How much (electric) power is needed?

Posted on January 8th, 2008 in Aerodynamics,Economy,electric,Opinion,Power by Julian Edgar

pug-trip.jpgA while ago I attended an electric car show held in Sydney. I made the 2000-odd kilometre trip in my Peugeot 405 diesel, a car that, incidentally, gained high Fives (in litres/100km) for the trip.

At the show I briefly sampled three of the home-converted electric cars – a very interesting experience. And on the long drive home to the Gold Coast, I had plenty of time to reflect on these cars.

The electric cars I drove each retained the original gearbox: the electric motor was bolted up to the ‘box and the ratios could be selected by the driver. Typically, the cars were started off in second gear and then third and fourth and fifth gears were used as appropriate. (I used first gear off the line and felt an immediate gain in starting performance.)

But none of the cars I drove had performance that came close to conventional petrol engine (or even commercial hybrids). Even when the electric motor was rated at a higher power than the original engine, the massive weight of batteries substantially dulled the resulting power/weight ratio.

Putting in a more powerful electric motor (or running two electric motors) would of course help solve that, but at the expense of greater electrical power consumption that in turn would need either more batteries or result in a shorter range (and none of the ranges were very good to start with!). However, all the cars could easily exceed the 110 km/h open-road speed limit.

Clearly, what is needed is an electric motor that has only enough power to do the job – but no more.

Performance nuts always talk acceleration times, but acceleration is enormously power-hungry. So what, in the real world, is sufficient power?

One answer is the power required to drive along a country road at the speed limit. In addition to the power required to overcome aero drag, you also require the additional power to climb hills.

Thinking along these lines as I drove open road kilometre after kilometre, I realised that the 1.9 litre diesel in the Peugeot was probably very close to the requirement. Oh no, not its peak power output, but instead the full-throttle power it was developing at about 2000 – 2500 rpm.

The Peugeot runs these revs in fifth gear at around the open road speed limit, and I found in the 1100-odd kg car that nearly every open-road hill could be climbed without a down-change (and so using higher revs). There was also enough power to battle headwinds, overtake slower cars, etc. No, you’re right – holding it in fifth and just flooring it didn’t make the Pug a performance car (not by a helluva long way!) but it was just competent.

And having dyno’d the Peugeot, I can tell you exactly how much power is available at the wheels at 2000 – 2500 rpm. The answer is about 30kW.

Now it starts getting tricky, but in round figures, I therefore reckon an at-the-wheels power/weight ratio of about 27 kW/tonne would be sufficient, if the powertrain was geared so that maximum electric motor speed was reached at the open-road speed limit.

Running an electric motor developing an at-the-wheels 27 kW/tonne and gearing it so that peak electric motor rpm occurred at 110 km/h in top gear would therefore give sufficient open-road performance. And, with the ability to then lower the other gear ratios, torque multiplication would be increased at all lower speeds.

This is not some profound breakthrough, but it does show two things.

If you’re prepared to live with a maximum speed of 110 km/h, and you have an aerodynamically slippery car, an at-the-flywheel power of (say) 35kW/tonne is probably enough for the real world.

And when you’re working with such low power outputs, and a motor that’s most efficient at its rated (high) speed, gearing is very important.

35 Responses to 'How much (electric) power is needed?'

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  1. Diego Valdez said,

    on January 18th, 2008 at 8:06 pm

    Hi, Julian. You are making a common mistake in calculating the power requirement for an electric car. If you were to apply the same math to an internal combustion (IC), be it petrol or diesel car, you would realise that they are completely inadequate for driving. That is due to IC low efficiency, which is around 25%, and the torque is only available at the certain max revs an engine is capable of, before it starts dropping the power as driver increases the speed. The fact is that we drive our IC cars with barely 10-12 kWs , regardless of their max “power” rating, in the city. And that is on 70-90kmh. So, the actual power of an electric motor required for a city car is around 25 kW. That would match the power of a 1600 ccm (like Hyundai Ecel for example) engine.

