# Permanent Magnet Rotary Generator

A device that converts primary rotary kinetic energy into electric energy by induction.

# Design

The design of the featured project is influenced by several authors on the internet, more specifically these and perhaps some other, less well-documented:

The design is based on an axial flux alternator, and is shown in Figure 1.

The assembly of “Axial-Flux Permanent Magnet 3-Phase Generator” is made of the following parts (Shown in Figures 1 and 2):

1. Copper coil (9 pcs)
2. Magnets 40×20×10 mm (24 pcs)
3. Magnet placer (2 pcs)
4. Shaft (1 pc)
5. Bearing holder (2 pcs)
6. Pin (6 pcs)
7. Housing rim (1 pc)
8. Custom weld-on nuts 1 (6 pcs)
9. Custom weld-on nuts 2 (6 pcs)
10. Housing rotor cover (2 pcs)
11. Shaft cover (1 pc)
12. DIN 471 circlips (4 pcs)
13. Bolts and nuts

The specifics of each part are visible on the Youtube video free of charge, or you can buy detailed plans for building the assembly on our eBay store.

## Stator assembly

### Winding coils

Stator is made of nine (9) star-connected copper coils wound from a 1.5 mm thick enameled copper wire. For lower currents one can use leaner wire, which is easier to wind. To wind the coil, you have two options:

1. You should make a tool, shaped as the inner profile of the coil, and use that as the winding tool for the coil as shown in Figure 3.
• Print the coiling tool, for which .stl file is available here for free “1031 – Coiling tool.stl” file for 3D printing.
• You can watch the Youtube video of winding a coil here: https://youtu.be/Cm4TSv-VyXY.

### Connecting coils into a star pattern

When nine coils are wound, it is time to connect the wires in the star pattern as shown in Figure 4 and place the coils in the stator pattern for resin molding.

Wires 1-1, 2-1 and 3-1 are the output wires of the three independent phases. Naming convention I used here, is 3-1 means Phase 3, Wire 1. Follow the connection diagram in Figure 4 and finish the connection of all three phases in the star point by connecting wires 1-6, 2-6 and 3-6 together.

### Stator resin pouring

Coils of the stator are molded in resin, which is poured over the coils. For the resin, you should choose from a number of resins and hardeners. I would recommend a low-viscosity resin, because it will flow in all crevasses and it looks better than one with bubbles. But performance-wise it should make no difference. The purpose of the resin is to hold the coils in place and provide a firm foundation for the generator.

For the build I used ViWood resin by Samson, but any 2-component resin should do.

There are several ways to produce the stator presented in this project, but after extensive planning, I decided to go for a wooden mold. The main reasons for the wooden mold are:

• the ability to make several more copies if needed,
• easy to do,
• This way the stator is a separate part and can be swapped for another, better version in the future or changed for maintenance.

The stator resin was directly poured in a wooden mold made of two parts, a base and the wall. The wooden mold was lacquered with acrylic water soluble lacquer. After the lacquer dried, the mold was lightly sanded to a smooth surface. Before molding, a special purpose separating wax and a separating PVA compound was used. The wax and the PVA that I used for this project can be found under the subsequent links, but any other separating compound(s) can be used.

Finally, the stator coils were placed in the mold and the resin poured over. The drawing of the stator as should be is shown in Figure 5.

The wood was lacquered with an acrylic water soluble lacquer and a PVA compound for easier mold disassembly was generously applied 3 times.

When the wooden positive was done, it was time to mold the silicone. It took 24 hours to reach full elasticity and the mold was ready to be disassembled. You can find a video of stator mold construction and stator resin pouring at this link: https://www.youtube.com/

After the stator was molded and checked for major deformations that would disable the generator from working, the stator needed some minor machining. I took molded stator to a friend’s lathe and he made the necessary machining for the bearings and through holes as shown in Figure 6.

## Rotor assembly

Buy plans (recommended price \$5.00): paypal.me/tilenthaler

Each of the two rotors contain 12 magnets that induce electric voltage in the coils of the stator. The magnets are placed equidistantly around the circumference of the rotor and glued to position. The poles are alternating as shown in Figure 7.

### Rotor plates (2 pcs)

Rotor plates (“1007 – Magnet placer”) are manufactured primarily from 10 mm thick aluminum sheet metal, laser cut and then machined according to Figure 8.

