GREEN STAMP – Every week, Automobile Propre answers your questions. This time one of our readers wondered about the difference between two types of electric motors, permanent magnet and wound rotor.
Our reader’s question
It was asked following our comparison between the MG 4 and the electric Renault Megane, which each have a type of electric motor: permanent magnet for the Chinese, wound rotor for the French. Frédéric’s question is simple: “Can you explain what is the difference between these two types of electric motors? “. And we add this question: is there a better one?
Automobile-Propre’s response
In electric cars, there are two main types of motors (or rather electric machines to use the real technical term): synchronous and asynchronous machines. Their name comes from the way the rotor turns depending on the magnetic field of the stator. In the family of synchronous machines, there are still two very different types: permanent magnets and wound rotor. Two blocks that interest us today.
Synchronous machines with permanent magnets (magnets as we know them) are by far the most widespread thanks to their operating principle and their energy efficiency, much better than with an asynchronous solution for example, more affordable but less efficient when She turns. Here, the current which circulates in the stator produces a magnetic field, which then induces a field on the magnets present in the rotor. This phenomenon then sets the rotor in motion, which turns at the same speed as the magnetic current.
The wound rotor motor uses exactly the same principle. With the difference that the rotor here is made up of copper coils which, supplied with current, then become electromagnets. The wound rotor thus has the same role as a rotor with permanent magnets, the latter always rotating at the same speed as the field of the stator in both cases.
In terms of operation, therefore, there is no difference. However, the wound rotor has as many advantages as disadvantages, depending on where you are studying it. There are visible advantages, others less.
Volkswagen ID.3 permanent magnet synchronous machine
First of all, it should be noted that the power supply to the rotor is more energy-intensive at low speed, but that the current can be reduced on the highway to limit consumption. If the winding requires strong engineering, the solution would be more affordable as a whole. And this in particular thanks to the absence of magnets, to increasingly expensive materials. This is the real advantage of this type of machine: by doing without traditional magnets, they do without the rare earths necessary for their manufacture. This is a group of metals that are rare only in name (one would find as many as copper used for coils for example), but their extraction and processing are known to be extremely polluting activities. !
Also, note that the rotor winding can be used to draw maximum AC charging power (at public terminals or at home), when other vehicles must use a more powerful charger. This is why the first Renault Zoé could accept up to 43 kW with their Continental engines. But this is no longer the case, and Renault even pays a high price for this advantage with the Megane!
But all is not so pretty with a wound rotor motor. On the one hand, the energy density is lower, and this requires complex development to offer characteristics similar to those of a permanent magnet machine. Not to be confused with the efficiency which is similar between the two technologies, although the powered rotor consumes energy. And the supply of the coils can only be done in one way: with brushes. These are the parts that send the current to the rotor. As with all stationary parts in contact with moving parts, there is wear. The wound rotor machine will therefore have to go through the maintenance box before the magnet machine. But it could also experience reliability issues due to dust and debris produced by the brushes. In short, longevity is mechanically more compromised.
Renault Megane e-Tech wound-rotor synchronous electric machine
It is not easy to mention the direct advantages or disadvantages of such and such a mechanical solution. Particularly in terms of performance and efficiency, the visible part of the iceberg, in other words consumption, which is also sensitive to other factors (aerodynamics, weight, efficiency of peripherals, etc.). The same applies when it comes to listing the financial savings that can be achieved, since many other parameters, at the manufacturer’s discretion or not, can quickly shatter the final price.
In any event, the energy advantages can be numerous with a wound rotor motor (less consumption at high speed, more regeneration, etc.). Still, in the eyes of the end customer, the benefits are to absent subscribers. Add €2,000 to the table to get 22kW AC charging with the Megane (11kW standard with the MG 4 64kWh), while there are wearing parts that can sacrifice reliability.
All he has for him is respect for the environment and a certain independence from strategic metals, the other name given to rare earths. Obviously, every saving is good to take. But we cannot say that the savings achieved (a magnet would be composed of 23% rare earths, mainly neodymium and samarium) is sufficient to justify the manufacturers’ insistence on their press releases. Especially not for a technology that uses copper (even infinitely recyclable), no doubt as rare as rare earths. In short, it’s like wanting to go on a diet by replacing Coke with Coke Zero…
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With this new Green Stamp section, Automobile-propre journalists are there to answer your questions on electromobility. Choice of a vehicle, need for technical information… Send us your questions in the comments below or by email at this address:
vosquestions[at]automobile-propre.com
