# Magnetic flux and coil

#### sonlinh

Am I understand correct that when an air coil pass through magnetic flux, it create current and emf? power = IV

If turns increase, do I get more power? why?
If power changes, then how to get out max power.

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#### davenn

Gold Member
Hi there Solinh
welcome to PF

rather than giving you the total answer straight out I will give you part and then ask you a question ... its good for you to think about a problem ;)

Am I understand correct that when an air coil pass through magnetic flux, it create current and emf? power = IV

If turns increase, do I get more power? why?
Yes you do
Now why do you think that would be ?
why would increasing the turns increase the current generated ?

If power changes, then how to get out max power.
that will mainly be answered when you figure out the answer to my question back to you :)

Dave

#### sonlinh

Dave,
According to Faraday's law if turns increase, V will increase. But Current should be reduce due to the coil increasing load (resitent). I think! Otherwise, we would have infinite output just by increasing turns!
I could not find the current formular. It would be great if one can point out.
Thanks.

#### Crazymechanic

Well basically @sonlinh if you have a constant magnetic flux "A" like in a transformer core and you then put a winding near it you can only get as much current out of that winding as the magnetic flux "A" is capable of giving you. if you will decrease the turns of your winding you will get more current less voltage if you will increase the turns you will get the opposite more voltage , less current.That's because of energy conservation.

So by a given time changing magnetic field you can get only a given current /voltage or power as you would say , just increasing the turns in a given situations doesn't give you more power rather increasing the magnetic flux strength will do that.

#### sonlinh

Thank you.

Now, If I have two coils and 1 magnet. If a magnet pass through between two coils, I will get two powers on two coils (1 through the N pole and the other to the S pole).

case 1: If two coils connect in series, I would get not thing because N and S cancel each other.
case 2: If two coils connect in parallel, I should get two powers. In this case, I should have turn =1 to have max current to have max power output of two coils. Maybe I also need a rectifier too.

Am I correct?

#### Crazymechanic

You don't get "two powers" or one power , power is the measure of how much work has been or is done in a given amount of time.A kid can move let's say ten bricks in one minute a grown man does like 25/30 in one minute.
If you have one magnet at a certain size and magnetic field strength then it can create just as much power as it can no more no less , arranging the coil in series or parallel just changes the voltage/current ratio nothing more than that. Well ofcourse the total power will increase also by moving the magnet through the coil at a faster rate, I mean the more complete periods of moving the magnet through the coil you make, the higher the frequency of the induced ac will be.

If you connect those coils in series and put the same magnet through them then nothing cancels out, magnets North and South don't just cancel out they can't. All that will happen putting the coils in series will be higher voltage less current.

the way to get max power out of a given coil and magnet depends on the magnet strength wire size and turns you can calculate those in simple equations.

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#### sonlinh

Assuming the rate of a magnet moving and the size of the coils are the same.
the coil is 1 in diameter and the inner diameter is 1/2 in
a cylinder magnet diameter 1/2 inch and 1/2 thick.

case 1: place 2 coils next to other. a magnet passing by say 1/2 inch apart.
case 2: same as case 1, but I put another coil #3 in the middle on top between 2 coils (total 3 coil). Now if I place (lift) the coil #3 to 1 inch apart from the other 2 coils. a magnet passing by in the middle b/w coil #3 and coil (1+2).

Should I have more power in case 2 as compare to 1?

#### Crazymechanic

@Sonlinh it is kinda hard to understand all your arrangements , but basically all the other coils who are just next to the one in which the magnet is are pretty much useless.
If you have one magnet and one coil and the magnet is moving through the coil creating a emf in the coil and a voltage, then to maximize the current and power gained from the magnet you either increase/decrease the wire turns , or for more current you get a thicker wire , bigger gauge.

Also you can take two isolated wires and wrap them together in each turn like they would be one wire and connect them electrically at the ends , this is called bifilar winding and they do this to increase the current capability of the wire and to increase the area which would be in the magnetic field of the magnet to maximize the energy that could be gained from the moving magnet.

But placing magnet out of the coil and having more empty coils near by won't do you any good, ofcourse some small amount of emf will be induced but if your looking for maximum this is not the way to achieve it.

#### sonlinh

Sorry I did not explain well. I Attached a picture of a stator for magnet AC alternator from windbluepower.com.

There are two layers of ring coils.

Based on the previous answer, the reason it has two layers becasue of space limited when a rotor spinning around the coils?

or something else.
Thanks.

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#### Crazymechanic

Well I'll be honest I haven't searched about this particular one or at all but from what i know is that it is not because there would be the lack of space rather because as I can tell by the three wires coming out it is a three phase generator and just as in three phase generators like in three phase motors the stator windings can overlay each other partly this is rather because of the phase angles and how the stator is wound to achieve better performance than a matter of space.

Take apart any electric motor AC induction type I have seen big ones , ones that are as high as 1m.
They all have these stator coils wound so that the next ones partly overlay the ones before and I think it has to do with the 120 phase angle in the three phase system and the motors magnetic field ability to do efficient work.Even though the three phase motor could use just three 120grade apart windings to maximize the efficiency and power output for given power input they wind more windings for each phase and split out the each of those 120 grade sections in the stator.Also the motor speed is in play here the ac asynchronous motor speed is not only affected from the ac frequency like 60/50 hz or above those when using a special semiconductor based speed controller.The speed is also dependant on how the motor is wound hence the coils wire diameter coil array so on.

http://www.me.ua.edu/me360/PDF/AC_Motors.pdf
http://en.wikipedia.org/wiki/Induction_motor

Here are some basic things about AC asynchronous induction motor.

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