Reluctance motor
Encyclopedia
A reluctance motor is a type of electric motor
that 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:
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 system
s for motor control. These control systems are typically based on microcontroller
s using control algorithms and real-time computing
to tailor drive waveforms according to rotor position and current or voltage feedback.
consists of multiple salient (i.e., 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 system
s challenge.
As the rotor is operating at synchronous speed and there are no current conducting parts in the rotor, the rotor losses are minimal compared to those of induction motor.
Once started at synchronous speed, the SynRM motor can operate with sinusoidal voltage, but the speed control requires an electronic frequency converter.
(SRM) is a form of stepper motor
that uses fewer poles. The SRM has the lowest construction cost of any industrial electric motor because of its simple structure. Common usages for an SRM include applications where the rotor must be held stationary for long periods and in potentially explosive environments
such as mining because it lacks a mechanical commutator.
The phase windings in a SRM are electrically isolated from each other, resulting in higher fault tolerance compared to inverter driven AC induction 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.
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
that induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance
Magnetic reluctance
Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is analogous to resistance in an electrical circuit, but rather than dissipating magnetic energy it stores magnetic energy...
.
A reluctance motor, in its various incarnations, may be known as a:
- Synchronous reluctance motor
- Variable reluctance motor
- Switched Reluctance MotorSwitched Reluctance MotorThe switched reluctance motor is an electric motor which runs by reluctance torque. It has wound field coils as in a DC motor for the stator windings. The rotor however has no magnets or coils attached. The rotor of the motor becomes aligned as soon as the opposite poles of the stator become...
- 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 system
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...
s for motor control. These control systems are typically based on microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
s using control algorithms and real-time computing
Real-time computing
In computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"— e.g. operational deadlines from event to system response. Real-time programs must guarantee response within strict time constraints...
to tailor drive waveforms according to rotor position and current or voltage feedback.
Design and operating fundamentals
The statorStator
The stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...
consists of multiple salient (i.e., projecting) electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
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
Magnetic reluctance
Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is analogous to resistance in an electrical circuit, but rather than dissipating magnetic energy it stores magnetic energy...
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
Solenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...
, or when picking up ferromagnetic metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
with a magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
.) 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
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
(see the synchronous reluctance variant below). Most modern designs are of the switched reluctance type, because electronic commutation
Commutator (electric)
A commutator is a rotary electrical switch in certain types of electric motors or electrical generators that periodically reverses the current direction between the rotor and the external circuit. In a motor, it applies power to the best location on the rotor, and in a generator, picks off power...
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
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
of each phase winding in the motor will vary with position, because the reluctance also varies with position. This presents a control system
Control system
A control system is a device, or set of devices to manage, command, direct or regulate the behavior of other devices or system.There are two common classes of control systems, with many variations and combinations: logic or sequential controls, and feedback or linear controls...
s challenge.
Synchronous reluctance
Synchronous reluctance motors do have an equal number of stator and rotor poles. The projections on the rotor are arranged to introduce internal flux “barriers“ i.e. holes which direct the magnetic flux along the so called direct axis. Typical pole numbers are 4 and 6.As the rotor is operating at synchronous speed and there are no current conducting parts in the rotor, the rotor losses are minimal compared to those of induction motor.
Once started at synchronous speed, the SynRM motor can operate with sinusoidal voltage, but the speed control requires an electronic frequency converter.
Switched reluctance or variable reluctance motor
The Switched Reluctance MotorSwitched Reluctance Motor
The switched reluctance motor is an electric motor which runs by reluctance torque. It has wound field coils as in a DC motor for the stator windings. The rotor however has no magnets or coils attached. The rotor of the motor becomes aligned as soon as the opposite poles of the stator become...
(SRM) is a form of stepper motor
Stepper motor
A stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely without any feedback mechanism , as long as the motor is carefully sized to the application...
that uses fewer poles. The SRM has the lowest construction cost of any industrial electric motor because of its simple structure. Common usages for an SRM include applications where the rotor must be held stationary for long periods and in potentially explosive environments
Electrical Equipment in Hazardous Areas
In electrical engineering, a hazardous location is defined as a place where concentrations of flammable gases, vapors, or dusts occur. Electrical equipment that must be installed in such locations is especially designed and tested to ensure it does not initiate an explosion, due to arcing contacts...
such as mining because it lacks a mechanical commutator.
The phase windings in a SRM are electrically isolated from each other, resulting in higher fault tolerance compared to inverter driven AC induction 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 machineWashing machineA washing machine is a machine designed to wash laundry, such as clothing, towels and sheets...
designs. - SRM's are commonly used in the control rod drive mechanisms of nuclear reactors.
- SRM's are used in some of the products of the Dyson company, the company refer to the motor as the Dyson Digital motor.
External links
- Real-Time Simulation of Switched Reluctance Motor Drives Technical Paper