    There was a small company in Sydney several years ago, Ida Techologies, if I remember correctly. They were designing conversion kits for your car or to your desire. I had an opportunity to see one of their cars, it was Hyundai Excel, with 35 kW brushless electric motor and with those chinese lithium batteries that would propell the little green bastard to 100 km in less than 8 seconds. That is because the electric motor is capable of delivering the max torque from zero to max revs per minute. IC cars can’t. And brushless electric motor is 95%+ efficient, so you have all the power available all the time, unlike an ordinary petrol car. By the way, the max speed they claimed was 160 km which the batteries would deliver for over an hour. I wish I kept their telephone because now I would love to convert my car and take advantage of CO2 buy-back scheme (they were doing that back then) and thus reduce the cost of conversion for about 40% from the full cost of the kit. If you know or anyone knows about them, I’d be grateful.


  2. Julian Edgar said,

    on January 18th, 2008 at 9:04 pm

    1) Comparing IC vs electric motor efficiencies is irrelevent when we are not talking fuel consumption but instead output power.

    2) I said what power the Pug needed on the highway: if you want to use a different driving requirement that’s fine, but it changes the discussion. kW are kW, however they’re produced.

    3) Sure, acceleration performance (especially off the line) will be better in an electric car than one of the same peak power that is powered by an IC engine, but that doesn’t change power required for hill-climbing on the open road, which is the greatest real world power requirement.

    4) The fact that a brushless motor might be 95 per cent efficient has nothing to do with its output power curve – it simply means you get more out of it for the power put into it.

    5) I stand by my major point that I think a power/weight ratio of about 35 kW/tonne is required for adequate real world performance.

    6) I don’t believe for one moment the claimed performance figures of the car – especially the 160 km/h for over 1 hour. I wish!

  3. Matt said,

    on January 20th, 2008 at 12:31 am

    If you want to look at some performance EVs there is a reasonably sized sub-culture in the US for electric drag racers. You can view some of the record holders they are avalable at 27kW seems pretty high, i have been told highway speeds take 10-15kW continous and solar cars manage 100km/h at about 1.5kW! There is a motor application guide for DC motors here: but generally speaking it isn’t the motor that limits performance its the batteries. Also with your point about bigger motors, a bigger motor in an EV will only draw slightly more power than a smaller one just because there is more wire and therefore resistance but it wouldn’t be very substantial. The efficiency isn’t adversely affected very much by a bigger motor. Your power consumption is actually prescribed very accurately by the controller; it decides how many amps go to the motor irrespective of how big it is. The motor rating only really decides how much power the motor can sustain without burning out. Anyway hope that made sense.

  4. Julian Edgar said,

    on January 20th, 2008 at 8:07 am

    As described in the post, 27 kW/tonne was based on the measured power that the Pug ACTUALLY NEEDS to perform adequately on the open road (that is, climb hills at the open road speed limit) – it’s not some made up figure… The only way that kW/tonne figure could be reduced is to lower aero drag very substantially.

    If you don’t have that power available, you won’t be able to climb hills at the open road speed limit, irrespective of available battery power.

  5. Wayne said,

    on January 21st, 2008 at 6:50 pm

    At the risk of shot down like the others, i dont believe power is relevant at all. The requirement to maintain speed up a fixed incline is available torque. You are probably correct in the case of your Pug with your guess, however changes in gearing could change that (for better or worse. More important though is the torque rating of the motor in comparison to the gearbox and final drive. If you have enough torque you will maintain speed, excess torque you will accelerate, less torque you will decelerate. Not Dependant on power.

  6. Julian Edgar said,

    on January 21st, 2008 at 6:57 pm

    Perhaps have a look at the series we’ve recently repeated in AutoSpeed on torque and power.

    If you increase torque at a given motor rpm, you have increased its power. You can easily increase torque at the wheels – just by changing gearing. If only torque was important, 1kW and incredibly low gearing would be all anyone ever needed, no?

