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Proper Speed Controller Circuit for 24 VDC motor : electric bycycle

Discussion in 'Electronic Basics' started by greenwanderer108, Sep 25, 2005.

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  1. Hi, I'm an American living in Bangkok, Thailand. I'm new to this forum an
    an absolute novice with basic electronics, I was hoping someone here ca
    assist me with a few basic questions.

    I'm planning to convert my bycycle electric in the near future (after som
    electrical enlightenment of course). I've basically narrowed the motor dow
    to a 24 VDC 250 watt motor. (Though 350 and 500 watt are also options)

    Using Ohm's law (I think), I calculated that such a motor's current dra
    would be aprox. 10.416 amps (250 watts / 24 volt = 10.416 amps), which
    assume from reading other threads here, is what the motor would draw a
    one hour under load. (Please don't hesitate to correct me--elaborate)

    As for power supply, I've decided that I want to rig six 12 volt 1
    amp/hour batteries in series-paralell for a total of 24 volts, 30 amp/hou
    power supply. This I figure should give me roughly two hours of motor wor
    between charges...

    Assuming my calculations are correct, my next task is to figure out th
    necesarry (speed) controller. I've browsed the many schematics/theorie
    for DC motor controllers and am lost. I'm assuming a Pulse Width Modulato
    circuit is what I'm looking for (rigged with a handle grip motorcycl
    throttle set-up for precise control). So, then, I would like to know i
    how to decide which particular circuit will suffice for the amp and vol
    consumption required for my particular project.

    Most every PWM circuit/schematics I've come across on the web are for lo
    amp consuming loads. As I mentioned, the motor will be drawing atleast 1
    amps. What is the theory and/or calculations for amp consumption in D
    motor controllers?

    Ideally, a universal controller that would work for 250, 350, and 500 wat
    24 VDC motor with minimal power loss would be best. Ofcourse, I would lik
    to know what determines/controls the current load/consumption in a contro
    circuit. There is so much to decipher among---MOSFET, other capacitors
    diodes, that I have still yet to learn their functions, purposes.

    While we're at it, how does adding more amp/hours (placing more cells i
    paralell) to the power supply (battery pack) affect the control circuit
    In other words, how must the control circuit be modified to accomadate th
    extra current without overloading/overheating the circuit, load, etc.?

    Thanks in advance,


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  2. That calculation has nothing to do with time. If the motor is rated
    at 250 watts and 24 volts then the only way it can consume 250 watts
    is if it passes 10.4 amperes. How long it does that depends on how
    large the battery is (ampere hour rating). You will also probably not
    ask the motor to produce that power continuously, but may run it at
    lower current (torque) much of the time.
    I think you have this right.
    I think the best drivability may come from an adjustable torque
    control (current control). This is because you feel the torque as
    acceleration, regardless of what speed you are traveling. So I would
    look for a PWM controller that included current feedback, and that
    used the throttle as a torque (current) set point.

    Current control also protects the motor from overload in the event
    that you come to a slope too steep for it to handle. If the setpoint
    can reach only the rated motor current (or slightly above), stalling
    the motor will not overheat it.
    That is just a matter of the current rating of the PWM controller.
    No. it just provides power for a longer time. The supply voltage
    affects the top speed.
  3. whatchu

    whatchu Guest

    maybe not the indepth answer you are hoping for, but heres my 2cents... :)
    10 amps, ok. Amperes are not per-unit time though (well technically you can define them by coulombs per second, but preferred is force between two parallel conductors.. nevermind!)
    You are maybe talking about amp-hours? battery capacity. Theoretically a 24V 10Ah battery would let you run your bike for 1 hour. But this is a bad theory, in practice the battery wont last that long.
    Looks like you understand that...
    2 parts to this. You need a throttle/PWM circuit, and a motor controller circuit.
    If it was me I would use a microcontroller with an ADC to read a potentiometer and output the PWM. Then you have the ability to program in other fun features.
    But you could also use something like a 555 timer IC I bet.

    As for the motor controller, I have seen combat robotics motor controller plans that would handle these specs. Try searching the web some more.
    I dont think you need a full H-Bridge though right? The bicycle only needs to go forward and the motor only needs to spin in one direction? That simplifies your motor controller greatly. Use a big N
    channel power MOSFET as a low side driver.

