Switched Reluctance Machine

Efficiency, cost and reliability are the key words which keep surfacing when customers are on the look-out for new motors or generators. These exact keywords are the very same which describe the SRM developed in CIPED.

The purpose of CIPED is to increase power density, specific power, efficiency and reliability while reducing costs. Initially, the problem was to develop a machine for an in-wheel drive train, which called for a machine which was not only able to deliver the necessary power, but to do it efficiently and in a small package. The experience gained during these initial exercises has been further refined, and the new switched reluctance technology demonstrates outstanding performance in all the areas mentioned.

What is a Switched Reluctance Machine?

For starters, the word “electric machine” is used to describe an electric device that has the ability to function both as a motor and a generator. A Switched Reluctance Machine (SRM) is a branch of electric machines, and it relies on the variation in reluctance when the rotor of the machine is moved with respect to the stator. The special thing about the SRM is the lack of permanent magnets and the very simple design, where the “active” parts consist of steel laminations and copper windings on the stator. The result is a simple, cheap and rugged machine.

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The figure shows the most common SRM. This machine is a three-phase machine with six stator poles and four rotor poles, often denoted as a three-phase 6/4 SRM. Around each stator pole, a coil is wound with the number of turns matched to the available voltage and required speed. The coils opposed to each other (e.g. the two red coils) are either connected in series or parallel and form one phase. In order to cause rotation of the rotor, current has to flow in the different phases in a pre-described way.

The special topology of the CIPED SRM has a better utilization of the lamination steel in the stator and rotor, which decreases loss and increases torque density compared to the conventional SRM. The result is a machine with a peak efficiency of 94 % and a power density of 2.9 kW/L which is comparable to PM machines with similar specifications. This peak-efficiency is not unheard of, however, the special thing about the SRM technology is that the efficiency is high in a very broad area of load points, being low and high speed as well as low and high load.

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As mentioned, the current in the phases of the SRM has a pre-described order of which it must flow. As all electric machines used for variable speed applications, the SRM is driven by an inverter to control the current. The inverter for an SRM is not a standard inverter, and through the development of the CIPED SRM, the development of appertaining inverters has followed. The unique topology of an SR inverter adds to the robust nature of the SRM, and the inverter developed in CIPED is no exception. Furthermore, the CIPED inverter is designed in a modular way, which helps decrease cost and increase flexibility.

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