size weight and performance of car. All the formulae are there if you can understand them. If not, then somebody has done it before and all you need do is use the same size motor for the same size car.
Don't make the mistake of fitting a motor which is too big, by comparing it with internal combustion power ratings. You are unliklely to need a motor bigger than 25 kw for any normal road going car and even this could be too big. If your motor is too big you will waste battery power for no extra speed, and get less range.

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Postby GregsGarage » Thu Aug 21, 2008 8:11 pm

qdos wrote:Whilst I agree there's less gear changes necessary it's easy to damage a gearbox quickly without a clutch especially when you don't have the noise of the ICE revs to judge the changes by

It's easy to abuse anything, I have seen expensive sports cars with blown up clutches and gearboxes with only a few thousand miles on the clock, because someone was pretending to be Jeremy Clarkson!

In my case I don't need to try and match gearbox to motor speed and here's why. For an upshift from 2nd to 3rd I release the throttle, pull the gear lever out of 2nd and gently start to push it into 3rd. I don't have to apply much pressure, just have patience. The motor is now spinning down and when it gets to the point that the gearbox and motor speeds match it slots into 3rd. Once it's in press the throttle and away I go. I have never had a missed shift and the gear change takes around 2 seconds.

For a downshift from 3rd to 2nd, I do this when stationary or nearly stationary. At slow speeds the syncros in the gearbox is able to spin up the motor quite quickly. Since the car pulls nicely in 3rd from 5 mph I can just leave it in third until I am stopped.

I won't say that going without a clutch is always preferable, it's not. There is quite a long discussion in another thread about slow speed control problems with very high voltage DC controllers.

Another case where a clutch would be preferable is if your controller doesn't allow you to coast. I believe that this is an issue with the Zapi controllers (I am just going by what I have read and have no experience of the Zapi controllers so please correct me if I am wrong) as they offer speed control, no matter what and will switch to regen when you let off of the throttle. This will stop the motor to quickly when changing gear and so it would be better with a clutch, or some way to make them coast.

My point is that a clutchless design is possible for some conversions and offers the advantages of being simple, lighter and less to go wrong (no clutch to slip or wear out). The disadvantages are longer gear change and the requirement for some driver retraining.
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Postby geekygrilli » Thu Aug 21, 2008 8:38 pm

My clutchless gear changes are pretty much the same as above...

To go from 2nd to 3rd I slowly let off the throttle whilst pulling out of 2nd. Then, fully off the throttle, let the contactor click open, count 'One, Two' then go into third and accelerate again. It works 100% without crunches and its easy to explain to other people when the go for a drive.

I can now go 2nd - 3rd just feathering the throttle, but its not 100% crunchless at the moment!

3rd to 2nd is the same as Greg; 90% of the time its when stationary, but I can do it on the move too with no problems, if I think about it properly.

For the driving I do I think leaving the clutch in would be better. All jouneys are around North London, and there is a lot of stop-starting and a few steep inclines. I think it will make the transition from ICE to EV easier for other drivers.

I do not believe I am putting any extra stress on the 'box by being clutchless, so long as I don't crunch the gears.

I think its depends on your circumstances. Look at your regular journeys, if there are more than 10 sets of lights then i think the clutch shole be left in, if gear changes are required.

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Postby hyve » Fri Aug 22, 2008 10:58 am

Now the "clutch or not" debate has taken over my thread ! HELP !

It's staggering how hard it is to get the idea across. Yes, there are all sorts of sources of info to help work out the requirements for an individual case, but I repeat, this is NOT what I'm trying to do. Read my last post again.

The original question was: what ratio of kw to GVW is required, roughly, to enable single speed clutchless design ?

I ask for kw KNOWING VERY WELL that kw only dictates top speed and that it is torque which produces acceleration, thankyou, all those who keep telling me this.
The reason for requesting kw is that most motors seem to put only this on their plates, and since output kw derives from torque and motor rpm, I would expect the relationship to be fairly consistent.
For torque data, which I fully understand is what governs acceleration, you need motor curves. This info may be available for new motors, but plenty of conversions are done using second hand motors with no such help. This IS the "Conversions" forum.

I suspect that an electric motor does not need to be as kw-powerful as the ICE it replaces because of it's much flatter torque curve, and comments above support this. But at what level does electric motor torque become sufficient to pull away on Malcolm's 1in 3 hill, which I agree is a good figure to include.
You're right, of course Malcolm to say that required top speed has to come into the question. I feel that unless people are being particularly stingy any commuter car with a need for no more than 40-45mph should need no more than one speed anyway. Christopher's experience seems to support this, along with many others.
So my query seems to imply automatically a vehicle needing to be able at least to keep up on A-roads; ie a 60mph cruise. This implies also a top speed of 70, to enable the car to get to 60 in decent time.