    Batteries in parallel allows you to run longer (or draw more current). No modifications should be necessary, your circuit will still draw the same amount of current. You might want a fuse.

    good luck
  4. kell

    kell Guest

    I'm planning to convert my bycycle electric in the near future )
    10 amp hours is the size of a motorcycle battery. I hope you can get
    deep cycle type batteries in that size. Deep cycle batteries are what
    you need. A deep cycle is designed to drive a steady load for a long
    time and thus will last much longer than a starter battery.
    The elements that actually carry the heavy current that the motor runs
    on will be either mosfets or a big igbt or igbts. These are modern
    transistors that do not require much power on the gate to turn them on
    and off, which simplifies the gate drive requirements. Thus the brains
    of the controller will consume a very, very small proportion of the
    total power. The major power concerns lie with the big semiconductors.
    If you are going to build this thing yourself you will need to know
    the differences between mosfets and igbts. Mosfets behave like
    resistors, and can be paralleled to carry heavier currents than a
    single mosfet can handle. On the other hand, igbts have a voltage drop
    across them instead of a resistance which means that power dissipation
    doesn't increase as much in an igbt at higher current levels as it does
    in a mosfet. In simple terms, power dissipation in an igbt is
    proportional to current (P=VI), while the mosfet's power dissipation
    increases with the square of the current (P=I^2 R). You should do some
    internet browsing of datasheets, app notes etc. What you're really
    looking for is a device designed for the purpose. There might be a
    motor-drive igbt on the market. There may design requirements like
    voltage clamping built in to it so that you don't have to design them
    into your circuit.
    If you have the perseverance to do the necessary research on the power
    side, you might even be able to do the PWM too.
  5. ehsjr

    ehsjr Guest

    Why? IGBTS don't even enter the the picture according to

    Mosfets behave like
    What the hell does that paragraph mean?
    Mosfets behave like resistors?
    IGBTS have a voltage drop across them instead of a resistance?
    Does "On the other hand" imply Mosfets don't have a voltage drop
    across them?
    The equations P=VI and P=I^2R are identical. Why do you think
    they are different?

    Whatever it is you have in mind isn't coming across.


    You should do some
  6. Rob

    Rob Guest

    Hello Steven
    I used to mess with this sort of thing some years back. In fact there
    is a 24V bicyle rusting away under the porch as we speak.
    Let me get back home tonight and I will dig through some of my old
    designs and see what I can modify to make suitable for your
  7. mike

    mike Guest

    A kid's electric scooter has a 20A 24V controller. About the size of a
    pack of cigarettes. They're made by the zillion, so expect that they're
    probably much cheaper than anything you could build. Do some research
    on the electric scooter sites.
    Don't know anything about Thailand, but here in the USA, you can buy the
    whole scooter at a garage sale under $10. Batteries will be dead, but...

    I'd rethink the batteries.
    If you're on flat land, most of the time you'll be using much less than
    the maximum power.

    You should shock mount the batteries somewhat. Bikes are very hard on

    There are also gearing issues. My scooter maxes out at about 10mph.
    And it still doesn't have enough torque to make it up my driveway.
    The slightest hill slows it down considerably.

    I'm finding that riding my bike is less hassle than riding the electric
    scooter. It's very easy to pedal on any slope that the electric can handle.


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  8. kell

    kell Guest

    Mosfets -- Rds (drain-source resistance).
    IGBTs -- collector-emitter on voltage.

    If you double the current going through a mosfet, you will quadruple
    the power dissipation. P = I^2 Rds

    If you double the current going through an IGBT, you won't quadruple
    the power dissipation. The power dissipation will increase to a lesser
    extent than in a mosfet because the voltage across an IGBT is not
    proportional to current.

    I oversimplified a little in my previous post with the relation P = IV
    suggesting that power dissipation in an IGBT rises in direct proportion
    to current. Collector-emitter voltage would have to remain constant
    for power dissipation to rise in direct proportion with the current,
    while in fact the collector-emitter voltage rises somewhat with
    increasing collector current. Still, the power dissipation at higher
    currents does increase much less in an IGBT than it does in a mosfet.
  9. ehsjr

    ehsjr Guest

    Thanks! Now I understand what you were saying.

    Unfortunately, I don't think the OP can make use of IGBTs in
    this case. IGBTs don't become practical until the voltage is
    250 or higher, per the information at the International
    Rectifier url posted earlier. The voltage is way too low to get
    into the benefit of the portion of the Vce drop curve that is
    relatively flat. In fact, I have no idea what the curve would
    look like at 24 volts.

