# Proper Speed Controller Circuit for 24 VDC motor : electric bycycle

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

1. ### greenwanderer108Guest

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

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

Steven

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2. ### John PopelishGuest

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. ### whatchuGuest

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. ### kellGuest

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
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. ### ehsjrGuest

Why? IGBTS don't even enter the the picture according to
http://www.irf.com/technical-info/whitepaper/choosewisely.pdf

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.

Ed

You should do some

6. ### RobGuest

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
application.
Regards
Robert

7. ### mikeGuest

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
stuff.

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.

mike

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8. ### kellGuest

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. ### ehsjrGuest

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
capability.

Ed

10. ### RobGuest

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.
Regards
Robert

11. ### RobGuest

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.
Regards
Robert

12. ### greenwanderer108Guest

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
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.

http://www.solorb.com/elect/solarcirc/pwm1/

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...

Cheers

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13. ### John PopelishGuest

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. ### ehsjrGuest

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