    In fact, the time it takes to lift a given mass a given vertical distance (your example) is easily quantified as the required power. You can’t work out how much torque it takes, because torque alone is irrelevent – you need both torque and speed, so giving power.

  7. Dennis said,

    on February 25th, 2008 at 4:45 pm

    I am going to say that your assumption of 35 KW / tonne is wrong. You made the error in using the term electric motor without any regard to what TYPE of DC electric motor (is it series-wound, shunt-wound, permanent magnet, or compound wound???) nor did you go into how it IS CONTROLLED. Is it set up for constant torque or constant horespower?

    You do realize that high horespower DC electric motors are not directly connected to the batteries but through a controller to: limit current as high horsepower motors have very low electrical resistance, regulate speed under varying loads like hill climbing or going on level ground, and provide speed control by some input like a variable resistor (the throttle). Also did you discuss the voltage level?

    Stick with gas engines which is what you know best………

    I’ll give you a little crash course in DC motor control:

  8. Julian Edgar said,

    on February 25th, 2008 at 7:21 pm

    You haven’t said why the 35 kW/tonne figure is wrong.

    That figure is based on the measured power needed in a real world driving situation, ie climbing hills at the Australian country road speed limit, in a car with a reasonably good aero and frictional drag.

    So why exactly is it wrong, whether applied to electric motors (series wound DC, compound wound DC, shunt wound DC, 3 phase AC, [etc, etc] controlled in whatever way you like) or for that matter, any other form of propulsion?

  9. Dennis said,

    on February 26th, 2008 at 2:55 am

    Because they all have vastly different torque curves from each other. Like in your combustion engine world, you would not say that a turbine jet engine is the same as a diesel engine or a gasoline engine because obviously they have totally different torque curves and are meant for specific applications. Likewise a series wound motor is extremely high in torque, but then it falls off very quickly as it speeds up were as a shunt wound motor torque curve does not.

    You also need to know that all electric motors are rated on continuous horespower rather than peak. For example my series-wound motor for my go kart is 4 horespower on the nameplate, but I can push up to 30 horespower for short durations (5 minutes). The beauty of electric motors is they are like sprint runners in that they can dish lots of horespower momentarily under severe loads as longs as they are provided with rest to cool down from the overload.

    Then there is the controller side that can control the torque, speed, and stall torque. Say for example you wanted a shunt wound motor to turn 3000 RPM under ANY load conditions (up to a limit of course). Well guess what YOU CAN have just that by using whats called feedback type controller. Feedback in this case is a tachometer sensor or current sensor that can detect changes from loads applied to the motor and then send the signal to the controller to compensate for the changes giving the motor superior speed regulation under varying loads like hill climbing.

    AC motors are recently able to perform DC motor like functions like speed control and constant torque or constant horespower that were not possible 40 years ago thanks to the flux vector controller. AC is far superior in reliability, super high RPMs, horespower density, and efficiency. Tesla uses a 3-phase AC motor controlled by a flux vector controller. On the motor you can see the tachometer that is there to give the motor excellent speed regulation under various loads.

    Here is a good picture of the tachometer:

  10. Julian Edgar said,

    on February 26th, 2008 at 9:55 am

    So what? You still have not addressed the point that I believe that you need a power/weight ratio of 35kW/tonne to achieve competent open-road performance.

    The torque curve, controller, etc, are completely irrelevant. (They might be relevant if we were discussing accelerative performance.)
    Whether the electric motor develops that as its peak short-term power or long term power is also irrelevant – I am talking about the ACTUAL power needed to decently propel the car, with the test situation being as described.

    It doesn’t matter if the engine is steam, turbine, electric, combustion, etc – the required power in that situation remains the same.

  11. on February 26th, 2008 at 10:50 pm

    Hello there Julian we have chatted in the distant past but i’m shure it was to long ago for you to remember, it was about electronic ignition controler you were involved with, but i digress sorry to the matter at hand.