    The smallest IGBT Vce drop I have seen in the datasheets is 1.2
    volts. Even if it applied at 24 volts, it's 12 watts wasted at
    10 amps. An 80 amp 60V P-channel mosfet such as an Infineon
    SPB80P06P with an RDSon of .023 would drop only .23 volts, and
    waste only 2.3 watts, and an N-channel such as the STP80NF10 with
    an RDSon of .012 would drop only .12 volts and waste about 1.2
    watts. Those, or equivalent, would be a good choice for the OP if
    he can get them. He definitely needs *way more* than 10 amp mosfets
    to accomodate a possible stall condition. As you pointed out, he
    could parallel mosfets and get lower RDSon and higher current

  10. Rob

    Rob Guest

    Hello Steven,
    I've now put some stuff on the website (go to the last item on the
    index page). First thing to look at is the motor you intend to use, as
    this will determin things like gear ratio of the bicycle. The bike I
    mentioned previously had a 750W motor.
  11. Rob

    Rob Guest

    Hello Steven,
    I've now put some stuff on the website (go to the last item on the
    index page). First thing to look at is the motor you intend to use, as
    this will determin things like gear ratio of the bicycle. The bike I
    mentioned previously had a 750W motor.
  12. Thank you everyone for input advice. This is all quite overwhelming at th
    moment---lots to grasp, learn, and consider in what I thought would be
    basic application in learning about circuits. Hopefully this thread wil
    be accessible for a while because I think it will take me a while befor
    I'm to the level of comprehending the relevant electrical components an
    how they work together in a circuit.

    Sure, I can easily buy a used bicycle speed controller/drive system a
    someone suggesteed. There was one available for the equivalent of 30 US
    here. However, it was a used part imported from Japan or China. Even if i
    did work, I wouldn't understand the power consumption, or gain an
    electrical knowledge for that matter. That's why I've adapted the goal t
    learn about how the speed control circuit works from the positive termina
    and everything in between, the actual path of the current..

    There are a few manufacturers of electric bicycles/scooters around here
    but unfortunately in Thailand, the market is so limited, so they won'
    sell individual circuits/parts/controllers as I'm guessing there cautiou
    of other competitors stealing the designs (which they most likely did fro
    Japanase/Chinese makes anyway).

    Can someone suggest a particular schematic of a controller / drive syste
    circuit that would be ideal for a 24 volt motor between 250 to 500 watts
    If you're confident enough, explain to me the path of electrical curren
    from positive to negative (or the other way around) i.e. from the positiv
    terminal the current comes to a ??k resister which .... then at ??amp
    ??volts comes to a diode which...etc. etc.

    What about this PWM circuit :

    Is it sufficient for my application. If not, which values will need to b

    This would benefit me (and a lot others) greatly, if anyone has th
    resources / capability to articulate, interprate the controller schematic
    particularly it's current path.

    I hope this isn't too much, but imagine it's just a short cut request fo
    my self-guided learning...


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  13. I worked up a PWM controller, recently for someone who wanted to drive
    an 8A 15V peltier cooler. A modification of that could be made into a
    24 volt 10 to 20A current regulator for your motor. But it would
    take a bit of rearranging and component substitution. It has the
    advantage of using two alternating phases (two PWM pulsers that take
    turns something like the way a two cylinder engine shares the load
    between two pistons) that reduce the ripple current drawn from the
    battery (reducing its self heating) and also reduces the ripple
    current to the motor, so a lower pulsing frequency can be used for
    lower switching losses. It also divides the current between twin
    drivers, so each can be made with smaller parts. I'll think a bit
    about what changes would be needed.
    It stinks. The PWM generator section is okay, but instead of having
    the throttle just changing the duty cycle (effectively just changing
    the fraction of the battery voltage the motor sees), I think you need
    to measure the motor current, somehow, and have the duty cycle
    automatically hold that current to the value set by the throttle pot.
    This, in effect, controls the motor torque, which is the force you
    feel. It also controls the current to a safe value (whatever the full
    throttle setting is) even if the motor is overloaded and the bike
    doesn't speed up. This will prevent blown fuses and burnt up motors.
  14. ehsjr

    ehsjr Guest

    You have to do *some* of the work! If you read the information
    on the site, you can answer the question "is it sufficient for
    my application" and the question "If not, which values need to be
    The site does a nice job explaining things. If you have specific
    questions after reading it, ask them.

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