A reluctance motor is an electric motor that produces temporary magnetic poles on its rotor. It is so named because it uses magnetic reluctance to generate torque. The primary advantage of this type of motor is that it typically produces a high power density for a given cost. This motor’s primary disadvantage is that it tends to generate torque ripple at low speed, which produces noise.
The use of reluctance motors has traditionally been limited by the complexity of their design and method of control. Advances in computer design tools have helped overcome the design limitations of these motors. The decreasing cost of embedded microprocessors has provided these motors with adequate control at an acceptable cost. These microprocessors use parameters such as rotor position, current and voltage to control the motor.
A reluctance motor’s stator and rotor are composed of a magnetic material that is highly malleable, such as silicon steel. The stator and rotor contain numerous projections, which produce magnetic poles. The rotor typically contains fewer poles that the stator. This prevents all of the poles from aligning at the same time, which prevents the motor from generating torque. The disparity between the number of rotor poles and the number of stator poles also reduces torque ripple.
The maximum amount of magnetic reluctance occurs when a rotor pole in a reluctance motor is exactly between two stator poles. This position is also known as the rotor pole’s fully unaligned position. The minimum amount of magnetic reluctance occurs when at least two rotor poles align with at least two stator poles. This position is known as the rotor pole’s aligned position.
The stator pole produces a magnetic field that pulls the nearest rotor pole from the fully unaligned position to an aligned position, thus generating torque. The stator’s magnetic field continues to rotate, which pulls the rotor with it. Most modern reluctance motors use switching to control aspects of the motor’s behaviors, such as starting it, operating it smoothly and specifying its speed. Some variations of this type of motor are able to use three-phase alternating current (AC) power.
A synchronous reluctance motor has the same number of stator poles and rotor poles. Holes in the rotor produce areas of low flux to achieve this equality between the stator and rotor. This type of reluctance motor typically contains four or six poles. The energy losses of the rotor are much less than those in induction motors because the rotor does not contain any parts that conduct electricity.