    I think the discussion so far your all discussing the problem as if you can have too powerful a motor! Within common sence anyhow!

    500kw might be a bit to much but to prove a point you can get a motor off the shelf and mod it to produce these sort of outputs granted for minutes/tens of seconds at a time max, info’s on the net from the states . The whole point is these motor’s is normaly rated at aprox 10 to 25kw as drive motors for fork lifts and arnt to efficient (brushed with field coils) but the 500kw motor isn’t significantly heavyer than the original and there a common swap into home converted cars and the effeciancy doesnt change significantly. So even with this primitive stone age electric motor you can smash your diff if you like you just need to power it !

    If you use a figure of 35kw/tonne continuous i think you’ll be very happy with the performance of your hypothetical car as most modern brushless motors with high tempurature rare earth magnets can be pushed considerably above there rated power outputs for minutes at a time 300% should be no problem with good cooling for 3 to 4 minutes being overdriven.

    The next issue is power to weight of the motor brushless motors can far exceed that of old field coil type motors (with a few exceptions perhaps) there efficiancys are commonly at or above 90% some as high as 98% (hub motors of low output 1.5 to 9 kw)
    With brushess motors as small as a can of coke exceeding 10kw granted at very high rpm prox 50000rpm and needing water cooling weighing in at less than a kilogram!

    Very high torque can be provided with large diameter motors sutable for direct drive to the wheels further reducing drag though gearbox loads (up to 30% loss as i’m sure you know) yet these motors are still capable of high rpms (not limited by brushes)

    If your battery is an issue due to cost or limit in capacity for long drives why not drive a similar motor as a generator with a high efficancy lpg or diesel and reduce you battery size yes i know hybrid.

    Hey I’m keen to do all this from home, and soon thinking i’ll put it in a early 80’s hilux lowrider just to upset the electric dogooders and the hot car seen as well! I’m planning on making my own morots from scratch (yes i’m aware of all the issues and have a local motor builder to assist) I’m going to make my own motor controler and i’m currently looking for a sutable petrol motor to convert to lpg I’m currently thinking along the lines of 660cc turbocharged diahatsu dohc etc do you know if there is an aluminium block equivelent about ex japan?

  12. Dennis said,

    on February 27th, 2008 at 4:38 pm


    You do realize that a 3-phase induction motor does not use brushes either…. It also does not require permanent magnets like your motor does. I’d say go with the induction motor for far superior reliability to either permanent magnet brushless motors or brushed motors. They already have been the dominant mechanical power plant in industry for 80+ years with DC motors only used for speed control applications (now AC motors can do what DC motors can today due to advancement in electronics)…


    You cannot deny that the majority of electric motors DO have the ability to produce peak horespower over what is stated on their nameplate and therefore your argument is just irrelevant. I know also in hydraulics that when a 3-phase motor is replaced by a diesel engine, that the diesel engine needs to be about 2.5 times the horespower because combustion engines cannot “over horespower” when more load is applied to them so they will stall if the horespower requirement exceeds their rating.

  13. Julian Edgar said,

    on February 27th, 2008 at 6:29 pm

    I don’t know what you are reading, but I never said anything at all about nameplate ratings versus any other ratings!

    What I said was that an at-the-wheels power figure of 27kW/tonne (say 35kW / tonne in front of any gearbox) is the minimum needed to give adequate performance in the most demanding real world situation – which is hill climbing at the open road speed limit.

    To make it really straightforward, if you have a 1 tonne car on the dyno, I think you need to be able to measure at least 27kW at the wheels at 110 km/h. That has absolutely zilch to do with nameplate ratings, type of electric motor, etc, etc.

    Nothing that you have said has disproved this.

