3. What is the difference between Switched Reluctance motor and variable reluctance stepper motor?(may12)

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1 EE6703 SPECIAL ELECTRICAL MACHINES UNIT III SWITCHED RELUCTANCE MOTOR PART A 1. What is switched reluctance motor? The switched reluctance motor is a doubly salient, singly excited motor. This means that it has salient poles on both the rotor and the stator, but only one member (usually the stator) carries windings. The rotor has no windings; magnet is or cage windings but is built up from stacks of salient pole laminations. 2. What are the advantages of Switched Reluctance motor?(dec 13, June 2016) Rotor is simple and it tends to have a low inertia, The stator is simple to wind, In most applications the bulk of the losses appear on the stator, which is relatively easy to cool, Due to the absence of magnet the maximum permissible rotor temperature may be higher than in PM motors, Under fault conditions the open circuit voltage and short circuit current are zero or varying small, Extreme by high speeds are possible. 3. What is the difference between Switched Reluctance motor and variable reluctance stepper motor?(may12) Switched Reluctance motor Variable reluctance stepper motor Conduction angle for phase current is Stepper motor is usually fed with a controlled and synchronized with the rotor square wave of phase current without position, usually by means of a shaft rotor position feedback. position sensor. The SRM is designed for high speed. It is usually designed with a limited speed. Closed loop control is necessary Closed loop control is required for high frequency operation 4. What are the applications of Switched Reluctance motor?(dec 2016) Precision position control system for Robotics and Low power servo motor. 5. Give basic features or characteristics of Switched Reluctance motor. (June 14, May 15) The switched reluctance motor is a doubly-salient, singly-excited motor. This means that it has salient poles on both the rotor and the stator but only one member (usually the stator) carries windings. The rotor has no windings, magnets, or cage windings, but is built up from a stack of salient-pole laminations. Low inertia and simple manufacturing, Losses appear only on the stator and easy to cool, No magnets and so permissible rotor temperature is higher than in PM motors, Torque is independent of the polarity of phase current. Reduction in no of

2 semiconductor devices in controller Open circuit voltage and Short circuit current are zero or very small under faulty condition, Immune from shoot through failure, High starting torque, extremely high speeds possible. 6. What are the disadvantages of a Switched Reluctance motor? The absence of free PM excitation imposes the burden of excitation on the stator windings and the controllers and increases the per unit copper losses, is limited, torque/ampere is limited, Non uniform nature of the torque production which leads to torque ripple and may contribute to acoustic noise. 7. Mention different modes of operation of SRM.(Dec 15) Low speed mode, High Speed mode 8. Mention the application of micro stepping VR stepper motor? (Dec 14) Application are Printing, photo type setting.v.r type stepper motor with µ stepping provides very smooth low speed operation and high resolution. 9. Write the relations between the speed and fundamental switching frequency. f=nnr=(r.p.m./60)nr Hz, Nr=No. of rotor poles, If there are q phases there are qnr steps per revolution and the step angle or stroke is given by ε=2π/(qnr) rad. The number of stator poles usually exceeds the number of rotor poles. 10. What is co-energy? In the ψ i curve of a motor, the area between the curve and horizontal i axis is the co-energy W and the other part is the stored field energy Wf. 11. Give the expression for torque of a Switched Reluctance motor.(may 12, June 13) The torque is given by T = [ W / θ] i=const Or by T = [ Wf / θ] ψ =const. With magnetic saturation negligible and with ψ i curve straight line,ψ=li, W =Wf =(½)Li 2z,T = (½)i 2 dl/dθ Nm where T is the torque, L is the inductance, Wf is the stored field energy. 12.Why rotor position sensor is essential for the operation of Switched Reluctance motor? (Dec 12, Dec 2016) It is normally necessary to use a rotor position sensor for communication and speed feedback. The turning ON and OFF operation of the various devices of power semiconductor switching circuit are influenced by signals obtained from rotor position sensor.

