Eng : Hazim Radwan Hadla

Eng : Hazim Radwan HadlaSwitched Reluctance MotorsCairo UniversityFaculty of EngineeringElectrical Power & Machines Dept.

Under the guidance of:Prof. Dr. Khairy F .AliPresented By:Switched Reluctance Motors1. Introduction3 . Torque production2. Operation principles4. Equivalent circuit5. SRM drive system6. Position sensors7.Power Converters for SRMsIntroductionThe reluctance motor is an electric motorin which torque is produced by the tendencyof its moveable part to move to a positionwhere the inductance of the excited windingis maximized.. Introduction Basic Structure

Fig.1 Switched reluctance motor configurationsIntroductionThe reluctance motor is a type of synchronous machine. It has wound field coils of a DC motor for its stator windings and has no coils or magnets on its rotor.. It can be seen that both the stator and rotor havesalient poles; hence, the machine is a doubly salient, singly excited machine.Operation Principles

Cross sectional model of a three phase VR motor, winding arrangement, and equilibrium position with phase 1 excitedOperation Principles

How rotor rotates when excitation is switched from Ph1 to Ph2Operation Principles

Rotor rotation as switching sequence proceeds in a three phase SR motorOperation PrinciplesTherefore, the conduction angle for the phase current is controlled and synchronized with the rotor position, usually by means of a shaft position sensor. Since the movement of the rotor, and hence the production of torque and power, involves a switching of currents into stator windings when there is a variation of reluctance, this variable speed SR motor is referred to as a switched reluctance motor (SRM).Torque ProductionThe torque production in SRM can be explained using the elementary principle of electromechanical energy conversion.The incremental mechanical energy in terms of the electromagnetic torque and change in rotor position is:

the incremental mechanical energy is equal to the change of magnetic energyTorque ProductionThe electromagnetic torque is then:where: L(, i) is the stator inductance at a particular position, and i the stator phase current.

Torque Production1. The torque is proportional to the square of the current and hence, the current can be unipolar to produce unidirectional torque.2. Since the torque is proportional to the square of the current, it has a good starting torque.3. The torque characteristics of SRM are dependent on the relationship between flux linkages and rotor position as a function of current.Torque Production

Fig.2 Inductance vs. rotor positionEquivalent CircuitElectrical equation

In this equation, the three terms on the right-hand side represent the resistive voltage drop, inductive voltage drop and induced emf, respectively. The induced emf, e, isexpressed as:

Equivalent CircuitFig.3 illustrates the equivalent circuit for one phase of the SRM.

Fig.3 Single phase equivalent circuit of SRMSRM Drive SystemA typical SRM drive system is shown in Fig.4. It is made up of four basic components: power converter, control logic circuit, position sensor and the switched reluctance motor.

Fig.4 Structure of SRM drive systemPower Converters for SRM Since the torque in SRM drives is independent of the excitation current polarity, the SRM drives require only one power switch per phase winding.Power Converters for SRMs

Fig.5 Asymmetric converter for SRMFig.5 shows the asymmetric bridge converter.Turning on the two power switches in each phase will circulate a current in that phase of SRM.If the current rises above thecommanded value, the switches are turned off.The energy stored in the motor phase winding will keep the current in the same direct until it is depleted.1.Asymmetric Bridge ConverterPower Converters for SRMs

operation waveformsPower Converters for SRMs2.(n+1) switches and diode configuration

Fig.6 (n+1) switches converter topology A more efficient converter topology is shown in Fig.6When T1 and T2 are turned on, phase A is energized by applying the source voltage across the phase winding. The current can be limited to the set level by controlling either T1 or T2, or both. the circuit provides restricted current control during overlapping phase currents.Power Converters for SRMs

3.Bifilar Type Drive Topology:Fig.7 Bifilar type driveFig.7 shows a converter configuration with one power switch and one diode per phase when T1turns off the current transfers to the bifilar secondary and freewheels through D1 and source hence transferring the energy rapidly.The disadvantages of this inverter is that it needs bifilar windings ,increase The copper losses and reduce the power density of the motorPosition Sensors In the SRM drives, rotor position is essential for the stator phase commutation and advanced angle control. The rotor position is usually acquired by the position sensors.Phototransistor Sensors:

Phototransistor position sensorconclusionsThe particular advantages are:(1) simple rotor structure, possibly with low inertia;(2) simple and reliable stator windings;(3) higher permissible rotor temperature since thereare no magnets;(4) high starting torque without inrush currents;(5) ability to run at extremely high speeds;conclusions There are several disadvantages of SRM:- The pulsed nature of torque production, which leads to torque ripple and acoustic noise.- Higher torque-volume ratio needs a small air gap betweenstator and rotor, which leads to increased acoustic noise and less manufacturing tolerances.- The motor is not self-commutated, it needs a converter and a commutation controller.