What is the Wattage Calculation for a DIY Generator with Multiple Coils?

[Alkemist]

Here it is - 1.1k for 13W
Ok..
P=V²/R
P=85*85/1100
P=6.5W
Maybe that's why it's not lighting.

Therefore, make sure what type of voltage you have measured. If it's the peak, then you will have to change the o/p of gen from 12v to 17v. That will give you 120V_rms on the bulb and it should be doing:thumbs:

I still have doubt
Let me present the case in different way. Let's consider that My single phase 12V o/p from generator is stepped up to 120V. Take a motor whose internal resistance is 1 ohm. If I apply this P = V*V/R here, it would be 14400. If you take other feasible resistance values... may be 0.3 or 3ohm, you will get P in the range of 48000 to 4800W.

What's wrong with me here?

 

[PhysicoRaj]

If I apply this P = V*V/R here,

That V in V*V/R is V_rms not 120V

 

[Alkemist]

That V in V*V/R is V_rms not 120V

Even in that case if I take it as 85*85/1=7225 W?

Are you sure about this

 

[PhysicoRaj]

Yes. That's right. But if you connect an underrated motor to over voltage of 120V or 85Vrms, that 7225 watts is going to fry your motor..
And... you have taken the resistance of motor as one ohm??

 

[Alkemist]

Yes. That's right. But if you connect an underrated motor to over voltage of 120V or 85Vrms, that 7225 watts is going to fry your motor..
And... you have taken the resistance of motor as one ohm??

There are motors in the market, whose resistance values are in the range of 0.02 - 1 ohm. Remember the earlier motor link I shared? It's resistance is below 0.05 Ohm.

 

[PhysicoRaj]

There are motors in the market, whose resistance values are in the range of 0.02 - 1 ohm. Remember the earlier motor link I shared? It's resistance is below 0.05 Ohm.

Then taking into account of your gen's int resistance as 0.8Ω the voltage across the load (whatever) will not be 120V.

 

[Alkemist]

Yes. That's right. But if you connect an underrated motor to over voltage of 120V or 85Vrms, that 7225 watts is going to fry your motor..
And... you have taken the resistance of motor as one ohm??

Either your Professor is Wrong or I'm Right

 

[Alkemist]

Then taking into account of your gen's int resistance as 0.8Ω the voltage across the load (whatever) will not be 120V.

We stepped it up to 120V, right? And as we put Transformer in between, it is decoupled from the generator too.

 

[PhysicoRaj]

The resistance as 'felt' by the generator will be different. Because it is electrically isolated from the load.
The apparent resistance felt by the generator is:
R_L=actualR_L*[(N_p/N_s)²]
So it is = 1.1k*0.1*0.1=11Ω
so V across this is 12*11/(11+0.8)
=11.186V
This gets stepped up. Hence the o/p at transformer is 11.186*10= 111volts. Not 120 volts! See there is lot of maths behind a simple generator directly connected as compared to through a transformer!

 

[Alkemist]

The resistance as 'felt' by the generator will be different. Because it is electrically isolated from the load.
The apparent resistance felt by the generator is:
R_L=actualR_L*[(N_p/N_s)²]
So it is = 1.1k*0.1*0.1=11Ω
so V across this is 12*11/(11+0.8)
=11.186V
This gets stepped up. Hence the o/p at transformer is 11.186*10= 111volts. Not 120 volts! See there is lot of maths behind a simple generator directly connected as compared to through a transformer!

My objection is not on the calculations. It's on the Power output in this scenario. You see we are getting Power o/p of 111*111/1 = 12321W from that 12 volt generator.

If you think you are Right on 12321 watt of o/p from 12V generator, then I think I should get admission in your school/college

Which one is it by the way?

 

[PhysicoRaj]

If you don't understand that, here it is: Since the gen has a finite internal resistance, exact 12v will not be applied at the input of the transformer. Hence obviously exact 120v cannot be expected at the output. Simple :)

 

[PhysicoRaj]

Wrong.

 

[Alkemist]

If you don't understand that, here it is: Since the gen has a finite internal resistance, exact 12v will not be applied at the input of the transformer. Hence obviously exact 120v cannot be expected at the output. Simple :)

Ok, let's re solve the problem. Generator has 0.6 ohm resistance. In Open circuit it gives 12V o/p. Now we connect it to transformer, whose internal resistance is same as generator. Then we step up that whatever voltage 10 times and then give it to the load of 0.05 ohm.
How does your calculation look like in this case?

For simplicity we can take 1ohm as resistance for generator as well as transformer and then for load. How does it sound in this case

 

[Alkemist]

Wrong.

Wrong For what?