    I also wrote about gearing and electric motor speeds, and I am happy to acknowledge that these statements may not be true with all types of motors and all types of controllers (but the fact remains that the electric motor must be able to develop the above described power at the open road speed limit – so gearing will often be relevant to the outcome.)

  14. James said,

    on February 29th, 2008 at 2:38 pm

    Since this is a discussion about power and stuff, I thought you may find this article from interesting. Also, I didn’t know where else to post it 🙂

    “Horsepower vs. mpg A timeline of changes in automotive fuel economy”

  15. Julian Edgar said,

    on February 29th, 2008 at 2:53 pm

    Yes an interesting column.

  16. Malcolm Scott said,

    on March 12th, 2008 at 12:27 pm

    By real world observation:
    GM EV-1, 102 Kw, 1322 Kg, = 77 Kw/tonne. 0 – 60 mph in high 7s. Considered to be a lively performer.
    Toyota RAV4 EV, 50 Kw, 1594 Kg, = 31 Kw/tonne. 0- 60 mph in 18 sec. Considered to be a leisurely performer.
    Honda EV Plus, 49 Kw, 1636 Kg, = 30 Kw/tonne. 0-60 mph in 17.7 sec
    Honda FCX Clarity, 100 Kw, 1628 Kg, = 61 Kw/tonne. 0-60 mph in low 7s.

    37 Kw/tonne mechanical power seems like a reasonable design target.

    Looking forward to the road test discussions on the capital cost / design / supply constraints on why BEV provide modest outcomes, especially in Australia.

  17. on March 12th, 2008 at 9:14 pm


    On a side note to this discusion I noticed you posted in another area you managed to aquire a prius front half cut with battery, who did you get it from and what kind of money are we talking ? Is it a practical way to access cheap batterys for my own hybrid?

  18. Julian Edgar said,

    on March 13th, 2008 at 7:43 am

    I have bought in the past an NHW10 Prius half cut plus battery, and a NHW20 HV battery pack. I wouldn’t bother with the NHW10 – batteries too old – but the current model stuff is a decent way of getting batteries. The NHW20 pack was bought on Australian eBay. This isn’t a ‘for sale’ section, but I have 15 or 16 NHW20 battery sticks from that pack for sale – I have decided not to use them.

  19. Dennis said,

    on March 13th, 2008 at 4:11 pm

    You know I still disagree with your horespower assessment until you study up on motor control and the different types of electric motors, Julian.

    I know this much though, a petrol car with the same weight (add leads weights to make it equal to battery weight of electric), horespower, gear ratios, drag coefficient, and same tires (basically a carbon copy of two exact cars with the same specs). would lose badly against an electric car with the exact same specs in both acceleration and top end. That’s a fact.

    I have already tested this with my electric go kart that weighs in at a hefty 650 pounds if I include my weight and I can put Briggs and Stratton Animal engine powered go karts to shame that are LIGHTER than me. Now just imagine how bad I would destroy them in a race if the weighed exactly the same as my go kart!!

    Oh yeah I am not lying about having an electric go kart. My Mypace has pictures when I was 48 volts. Now I am at 72 volts and seriously kill the gassers now.

  20. Julian Edgar said,

    on March 13th, 2008 at 4:47 pm

    Dennis, do you truly believe that petrol engine kilowatts are different from electric motor kilowatts, and both are different from the kilowatts required to propel a car of a given drag coefficient up a given grade (which all along has been my point)? If you think that kilowatts vary depending on the motive power, that’s exactly like saying litres of milk are different volumes to litres of water. Or a tonne of feathers weighs more than a tonne of lead.

    Perhaps start at the definition of a ‘watt’?

    You’re constantly confusing power at one speed with power achieved across a rev range, where a high average power will result in much better acceleration than an engine with low average power.

    To (again) make it really straightforward, if you have a 1 tonne car on the dyno, I think you need to be able to measure at least 27kW at the wheels at 110 km/h.

