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Synchronous Reluctance Motor Technology

A synchronous reluctance motor is a three-phase electric motor with a magnetically anisotropic rotor structure. In the four-pole version, the rotor has four high and four low permeance axes. High permeance means high magnetic conductivity and higher inductance, while low permeance means lower inductance. Reluctance is the inverse of permeance and is, in practical terms, magnetic resistance; high reluctance results in low inductance. The axes with high permeance can be referred to as the direct, or d-axis, while the axes with high reluctance can be referred to as the quadrature, or q-axis.

When a magnetic field is produced in the air gap by applying exciting currents to the stator windings, the rotor will strive to align its most magnetically conductive axis, the d-axis, with the applied field, in order to minimize the reluctance in the magnetic circuit. In other words, torque is produced in the air gap between the stator and rotor whenever the applied field vector and the d-axis of the rotor are not aligned.

The magnitude of the vector field and the speed of its rotation can be controlled by a frequency converter. The high saliency of the rotor means that its angular position can be simply detected by a sensorless control. Expensive absolute encoders, resolvers and other rotational sensors are therefore not required.

Since performance is dependent on information about the position of the rotor, the motor always needs a frequency converter – it will not operate properly direct-on-line. The rotor runs in synchronism with the applied vector field, striving to minimize reluctance in the magnetic circuit that is present, and this functional principle has given its name to the technology – synchronous reluctance.

Magnet-free synchronous reluctance technology can deliver similar benefits compared to more commonly known permanent magnet technology. Additionally synchronous reluctance motors are as easy to service and as cost-efficient as induction motors, making them an excellent new technology motor alternative. While having good efficiency at nominal load, the partial load efficiency is better compared to traditional induction motors. This is particularly important in variable speed applications where you should save energy by controlling the speed when full output of the motor is not needed.

The newest generation of synchronous reluctance motors features a rotor combined with a conventional induction motor stator. The rotor has no windings which means that rotor power losses are virtually non-existent. This not only increases efficiency but also ensures that the rotor runs cool, which keeps the bearing temperature low and improves bearing life.

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