 

[PhysicoRaj]

Wrong in my calculations

 

[PhysicoRaj]

111V is for that 13w bulb. Sorry i had To consider your 1Ω motor right? that will give you 1.48 volts. Implies not 12321W So it does not cross the max power rating of your gen I gave as 45W. And you r not goin to get admission in my college.:tongue2:

 

[Alkemist]

111V is for that 13w bulb. Sorry i had To consider your 1Ω motor right? that will give you 1.48 volts. Implies not 12321W So it does not cross the max power rating of your gen I gave as 45W. And you r not goin to get admission in my college.:tongue2:

So that's what my point was. From 12V battery, you can draw very high power, may be 1kw, 2kw, 3kw or more. But from generator, you can't

And surely, there no school in this world where I can't get admission

 

[PhysicoRaj]

So that's what my point was. From 12V battery, you can draw very high power, may be 1kw, 2kw, 3kw or more. But from generator, you can't

Surely it depends on the max wattage of the generator,yes.. But you told that your 45W generator cannot light a 13W bulb if simply the voltage was stepped up. I told this was wrong.

If 12V cannot lit the bulb, then it won't lit, even if I step up my generator to 120V.

Your notion that whatever be the voltage the power consumed is same - was wrong.

Have you thought of any permanent load for your generator?

 

[Alkemist]

Load will be some 12V or 24V rechargeable battery, which will run bulbs or small motor using inverter. Once finished this setup will be donated to one of the Farmer or I'll keep this in my backyard as show piece Wind generator, if no one takes it. But for now I'm thinking of having some fun calculations on this generator as well as check how much power human can generate.

If internal resistance = load resistance = max power transfer. Then we loose exactly half power in generator itself.

So in short circuit condition one coil generates I*I*R = 16*16*0.8 = 204.8 Watt. This is all internal power dissipation, but that's what generator can generate at the max.

If I have 18 coils short circuited independently, generator generates 204.8 * 18 = 3686.4 [I know I can't turn it by hand any more, neither wind can, this is just for calculations]. So out of this I can extract max useful power of 1843.2 watt from generator.

Now if I connect them in series I get 12*18 = 216V, and P = 3240W.

Let's connect them in Parallel. Resulting resistance is = 0.8/18 = 0.044ohm. If I calculate, 16*18 = 288 Amp, so total power = 288*288*0.044 = 3649.53W.

 

[Simon Bridge]

I just want to make sure:

A generator converts one form of energy into another - in the example, mechanical energy is being converted to electrical. It does not create energy.

You cannot get more energy out of a generator than you put in - you cannot get it out at a faster rate than it goes in. You can run the generator to some sort of storage device and draw energy quite fast from that.

If the generator delivers a certain max current (for a fixed mechanical power in), for a particular load, then naively stepping up the voltage via a transformer will just reduce the max current - this is because the power input is fixed.

You could just pedal harder.

For something like a wind-driven generator, increasing the load usually slows the windmill down.
Usually the setup is such as to overpower the generator for the expected loads and then use some sort of gearing system to get a steady energy output.

The ratio of power delivered to the load to power delivered to the generator is the efficiency.
The efficiency may vary with the load - it is always less than 1, usually a lot less than 1.

A generator rated at 45W may not be able to light up a 25W bulb - it depends on how the generator was rated, how the power delivered varies with the load.

A "25W bulb" just means that for a mains rms voltage it will dissipate energy at the rate of about 25W.
At lower voltages it will dissipate less - but bulbs are not Ohmic resistors, and, anyway, below a certain voltage they would dissipate energy almost entirely outside the visible spectrum. If the goal is to run the bulb at it's rated power, then you may have to step-up the voltage by quite a lot.

The transformer itself is an extra load on the generator. It would be worth looking up the operation of a power transformer.

But the lynchpin is still how that 45W rating for the generator was arrived at.
If there was any statement about that, I missed it. Arn't we supposed to be discussing a specific home-built generator?
It may be helpful to restrict discussions to that.

Right - glad I got that off my chest.
Carry on :)

 

[Alkemist]

But the lynchpin is still how that 45W rating for the generator was arrived at.
If there was any statement about that, I missed it. Arn't we supposed to be discussing a specific home-built generator?
It may be helpful to restrict discussions to that.

Right - glad I got that off my chest.
Carry on :)

My apology Simon, if I sounded/acted off subject...

45W is the max power we can extract from this generator at given rpm. It was calculated based on internal resistance = load resistance and based on that Raj got 45W [Can we call it as rating?].

Rotating this 300 turn coil at 2000 rpm generates 12v (as you have stated) so V=12v; the load you have to connect in order to draw maximum power is same as the internal resistance so R=0.8Ω; then the max wattage is
P_m=V²/4R
=(12)²/4*0.8
=45W.

What I mentioned is without putting load, in short circuit condition, power dissipated in generator is 16 Amp * 16 Amp * 0.8 Ohm = 204.8W, Which is not a useful power, but that's what is being converted as a result of mechanical energy to electrical energy.

In short whatever muscle power I'm supplying to rotate the generator is = 204.8W + energy loss in friction. Out of these 204.8W energy, only 45W max can be delivered to the load.

If I take 2 coils in series it becomes like this:
24V Open circuit voltage. 1.6 Ohm internal resistance. 15 Amp short circuit current. Which is 360W. Here we can deliver 90W to load.