    That has absolutely zilch to do with electric motor nameplate ratings, type of electric motor, control gear, theory of electric motor design – or whether those kilowatts are provided by horses, a gas turbine, a Stirling engine, the wind, solar power, burning cane toads, etc, etc.

    27kW/tonne at the wheels – irrespective of the means by which it is developed – will propel a given car up a given incline at the given speed. Any less than that will not do so.

    In the case of the example I gave in the original blog, I consider that to be the minimum practical power for normal country road hillclimbing at the Australian country road speed limit.

  21. on March 13th, 2008 at 10:10 pm

    Sorry to take sides Dennis but Julian is on the right track with the power needed for what he describes, But why limit yourself to such a low figure when and electric motor can provide much more than the average petrol car motor can provide at least on a short term basis?

    You mentioned in an earlyer post about using a three phase induction motor there rating’s are generaly easy to exceed by at least 100% for considerable time ie 30 min as the power is increased the time avalable till destruction drops, 1000% increase time in seconds!

    Yep your right there great motors but one big problem there just way to heavy n big! Look at a induction motor install on a performance car if it has a good power to weight ratio the motor is as big as a small ic motor and weight to match ie motor 45kw induction continus rating, at least 60kg granted a lot lighter than a 150kw V eight but still heavy and yes no G box I know but still very heavy not a good thing for an electric car!

    Brushless motor is just a three phase induction motor with out the induction needed as there are are very powerful (n low weight) magnets instead, A 45kw continus rated very high torque brushless motor (no G box) weight under 10kg! very relitively indestructable and easly over driven up to 300% (within thermal/time limits)

    The only limmit to having a 300kw continus motor in a electric car is the batery powering it, it will operate at a resonable efficiency at 35kw to make Julian happy and weigh about the same as the 45kw induction motor!

  22. Dennis said,

    on March 14th, 2008 at 6:28 am

    “You’re constantly confusing power at one speed with power achieved across a rev range, where a high average power will result in much better acceleration than an engine with low average power.”

    FINALLY, Julian, you understand. Guess what, it is the AVERAGE overall power over the rev range that an electric motor destroys a gas engine. Why not have a look at a flux vector controlled 3-phase motor’s horespower curve then maybe you will understand how bad it destroys a gas engine in the overall horespower over the entire rev range.

    “Yep your right there great motors but one big problem there just way to heavy n big! Look at a induction motor install on a performance car if it has a good power to weight ratio the motor is as big as a small ic motor and weight to match ie motor 45kw induction continuous rating, at least 60kg granted a lot lighter than a 150kw V eight but still heavy and yes no G box I know but still very heavy not a good thing for an electric car!”

    Adam, one thing you fail to realize is you are talking about an INDUSTRIAL 60/50 HERTZ 3-phase motor with many poles designed to be for STATIONARY applications, not moving applications. A HIGH FREQUENCY designed 3-phase motor with 4 poles or 2 poles controlled by a variable frequency drive called a flux vector controller is much lighter than a 60/50 hertz motor. Tesla Roadster uses a 400 hertz designed 3-phase motor with 4 poles and it weighs only 70 pounds, but can dish out 185,008 watts. Oh yeah you do realize an induction motor is brushless as well.. Maybe you do not know what induction means…or maybe you never looked inside a 3-phase motor.

    You can substantially push well beyond the horespower limits as well in an induction motor by water cooling it by way of its outer frame. They have such a motor frame setup called “Totally Enclosed Water Cooled” or TEWC.

  23. Julian Edgar said,

    on March 14th, 2008 at 6:59 am


    Read what I have written. I have ALWAYS been talking about power at one speed, ie hillclimbing ability at the country road speed limit.

    I have NEVER mentioned acceleration performance, because that was not the point the blog was about!

    I can’t help it if you read my blog, decided I was talking about something else, and then ran with that.

