Power/weight ratios

Have you made or bought a converted vehicle if so this is for you
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Postby MaryRCrumpton » Thu Nov 19, 2009 3:04 pm

qdos wrote:
Been busy over the summer building some vehicles ...

Hello qdos,

good to see you.

This thread is probably the wrong place for it, but how about a post from you with some pics and descriptions of what you've been working on this summer?

You're a bit far away for some of us to travel to. :wink:

I had a look at your website, but it didn't look like you had any of your summer work on there, unless I missed it? If I was just being clueless, then do post a link to where the info is.

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power:weight ratios

Postby hyve » Sat Nov 21, 2009 5:46 pm

EVguru wrote:
my original query was as to some sort of rule of thumb for motor power (meaning torque of course) to vehicle all up weight which will allow single speed transmission.

I'm very sorry, you can't have one.

Cedric can give you an estimate for his motor because he knows all the performance curves, but your whole approach seems to be based around finding a surplus motor and trying to determine if it might do the job.

With motor torque rarely being quoted in specifications but knowing that total output in kw is in fact a function of torque and rpm it did not seem unreasonable to me to ask for a rough ratio of kw to kgs to permit the use of a nice simple single speed conversion

Unfortunately it IS unreasonable, there are just too many factors to take into account. Two motors with the same Kw ratings can have such wildly different characteristics that one will do the job just fine, whilst the other will burn out almost immediately. Maximum safe rpm, maximum safe voltage, the ability to handle overload currents, cooling, thermal mass, etc. are all factors that will affect the suitability of a motor for a particular application.

If you go through the archives of the Electric Vehicle Discussion email list you will find countless stories of burnt out motors, controllers and failed batteries. Many of these failures will have been becuase someone followed a 'rule of thumb' without understanding why that rule did not apply in their particular case.

Let me ask you, 'how many horsepower (Internal combustion engine) do I need for a 1000Kg vehicle?'
Many thanks for all the foregoing. your guess as to my motivation( trying to see whether a surplus motor might be suitable) probably sums it up,but there was no need for the insulting final question.nobody would try to make an ICE drive a car without some sort of transmission; I expect you were trying to make me look foolish by asking that. of course Drag racers have been getting away with it for decades by making clutch slip and wheelspin substitute for a starting gear and that is an extreme example but your question implies that I ought to be able to produce a reasonable answer simply because I ask for something similar for an electric motor.this I feel is an insult and I'm not prepared to spend further time on this argument if it is going to descend into trading abuse.For those who think there ought to be an answer I used to own an old Jeep from the '50's which made such good torque that it almost could get away without gears. this made around 50 Kw and was moving 1250Kg but I would not use this for a rule of thumb because it's final drive ratio was such that top speed was quite restricted.I understand that similar considerations will apply to electric motors, depending on age too because just as it would be quite easy to find a modern ICE making 50Kw which would have no chance of shifting a tonne without a gearbox, I expect modern designs of electric motor equally have been tuned to make more revs.The argument that giving a 'rule of thumb' can lead to people making serious mistakes is I feel a red herring, as the whole object was to establish a simplifying notion for those who are prepared to investigate thoroughly what they are about to do and almost inevitably would be applied to older series wound DC motors,being the most commonly available.But having had lengthy explanations now as to why my request is unreasonable I shall herewith desist and recommend any who have followed the argument thus far to get themselves an outside education in electric motor design and application rather than hope that any of the self-proclaimed experts in this organisation will assist them much in that direction. You cannot do it yourself seems to be the attitude.It reminds me so much of the attitude of older journeymen when I was an aprentice metalworker 50 years ago.DO NOT ASK DIFFICULT QUESTIONS! You are not allowed to try to make short-cuts or do anything which might show up their ignorance. Good luck!
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Postby EVguru » Sat Nov 21, 2009 9:15 pm

I put the question simply to make you consider the validity of your original question.

I said nothing in my question about a single ratio transmission.