If I take 2 coils in parallel it becomes like this:

12 V open source voltage. 0.4 Ohm internal resistance as coils are in parallel. 30 Amp is short circuit current. So max power which can be delivered to load is 90W.

What I'm concluding is : Input mechanical/muscle power to generator is greater than 360W. And the Max O/p a generator can deliver to load is 90W in this case, which says o/p is 4 times less than input, what a efficiency...

 

[Simon Bridge]

My apology Simon, if I sounded/acted off subject...

You didn;t - I just got a bit concerned that the thread was going in circles.

45W is the max power we can extract from this generator at given rpm. It was calculated based on internal resistance = load resistance and based on that Raj got 45W [Can we call it as rating?].

You can choose any rating system you like (it's your machine) but you have to be clear about what it means.

iirc. That power was dissipated internally - the generator gets hot. Or was that the impedence-matched condition (load = internal resistance)?

What really counts is the power dissipated in the load.
The hypothetical situation concerns a load that is made of a step-up transformer and a light-bulb.

You can figure what to expect by constructing the Thevinin equivalent for the generator and then drawing out the circuit - analyse the circuit normally. (This would be )

Since you have built it already though, you can probably work out the efficiency for different loads experimentally - this would involve driving the crank from a dropped weight and tracking the voltage across the load over time. That would mean getting a recording oscilloscope - iirc: audio-frequency oscilloscope programs can be obtained that use the computer's mic input.

Access to this sort of stuff is pretty much de-rigeur for the sort of things you are attempting.

Of course, the alternative is just to try it and see - then all this discussion is moot ;)
It is a wonder you have not been experimenting - after all, what you really want to know is how it will perform with the wind blowing. Instead of working out how generators get designed, try investigating the properties of the one you built - you'll learn more.

Power generation is quite complicated - the bottom line is that any theories must be checked against real life.

 

[PhysicoRaj]

Load will be some 12V or 24V rechargeable battery

If it's a battery then you will have to rectify the output of the generator to DC.

Load will be some 12V or 24V rechargeable battery

You can't be ambiguous about that. A 12V output cannot charge a 24V battery to 24V.

 

[Alkemist]

If it's a battery then you will have to rectify the output of the generator to DC.



You can't be ambiguous about that. A 12V output cannot charge a 24V battery to 24V.

You are correct. As of now I'm playing with only one coil, which O/P only 12 V. But when I subsequently complete other coils, It will have more Voltage O/P. Then I can decide the series or parallel combination of coils for appropriate battery resistance.

But before adding the load, I was thinking I can test muscle power o/p by short circuiting the coils.

 

[Alkemist]

Here is the video of my Genny.
The gap in between Coil and Magnet is more than 0.8 inch but still I can get more than 12V out of this coil. I believe this coil can give 25-30V or may be more, if the gap is reduced to minimum possible. For now I have only 3 coils ready, not attached in shown video.

Video showing output can be viewed by clickinghttps://picasaweb.google.com/116297647808736459252/Mar92014?authkey=Gv1sRgCO2oqueouPGnUw#slideshow/5988894475777428818

 

[PhysicoRaj]

Hmm.. By seeing the arrangement, I think your gen has a lot more potential than what is being shown on your meter. It seems the fact is that the way you have placed the coil is wasting a lot of flux from the magnet. I understand it's a simple way. And if you really place more coils around, that would make a serious generator!
To charge a battery, you will need a constant source.
Look up these:
Full wave rectification
http://en.wikipedia.org/wiki/Rectifier#Rectifier_output_smoothing
Filters .

 

[Alkemist]

Hmm.. By seeing the arrangement, I think your gen has a lot more potential than what is being shown on your meter. It seems the fact is that the way you have placed the coil is wasting a lot of flux from the magnet. I understand it's a simple way. And if you really place more coils around, that would make a serious generator!
To charge a battery, you will need a constant source.
Look up these:
Full wave rectification
http://en.wikipedia.org/wiki/Rectifier#Rectifier_output_smoothing
Filters .

Yes lot of flux is wasted. In fact I might be using only 20-25% of useful flux as the gap is more than 0.8 inch in between coil and magnet. But that was the simplest and fastest way to get the prototype done. I'll have to construct custom rotor/stator assembly to reduce the gap.
But considering I could reach up to 30V per coil, and can fit 6 coils on that generator, it gives me 180V assuming they all are in phase. Since per coil resistance is 0.6 ohm, I wonder whether parallel or series arrangement or their combination would give me better results in ideal condition [considering load have same resistance as internal resistance]?

I do have High Power rating Diodes, but I'm not there yet to assemble everything together, will do it once I finish with this Genny construction first.

 

[PhysicoRaj]

Since you can always step up the voltage, its better to increase the ampereage of your genny.
Connect them in parallel. You'll get a low internal resistance, high ampereage so more power output.
You can step up/down the voltage to whatever value you need and use a rectifier-filter to power a DC load.