  24. on March 27th, 2008 at 12:04 pm

    Dennis , 2.2 pounds per kilo 70 pounds = 31.81Kg motor@ 185Kw continus ? only if water cooled i’d bet the 45Kw 10Kg motor I speek of will happly run at that power level water cooled i’d bet (45 Kw rating inside a sealed aluminium gearbox!) Yes i do realize what an induction motor is i’ve qorked on many over the years and yes a high frequency motor can be more efficiant in this type of aplication than a low frequency unit,but do you relize no matter how much better it becomes the FACT is that the design uses some of the energy injected into the motor to produce work to energize the field inside the armiture (loss!) In almost every other way a magnet type brushless motor is identical to your beloved high frequencey induction motor it just doesn’t waste as much energy and is always lighter for the same power output!

    The main problem I see is that most conversion’s done at this stage on coventional cars is they use brushed D.C motors or industrial induction motors , both very good at there job but far from ideal for the conversion of cars to electric power. Yes there are new high frequency induction motors about but you go cost one you’ll get a fright! Then you need a controler as well and i bet the price of that scares you just as much!

    There are examples of low weight (rare earth magnet) brushless motors about now with power levels suitable for cars if you look in the right places(china will have a manufactuer that will make it very cheap!) If you want it now, how about make your own as I am (relitively easy, look up the web) or just strip a prius or two for it’s 35Kw motor.

  25. Dennis said,

    on March 27th, 2008 at 3:42 pm

    Adam, the rest of the motor industry disagrees with you as well on using permanent magnet brushless motors. Every motor manufacturer flaunts there 3-phase AC induction motors on their front pages of their websites, but not your precious permanent magnet brushless motor that loses magnetism as the magnets age. Until they can make magnets that never lose their magnetic strength as they age, then the 3-phase AC induction is the most reliable motor to use PERIOD. There is a saying that the second most reliable piece of equipment in industry is the 3-phase AC induction motor. The first is the transformer. hmm no mention of your precious motor as being reliable…….

    No where did I say that the 185kW was continuous motor power for the Tesla Roadster. It was to prove a point that another way to make a motor light is to have an intermittent duty cycle of power it can put out for short durations for say acceleration. Now a REAL 185Kw motor will be much larger and heavier since it was designed to put out that much power 24/7 for 10 years non stop running.

    Now water cooling can increase the duration that an intermittent power output type motor can sustain. Tesla just uses aluminum fins protruding from the motor frame and rely on the air passing through the grille of the car to cool it down instead of a water compatible frame….

  26. on March 28th, 2008 at 5:44 am

    Dennis motor manufactures may flaunt there 3-phase motor on their website’s but you still cant get past the fact that any realworld comparison between our beloved motors comes out like this

    :- Permanant magnet brushless motor compared to an equaly rated continus power output induction motor of any description will result with these facts

    magnet:- Lower weight (by considerable amount aproximatly 50%)
    magnet:- higher effeciency
    magnet:- higher torque (given similar rotating diameter)
    magnet:- currently lower cost (granted will change to opostite with increase in supply of high frequency induction motors)
    magnet:- easy to setup regenrative braking (yes induction motor will to but more involved in electronics)

    Sorry to say but at the moment the only real benifit to a induction motor over a rare earth magnet brushless motor that I can see is as you point out, long life with limited loss of effeciency!

    Yes a rare earth magnet will loose magnetic strength over time (very quickly if over temped currenty anything over 120 deg)

    Permanent magnet brushless motors have taken off in many areas of the hobbie and industral world and have proven themsevels very reliable over the last 5 years in the hobbie world they are most often abused far beond their design limits (I have myself on regular occasions driven a now 4 year old motor past its rated limits by over 1000% to thermal shutdown with only modest reductions in effecency) and yes i’ve killed a few but all were driven far past their working limits.

    As far as I’m concerned the electric motor industry seems very keen on the permant magnet brushless motor also, alot of auto manufactures are using them and yes induction motors to ! So there must be benifits to both or they would all be using induction motors wouldn’t they?

