A reluctance motor is a type of synchronous electric motor which induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance.

A reluctance motor, in its various incarnations, may be known as a:

• synchronous reluctance motor
• variable reluctance motor
• switched reluctance motor
• variable reluctance stepping motor

Reluctance motors can have very high power density at low-cost, making them ideal for many applications. Disadvantages are high torque ripple when operated at low speed, and noise caused by torque ripple. Until recently, their use has been limited by the complexity inherent in both designing the motors and controlling them. These challenges are being overcome by advances in the theory, by the use of sophisticated computer design tools, and by the use of low-cost embedded systems for motor control. These control systems are typically based on microcontrollers using control algorithms and real-time computing to tailor drive waveforms according to rotor position and current or voltage feedback.

Design and operating fundamentals

The stator consists of multiple salient (ie. projecting) electromagnet poles, similar to a wound field brushed DC motor. The rotor consists of soft magnetic material, such as laminated silicon steel, which has multiple projections acting as salient magnetic poles through magnetic reluctance. The number of rotor poles is typically less than the number of stator poles, which minimizes torque ripple and prevents the poles from all aligning simultaneously -- a position which can not generate torque.

When a rotor pole is equidistant from the two adjacent stator poles, the rotor pole is said to be in the "fully unaligned position". This is the position of maximum magnetic reluctance for the rotor pole. In the "aligned position", two (or more) rotor poles are fully aligned with two (or more) stator poles, (which means the rotor poles completely face the stator poles) and is a position of minimum reluctance.

When a stator pole is energized, the rotor torque is in the direction that will reduce reluctance. Thus the nearest rotor pole is pulled from the unaligned position into alignment with the stator field (a position of less reluctance). (This is the same effect used by a solenoid, or when picking up ferromagnetic metal with a magnet.) In order to sustain rotation, the stator field must rotate in advance of the rotor poles, thus constantly "pulling" the rotor along. Some motor variants will run on 3-phase AC power (see the synchronous reluctance variant below). Most modern designs are of the switched reluctance type, because electronic commutation gives significant control advantages for motor starting, speed control, and smooth operation (low torque ripple).

Dual-rotor layouts provide more torque at lower price per volume or per mass.

The inductance of each phase winding in the motor will vary with position, because the reluctance also varies with position. This presents a control systems challenge.

Types of Reluctance motors

Synchronous reluctance

This type is a hybrid inductance/reluctance motor which develops a small amount of synchronous torque. This is an old type which predates modern control systems.

Switched reluctance or variable reluctance motor

Switched reluctance motors (SRM), are a specialized form of stepper motor, distinguished mainly by having fewer poles. The SRM has the lowest construction cost of any industrial electric motor due to its lack of magnets and simple structure. The most common usage for an SRM is in applications where the stator must be held stationary for long periods.

The phase windings in a SRM are electrically isolated from each other, resulting in higher fault tolerance compared to inverter driven AC motors. The optimal drive waveform is not a pure sinusoid, due to the non-linear torque relative to rotor displacement, and the highly position dependent inductance of the stator phase windings.

Applications

• SRM's are used in some washing machine designs.
• SRM's are commonly used in the control rod drive mechanisms of nuclear reactors.

See also