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Author Topic: Electric bicycle  (Read 6608 times)

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Offline newbie_teachTopic starter

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Re: Electric bicycle
« Reply #30 on: January 30, 2015, 12:22:25 PM »
So yeah, i'll order those motors first and then get back(maybe for help regarding mounting), and further discussions on batteries and esc
P.S. :- Any other motor's link will also be fine :) (in the same price range though)

Offline newbie_teachTopic starter

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Re: Electric bicycle
« Reply #31 on: February 11, 2015, 03:13:31 AM »
I will get back to this project soon, keep me posted on any suitable motor for this application (the ebay motor takes double the money for shipping 35$ for motor and 49$ for shipping)


Offline newbie_teachTopic starter

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Re: Electric bicycle
« Reply #33 on: March 29, 2015, 04:55:31 AM »
Hey guys sorry for the inactivity, (all exams and stuff killed me).
Now back to the project. While these days I found quite a strong motor in a nearby workshop for fairly cheap so I got super tempted to buy it and I bought it. So I'll be using this motor (if this fails I'll quietly turn back to ebay solution).
About the motor its a 40V DC brushed motor no further specifics on that but has a lot of torque. As said earlier ESC /motor driver will be made at home...any suggestions on how to make one?
And any idea about mounting the motor on to the bicycle frame?

Offline Schlayer

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Re: Electric bicycle
« Reply #34 on: March 31, 2015, 01:13:32 PM »
     40V is quite a lot of power. I have no idea how you plan on making custom electronics to drive that motor without knowing more about it. You need to know the stall current of the motor to even begin figuring out how to construct a speed controller for it. I admit i'm not such an electronics whiz as to know how to build a traditional electronic speed controller, but I do know how you can make a manual speed controller using a massive rheostat/potentiometer and a use servo to control it. Problem is... well, pretty much all of it. for starters, a potentiometer that big is difficult to find and rather expensive. I had to use a pot for controlling the speed of a motor which drew only around 75W, and the cheapest we could find was still over $50. a 40V motor? Easily drawing 10A stall current (that's probably a generous underestimate) making your power draw a startling 400W. There is no way it's worth spending $150 on a rheostat to avoid paying $60 for a motor. And I don't have the slightest clue where you can get battery packs over 24V for under $100 which will last for any good amount of time. There are some results which come up when I search for speed controllers that handle 100s of Watts of power, but none have detailed spec sheets at all. The best looking thing is this: http://www.newegg.com/Product/Product.aspx?Item=9SIA35C1932019&nm_mc=KNC-GoogleMKP-PC&cm_mmc=KNC-GoogleMKP-PC-_-pla-_-Eco+Gadgets-_-9SIA35C1932019&gclid=Cj0KEQjwi-moBRDL4Omf9d_LndMBEiQAQtFf86cGN1HKSAHPY06J7ze9RPwRdxT0cQZEQJaGk3lVyc0aAnjN8P8HAQ&gclsrc=aw.ds And that can't even handle 40V. It too requires a potentiometer to be rotated, but uses electronics within to manage power output in a more efficient way than one single huge rheostat.

     The short of it is, any kind of system where you need to safely control on the order of 100s of watts of power is gonna be very, very expensive, and if it isn't, it will be very, very dangerous :(.  Even my 75W motor with under 2 ohms of resistance generated too much heat to touch. I'm sorry, but the bottom line is, when you start out on an ambitious project like this, you need to start smaller and work your way up.  Believe me, I have run into this sort of problem numerous times in the past. I know it's not the best news to hear, but I think you need to downsize considerably. I'd still like to help you complete this project, but it might mean altering your plans a fair bit. Let me know of any further updates :)