  27. hugo said,

    on June 5th, 2008 at 10:07 pm

    Julian Edgar, do you want to get rid of the 10 series Prius battery??

  28. Fred said,

    on July 15th, 2008 at 3:57 am

    I’m trying to figure out how many kw-hours a car uses in order to see if it makes sense for me to install solar collectors on my house to supply an electric car. The above discussion is mostly too technical for me. But, if we take the 27 kW/tonne figure as given, how would that translate into kw-hrs/kilometer?

  29. on July 15th, 2008 at 3:21 pm

    Hello Fred! The 27kw/tonne figure is a good one for power output of the motor for your car but the size of your solar array will have little to do with this figure in the real world.

    The things you need to know are how far are you planning on going in your electric car on a regular basis and how much energy will it need for that!

    Things you will need to know include (if your car isn’t off the shelf n have the figures handy! ha ha ) Average distance your going to travel, average speeds, power consumption at those speeds, battery effeciency at those current draws etc etc lots to work out!

    If your looking for a figure of some sort to guestimate stuff a VN commadore sedan uses about 12kw to cruse at 100km/h on level ground on a windless day! A different car (different wind resistance n rolling resistance ) will use more or less . So you would say 12 kw for an hour at 100km/h right would get you 100km right? sorry wrong you have to factor in your losses charging the battery for a start then losses as you use the power driving the car from large current draw etc if you assume 80% loss at both ends (very conservative) you loose alot before you even worry about hills n stuff!

    You will need a huge solar array or a very small car ! Look up the solar taxi that went through Aus recently for an idea of the size of array for even a very effecient (small) car.

  30. jake c said,

    on July 15th, 2008 at 4:22 pm

    fred, assuming your motor is supplying 20kw of power at 100kph, for one hour, then it is simply 20/100 = 0.2kWh/km

  31. on July 17th, 2008 at 10:00 am

    Sorry to say, jake you got you got it backwards mate! 20kw for an hour is 20kw/h not 0.2 kw/h big difference, power divided by time not speed gives watt hours! Also If Motor is suplying 20kw actual output power, it will use more electrical energy than this prob in the order of 23kw!

  32. on July 17th, 2008 at 10:04 am

    My opologie’s Jake miss read your statement ! agree with your 0.2kwh/km statement assuming motor draws 20kw power! very sorry to tired to read properly need some sleep i think!

  33. Paul said,

    on October 30th, 2008 at 10:41 am

    It’s a pretty well known number really. 200wh/mi

    Most AC EVs use around 200wh/mi. (approx 125wh/km) That’s the figure given for the GM Volt, Wrightspeed X1 and a numer of others. The tzero uses around 155wh/mi. So @ $0.10 per kw/hr you’ll get 5km range. Comparing the per mi/km cost to an ICE gives you 1/20th and beyond. This doesn’t apply to brushed DC as they do not have regen.

    What’s even more mind blowing is how little power something like the Killacycle uses in a 1/4 mile run. It has 500hp, does an ET of 8 seconds and uses $0.07 worth of electricity per run including the burnout and the drive back to the pits. The truth is 8 seconds is only 1/450th of an hour so the faster it goes the less it costs (well…. almost) LOL

  34. Eric said,

    on September 19th, 2009 at 2:49 am

    Hi Julian. Mating electric motors to ICE transmissions is not a good idea. I believe the best way is get rid of the trans and hook the motor directly to the wheels. You would get better performance. A 30kw motor with a peak of 60Kw is adequate.

  35. Ben said,

    on October 6th, 2009 at 8:50 am

    that only applies if the peak speed of the motor is the same as the peak speed of the wheels (ie whatever wheel rpm equate to 130km/h). If it is higher or lower you get either less available power or a lower speed.

    Then you also have to realise that a motor capable of directly driving a wheel is going to be quite large and heavy (simply because of the sheer torque it has to generate). Smaller motors capable of higher rpm (but less torque) through a gearbox are a lighter solution.