AC Generator Simulation with FEMM.

2D Finite Element Animation.

In this chapter I will describe you a metod to obtain an animation using a Lua script file in FEMM 4.0.

We start from the femm file that we want animate.

We need to create a group with all the components of the rotor. To do this select the components ( arcs, lines, etc ... ) and write the number "1" ( you can use one number for each group that you need in your design ) in the field "In Group" from the "Properties for the selectec block" dialog window. This let us select it in the Lua script file. In this case in the Group number 1.

Then create the Lua Script file as is:

-- dAlfa=5	-- Increase of Alfa ( Degrees of rotor rotation by step )
dAlfa=9	-- Reduce dAlfa to increase number of created images.
-- dAlfa=18
-- dAlfa=36
-- dAlfa=45
-- dAlfa=90
-- dAlfa=180

open("ACGenerator.fem")
mi_saveas("temp.fem")	-- Temp file for save the original
mi_seteditmode("group")

for n=1,(180/dAlfa+1) do

	mi_analyse()
	mi_loadsolution()

	mo_showdensityplot(0,0,0.5,0,"mag") 	   -- Obtain Image
	alfa=dAlfa*(n-1)
	mo_savebitmap(format("acgen_%1$d.bmp",alfa))  -- Save Image
	mo_close()

	mi_selectgroup(1)			-- Select Rotor
	mi_move_rotate(0,0,dAlfa)		-- Rotate the rotor
end

Adjust the Femm window to the area that you want see in the animation, and open the Lua Script.

Waint a moment.

If all is right, we have several BMP files, acgen_0.bmp, acgen_18.bmp, acgen_36.bmp, etc...

You can convert these files to *.gif using IrfanView http://www.irfanview.com

Now, that we have *.gif files we can join it to one file using Microsoft Gif Animator.

To do this: Make a copy of acgen_0.gif and rename it to acgen.gif

Open it with the MS Gif Animator.

Intert the other gif files.

Set the Image Duration to 10 for all the images. Set the Animation Looping to Repeat Forever. Then save you acgen.gif.

Now you can publish you 2D Finite Element Animation on the Web.

To create your own 2D Finite Element Animation, you only need to create the model that you want to animate, solve the problem, and save the solution for each step of the animation. With this you can make an animation of moving parts ( rotating or linear movement, of blocks ) or shape modification ( moving points or lines ).

Open Circuit Voltage.

You can simulate the open circuit voltage of your generator design using the Finite Element Method Magnetics Software ( FEMM ) and with the help of the Lua Programing Language.

As the Voltage Inducend in a coil is governed by the mathematical experssion, Vind = N * d Phi /dt. Where N is number of turns in the estator coil, and Phi is the magnetic flux.

You can create a Lua scipt file and obtain the magnetic flux trough the stator poles for each position of the Permanent Magnet Rotor. And then apply a relation between the angle position and time ( rotational speed ) to obtain the variation of Phi over time ( d Phi / dt ).

The I have obtained a Time-Domain or Transient Simulation of an Electric Generator from a Finite Element Model.

With this you can change the dimensions and form of the permanent magnets in the rotor, the generator dimensions, the speed of rotation, the number of turns in the stator coils, and more variables to see what happen with wave form of Induced Voltage, and adjust it to your application.

For example:

When I have changed the rotor by this new shape.

I have obtained a sinusoidal voltage output.

Now we can see what happen with change of units, from millimeters to centimeters, rotor poles from 2 to 4, stator poles from 2 to 4, number of turns in stator coils to 5000 ( 2500 x 2 ) and rotation speed to 5500 rpm.

Here we have a senoidal voltage with 313 Volts of pic, this means 220 V eff, aprox.

The frequency is 18.3 Hz only. But can work fine for battery charge, bulbs and electric heaters.

Of course, we can change the frequency by increasing the number of poles in the rotor, and/or the rotatonial speed. Then we can reduce the number of turns in the coil and obtain the same out voltage.

Open Circuit Torque.

Another example of what you can do with Finite Element Analysis. Is calculate the torque in Open Circuit conditions, without loading the generator. This Open Circuit Torque is due to the attraction between the permanent magnet rotor and the ferromagnetic estator when the current in the outcoils coils is equal to zero.

With this you can change the dimension and shape of the rotor permanent magnets and see the effects over torque. For non zero current in the output coils we need a more complex transient algorithm, see the section, Simulation of Loaded AC Generator.

With the last Generator Finite Element Model I have obtained this torque in rotor.

That now is senoidla, becose the distance between the permanents magnets in the rotor is zero.

Simulation of Loaded AC Generator.

We can model the Electric Heaters and Electric Bulbs as simple resistive loads, that we can test the effect over the torque required to rotate the generator.

Modeling Electric Heaters and Electric Bulbs.

For this we only need obtain the Resistance from the Power and Voltage using the mathematical expression R = V^2 / P, that we can obtain from I = V / R and P = V * I, were P = Power in Watts, V = Voltage in Volts, I = Current in Amps and R = Resistance in Ohms

Bulb Examples:

24 V, 50 W, 11.52 Ohms

24 V, 20 W, 28.8 Ohms

12 V, 10 W, 14.4 Ohms

12 V, 20 W, 7.2 Ohms

12 V, 50 W, 2.88 Ohms

12 V, 75 W, 1.92 Ohms

6 V, 10 W, 3.6 Ohms

6 V, 20 W, 1.8 Ohms

Heater Examples:

4.5 kW, 240 V, 12.8 Ohms

2.2 kW, 240 V, 26.2 Ohms

1.5 kW, 240 V, 38.4 Ohms

250 W, 240 V, 230.4 Ohms

350 W, 240 V, 164,5 Ohms

450 W, 240 V, 128 Ohms

540 W, 240 V, 106.6 Ohms

635 W, 240 V, 90.7 Ohms

750 W, 240 V, 76.8 Ohms

With this you can see the effects over your ac generator design when you connect all the bulbs of your home in the generator. Well you need calculate the equivalent resistance of all the bulbs connected in parallel, and apply this equivalent resistance to the mathematical model of the generator.

Additionally you can see the effects in you bulbs vs the working conditions of the generators, rotational speed, shape, ferromagnetic material, dimensions, permanent magnet in the rotor, etc...

You can change the design and see what happen.