# Big rotor vs multiple small rotors in a Permanent Magnet Generator

• RobertSll
In summary, the conversation discusses two possible configurations for building a permanent magnet generator, with the main difference being the placement of the magnets and coils. In theory, both configurations would result in the same number of magnets passing over the same number of coils in one complete revolution. However, configuration A, with higher speed magnets, would generate a higher voltage in an open circuit. When a load is added, the rotor in configuration A would be more easily slowed down due to the force being applied further away from the axis. The speaker then asks if both configurations would have the same efficiency, with the advantage of higher voltage leading to lower current and lower I^2R losses in the windings.
RobertSll
Hello guys,
I am trying to build a permanent magnet generator, I have 48 magnets and 18 coils, and I am hesitating betwen 2 configurations.
Configuration A: 2 huge steel plates with 24 magnets on each and an 18 coil stator between them or
Configuration B: 4 smaller plates stacked to sandwich 3 stators, same total number of magnets and coils.
In theory, for both configurations, over 1 complete revolution the same number of magnets would pass over the same number of coils. Since voltage is proportional to the variation of flux in time, in config A the speed of the magnets being higher, a higher voltage is obtained, in an open circuit at least.
But when I add a load current will start flowing and that would slow the rotor. In config A the force will be applied far away from the axis making it easier to slow down the rotor while in config B it will have a smaller impact since it is closer to the axis. Having said all that do you think that I would have the same efficiency for both configurations?

For the same power, higher voltage means lower current and lower I^2R losses in the windings.

## 1. What is the difference between a big rotor and multiple small rotors in a Permanent Magnet Generator?

The main difference between a big rotor and multiple small rotors in a Permanent Magnet Generator is the size and number of rotors used. A big rotor refers to a single, large rotor while multiple small rotors refer to several smaller rotors. This impacts the efficiency, power output, and cost of the generator.

## 2. Which type of rotor is more efficient in a Permanent Magnet Generator?

In general, multiple small rotors tend to be more efficient in a Permanent Magnet Generator. This is because having multiple rotors allows for a larger surface area for the magnets to interact with the stator, resulting in a stronger magnetic field and higher power output.

## 3. How does the size and number of rotors affect the power output of a Permanent Magnet Generator?

The size and number of rotors directly impact the power output of a Permanent Magnet Generator. A big rotor has a larger surface area for the magnets to interact with the stator, resulting in a stronger magnetic field and higher power output. However, multiple small rotors can also achieve a higher power output due to the combined effect of their individual magnetic fields.

## 4. Which type of rotor is more cost-effective in a Permanent Magnet Generator?

Generally, multiple small rotors are more cost-effective in a Permanent Magnet Generator. This is because they are easier and cheaper to manufacture than a big rotor, which requires more materials and precision. Additionally, the combined power output of multiple small rotors can be comparable to that of a big rotor at a lower cost.

## 5. Are there any other factors besides efficiency and cost that should be considered when choosing between a big rotor and multiple small rotors in a Permanent Magnet Generator?

Yes, there are other factors to consider when choosing between a big rotor and multiple small rotors in a Permanent Magnet Generator. These include the overall size and weight of the generator, maintenance and repair costs, and the specific application and power requirements. It is important to carefully evaluate all factors to determine the best rotor design for a particular use case.

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