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  • OT calling all electrical guru's

    Im building my bike trainer, i have it all hooked up but im getting too much variable and fluctuation in my volts output, my cadence is about 90rpm and this makes my digital volt meter fluctuate between 5.9 and 6.1 volts if im trying to hold 6 --- this is the result of the uneven power strokes that pedaling a bike produces,,, My generator is a brush type perm. mag. motor, right now im just useing a couple of bulbs for resistance soon to be replaced by a fan motor that helps keep me cool, my question to you guys is this, what in the world can i put in line with this power output to even out the fluctuation? iv thought about a needle type volt meter but they can get even worse if they run into a rythem and start "bouncing" --- iv thought about just putting a battery in to charge but i want better responce than that, i want something thats stable yet will record the average change over the time limit of aprx. 3 to 5 seconds, My bro who is an electrician recomended some kind of filter? its way out of my league, any help is apreciated.

  • #2
    A filter at the meter would be the solution. Connect a capacitor across the meter leads. The negative lead of the capacitor is usually marked on the case. Connect the negative meter lead to the negative output of the generator. Connect the positive meter lead to one end of a resistor and connect the positive lead of the generator to the other end of the resistor. This is the charging resistor. A second resistor is placed in parallel with the capacitor to discharge the capacitor. The charging resistor determines how much current flows to charge the capacitor. More resistance slows the charging rate as does more capacitance. This will even out the power strokes. The discharge resistor slowly discharges the capacitor as you slow down, but not so much between power strokes.

    I did a quick test with a 220 microfarad electrolytic capacitor and a 4700 Ohm resistor. Came pretty close. If you get a 220 mfd, 30 volt capacitor and a couple of 5 or 10K potentiometers (variable resistors, volume controls, etc.) you can adjust the charge and discharge rates of the capacitor to suit your needs. Connection to the potentiometers would be to use the center terminal and one end terminal and leave the remaining terminal open. Just be sure not to turn both potentiometers to minimum resistance at the same time as this allows maximum current to flow and might burn them and/or the generator out.

    Maybe I can post a schematic and some photos tomorrow, if you need them.
    Last edited by Weston Bye; 11-24-2006, 10:44 PM.
    Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
    ~Practitioner of the Electromechanical Arts~

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    • #3
      I just deleted a multi-paragraph dissertation on voltage regulators and perm-mag generators. Bottom line is, any regulator is going to waste power and you don't want that - you want to turn that fan!

      Because such generators produce d.c. pulses (on a scope they have the same shape as the McDonald's Golden Arches , and because the regulators provide output only when the input voltage exceeds the output voltage what you get is pulsed d.c. out (imagine the top half of those arches being the conducting part of the generator output). If you don't also put some filtration there you get switching noise when those pulses dump into the load. The fact is you really don't need to filter anything except the volt meter.

      Go to Radio Shack and buy a 250uf capacitor (voltage rated at twice your generator output) and a 47k ohm resistor. Wire the resistor and capacitor in series and attach the resistor to the generator output (assume positive pole), and the negative lead of the capacitor to ground, or negative. Put your voltmeter across the capacitor and it should clean up the measured voltage rather well. If it is still bouncing, try a 100k resistor. Or even higher. The digital voltmeters are very high impedance devices so even using a 1 meg resistor is viable.

      What you have at this point is a simple single pole low-pass filter that should damp the variations caused by your irregular stroke.

      Edit: Or do what Wes said Great minds and all that!
      Last edited by dp; 11-24-2006, 10:58 PM.

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      • #4
        Ah, who cares about the volts. Just rig up a motor and gear train and have it lift and pour a glass of beer. Run it off your generator and once you've done enough pedalling, your beer's served.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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        • #5
          Use a silicon photocell and put it next to one of the bulbs. Measure the voltage output of the cell. The lag in the bulbs will smooth it nicely. Amorphous silicon cells (the reddish cheap ones) work best with incandescent lights.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            I'm not sure why you need such a constant voltage.

            BTAIM, the first thought I had was a nice, big, mechanical flywheel.

            As for electric/electronic means, it would be possible to build a filter that had a time constant of several seconds, but the coils and capacitors would be really big. An electronic filter is possible and it's design would be an offshoot of the voltage regulators found in most regulated power supplies. You would want a filter with good short term but poor long term regulation. It seems like a lot of work and expense. I really am curious about your reasons for this kind of performance. I suspect that there is a better way to reach your final result.
            Paul A.

            Make it fit.
            You can't win and there is a penalty for trying!

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            • #7
              what a site! ask for a few guru and get several,,, you guys are amazing even giving me the specs (does what kind of amps im putting out matter for what kind of filter and resistors I need to buy)

              Evan im going to end up with a fan so the photocell wont work but thanks.

              Paul im dynamometer testing my experimental bike cranks and really need a stable readout, I hold a rock solid predetermined voltage for a set period of time in a set gear and then check out the average heart rate for that duration, I now have a lockout mech. on my experimentals that enables me to imediatly make them a standard crank set so I get to use the exact bicycle and wheels and everything for testing...

              Darryl your method sounds fun but i wouldnt get any testing done...

