Simulating Permanent Magnet Generators
Fanny Littmarck | November 6, 2012
Permanent magnet generators, or PM generators as they are also called, generate power without batteries. PM generators consist of a magnetic stator coiled with wire and a wheel with permanent magnets rotating inside the stator. From motorcycles to wind farms, PM generators can be used in many electrical machinery applications. Let’s take a look at how these types of generators work and how they can be simulated.
How Permanent Magnet Generators Work
Essentially, a wire is wound around a stator made of material with high relative permeability. Inside the stator you have a wheel, or rotor, which consists of a center (made up of the same material as the stator) and permanent magnets that create a strong magnetic field. These permanent magnets are typically rare-earth elements, such as samarium for example.
When the rotor is set in motion a current is induced. That is because the electromagnetic fields (EMF) of the permanent magnets on the rotor move past the coiled stator. As the magnets are spaced out like teeth on the rotor, the strength of the EMF fluctuates up and down as the rotor spins. It is this continuous flux that induces the current into the stator wire. Naturally, the faster the rotor spins, the higher the voltage output.
2D Simulation of a PM Generator in COMSOL Multiphysics
I came across a great example of a PM generator when browsing our online Model Gallery. The model shows how the rotation of the rotor with permanent magnets generates an induced EMF in the winding. The voltage is calculated as a function of time during the rotation. Also depicted in the model is the influence on the voltage from material parameters, rotation velocity, and number of turns in the wire winding.
The simulation below was produced by my colleague Valerio so that you can see the model in action:
A brighter color can be observed where the permanent magnets are positioned on the rotor.
As usual, if you have a COMSOL Access account you can download the model documentation of the generator model to read more about it. If you also have COMSOL Multiphysics and the AC/DC Module you may download the model files and explore this multiphysics problem further.
Applications of PM Generators
According to an article published on windpowerengineering.com (Generators 101) permanent magnet generators are used in the wind industry mainly because they do not require a battery to run. The wind is used to set the rotating wheel in motion, and off it goes generating a current. The zero-battery aspect is so important because wind farms tend to be located in remote areas. Harnessing wind power via PM generators requires less maintenance than generators that contain a battery that needs to be replaced every so often.
If you’re interested in motorcycles, you might know that PM generators can also be used to power your bike. I came across an interesting article on how to assemble a permanent magnet alternator for your motorcycle. Thought I’d share it with those of you who are handy in the shop.