Electric Skateboard

Discussion in 'The Projects Forum' started by Suladan, Nov 2, 2008.

  1. Suladan

    Thread Starter New Member

    Nov 2, 2008
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    Hiya, I am currently 15 years old and am taking a GCSE in electronics. I will be building an electric skateboard, and for this will require a PWM circuit to drive my motor. I've built some test circuits using 2 x 555 timer, and another one using a Pic chip, but these have all been used with fairly small voltage motors. The motor I'm looking at is a 36 volt 27amp 800 watt electric motor. I was wondering if anyone could help me make a circuit or link me to any premade circuits. I have seen lots of PWM circuits, but nothing powerful enough to drive this motor.

    On a separate note I was wondering how hard it would be to charge the batteries when going downhill. Would this require a very complex circuit, and once again could you link me to any premade schematics.

    Thanks
    Denis
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    To drive the motor with PWM use a power MOSFET transistor. I bought some from e-bay at a good price. I bought IRFP2907 which can handle your motor, actually is more powerful than that but its ok to use it. If you want to see more search in ebay for MOSETs.
     
  3. John P

    AAC Fanatic!

    Oct 14, 2008
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    Have you thought this project entirely through? I wouldn't know where to begin to find a 36V battery. 3 12 V batteries in series? That starts to seem like a lot of stuff to put on a skateboard. And a battery that can supply 27A is not a small battery.

    Then, 800 watts! Sorry for the medieval units, but that's about 1 horsepower. It could lift you vertically at about 3ft/second, and who knows what it would do on a flat surface. Shoot out from under you, most likely. Do Mum and Dad know that you're contemplating this?

    My instinct is to say you need to scale down the motor, and the electronics will become a lot simpler.
     
  4. scubasteve_911

    Senior Member

    Dec 27, 2007
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    John has an excellent point, one which you need to listen to. Many people that do not understand the physics behind such devices tend to 'overkill' the parts used. I have done that a lot in the past, so I recognize the error well.

    The work-energy principal states: Wnet = 0.5*m*Vf^2 - 0.5*m*Vi^2
    Where, Wnet is total work, Vf = velocity final, Vi = Velocity initial, m is mass

    So, if you wanted to get to 7.5m/s (27Km/h) in 30s from a stop and you weigh about 100Kg, You'd need
    = 0.5*100*7.5^2 = 2812.5 Joules

    A watt is joules/second, so 2812.5 Joules in 30 seconds is 93.75W. Clearly, there are other factors that will increase this ideal number. Wind resistance and friction are two losses. You need to understand the torque/speed curve of your motor to deliver the necessary power. A PMDC motor will put out a peak power under a certain torque/speed condition. Which means, you cannot just hook up the motors without any gearing. This is why cars have transmissions.

    [​IMG]

    Goodluck!,

    Steve
     
  5. scubasteve_911

    Senior Member

    Dec 27, 2007
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    A couple more things on the electronics side. If you only seek unidirectional control, then you can use a single MOSFET that is designed for that kind of current. Hopefully, from my previous post, you will choose a 200-300W motor, instead of that monster.

    This page is excellent, I suggest you read it.
    http://www.4qdtec.com/pwm-01.html

    As far as charging goes, I am a little uncertain of the best way to do this. Someone may have a suggestion.

    Steve
     
  6. floomdoggle

    Senior Member

    Sep 1, 2008
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    How are you going to manually control the motor? With handle bars, or a wire connected hand switch? Either one could have an off switch to the motor, then a dc-dc voltage inverter to increase the voltage. Then straight to the battery to charge it.
    I agree with Mr. Steve up there 200 watts,1/8 horsepower to start. If you can control that, you can step up. It's not the speed, it's the side to side control.
    Dan
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
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    Here's another point. If you need to reduce the speed of the motor output, as you almost certainly will, you'll have to use some sort of power transmission--gears or belts/chains--with small gears driving larger ones. But a skateboard has tiny wheels, and your last gear has to be smaller still, or it'll dig into the ground. That doesn't make the job impossible, but it suggests that you'll need a lot of mechanical parts, and they'll need to be installed with enough precision to function poperly, not to mention the fact that the assembly needs to have some strength.

    I hate to keep pointing out difficulties, but if this is an educational project, part of what you might end up learning is how a machine has different components that each need to be fitted into an overall design.
     
  8. Suladan

    Thread Starter New Member

    Nov 2, 2008
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    Thanks a lot guys. The main reason I came up with these figures was from my market research. There are currently products on the market which are similar to mine, and have 800 watt motors, and reach 35km/h, with a Motor: 800W, 36V and a Battery: 9Ah, 3pcs, 12V

    http://www.skateelectric.com/index.php?option=com_content&task=view&id=38&Itemid=45

    About the comment on the small wheels of the skateboard, I plan to make the trucks wider and then have higher off road wheels. About the control, I plan to use a hand switch
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I found that 12V/9Ah weigs 2.6kg, so three of them is 8kg. Plus chassis, wheels, control circuitry.. That is quite a heavy load for a skateboard. You definitely won´t like if you ran out of batteries without any possibility to recharge them.
     
  10. scubasteve_911

    Senior Member

    Dec 27, 2007
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    Suladan,

    That's a logical approach, but I think you have some things to learn about advertised specifications. Almost always, when things are specified in watts, that is a peak value. So, at some instantanious moment, under ideal conditions, it could, theoretically output 800W of power. This extends to audio amplifiers, speaker power handling, etc.

    The suggested ~200W motor needs to be rated for continuous duty. What type of motor is it anyways? You would want to get something with a lot of power density by weight and volume, such as a brushless DC.

    Steve
     
  11. Suladan

    Thread Starter New Member

    Nov 2, 2008
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  12. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    How do you want to attach such a big motor under the skateborad?
    What about the chassis, how will the motor drive the weels?

    And the switch you planned won´t probably work, you have to use some kind of adjustable power control because simply plugging the motor in while standing on the skate will knock you off.
     
  13. steveb

    Senior Member

    Jul 3, 2008
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    Can you tell us what kind of motor it is; permanent magnet, induction, field-synchronous etc. I couldn't tell from the web-site.

    Also, I think some of the previous comments about the required power are neglecting the issue of going uphill which requires an increase in potential energy. If you want to go 10-20 m/hr uphill, the power you suggest may be necessary. Although, I haven't done the calculation yet.

    However, I agree with those cautionary warnings, you need a good design to have control when that power is not needed! It does seem like a lot to put on a skateboard, perhaps what you are contemplating could be called a "street-raft". - :) Just kidding, I was young once too. Be careful.
     
  14. scubasteve_911

    Senior Member

    Dec 27, 2007
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    Steveb brings up a good point. Technically, there are a ton of calculations that can be done. Drag equations, energy equations, etc. We can do a potential energy calculation for a slope, or there are ways to derive the energy which includes kinetic and potential from force, etc.

    Approximately, if you are to raise 25m in 30seconds, then

    Wpot = mgh
    = 100Kg * 9.81 * 25m
    = 24525 Joules, then divide by 30s, 817.5W !

    So, the scenario I previously illustrated, if you are to raise 25m in that amount of time, then you would need about 1KW of energy. Or, in terms of battery capacity, you have 9X3X12 = 324W/h of energy, this process consumes 8.33W/h, so you can do this 39 times before your batteries would die. Let alone I am not sure if these batteries would be able to supply that amount of sustained current.

    Steve
     
  15. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    You might also look at some of the off-the-shelf stepper motor drivers. Dragon Drivers are great for this sort of thing...but perhaps a bit on the pricey side.
     
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