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              • #8
                When you add the fan, your load is increasing and the fluctuations could be even worse. If the output of the generator is still good enough for you at this point and you just want to smooth the meter ...

                Get a schottky rectifier diode (maybe from the Shack) as a schottky voltage drop will be lower (around 0.3 volt range vs 0.7 for regular rectifier at light load). Put that in series with your meter connection, cathode (band end) toward the meter + side. Directly across the meter, put a capacitor AND resistor in parallel.

                The diode will create a dirt cheap peak detecting circuit while the RC will smooth it out to ANY value you want without large capacitors (thanks to the meter's 10 Meg ohm or so input).

                For 5 seconds, you could use a 100uf cap and a 50,000 ohm resistor.

                Just noticed dp's post. Difference here is that the diode isolates you from your load ... possibly a good thing in your case as it will definitely remove peaks and valleys.

                Den

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                • #9
                  There is no reason you can't use a small 6 or 12 volt incandescent pilot lamp coupled to a photocell even though you want to power a fan. All you need is a relative output indicator and that is a dead simple way to do it. Devices like that are actually commercially available for producing average readings from complex waveforms.
                  Free software for calculating bolt circles and similar: Click Here

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                  • #10
                    Its a whole nuther world to me, and one im not comfortable at all in -- i think its really cool that you guys can pull this stuff right off the top of your heads, dammit i wish you all agree'd and this would make it much easier!

                    Is a filter a capacitor?

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                    • #11
                      Off to radio shack

                      Im off to the shack, thanks for the reply's, I think you all know what your talking about but Im going to go with DP's specs because one of the important things to me is to have all available power going to the fan and just be able to "scoop" a reading off of the gen. without any draw, He put emphisis on this and it is important to me because of the limited power that is produced mostly due to the type of trainer i have is designed to scrubb enough H.P. so as to not need any added resistance... so there is very limited power to convert.


                      I will briefly give you guys the Gen, specs and tell me if it should be enough to power a light duty fan if you want, i hooked the gen up to the china hoe and got some variables to work with to try and get an est. of what kind of amps i may be producing -- china hoe is a step pulley machine so here ya go


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                      I couldnt get an amp reading after 610rpm because i knew it was going to go over my meters 10 amp limit and i didnt want to puff a fuse, but if the chart is fairly consistant in its progression then 6 volts which is about what i train at should be close to 20+ amps no? and this should be enough to power a fan right? also keep in mind that this was a free run with no resistance --- does that matter? well --- thanks -- im off to radio shack- im excited and i like doing something i dont know a thing about because im learning, thanks again, you guys are great...

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                      • #12
                        The generator power output capacity in watts is dependent upon rpm. Its voltage output is dependent upon load resistance and internal resistance as well as rpm. The testing you do should be with fixed loads if possible as lamps vary in resistance a lot as they heat up. Useful to know from a science/engineering view is the open circuit voltage over the rpm range, the volt/current output at various fixed loads, and the short circuit current capacity (this last part may be depended upon to destroy the generator). There are some characteristics that can be inferred from knowing the open circuit voltage, and the current output when a load is impressed that drops the voltage to half the open circuit voltage (demonstrates the internal impedance of the generator) but this is not entirely reliable and also not particularly useful for your application

                        However - what you need to know is the power requirements of the fan as that is your load. The generator is certainly capable of putting out adequate power for a fan but may not be creating the power in the way the fan requires it. That principally being the correct voltage. The thing to do if you haven't yet is to attach the fan to the generator and spin up the generator on your lathe or mill and see that it does. A little emperical engineering answers a lot of questions quickly.

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                        • #13
                          An electric radiator fan from a small car would be about the right size. You can put out about 1/2 hp on a contious basis and with conversion losses that will amount to maybe 1/4 hp at the fan motor. Those fans draw about 1/4 to 1/3 hp (10 to 30 amps at 13.5 volts) so should be a pretty good match to the generator.

                          You can still monitor relative output with a small pilot lamp and photocell as the drain of the lamp is miniscule and won't affect the linearity of the system.
                          Free software for calculating bolt circles and similar: Click Here

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                          • #14
                            Add to the novelty of the project by using one of these: http://www.intowind.com/windmeter-overview.html

                            It will directly the effectiveness of your effort which is to generate a breeze

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                            • #15
                              Koool, all i could find at the shack was a 220UF cap. and its rated to 35 volts --- I did however find a 47k ohm resistor (actually 5 of them to a pack)

                              Gots me a gurl/friend coming over and we are gonna hook the stuff up and have a little holiday cheer (carlo rossi paisano)

                              If any of you blokes get this let me know whats + and - on the cap. and resistor...

                              Evan, your electric car fan idea is what iv been looking for, if i could use 12 volts id have it made because they are everywhere, my max output will be around 8 or 9 --- my sustained will be 6 to 7, so im looking for a rare 6 volt electric rad fan, one went for too much money on e-bay last week, i can get new for 80 bucks,, they make them aftermarket for the old cars so they are out there, if not i will have to go new,,,

                              DP total cost of cap. and resistor was $2.53,,, thanks , im stoked...

                              even if i blow sumthin up and start a fire it will still be fun

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