## Shaft

Shaft will hold the rotors in place relatively to the stator, guided by two deep-groove ball bearings DIN 625 T1 – 61806 – 30×42×7. The dimensions of the Shaft are shown in Figure 9.

## Bearing holder

Bearing holder holds the bearings fixed in axial direction relatively to the stator.

## Pin (6 pcs)

Pins transfer rotary motion of the shaft to the rotors.

## Housing rim

Housing rim is placed around the perimeter of the stator and is planned to be made from a sheet metal strip, welded into a rim. The purpose of the hole is to guide 3 phase wires from the coils. Here is a video showing how-to bend a 4 mm thick sheet metal strip into a rim: https://www.youtube.com

## Custom weld-on nuts (2 × 6 pcs)

6 pieces of the weld-on nuts hold the rotor covers in place, the other six, equidistantly apart, hold the entire assembly to a prime mover and/or to the mechanical ground. Here is a video showing how to weld the custom-weld-on nuts to the housing rim: https://www.youtube.com.

## Rotor housing (2 pcs)

Rotor housing prevents rotating parts from posing danger to a person. The rotor housing should be made of metal if you wish to use it as a tool for positioning the custom weld-on nuts [recommended].

## Shaft cover

Shaft cover prevents one side of the shaft from endangering a person while in rotation. There is only one cover, since the other side is attached to a prime mover, for example a shaft of a wind turbine.

Figure 15: Shaft cover drawing.

# Estimation of power output

Electric power output is calculated as

$Latex formula$

where Pel is electric power, URMS is root mean square or effective voltage and IRMS is root mean square or effective electric current. The electric current is what heats up the coils, and for that reason it has to be limited.

## Current estimation

I estimated, that for the 2.08 mm or 15 AWG wire, which I am using in my design, IRMS = 10 A is enough. (Temperature of ambient air 50°C, PU coating, bundle in confined air of 20 wires.)

The coils are set in epoxy and don’t have the best means of cooling. I think it is a conservative estimate, but this is what I will work with in the following sections.

## Voltage estimation

OK, we have the current, now we also need voltage VRMS. Voltage is induced in the coil of N = 20 turns. The law describing the electromotive force emf [V] is the Faraday’s law of induction

$Latex formula$

where Φm is magnetic flux and t is time. Magnetic flux is further defined as

$Latex formula$

where B is magnetic flux density vector and A is area vector. In featured example, the magnetic flux density B is approximately B = 1.3 T (N42 magnet). Area protruding the magnetic flux is approximately A = 10 × 40 mm2, which is the maximal area of the coil under the magnet at any one time.

In the featured design, there are three coils wired together, so the area is 3-times 400 mm2, A = 1200 mm2.

Because the rotors are spinning, the magnetic flux is approximately sinusoidal,

$Latex formula$

where ω is the angular frequency of magnets passing a coil. There are 6 pairs of magnets on the rotor, which means, that the magnets are passing at six-times the frequency of the rotor spinning around its axis, ω = 6 ωrotation.

With all this in mind, the emf or voltage U takes form

$Latex formula$

Let’s analyze this equation in more detail. Voltage U [V] will be increased proportionally, if any of the factors; number of coil turns N, Magnetic flux density B, Area of the coil A or frequency of coils passing the magnets ω, is increased.

Let’s use some real data to calculate the amplitude voltage output of this generator at frequency of rotation f = 20 Hz.

$Latex formula$

Root mean squareof a sinusoidal signal is calculated as

$Latex formula$

## Power estimate

We can conclude with power estimate for the featured generator design,

$Latex formula$

at 20 Hz rotor spinning. If the rotor is spinning faster, we get proportionally more power.

# Assembling

Buy plans (recommended price \$5.00): paypal.me/tilenthaler

Video coming soon.

# Conclusion

Permanent Magnet 3-Phase Axial Flux Rotary Generator is a device that enables transformation of kinetic energy of a primary source to 3-phase electric energy by induction. The featured design is published under TAPR open hardware license and enables a person to create one by herself.

This solution is designed to be used with other design solutions for transforming primary natural forces like air or water flow into rotary kinetic energy (wind turbines and water turbines).

The purpose of these designs is to provide people with plans to become as energy independent as possible.

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