Reluctance motor

A reluctance motor is a design of an electric motor in which the torque is produced solely by means of the reluctance in the rotor and not substantial proportions by the Lorentz force, as is the case in magnetically excited machine. This means that the machine is equipped with either permanent magnets or electrical coils of the rotor are. This inherently also eliminates any kind of susceptible to wear slip rings and brushes. The rotor has salient poles, and consisting of a high permeability soft magnetic material such as electrical steel.

In comparison with permanent magnet excited machines such as the synchronous machine, the reluctance to low torque density. That is, the nominal torque based on the volume of the machine is reduced. However, since the type of motor in the synchronous mode of operation by means of a frequency converter has a higher efficiency than other asynchronous and synchronous motors, it is still economical.

Basic principle

The move comes due to the fact that the system strives for minimal magnetic resistance ( reluctance ). The model can be thought of a toroidal coil in which one leg is loose ( see picture). The reluctance of the loose leg is seeking position in which the reluctance reaches its minimum (that is, the inductance reaches its maximum ). The reluctance motor always two poles of the stator and two poles of the rotor thus forming a "ring " that the rotor so it appears that the reluctance is as low as possible ( ie parallel to the field lines ).

An alternative (slightly more mathematical ) approach is that the magnetic field vector that passes through the rotor, is broken down into several components. Since the rotor is in saturation, the sub-vectors does not reach full " length", but only a maximum, which is higher in the direction of the poles ( for lower reluctance ) than in other directions. Adding up now so " shortened " components vectorially, it shows their vectorial sum in a different direction than the causal field vector of the stator. The rotor then rotates until the vectors point in the same direction.

By the stator poles are magnetized with a time delay, the force always acts in a different direction, and it comes about a rotation.

Benefits

The advantage of a reluctance motor is the fact that losses are practically only in the resting and thus well be cooled from the outside stator. Consequently, suitably built reluctance motors are tolerant of short term overload. Due to the relatively simple structure of the rotor without coils or special materials ( there is no permanent magnets, and no materials such as rare earths needed ), the rotor can be performed tolerant against overspeed robust and with appropriate design.

Disadvantages

A disadvantage of reluctance motor is mainly the cogging torque, which affects particularly at smaller number of stator poles. Other disadvantages are pulsating radial forces between the rotor and stator, which stress the bearings, and are responsible for a relatively high noise levels. Also, a reactive current is for the structure of the rotating field of the induction machine as required. This reactive current apparent power electronic converters increases.

Types of reluctance motors

There are three types of reluctance motors:

• The synchronous reluctance motor has a wound polyphase stator (stator ) as an asynchronous machine. However, the rotor (rotor ) is not round, but has salient poles on. It is usually driven by a frequency converter, so that the advantages of high efficiency can be used without the disadvantages ( need for reactive power ).

If you want to dispense with the frequency converter, the motor is usually as an asynchronous machine equipped with a squirrel-cage induction motor and is then called with reluctance torque. He then operates like an induction motor up to the vicinity of the asynchronous equilibrium speed. Then outweighs the reluctance, and the rotor rotates in synchronism with the rotating field. So lets build a simple and relatively inexpensive way, with the stator synchronously running AC motor, but has this design compared to its operation with frequency converter as disadvantages compared to synchronous and asynchronous motors with a lower efficiency, lower power density, and above all a high reactive power demand on.

For this reason, it is useful to control the synchronous reluctance motor with a frequency converter.

• Switched reluctance machine ( SRM short, of English: switched reluctance motor, SR -drive): Such reluctance motors have as the other types have a different number marked teeth on the rotor and stator. The stator teeth are wound with coils that are alternately switched on and off. The teeth with the energized windings each draw to the nearest teeth of the rotor as an electromagnet and are switched off when (or just before ) the teeth of the rotor facing the stator teeth it attractive. In this position, the next phase is switched to the other stator, the rotor attracts other teeth. In general, a switched reluctance motor, three or more phases. There are also special designs with only two or phase. To switch at the right time, the machine is usually provided with a rotor position encoder. But there are also sensorless control method based on the stator current or the torque. Reluctance motors of this type are characterized by high robustness and low construction costs. As asynchronous form in the non-energized state when no rotation torque. Often still a residual magnetization leads to a small cogging torque in de-energized state. At low speeds, they are superior induction motors torque density due to the high numbers of pole pairs produced, clearly inferior at higher. Permanent magnet synchronous machines they are in this respect inferior in any case.
• Reluctance stepper motor: A reluctance stepper motor can be constructed as a switched reluctance motor in principle the same. It is contrary to this connected but without knowing the rotor position, thus is simpler but less reliable (step losses). In contrast to other stepping motors of the variable reluctance stepper motor is not able to keep its position in the currentless state.

Applications

Reluctance motors are ideal for medium-sized drives ( diameter of 100 to 300 mm) with low turn-on. Due to its simple and robust design (eg no rotor windings or magnets) they are very well suited for operation in harsh environments. For small motors, they are ruled out due to low power density and low efficiency, and large due to low energy efficiency and high reactive power demand. Currently, versions are known up to 400 kVA.

Another field of application for synchronous reluctance motors running can be found in the textile industry for synchronous unwinding of yarn.

Switched reluctance motors have been used in hybrid electric vehicles as a parallel hybrid drive, because, in contrast to the permanent-magnet motors run without losses in the drive by the gasoline engine and unlike induction motors have more torque, especially in the start-up.

One advantage of running synchronously switched reluctance motors as is the cost-effective manufacture of the engine. For switched reluctance motors, the control electronics is more expensive than for other technologies due to the high motor reactive power requirement. As a result of lower prices for electronic components, they are nevertheless now also for use in larger domestic appliances (washing machines, cleaning pumps) attractive.