Proper Speed Controller Circuit for 24 VDC motor : electric bycycle

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

I'm planning to convert my bycycle electric in the near future (after some
electrical enlightenment of course). I've basically narrowed the motor down
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 draw
would be aprox. 10.416 amps (250 watts / 24 volt = 10.416 amps), which I
assume from reading other threads here, is what the motor would draw an
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 10
amp/hour batteries in series-paralell for a total of 24 volts, 30 amp/hour
power supply. This I figure should give me roughly two hours of motor work
between charges...

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

Most every PWM circuit/schematics I've come across on the web are for low
amp consuming loads. As I mentioned, the motor will be drawing atleast 10
amps. What is the theory and/or calculations for amp consumption in DC
motor controllers?

Ideally, a universal controller that would work for 250, 350, and 500 watt
24 VDC motor with minimal power loss would be best. Ofcourse, I would like
to know what determines/controls the current load/consumption in a control
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 in
paralell) to the power supply (battery pack) affect the control circuit?
In other words, how must the control circuit be modified to accomadate the
extra current without overloading/overheating the circuit, load, etc.?

Thanks in advance,

Steven



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