Offline newbie_teachTopic starter

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Re: Electric bicycle
« Reply #35 on: April 06, 2015, 03:47:23 PM »
Frankly I never debate/argue with pro engineers/makers who have a ton of knowledge than me. But let me take a chance and clear this concept in my mind. I "need" to know the stall current you said,can't we make assumptions? How much can be the max stall current? Not more than 220A right? Just use a MOSFET IRF3205 has a drain current of 110A continuous and 220A burst VDS of 55v. Isn't that enough? By your potentiometer/rheostat method do u mean to power it up directly? No way that will be done. Your correct.
Basically a small potentiometer will be wired up with an arduino and just like the fade sketch where pot controls the LED's brightness instead of led there will be that motor driver circuit (no h bridge since only one direction is required).
Please correct me where I am wrong. I like being corrected.
Thank you for bearing and helping me :)

Offline Schlayer

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Re: Electric bicycle
« Reply #36 on: April 06, 2015, 04:44:22 PM »
Well... The max stall current should definitely be less than 220 Amps, yes. I highly doubt the continuous drain would be over 110A also, but those number seems extremely high for a MOSFET at that price. If its true to its datasheet, that should indeed work. I don;t know a ton about transistors, but if I were you I'd make sure you have a fuse between the positive lead of your battery and whatever circuitry you have in that pot/controller circuit. You'll want one that will trigger at 200A like this: http://www.ebay.com/itm/Gold-ANL-Fuse-Holder-1-Foot-2-Gauge-Wire-200A-Fuse-/390461024295?pt=LH_DefaultDomain_0&hash=item5ae94a6827&vxp=mtr
My concerns about the MOSFET are mainly due to its size. You see that fuse has 2 gauge wire on it. 2! That's gigantic. Considerably smaller would melt under the heat of 100-200A of current. I'm really unsure how a MOSFET with such tiny pins could handle 110A continuous, but you are right that in all likelihood the motor draws well under that. I guess you may as well go for it and test it, but make sure you trigger it from a distance in case the MOSFET explodes!  I'll talk to a friend who is more familiar with MOSFETs and get back to you.

Offline newbie_teachTopic starter

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Re: Electric bicycle
« Reply #37 on: April 08, 2015, 04:21:34 AM »
http://www.futurlec.com/Transistors/IRF3205.shtml
At least according to this drain current is 110A I read a several others too and they mentioned the same specs as well. I agree to your point that to carry 200A of current the fuse uses 2 guage wire which it nice and fat whereas the MOSFET pins use small tiny pins... Suspicious. Maybe it uses some special kind of metal. Please find out from your friend and let me know,I'm eager to know how this works.

Offline Schlayer

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Re: Electric bicycle
« Reply #38 on: April 08, 2015, 05:57:37 AM »
    The spec sheet does reveal one fact about this little MOSFET which could explain the current capacity: its operating temperature. The thing can safely operate up to 175 C. My MOSFET knowledgeable friend says this checks out, it should probably work OK. however, he agrees that the stall current of a 40V motor could be monstrously high and still potentially kill those MOSFETs simply by being over 100A. To make sure this doesn't happen you will need a slightly expensive piece of equipment.
http://www.amazon.com/ACM03-Range-Digital-Current-Tester/dp/B00K4FNSX2/ref=sr_1_1?s=hi&ie=UTF8&qid=1428493725&sr=1-1&keywords=digital+clamp+DC+current
My friend says this is what you'll need. It is a clamping multimeter. Any regular multimeter would fry under currents over maybe 10-20A. This gets up to 400A DC by using magnets and a clamping design to sense current without being plugged into it. There are cheaper meters out there but this is the least expensive some searching got me that measurers high DC current, the others gave only max AC current readings.
    You need to actually plug the motor up to some large battery pack (I recommend three 12V car batteries in series if you happens to have those around) and actually test running the motor and checking what stall current it maxes at. It will be difficult to get a proper rating even with this meter since the stall current is reached to suddenly, you will need to see if the meter has a mode for finding a maximum spike, or a graph, or something. Worst case scenario, stare at the meter while repeatedly power cycling the motor (on, off, on off, on, etc.) and assume the largest number you see is a bit under the stall current. That's the only way to play it safe using those MOSFETs and any other electronics you might want to interface with the motor. Make sure the batteries/power supply you are using to test have extremely high current draw ratings! EDIT: Maximum current draw ratings.
« Last Edit: April 09, 2015, 02:40:40 PM by Schlayer »

