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-   -   Capacitor Question (http://www.chiefdelphi.com/forums/showthread.php?t=54495)

Al Skierkiewicz 20-02-2007 12:18

Re: Capacitor Question
 
Jose,
Each motor is designed for a specific current based on the strength of the magnet. Automatically dumping in twice the rated current may get you nothing more than additional heat. If your motor was rated for 3-6 volts then you are going to see an increase in power with twice the input. All things being equal, power in equals the square of the current times the load resistance. Since you are doubling the current, the power would go up by a factor of four less the losses.
Based on the fuel cell providing one volt, you can use a voltage boost convertor to charge the caps. The charge voltage curve is the inverse of the discharge curve, but remember that at the start of charge, the cap is taking a lot of current. You could then use a timer to place the cap in series with the fuel cell so you get that extra boost (cap voltage plus fuel cell) to start and then switch to fuel cell alone for the remaining time. I bet that if you looked at the 555 timer circuits and you used the capacitor bank as the timing component you could simplify design somewhat.
You say the rules allow charging before the run? Is so, I think you may be able to get the caps charged up. I was thinking, can you also use mechanical storage? i.e. use a motor to run up a flywheel and let the stored energy in the flywheel drive the base.

Ronald_raygun 21-02-2007 23:15

Re: Capacitor Question
 
Al,

I can't thank you enough for your effort in providing me the help I need to get this project off the ground. I know I can't say this enough, but thank you for your response.

As for the motor, presumably dumping twice the voltage into the motor would also have the motor draw twice the amps, which is where I get the "4 times the horsepower figure" "When you double the voltage, you also double the amperage that a motor can draw. This means that the power that can be produced goes up by a factor of four."

I still don't quite understand the charge/discharge characteristics of a general capacitor (supercap or otherwise).

Do I need to charge it at the rated voltage? (i.e. 1v source into a 2v cap)
Can I discharge it at a different voltage? (i.e. 50V cap into a 6v motor, or 2v cap into a 6v motor)

As for other forms of energy on the car...As long as its powered by the fuel cell, it's generally allowed, within reason (No fusion/fission reactors, rubber bands, etc.) As for flywheels, they are definitely easier to implement for me, but engaging them to the drive wheel doesn't seem too feasible given the size and power limitations. (Weight as you remember is unlimited)

All this circuit talk has my head spinning. The 555 Timer circuit I know is pretty common, but I have no idea how it works. Is there some sort of simulator or circuit-building program that you recommend? That might make things easier.

I just need to design a circuit to incorporate a capacitor into my design so that I get a boost at the start of a run.n

Al Skierkiewicz 22-02-2007 08:06

Re: Capacitor Question
 
Jose,
You can charge a capacitor to any voltage. It only becomes a problem when you are trying to charge it to a voltage that is higher than it's rating. You can use the 1 volt fuel cell to power a voltage multiplier much like the article in your post above. Maxim has a few handy ICs that are designed for these applications. If your motor is rated for 6 volts then charge a cap up to 6 volts and then dump it into the motor. 6 volt capacitors are generally smaller in size than a 16 or 25 volt cap but you can still use the higher rated cap charged to 6 volts.
Capacitors are funny little beasts. When charging, the voltage starts out very small and rises exponentially until max charge is reached. This takes about 6 time constants to achieve, where t=R*C. The voltage that appears at the terminals will be V(cap) =Vin(1 - e^{-t/RC}) if you are solving for time. The charge current is the inverse of this. Current is maximum at the start of charge and falls off to zero when a cap is fully charged. In discharge both current and voltage are reversed. The cap will deliver max voltage and current at the start of discharge both falling to zero at the end of discharge, again as an exponential function. this link has a good curve and explanation http://hyperphysics.phy-astr.gsu.edu...ic/capchg.html
The 555 timer is a special circuit that uses the function of RC timing (cap charge/discharge) to perform timing functions. If you were to use the same cap for both motor function and timing, that would be pretty cool.
Once you take a look at the charge discharge curves, you will start to get a feel for the action of the cap in your application. During charge you can have one time constant that is determined by the charging circuit series resistance and chosen so that you do not exceed the fuel cell 500 ma max current. On the discharge side, your time constant is now determined by the series resistance of the motor and wires. The discharge constant will be very short due to the low series resistance of the motor. Say you were using a motor like the Fisher-Price motor. It has a resistance of about 0.12 ohms. With a bank of caps rated at 0.5 farad the bank would be fully discharged in 6 time constants or 6*0.5*0.12=.36 seconds. But it would only produce some useful current for perhaps 1 or 2 time constants. Remember that the using the equation above, one time constant will discharge the cap to 37% of it's fully charged value. So in 0.06 seconds the voltage in the cap will be down to 2.2 volts but the current will be as high as 50 amps. With all things considered, you might find that in 0.06 seconds, you may have not been able to transfer enough current to get the motor turning. Only experimentation will tell, different motors will have different resistance and power curves. Good luck.
As to the flywheel, all you have to do is have a pulley attached to your drive wheels that pivots against the flywheel. When the race starts, just allow the pulley to fall against the flywheel to conduct the flywheel energy to the drive wheels. A spring would help do that as well as having a grippy surface on the pulley. Then use the fuel cell to drive a motor that spins up the flywheel.


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