# Magnets & Eddy Current Drag help please

• Mr Nobody
A well designed home built generator might be 50% efficient, so the power to generate 30 amps at 240 volts would be 30 X 240 / 50% = 14,400 watts, or about 20 hp.f

#### Mr Nobody

Hello everyone,
I'm working on a home project to create AC current, using magnets (AC Generator).
I understand the principles of how to generate AC current however, I lack the complex knowledge of the physics side.
I have searched and watched various Youtube videos etc, but none of them answer my questions, which are;

1. What can counter act the Eddy Current Drag when rotating N & S pole magnets around a copper coil. From my basic understanding, when achieving 50hz on rotation, as soon as I apply a load to the circuit Eddy fields will cause a drag thus slowing down the rotation and lessening the 50hz.
2. Is there any reason why copper wire coils are used instead of a solid copper bar?
3. How do I calculate the correct amount of copper and magnetic force required to achieve 240vAC / 50hz and say 30amps?

I have an understanding of electrics (NVQ Electrical Installation qualification) but not to this level and frankly need some expert advice.

Thank you, any guidance would be gratefully appreciated.

The only way to counteract the drag from applying a load is to put more power into the generator. Power in is equal to power out plus power lost due to inefficiency. A well designed home built generator might be 50% efficient, so the power to generate 30 amps at 240 volts would be 30 X 240 / 50% = 14,400 watts, or about 20 hp.

Wire size is determined by speed, voltage, and current. You need number of turns to get the voltage, and wire diameter to get the current without generating too much heat.

Search terms permanent magnet generator design will help you to find a lot of good information. Most of the hits seem to be for low speed generators for wind turbines, but the principles equally apply to high speed generators.

DaveE and phinds
Good project. Bravo.
I understand the principles of how to generate AC current however, I lack the complex knowledge of the physics side.
OK, you understand the principles. What is your next goal in this project? Is it to make a more efficient generator? a more powerful generator? To advance your understanding?

2. Is there any reason why copper wire coils are used instead of a solid copper bar?
Why do you think we have coils at all? Why not just a straight piece of wire? That is a very important principle.

The only way to counteract the drag from applying a load is to put more power into the generator. Power in is equal to power out plus power lost due to inefficiency. A well designed home built generator might be 50% efficient, so the power to generate 30 amps at 240 volts would be 30 X 240 / 50% = 14,400 watts, or about 20 hp.

Wire size is determined by speed, voltage, and current. You need number of turns to get the voltage, and wire diameter to get the current without generating too much heat.

Search terms permanent magnet generator design will help you to find a lot of good information. Most of the hits seem to be for low speed generators for wind turbines, but the principles equally apply to high speed generators.
Thank you jrmichler.

Good project. Bravo.

OK, you understand the principles. What is your next goal in this project? Is it to make a more efficient generator? a more powerful generator? To advance your understanding?

Why do you think we have coils at all? Why not just a straight piece of wire? That is a very important principle.
Hi anorlunda,

My goal is to make a self perpetuating and sustaining AC generator.

I know this sounds strange but, I have had this nagging urge to build one in my head for quite a while now. I often get sporadic ideas just come to me, usually in the middle of the night.

When this happens I normally have the knowledge and resources to make it. Certain aspects of this idea is beyond my knowledge, which is why I am asking for your help.

The problem I have is induced eddy current drag. I'm toying with the idea of finding a way to neutralise these by either shielding or creating a mechanism to increase rotation when I drop below 50hz.

It would be easy to make a DC Generator and convert to artificial AC, but I want to make an AC Generator which is free of as many potential parts which could cause a failure. Simple is best!

I think you missed an important point in:
A well designed home built generator might be 50% efficient
He is stating that for every 100 mechanical Watts you use to drive the generator you will get 50 electrical Watts out.

The extra input energy is needed to overcome friction losses in the moving parts (bearings, air drag of the rotating parts, etc.), the electrical losses in the wires and slip rings in the generator, and if it is an electrically excited generator, the wasted magnetic field where not all of the field lines go thru the windings.

The end result is if you put 100 Watts into spin the generator, you have 50 Watts coming out to feed back and try to keep it going, unfortunately a losing proposition! The next time around the loop you have 25 Watts to feed back and 12.5 Watts out!

Oh Well.

In my understanding, there are two competing effects here: applying a force to move a conductor in a magnetic field will create a current (your generator) but having a current moving through a magnetic field a magnetic field will create an opposing force. Drawing a load reduces the frequency unless, as otehrs have noted, increased energy is input to counter the opposing force. Eddy currents are reduced by using coils rather than bars because there is less room for the current to circulate.

I think you missed an important point in:

He is stating that for every 100 mechanical Watts you use to drive the generator you will get 50 electrical Watts out.

The extra input energy is needed to overcome friction losses in the moving parts (bearings, air drag of the rotating parts, etc.), the electrical losses in the wires and slip rings in the generator, and if it is an electrically excited generator, the wasted magnetic field where not all of the field lines go thru the windings.

The end result is if you put 100 Watts into spin the generator, you have 50 Watts coming out to feed back and try to keep it going, unfortunately a losing proposition! The next time around the loop you have 25 Watts to feed back and 12.5 Watts out!

Oh Well.
Hi Tom.G,
I'll be using frictionless magnetic bearings and creating rotation with same pole repelling magnets.

New poster has been reminded of the PF Forbidden Topics list, which includes PMMs
In my understanding, there are two competing effects here: applying a force to move a conductor in a magnetic field will create a current (your generator) but having a current moving through a magnetic field a magnetic field will create an opposing force. Drawing a load reduces the frequency unless, as otehrs have noted, increased energy is input to counter the opposing force. Eddy currents are reduced by using coils rather than bars because there is less room for the current to circulate.
Hi Jon Tarrant,
Very true, so how is this mitigated against in an AC Generator?
I know there's a trade off, I'm just trying to minimise this trade off and increase efficiency. There must be a way, maybe introducing perpetual motion, centrifugal or gyroscopic forces. I know the solution is out there, I just need to find it!
As you can probably guess, I am quite persistent and relish challenging the status quo.