Offline newbie_teachTopic starter

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Re: Electric bicycle
« Reply #39 on: April 09, 2015, 12:40:49 PM »
I don't understand how my motor can draw over 100A if my battery can't supply that much. Lead acid batteries don't have much high discharged rates or energy density or whatever its called can someone please explain what's up with these batteries in detail? I feel confused how can a battery rated at 20Ah give 100Ah? Some people say lithium batteries can go up to several 100Amps? How does it work and is it completely true?

Offline Schlayer

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Re: Electric bicycle
« Reply #40 on: April 09, 2015, 02:39:26 PM »
  A and Ah or Amps and Amp-Hours are totally different units. Amps (A), or Ampheres, are a measure of current. Ampere-Hours or Amp-Hours (Ah) are a measure of energy. Current is a discharge rate and Energy is a current capacity.  Amp-hours as a using of energy are pretty intuitive, though the unit names are confusing. Example: you have a 2Ah lithium battery. It can discharge at up to 20A max. If you draw 20A from this battery continuously, it will be drained after 2A/20Ah = 1/10h or 1/10th of an hour (6 minutes). There is no direct relationship whatsoever between the discharge rate (or current) of a battery and its current capacity! All batteries have limitations of how much current can be drawn at once, but this depends on the type and configuration of battery.

  You might fined a large lithium battery can discharge at up to 40A, but an SLA might only be able to deliver 5A even one of the same physical size, since they are using totally different chemical processes. If you try and draw more than the max from a battery (E.g. by using a motor with a 50A stall current) you will damage the battery and it might melt or have other bad things happen to it. Be careful about mixing up these units, it can be confusing rather often, but that's just one more thing to look out for :)

Offline bdeuell

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Re: Electric bicycle
« Reply #41 on: April 11, 2015, 04:29:50 PM »
the maximum discharge rate of a battery is its maximum discharge rate without risk of damaging or destroying (depending on the battery chemistry explosion or fire can also be a very serious concern) the battery is capable of supplying much greater currents. Unless there is a battery protection circuit/device the current it can supply will be limited by the internal resistance of the battery. The maximum discharge spec may be listed with several values depending on length of time i.e. it may be capable of handling a higher current for 30 seconds than one hour. Higher discharge rates will also negatively impact battery life and capacity. the discharge rate is often listed in terms of C (for example maximum continuous discharge rate of 2C) where C is equal to the batteries capacity (for example a 12Ah battery C=12A).

Remember that brushed DC motors draw the full stall current when the speed is zero (regardless of load) which will occur every time your motor starts. However under a light load it is likely that your motor will accelerate quickly and the current draw will correspondingly decrease.

Also assuming you will be driving your motor with PWM (which relies on the inductance and resistance of your motor to produce an analog voltage) you must consider how the frequency will impact your motor controller. low pwm frequencies will not be filtered by the motor (acting as a LR filter) and high frequencies may result in excessive losses within the transistors.

I am by no means an expert in designing motor controllers /H-bridges but I suggest if you plan to build your own for this project you do some extensive research on proper design. you are dealing with some pretty extreme currents that can easily blow up components or melt wires. I haven't built a high powered h-bridge  like this myself but I worked on a project where the electrical engineers built their own for some motors with 110A stall currents... we had lots of problems smoking components and literally blowing up the transistors (pieces of the component flying)...not sure what the problem was but everything can have an impact including the firmware driving the motor controller.

in addition to all the design that goes into the electrical circuits you will also have to consider the thermal design.

I provide this information not in an attempt to discourage you from building your own controller but to share my knowledge of the complexity that is involved in developing a robust motor controller (especially for higher current requirements).



 


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