There's probably here the beginnings of another debate to precede this one, over what sort of top speed is sensible to aim for with an EV built around existing technology. A very real question, I can see, but for the moment let's work on my figure above: 70mph.
It seems to me from what I've seen so far that a motor powerful enough to give that top speed ought to have enough torque to do it with just one gear, but there are other complications. Modern car bodies have quite good drag figures but have become very heavy with all the luxuries and safety gear added. It could well be that for very recent designs the motor output needed to reach my 70mph does NOT provide enough torque to pull from zero all the way to that speed; certainly not starting on a 1 in 3 hill.
This does not mean that an approximate kw:GVW ratio such as I'm seeking is not calculable, only that the potential top speed resulting will be higher. This means that, instead of allowing say 10mph cushion above the desired cruise speed we can expect that the car will accelerate briskly enough all the way to maximum motor rpm and design that to coincide with our cruise speed, because aerodynamic drag is not increasing as rapidly as with some earlier, lighter vehicle. The lighter requires less torque, but it's shape needs more kw. Therefore it has to have that top speed margin.

Being an approximate ratio only means there have to be some assumptions. Few will want to convert some older car, due to rust questions - even though the lower mass is significant. For example, a VW Polo weighs around 1170kg from memory: my Panda 700, or a BMC Mini 600. But is that saving enough to justify all the rebuilding first needed ?
Another assumption is that almost nobody will be converting say, a Mondeo or other midsized (or larger !) model, simply because the cost of batteries to haul the extra load will be out of reach.
So the objective is an approximate guide for the typical expected conversion: a small car of around 1000-1100kg kerb weight, loaded to it's normal GVW and typically needing maybe 25 kw to hold around 65mph ?

I very much suspect that such a load will call for such a powerful motor to cope with one speed plus 1 in 3 hill starting that a gearbox will be inevitably more efficient.
Maybe what I'm looking for instead of a kw:GVW ratio is a simple maximum GVW for my typical example, before a gearbox becomes sensible ! But have a look at the discussion which Jeremy has started in the "General" forum on multi- motors and controllers. The kind of motor power:weight:cost figures we're looking at there are starting to make a high power/one speed option sound more attractive.

As a final point here, all the graphs I've seen for motors of all sorts so far indicate that electric motors are most efficient operating at a lot less than full power. It therefore seems to me that it makes sense for the motor to have a very large surplus at cruise speed.
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Postby MalcolmB » Fri Aug 22, 2008 12:11 pm

I think the problem is with your requirement for a single-speed conversion.

It would be much easier to give a single bhp per tonne figure for a multispeed conversion, although I don't have enough experience to suggest one.

The problem is when you go to single speed you have to make a big compromise. To achieve your top speed of 70 mph, taking into account the useful rpm limit of the motor, you will probably need a reduction of around 5:1. This is equivalent to setting off in third gear in the average car. This in turn means that the limiting factor is starting torque. To provide sufficient starting torque you need an oversized motor and you need to dump a lot of current into it. If you can achieve the required starting torque and the voltage of your system is high enough (say 100 volts plus for a DC motor) then achieving 70 mph should be no problem at all, since your system is overpowered anyway.

The picture changes if you keep the gearbox, since starting torque is no longer the limiting factor and you can use a much smaller motor. In this case it makes sense to ask for a rough kW/tonne guide figure.

In my case I'm choosing to use two motors because I got them cheap and because I don't have the machining skills or resources to mate a single motor to a gearbox. If I did use a gearbox I would probably only need one motor, but since I'm not I need to use two motors simply to get enough low-end torque to pull away on a hill. I reckon it's a fair trade-off though, since the second motor weighs only a little more than the gearbox, and it means I should have a good reserve of torque for acceleration at higher speeds.

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Postby EVguru » Fri Aug 22, 2008 1:43 pm

The original question was: what ratio of kw to GVW is required, roughly, to enable single speed clutchless design ?

The answer is;

It depends!

Sorry, that's just how it is.

The reason for requesting kw is that most motors seem to put only this on their plates, and since output kw derives from torque and motor rpm, I would expect the relationship to be fairly consistent.

They usually also put the voltage and current that go with the power and rpm rating.

I wrote in the 'Finished' thread;

Motors are just energy conversion devices, you can pick all sorts of operating points.

If a series motor was rated at say 10hp at 3000 rpm on 72volt and 130amp (approx 80% efficiency), then the rpm per volt at that current would be 41.666.

You could run it at 120volt and at the same torque output it would rev to 5000rpm (41.666 * 120).

The torque output at 10hp would be 10 * 5252 / 3000 = 17.5 ft/lb, so the power at 120 volt and 130 amp (same torque) would be 17.5 * 5000 / 5252 = 16.66hp.

The heat produced in the motor is mostly due to copper losses (I2R), which are the same as before, but you may well have more cooling from an internal fan at higher rpm.

