Reluctance motor
From Wikipedia, the free encyclopedia
Cross-section of switched reluctance machine with 6 stator and 4 rotor poles. Notice the concentrated windings
on the stator poles.
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.
There are various types of reluctance motor:




Synchronous reluctance motor
Variable reluctance motor
Switched reluctance motor
Variable reluctance stepping motor.
Reluctance motors can deliver very high power density at low cost, making them ideal for many
applications. Disadvantages are hightorque ripple (the difference between maximum and minimum
torque during one revolution) when operated at low speed, and noise[clarification needed] caused by torque
ripple. Until the early twenty-first century their use was limited by the complexity of designing 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 control,
typically based on microcontrollers using control algorithms and real-time computingto tailor drive
waveforms according to rotor position and current or voltage feedback. Before the development
of large-scale integrated circuits the control electronics would have been prohibitively costly.
Contents
[hide]





1Design and operating fundamentals of reluctance motors
2Types of reluctance motor
o 2.1Synchronous reluctance
o 2.2Switched reluctance or variable reluctance motor
 2.2.1Applications
3See also
4References
5External links
Design and operating fundamentals of reluctance motors[edit]
The stator consists of multiple projecting (salient) electromagnet poles, similar to a wound field
brushed DC motor. The rotor consists of soft magnetic material, such as laminatedsilicon steel,
which has multiple projections acting as salient magnetic poles through magnetic reluctance. For
switched reluctance motors, 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 cannot 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.[citation needed]
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 motor[edit]
Synchronous reluctance[edit]
Synchronous reluctance motors have an equal number of stator and rotor poles. The projections on
the rotor are arranged to introduce internal flux “barriers“, 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, rotor losses are minimal compared to those of an induction motor.
Once started at synchronous speed, the motor can operate with sinusoidal voltage. Speed control
requires a variable-frequency drive.
Switched reluctance or variable reluctance motor[edit]
Main article: Switched reluctance motor
The switched reluctance motor (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[citation
needed]
. Common uses for an SRM include applications where the rotor must be held stationary for long
periods, and in potentially explosive environmentssuch as mining because it does not have a
mechanical commutator.
The phase windings in a SRM are electrically isolated from each other, resulting in higher fault
tolerance than 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 positiondependent inductance of the stator phase windings.
Applications[edit]




Analog electric meters.
Some washing machine designs.
Control rod drive mechanisms of nuclear reactors.
Hard disk drive motor.
Torque ripple
From Wikipedia, the free encyclopedia
Torque ripple is an effect seen in many electric motor designs, referring to a periodic increase or
decrease in output torque as the motor shaft rotates. It is measured as the difference in maximum
and minimum torque over one complete revolution, generally expressed as a percentage.
Examples[edit]
A common example is "cogging torque" due to slight asymmetries in the magnetic field generated by
the motor windings, which causes variations in the reluctance depending on the rotor position. This
effect can be reduced by careful selection of the winding layout of the motor, or through the use of
realtime controls to the power delivery.
Torque ripple
From Wikipedia, the free encyclopedia
Torque ripple is an effect seen in many electric motor designs, referring to a periodic increase or
decrease in output torque as the motor shaft rotates. It is measured as the difference in maximum
and minimum torque over one complete revolution, generally expressed as a percentage.
Examples[edit]
A common example is "cogging torque" due to slight asymmetries in the magnetic field generated by
the motor windings, which causes variations in the reluctance depending on the rotor position. This
effect can be reduced by careful selection of the winding layout of the motor, or through the use of
realtime controls to the power delivery.
Magnetic reluctance:
Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic
circuits. It is analogous to resistance in anelectrical circuit, but rather than dissipating electric energy
it stores magnetic energy. In likeness to the way an electric field causes anelectric current to follow
the path of least resistance, a magnetic field causes magnetic flux to follow the path of least
magnetic reluctance. It is a scalar, extensive quantity, akin to electrical resistance. The unit for
magnetic reluctance is inverse henry, H−1.
Definition[edit]
In a DC field, the reluctance is the ratio of the "magnetomotive force” (MMF) in a magnetic circuit to
the magnetic flux in this circuit. In a pulsating DC or AC field, the reluctance is the ratio of the
amplitude of the "magnetomotive force” (MMF) in a magnetic circuit to the amplitude of the magnetic
flux in this circuit (see phasors).
The definition can be expressed as follows:
where
("R") is the reluctance in ampere-turns per weber (a unit that is equivalent to turns
per henry). "Turns" refers to the winding number of an electrical conductor comprising an
inductor.
("F") is the magnetomotive force (MMF) in ampere-turns
Φ ("Phi") is the magnetic flux in webers.
It is sometimes known as Hopkinson's law and is analogous to Ohm's Law with
resistance replaced by reluctance, voltage by MMF and current by magnetic flux.