3 13. Define: Chopping mode of operation of Switched Reluctance motor? In this mode, also called low speed mode, each phase winding gets excited for a period which is sufficiently long. 14. What are the types of power controllers used for Switched Reluctance motor? (Dec 15) Using two power semiconductors and two diodes per phase, (n + 1) power switching devices and (n+ 1) diodes per phase, Phase windings using Bifilar wires, Dump C converter, Split power supply converter. 15. What are the two types of current control techniques? Hysteresis type control, PWM type control. 16. What is the step angle of an 5 phase Switched Reluctance motor and commutation frequency in each phase for the speed of 6000 rpm. SRM having 10 stator poles and 4 rotor poles. Solution: Step angle ( )=(2π/qNr)=(360 /5*4)=18. Commutation frequency at each phase=(nr*ω)/2π=(4*6000)/60=400hz.[ω = 2πN]. 17. What are the merits of Dump C Converter? This topology uses lower number of switching devices and has only one switch voltage drop, the converter has full regenerative capability, and there is faster demagnetization of phases during commutation. 18. What are the merits of split power supply Converter? It requires lower number of switching devices, there is faster demagnetization of phases during commutation. 19. What are the merits of classic converter or power controller in SRM? (May 12) Control of each phase is completely independent of the other phases; the energy from the off going phase is feedback to the source, which results in useful utilization of the energy. 20. Why SR machines popular in adjustable speed drives?(dec 12) Rotor is simple and it tends to have a low inertia, The stator is simple to wind, In most applications the bulk of the losses appear on the stator, which is relatively easy to cool, Due to the absence of magnet the maximum permissible rotor temperature may be higher than in PM motors, Under fault conditions the open circuit voltage and short circuit current are zero or varying small, Extreme by high speeds are possible.

4 21. Mention some position sensors used in switched reluctance motor.(june 13) Optical encoder, resolver, Speed sensors and Hall Effect sensor. 22. What is the significance of closed loop control in switched reluctance motor.(dec 13, June 2016) Switched reluctance motor is always operated with closed loop control. Normally we have to use a rotor position sensor for commutation and speed feedback. Here the phase windings are energized by using power semiconductor circuit. He turning on and off operation of the various semiconductor devices are influenced by signals obtained from rotor position sensor. It is the main significance of closed loop control in SR motor. 23. Give the advantages of sensorless operation of switched reluctance motor?(june 14) Low cost, Reliable and it avoids additional cost size. 24. List out the advantages and disadvantages of the converter circuit with two power semiconductor devices and two diodes per phase? (Dec 14) Advantages The converter has low number of switching devices which reduces the cost of the converter. The converter is able to freewheel during the chopping thus reducing the switching frequency and losses. Disadvantages The common switch conducts for all phases and thus have higher switching stress. Disability to magnetize a phase while the off going phase is still demagnetizing which results in high torque ripple during commutation. 25. What is Hysteresis current control? This type of current controller maintains a more or less constant throughout the conduction period in each phase. This controller is called hysteresis type controller. PART B 1.Explain the construction and working of Switched Reluctance motor? (May 12, Dec 13) Construction of Switched Reluctance motor

5 working of Switched Reluctance motor Construction: The switched reluctance motor is a doubly salient, singly excited motor. This means that it has salient poles on both the rotor and the stator, but only one member (usually the stator) carries windings. The rotor has no windings; magnet is or cage windings but is built up from stacks of salient pole laminations. The stator is made up of silicon steel stampings with inward projecting poles. The number of poles in the stator is either an even number or odd number. Most of the motors have even number of stator poles. All these stator poles carry field coils. The rotor is made up of silicon steel stampings with outward projecting poles. Number of poles of rotor is different from the number of poles of stator. The rotor shaft carries a position sensor. Principle: Faradays law of electromagnetic interaction. Whenever a current carrying conductor is placed in a magnetic field, it experiences a force. The direction of force given by Flemings left hand rule.the SRM develops an torque due to variable reluctance

6 2.With a block diagram explain the importance of closed loop control of Switched Reluctance motor. (Dec 13,June 14,Dec 14) Switched reluctance motor is always operated with closed loop control. Normally we have to use a rotor position sensor for commutation and speed feedback. Here the phase windings are energized by using power semiconductor circuit. He turning on and off operation of the various semiconductor devices are influenced by signals obtained from rotor position sensor. It is the main significance of closed loop control in SR motor 3.Describe the Hysteresis type and PWM type current regulator for one phase of Switched Reluctance motor.(dec 14) The control methods available for SRM are two types 1.hysteresis type current regulator This type of current controller maintains a more or less constant throughout the conduction period in each phase. This controller is called hysteresis type controller. 2.voltage PWM control or duty cycle control The circuit consists of two transistors T1 and T2, two energy feedback diodes D1 and D2, phase windings A and control circuit. The transistor T2 is turned on by applying base signal from rotor position sensor. The operational amplifier compares reference voltage signal and actual voltage signal. The output of the error signal is fed to the monostable multivibrator.