You can keep increasing the voltage until the on-load revs are too high (you have to limit no load revs with a series motor whatever the voltage).

The common second hand motor suitable for an EV conversion is a series wound type. They have no fixed relationship between voltage and rpm, at no load they can rev to destruction on quite low voltages. The relationship between current and torque has a major exponential component.

The closest I can come to an answer for you, is to say that you need a series motor of at least 8" diameter and about 18" long to achieve your required speed from a vehicle of that weight. Given that you want to go with a single ratio, you need a bigger motor because you'll be needing it to produce more torque.

My VW Scirocco could just about meet your requirements. It weighed 1200Kg with 320Kg of battery. The motor was a 9" diameter GE motor rated at 23hp, 3285 rpm @ 99.4 volt and 201 amp continuous. Quoted weight was 170lb. When driven from a 192 volt battery via a 680 amp controller, it COULD be driven in 3rd gear from standstill to 70mph+ (self imposed rev limit). Compared to using 2nd gear up to 40mph, the acceleration suffered and the motor and controller ran hotter. The efficiency was less, but I can't tell you by how much. I coud keep up with pretty much all trafic even when limiting the battery draw to 40Kw peak. The secret was the ammount of area under the torquw curve I could get with the high voltage.

At lower voltages and/or with a heavier vehicle it becomes much harder to do a single ratio drive , particularly with a series wound motor. The Enfield car with a weight of around 1000Kg and running at 72 volt had a top speed of about 50mph. The motor was considerably larger and heavier than the one in the Scirocco.


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Postby GregsGarage » Fri Aug 22, 2008 3:06 pm

Good explanation Paul.

The closest I have seen to a KW per kg figure is Cedrics formula for his motors. He multiplies vehicle weight in KG by top speed in KPH and if the result is 30,000 or less then 1 motor is o.k. The motor is rated at 9KW so 1KW=3333. divide your result by this amount to get your KW figure.

The problem with this is that you would be foolish to commit yourself to building a EV on this basis alone. With only a single speed you need to get it right because it is hard to change if wrong. You really need to calculate torque required at the wheel and then look at motor specs. A series wound motor will deliver the same torque at 12 volts as it will at 120 volts for a given current, the difference is that at 120 volts it will be spinning a lot faster. So when you know what current you require to generate the torque to pull away you then need to figure how much voltage will be required to get the motor spinning to the required rpm for your top speed. Once you have those figures you have your KW figure, volts x amps. :D

Regarding name plate ratings on motors, they can have more to do with the original customers requirements than the capabilities of the motor. An example was given by Paul that the early ADC motors had a 72v plate. When they were asked for a 144 volt motor they slapped a 144 volt plate on the motor. Thats why size does matter in a motor. If you are using an old motor with no published specs, find as similar a motor as you can with published specs and use that, it should be close enough.
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Postby MalcolmB » Fri Aug 22, 2008 5:24 pm

Talking about kW, I just came across this very neat performance calculator. The clever part is that you can enter your own torque curve, or known torque points. http://vlsicad.ucsd.edu/~sharma/Potpourri/perf_est.html

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Postby hyve » Sat Aug 23, 2008 9:10 am

Well I do appreciate all the time everyone has spent on this. A quick response to Greg: I most certainly would NOT be committing myself to a conversion on the basis of such a figure (kw:gvw) As I said several times, the idea was just to get a starting point, etc etc etc.

Thanks for going over your earlier sums again, Paul. I don't quite see how I can calculate torque from volts & amps, but the reminder that a series motor does NOT have a fixed volt/rpm relationship was valuable.
I recognised that different motor configurations would give different results, though felt that generally they all have torque curves of similar shape. Series motors are a bit special though and since their starting torque is so good, would be the best candidate for single-speed at first glance.
So trying to arrive at a general minimum power/weight ratio, even for a set top speed, seems to be asking too much. I concede.

Malcolm questioned my rationale for attempting one for a single speed clutchless design, then proceeded to answer that himself ! Getting rid of as much as possible of the mechanical weight and complexity associated with engines which cannot set off from zero rpm was the objective.
I felt this was worthwile because so much extra mass has to be added to an EV instead, in the form of batteries. It's not easy though to guage the difference between savings that way and having to use a larger motor - or several - plus the extra battery capacity required.
Unfortunately with one motor only a car still needs a differential and as Paul says, probably a 5:1 step down. This section of the modern fwd transmission is a large chunk of it's mass, so there is not so much to be saved there after all. Adding a clutch puts it back up of course, but I'm not setting off down that road again !!!

It strikes me I could do a huge lot of sums around all these factors and get well tied in mathematical knots. I try to avoid this; I'm not good at it, but it's clear I need to think some more. Maths is the way to resolve this.

Thanks everybody.
Peter Ph

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