http://forum.allaboutcircuits.com/showthread.php?t=45277
this was my previous thread

Hi guys i have set up my bicycle generator
and what i'm currently doing is linking up the dc output of my generator straight to a voltmeter
the rating for my generator
Voltage 24V
Output Power 300W
No load current 1.8 Loaded current 16A
loaded speed 2750rpm no load speed 3400 rpm
efficiency 77%

what ive done now is basically linking the output of generator to my voltmeter and when cycling im able to get around 17-20+ volt

What i understand is that you need a sort of circuit to calculate the current and in return get the power.
If i were to directly connect an ammeter to my generator output and connect my voltmeter together would i be able to get a power rating?

do you guys have a simple circuit that involves a light bulb? the wattage that i get does not actually matter. I just want to be able to get the power output.
I'm using a laser tachometer to analyse the speed of tyre and the generator itself.

 

You can multiply the current A * voltage V to get wattage W.

 

Measuring power output requires a load of some kind. You could use power resistors, but they would be big and fairly expensive in the sizes you would need. Perhaps 12V headlamp bulbs would be suitable. These are available with ratings of around 55W, so a combination of two or three of them in parallel might do the job. You could go straight to charging a 12V battery, but this may not be the safest option.

For charging batteries, a fuse is obligatory. So is an anti-discharge diode (otherwise, quite apart from running the battery down, the bike will end up running by itself - seriously dangerous). You might want to get these things arranged right away as their losses will not be negligible, particularly the diode which could burn about 10W.

If using lamps, you would want to monitor the voltage across them, and ideally also measure the current flowing into them using another meter. If you can't get a 20A or more full-scale ammeter, you might check the current of each bulb one at a time at your target voltage, which would need to be about 14.5V if charging a 12V lead-acid battery is your ultimate aim. Knowing how much each bulb draws at 14.5V, you would set up a combination of two or more in parallel then try to pedal so as to get that voltage.

Personally I would be surprised if you could get keep even two 55W headlamp bulbs at full brilliance for long. An electrical output of 110W with 77% generator efficiency implies 143W mechanical input, without allowing for any mechanical losses in driving the generator. You may also find that the efficiency of your 24V motor will be considerably worse when used as a 12V generator.

 

let say i get 2, 12V 55W lamps on a parallel connection directtly connecting to the generator output
P=VI V=IR so i get R=2.6 and I=4.6A
1/r= 1/R + 1/R
How much power do you actually need to power up two 12V 55W lamps on a parallel connection? Isit okay if the voltage is above the 12V lamp?
Current is changes in parallel while voltage is constant
So i'll be putting my voltmeter somewhere between the parralel connection of the lamps and the 20 A Ammeter at the start and end of the circuit.

 

How much power do you actually need to power up two 12V 55W lamps on a parallel connection?

I think you'll see the filaments start to glow with as little as 2v or so. And if 110 watts lights them both fully, I'd guess that 5-10 watts would be enough to make some glow.

Is it okay if the voltage is above the 12V lamp?

An automotive bulb will be fine up to 16v or so, since that's not uncommon in a car. Above that, it's just a matter of lifetime. Higher voltage increases the probability of burnout. But once you connect the load, I doubt that over-voltage will be your problem.

So i'll be putting my voltmeter somewhere between the parralel connection of the lamps and the 20 A Ammeter at the start and end of the circuit.

Voltmeter goes in parallel to your load, ammeter goes in series with it.

 

In the end i connected the generator to another generator same type
in series with an ammeter and in parallel with a voltmeter
ammeter shows around 4 Amps
and voltage around 7
power is 28 watts
I know this may be due to the load that i put at the generator
thing is the dc motor wont move with a higher load is this because my dc motor is a shunt wound? how do i explain the value of current and voltage that i got?
I'm abit confused why my dc motor won't start to move because of a heavier load
but after it speed up under the lesser load i think it would still move
my only conclusion is that it's a shunt wound dc motor charactherictic that you can only add load after it moves.. am i correct guys?