If the vehicle was a cricket pitch roller, then an engine of 3hp might suffice. A forklift might use 20hp, whilst a town car with a top speed barely above 30mph might get away with only 10hp. A lightweight sportscar would get interesting at around 60hp and a Le Mans prototype might be 500hp.

The point is that that it is not only the weight, but the top speed and acceleration that determine the power requirements for an ICE vehicle. The power requirements for an electric vehicle are different, both because of the different delivery curves and becuase of the way that motor/drive systems are rated, but they are effected by the same factors.

The electric race car I used to be involved with weighed about 1.5 Ton and used a motor a motor rated at something like 10hp (continuous). One of our rival teams with the same Porsce 914 base used a motor rated at 20hp (continuous). Their top speed was something like 90mph, whilst ours was 135mph. Their motor ran pretty hot (you could smell the varnish), whilst ours ran only warm (you could keep you hand on the casing indefinitely). They used 2 and sometimes 3 gears (a disadvantage as the shift on a 914 is truly awful) whilst we could get away with a single ratio.

If you only knew the stated ratings, which motor would you think would work best?

Their motor was an ADC FB1-4001 and was pretty much identical to our ADC FB1-4001. The only real difference between them was the spec. plate riveted to the side. Ours was rated at 72volt and theirs at 144volt.

They were running a 144volt battery pack, whilst we were at 288volt, although we limited the voltage at the motor terminals to not much more than 200. What our rivals really struggled to understand was how we were using about 50% lower peak power (motor input) to go considerably faster.

You expect me to come up with a rule of thumb that will determine the motor power (as displayed on the spec. plate) needed to propel a given weight vehicle at an unknown speed and with an unknown hillclimbing/acceleration ability.

I'm not guarding secret knowledge or deliberately trying to obstruct you, I simply don't know how to give you the answer you want, and I don't think anyone else does.

It reminds me so much of the attitude of older journeymen when I was an aprentice metalworker 50 years ago.DO NOT ASK DIFFICULT QUESTIONS!

Don't ask complicated questions unless you're prepared to accept complicated answers, or get questions back in return.


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Postby GregsGarage » Sun Nov 22, 2009 4:09 pm

You may actually be able to get a useful idea of a motors suitability from it's rating plate. Paul's example of 2 identical motors with a very different horsepower rating gives the clue. He has shown how doubling the voltage doubles the horsepower of the motor. On a series DC motor, amps=torque and volts=rpm. So by doubling the voltage you double the rpm. The amount of amps a motor can handle is determined by how much heat it can dissipate. So the increased voltage gives double the rpm but no increase in torque. If, on the nameplate, you have voltage, amps, rpm and horsepower it should then be possible to calculate the torque the motor is generating at that point. This then gives you a fixed point on a graph of torque x amps. You can use one other fixed point, that is 0 amps = 0 torque, and hey presto you have a crude graph of the torque curve of the motor. This works because a series dc motor has a nearly linear relationship to torque vs. amps. It's not perfect, but will give you a good approximation of torque. Then you can get an idea of how much voltage you will require from rpm you need the motor to achieve.

http://www.go-ev.com/ has performance graphs of their motors on their website. All the graphs are for 72v, just adjust the rpm figures for different voltages, double them for 144v, etc. They could be useful if you find a similair motor and the rating plate closely matches a point on one of their graphs.

Edit. Torque can be calculated from rpm and horsepower. Then you know how many amps for that torque figure.
Last edited by GregsGarage on Mon Nov 23, 2009 7:59 pm, edited 1 time in total.
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power/weight ratios

Postby hyve » Mon Nov 23, 2009 3:05 pm

Greg; many thanks for all that, which seems to answer my question more than adequately, and my apologies for being abusive to Paul and anyone else whom I might have offended with my last post. As mentioned I have been suffering from health issues this year and such things do make one a bit shirty at times. sorry everybody.
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Postby qdos » Tue Nov 24, 2009 5:21 pm

Righty ho here's a pic of what we've been working on all these months


Yes yes yes it's not finished but it's almost there and we thought folk would like to see it

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