Why are magnetic motors not efficient

Interesting facts about permanent magnet motor technology in compressors

In our interview, Professor Gerd Terörde explains the advantages and disadvantages of permanent magnet motor technology and reluctance motors in industrial compressors.

Gerd Terörde, professor of electrical drive technology, together with his students from the University of Applied Sciences Osnabrück and an industrial partner, set a world record for the acceleration of an electrically powered kart in 2015: from zero to one hundred in 2.635 seconds.

That was also worth an entry in the Guinness Book of Records. So the professor knows what he's talking about, especially when it comes to permanent magnet motors in compressors.

Professor Terörde, we would like to talk to you today about permanent magnet motors versus reluctance motors.

What is the main advantage of a permanent magnet motor compared to a conventional three-phase asynchronous motor?

Terörde: The main advantage of a permanent synchronous motor is based on the fact that there are virtually no losses in the rotor because, in contrast to asynchronous machines, no currents flow in the rotor. I have permanent magnets there. Likewise, there is no field there that induces losses, such as with induction cookers at home or with eddy current brakes in trains, where high losses occur.

Can you briefly explain how a reluctance motor works?

Terörde: The reluctance motor works with the attractive force of magnets on iron, as is known, for example, from kitchen magnets. For this I need a slight asymmetry in the rotor, so that the magnetic parts are attracted by magnetic fields, which in turn are generated by currents.

What are the main differences between a permanent magnet motor and a reluctance motor?

Terörde: The main differences lie in the power to weight ratio, which is significantly greater with the permanently excited synchronous machine. Secondly, the synchronous motor has a slightly higher degree of efficiency, especially at full load, and thirdly, the noise development in the reluctance machine is of course not negligible. In addition, the interaction with a frequency converter plays a decisive role for the entire control. This is much easier with the synchronous machine and has several degrees of freedom, which can further increase efficiency.

And what are the disadvantages of a reluctance motor?

Terörde: The disadvantages of a reluctance motor are both the lower power-to-weight ratio and the somewhat lower efficiency, especially under full load, and because the reluctance motor is not the measure of all things for very high speeds either.

How long have reluctance motors been on the market?

Terörde: Reluctance motors were invented in 1930, which means for a relatively long time. There were many problems, especially with the noise development, and it only had a low power-to-weight ratio. For about 20 years these problems have been more or less reduced, so that nowadays reluctance motors naturally also have their raison d'etre.

What applications are there currently for reluctance motors?

Terörde: Reluctance motors are found on a small scale in household appliances, fans and compressors. Everywhere where the power-to-weight ratio does not play a decisive role and where noise development is not decisive.

Where are permanent magnet motors used?

Terörde: Permanent magnet motors are used in particular where a high power-to-weight ratio is required and where only limited installation space is available. These include the automotive industry, electric motors, turbo runners and where particularly high efficiency is important.

Of course, permanent magnet motors are also used in industrial compressors. This makes them particularly efficient.

Find out more about the latest generation of compressors with permanent magnet motor technology in this article:

New oil-free compressors with permanent magnet motor technology now meet the IE5 standard

Is there a risk that a permanent rotor will be demagnetized?

Terörde: Not anymore today. Because the permanent magnets are built into the rotor, there are magnetic paths so that demagnetization can no longer take place. The problem of corrosion has also been resolved. And thirdly, temperature monitoring. Overcurrents are monitored in the converter.

How many rare earths are needed?

Terörde: That can be compared to seasoning food. The seasoning is the most important thing, but very little of it is needed, which means that very little of this material is used in all high-tech devices.

Can rare earths be recycled?

Terörde: For some years now, new methods have been on the market to recycle the material on a large scale, especially in laptops, cell phones and so on, large amounts of it are built in, which one now wants to make usable again.

Professor Terörde, thank you for talking to us.

You can find more information about the latest generation of industrial compressors with permanent magnet motors here: