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Old 06-09-2011, 09:19 AM   #1 (permalink)
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Need some ideas for current limiting

I would like to add an electric assist to my car. It is a BMW 528I and has the room to add a belt driven motor. This motor would run to the front crank. I have the skills and equipment to fabricate the set up. Will be for assist only with a 48 volt lead acid pack in the rear to start, will go lithium later if it works well. I only drive about 20 mile a day so range should not be a problem. Wish to link the throttle and the ESC together so the car will drive with standard controls, no seperate throttles. Problem is I am not sure how to control current. Would like to set it up so that when I accelerate the motor will apply up to a certain current, say around 150 amps. However this would be applied with in the first say 10% of throttle then stay at that current level up to full throttle. Anyone have any ideas on how to do this. Thanks for your time and assistant.

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Old 06-09-2011, 09:28 AM   #2 (permalink)
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The first step is to describe the electric motor as some use AC and others DC. That in turn drives the type of current control circuit. But let me suggest you might study "buck" switching circuits as micro-controller driven switching allows very sophisticated, control.

BTW, I would recommend planning and doing your first work on a bench with serious consideration of including a data recording system. Trying to operate a car while monitoring engineering data usually has a bad outcome.

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Old 06-09-2011, 09:35 AM   #3 (permalink)
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I have not decided whether to go ac or dc. Regen is not a big consideration but drag with the motor turned off is. If I do not use regen I could use a sprag so the motor would not spin on longer trips. Would prefer using available products. Don't want to make it too complicated. Budget has to be reasonable, just because it may or may not provide a big mpg improvement.

Last edited by Yellowbird911; 06-09-2011 at 09:42 AM..
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Old 06-09-2011, 09:59 AM   #4 (permalink)
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I don't know your budget but perhaps you might consider getting a used hybrid that already has the hardware and control systems built-in to the vehicle. This avoids having to go 'cold turkey' in to dealing with power levels and mechanical issues, not to mention the control law issues.

Another approach would be a 'hillbilly' plug-in that uses a battery to provide operating power and current for the 12V system. This can off-load the alternator drag on the engine and let utility power provide the power for the car systems. Best of all 48V to 13-14V power supplies at 500+ watts are 'off the shelf' available.

If you still want to put a motor in the car, the power supply could provide some measure of DC conversion, again without having to take the great of a leap of jumping straight into hybrid technology.

An intermediate step would be to replace the alternator with a motor-generator. This begins to look like the micro-hybrids GM still sells. The beauty is the alternator mount, belt, and wiring is already there. Certainly a lower weight penalty than a second, partially used, motor.

Just trying to offer easily accomplished efficiency improvement ideas.

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Old 06-09-2011, 10:37 AM   #5 (permalink)
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In addition, are you applying power at the front or rear of the crank?
The snout tends to be much smaller and cannot handle much of a torque input.
Yes, I have snapped the end off of cranks. Long unrelated attempt.
Applying power at the flywheel gives a full thickness crank shaft.
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Old 06-09-2011, 02:09 PM   #6 (permalink)
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Quote:
Originally Posted by Yellowbird911 View Post
Anyone have any ideas on how to do this. Thanks for your time and assistant.
Have you thought about making it full manual control at first? Like a button you push when you want assist, that cuts assist when you let go?

Super easy to implement, and you can work out a lot of kinks before you start messing with more complicated stuff that you have less experience with.
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Old 06-09-2011, 04:15 PM   #7 (permalink)
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What we're trying to suggest are some more 'bite-sized', mini-projects to move you towards your goal without having to take a big-bite of some tricky technologies. No one wants to hear that "the smoke escaped!"

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Old 06-09-2011, 04:45 PM   #8 (permalink)
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You'll have a heck of a time finding a 48V AC motor controller (and motor). On the other hand, a 48V DC motor controller is quite easy to find and will probably even work off the TPS sensor on the throttle body.

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