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dukeleto said:
Powering the motor taxed my limited elctronics skills rather substantially, but I think I've got that part working 'OK'.
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Are you converting the DC from the battery to a square wave or to a pseudo sine wave? The latter is more difficult to do but makes the motor less noisy (and generates less EMI).
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- is the maximum braking power fixed just by the choice of motor?
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In a word, yes. To give an easy to understand analogy think of a wheel-mounted bicycle generator. This is really just a small motor running in reverse, so to speak. Even with maximum load on the generator output there is only so much power it will produce, and therefore only so much braking action. To get more, you need a larger generator (or motor in your case). A second limit is how fast you can absorb the energy from the motor. A reasonable-sized motor might be able to generate a few kilowatts of power at, say, 30 mph, but if the battery pack can't absorb it fast enough you're left with two alternatives-dissipate it in a resistor grid or use less regenerative braking. This is a problem electric trains encounter all the time. If they try to feed back power into the catenary but there are no loads then the power must be dissipated in an onboard resistor grid. A third limit not usually encountered is the wheel adhesion. You can only generate power up until the point that the wheel starts sliding and then you've reached a hard limit. However, for typical bicycle tires this is over 10 kW at 30 mph.
One plus is that a motor which may produce only 1 kW for traction can produce 10 kW or more for braking because it is by definition short term. The motor just doesn't have a chance to heat up as it would running for many minutes producing tractive power.
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- the only way I've found of adjusting the instantaneous braking power is by feeding some current back into the circuit. Is that how it should be done?
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I wish I could help you there. I know since it's an AC motor if you don't energize the coils all that will happen is that the armature will freewheel. I also know that putting straight DC to the coils makes an AC motor operate as a non-regenerative brake. You may get the best results by driving the coils with a sine wave closely matched in frequency to the rotor speed. The greater the speed difference the more the regenerative braking action but this is just a guess.
I also have an interest in creating an electric drive one day for a human-powered vehicle (HPV). Not anytime soon, of course, but someday. My reason is simple. Superior aerodynamics have resulting in HPVs breaking 80 mph in the last few years. I think eventually we'll be able to reach 100 mph or more under human power alone, and maybe be able to maintain average cruising speeds of 80+ mph for one or two hours. A big problem keeping these vehicles from being mass produced is the drivetrain. An HPV with a 100 mph speed range needs a very cumbersome mechanical drivetrain in order to cope with hills, acceleration, and high-speed cruising. If this could be replaced with a pedal-powered generator driving a multi-pole electric motor connected directed to the back wheel (i.e. a very thin motor with a very large diameter) then the problem is solved and these things can be made available to the public for well under $1000. Additionally, you can have a small amount of energy storage (maybe 0.1 KW-hr) to allow for quick acceleration to cruising speeds since it takes even a strong human half a minute or more to reach 60 mph from a dead stop using only their own power before you even account for rolling and aerodynamic drag. Energy storage makes high-speed merging that much safer and lessens the strain on the rider.
And in case anyone suggests it, yes, a vehicle like I described would make an ideal platform to convert to a highly efficient, ultra-long range, one-person BEV for those with no desire to make their own power. /ubbthreads/images/graemlins/grin.gif
