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Switched Reluctance

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  • FeaturesDoubly salient, singly excited machineSalient poles on both the stator and rotorOnly one member (stator) carries windings.

  • Difference from stepper motorCurrent in the phase flows when the inductance is increasing , current weak or absent when L is reducing.Designed for efficient power conversions at high speeds.Improved pf.Improved power efficiency.

  • AdvantagesRotor simple, requires less manufacturing steps.Absence of brushes.High efficiency.

  • DisadvantagesPulsed or non-uniform nature of torque production which leads to torque ripple and acoustic noise.Need for position measurement.

  • Principle of operationSR motor with 8 stator poles and 6 rotor poles.The stator and rotor poles seek minimum reluctance position.Hence when current flows in SR motor stator phase, produces a magnetic field , the nearest rotor will tend to position itself with the direction of magnetic field- Aligned position.

  • Aligned position rotor central axis aligned with stator pole center axis.Position of minimum reluctancePosition of maximum inductanceUnaligned positionrotor inter-pole axis aligned with stator pole axis.Position of minimum inductance

  • To achieve continuous rotation SR motor with 8 stator poles and 6 rotor teeth.8 stator coils connected to give 4 phases.One or two phase energized at a time.At a time 1 -1 alone energized.Stable position where the inductance of phase 1-1 maximum.Slight disturbance of the rotor from this position is followed for the development of an electromagnetic torque trying to pull back the rotor to the original position.Now let phase 1-1 de-energised ,phase 2-2 switched ON.The rotor moves to a new stable position 15in the CCW direction.

  • Inductance variation and torque distribution

  • Torque derivation

  • Torque derivation

  • Inductance and corresponding torque

  • Inductance of each phase increases -remains constant increases cyclically-depending on the amount of overlap between the stator pole and rotor tooth.Permeance directly proportional to the overlap.It is necessary to force a current in the winding when the rotor tooth about to enter the stator pole so that i2 (dL/d) is high.Hence essential to know the precise position of the rotor at any instant of time.

  • Region A- begins at rotor angle 1- first edge of the rotor meets the first edge of the stator pole. Inductance rise in a linear fashion until the poles of the rotor and stator completely overlapped at an angle 2.

  • Region B- from 2 to 3.Inductance remains constant because rotor pole completely overlapped by stator pole. At rotor angle 3, the edge of the rotor pole reaches stator pole overlap region. The area of overlap will begin to decrease.

  • Region C- when the rotor moves past 3 , the rotor pole leading edge begins to leave the pole overlap region, and region C begins. Inductance begins to linearly decrease, until at 4,the rotor pole has completely left the stator pole face overlap region. Inductance minimum once more.

  • Region D- rotor and stator have no overlap-inductance remains constant-at the minimum level.

  • Torque can be controlled to give a resultant which is positive(motoring) or negative(generating action) by switching the coil on and off at appropriate inductance cycle.

  • Step angle

  • Speed and switching frequencySpeed= (f* step angle in degrees* number of stator poles* 60)/360

  • VR machine drive

  • Low to medium speed drivesFor motoring-square pulse of current flowing only during the increasing inductance region.In practice, current waveform not a square pulse-inductance of stator phase winding would delay the rise and fall of the current at pulse edges. Closely approximated using hysteresis current control(chopping mode control).

  • During time of conduction (between turn-on and turn-off angles)- current maintained within hysteresis band by switching on and off of the phase voltage by the inverter when the phase current reaches maximum and minimum hysteresis band.Turn on region (angle between the turn-on angle on and turn off angle off .on placed little before rising inductance region.off placed little before maximum inductance region

  • Power electronic controllerThe direction of torque independent of sign of current.So the current may be unipolar. This mode of operation has lower iron losses and simpler.Controller must apply unipolar current pulses ,relative to the rotor position.

  • Power Electronic Controller

  • Power Electronic Controller2 transistors /phaseMaximum control flexibility and efficiency , with a minimum of passive components.

  • Contol System structure

  • Control System StructureInput current to the SR motor controlled by the SR controller based on the speed error.Speed error-difference between the reference(desired) speed and the actual speed.Actual speed derived from the speed transducer coupled to the load on the motor.

  • LMB1008 control IC28 pin, 24V IC.

  • LMB1008 controller based SR motor

  • LMB1008 controller based SR motorSpeed error amplifier produces a pulse train whose duty cycle proportional to speed error.Used to chop the power transistors at fixed frequency.Current limit input compares with external current sensor signals to turn the transistor off if the current exceeds a preset value.Commutation based on shaft position signal.

  • General Controller Architecture

  • Firing angle variation with speed

  • Firing angle variation with speedAt the initial speed,firing angles are turn-on angle,0 and turn-off angle c.c - 0 conduction angle or dwell angleAs the speed increases, angles are both advanced till the speed reaches the reference speed. At this speed firing angles are maximum 0R and cR . If the speed exceeds the reference speed, firing angles are suddenly retarded to fixed values 0B and cB .

  • Sensorless control Main reasons for eliminating the shaft position sensors to reduce costTo increase reliability and adapt the drive for operation in harsh environments, such as high-temperature environments

  • Sensorless control schemesActive probingState observersOpen-loop control with some form of additional stabilizationPassive waveform detection

  • Active probing

  • Active probing

  • Active ProbingA square wave voltage is applied to unexcited phase.Current pulses increase as the phase inductance decreases reaching a maximum at unaligned position.Then the current pulses decrease as the inductance increases.By detecting when the current pulses fall below a particular threshold ,a particular rotor position detected and used for switching on or commutation of the power transistors.

  • State observersState observer-Mathematical simulation of the motor running on line on a microcomputer in parallel with actual drive.Measured current and voltages from the actual drive fed to the model.Model formulated in such a way that explicit signals representing rotor position and speed available from it.

  • Open-loop control with some form of additional stabilization

  • Open-loop control with some form of additional stabilizationDwell angle ( c - 0) and commutation frequency is controlled.Stabilizing circuitry provided to detect any departure from stable synchronous operation and take corrective action.Increase the dwell angle in response to sudden increase in DC link current ,which is taken to indicate an increase in load torque.

  • Passive waveform detectionRelies on natural points on inflection ,or maxima or minima, in the phase current waveform as the rotor passes certain positions such as aligned position or unaligned position.

  • Microprocessor based controller Main supply: provide circuit energyMicroprocessor supply: Regulates voltage and current for microprocessorMicroprocessor: Produces accurate switching signals MOSFET in the driver circuit.Driver: switches the power necessary for motor phases.Motor feedback: Gives continuously information about the position of the rotor, speed and spinning direction.

  • Microprocessor based controllerMain supplyDriverMotor FeedbackMicroprocessorMicroprocessor power supplyM

  • System configuration

  • System configurationMicroprocessor runs the main control algorithm. It generates PWM output signals for SRM.Rotor alignment and initialization of control algorithm is performed.Stabilization takes less than 1 sec.After this motor stable enough to reliably start the motor in any direction of rotation.

  • Rotor position evaluated using an external encoder.Timer counts the external no. of pulses provided by the encoder and the required value is loaded to the counter register.Encoder provides index signal-occurs once in a revolution.Reference speed set in potentiometer.Actual speed and reference speed compared and fed to the speed controller to evaluate the voltage applied to each phase winding.

  • Commutation algorithm includes calculation of commutation angles and phases to be commutated.When the actual position of the motor equal to the reference position, the actual phase is turned off and the following phase is turned on.