7 4.Explain in detail about microprocessor based control of Switched Reluctance motor.(may 12, Dec12,Dec 14, Dec 2016) The input DC supply is fed to the power semiconductor switching circuits. The power semiconductor devices are turned on and off by control circuit. The microprocessor or computer functions can be general be categorized as follows 1) control of feed back loops2) Optimal and adaptive control3) Estimation of feedback signals4)general sequencing control5)protection and fault overriding control 5.a.Describe the various power controller circuits to Switched Reluctance motor and explain the operation of any one scheme with suitable circuit diagram. (June 2016, Dec 2016)

8 Classic Converter Merits: 1. Control of each phase is independent of other phase. 2. The converter is able to freewheel during the chopping period. 3. The energy from the off going phase is feedback to the source. Demerits: 1. Higher number of switches required in each phase which makes the converter expensive. b. State the advantages of sensorless operation. (June 2016) The method works in a speed range from stand-still to rated speed (high-speed sensorless operation is still under research). For low-dynamic applications (estimation of commutation angles only), the method is highly robust against heavy saturation of the machine. The position information can be extracted from a large-amplitude voltage signal. Depending on the bus bar voltage used, the measured signal can have an amplitude of hundreds of Volts. Such a signal is less prone to disturbances compared to smallamplitude voltage or current signals.

9 Due to the very short test pulses, no current flows in the idle phase winding. Therefore, no torque disturbance is generated. The additional loss due to the voltage resonances is negligible. 6.Describe the various operating modes of Switched Reluctance motor.(dec 12,Dec 14) There are two types of operating modes of SRM. 1. Single pulse mode 2. PWM mode Single pulse mode: Single pulse mode is also called as high speed mode, the current rise is within limits during small time interval of each phase excitation. PWM mode: It is also called as low speed mode, each phase winding gets excited for a period which is sufficiently long. The current build up during this long period of time may be prohibitively high.

10 7.Derive the voltage and torque equation of Switched Reluctance motor and also explain torque speed characteristics of Switched Reluctance motor.(dec 12,Dec 14, Dec 2016) Voltage equation of SRM= ir+ λ/ t; V= ir+l( i/ t)+iω( L/ θ ) λ/ t-slope of magnetic curve L-incremental inductance ir-resistive drop L( i/ t- emf due to incremental inductance iω( L/ θ- Self emf depends on current,speed and rate of change of inductance with rotor angle. Torque equation The torque is given by T = [ W / θ] i=const Or by T = [ Wf / θ] ψ =const With magnetic saturation negligible and with ψ i curve straight line, ψ = L i,w = Wf = (½) L i 2,T = (½) i 2 dl /d θ Nm b. Discuss the need of rotor position sensor in SRM.

11 CURRENT SENSING CURRENT SENSING METHODS Current control could be inexpensively achieved both with sensors and without sensors. A number of options emerge as indicated in the flow diagram shown in above Figure. Direct measurement with galvanically isolated current transducer is still an expensive solution. Non-inductive, low-ohmic resistors can be used to monitor the current by measuring the voltage drop across them. They are not galvanically isolated, as in many applications galvanic isolation is not a strict requirement. This method provides the most accurate current measurement with the least amount of investment in materials and processing circuitry, but it does introduce an external resistor component in the circuit that consumes power on the order of a watt or less. The indirect method, such as the one utilizing the resistive nature of the drain-to-source conduction path of a MOSFET for monitoring the current, is an attractive alternative. It also does not require any additional components to serve as a transducer, as the phase power switches could be used for monitoring indirectly 28 the currents. Special MOSFETs, called sense FETs, are available with processed current signals that minimize additional control electronic circuitry to transform the drain source conduction voltage drop to the control level and keep it isolated during its off state from the control circuit. Otherwise, the drain voltage at turn-off will be at least equal to the dc source voltage, which is multiple times greater than the control supply voltage and will damage the control circuitry. CURRENT SENSING WITH RESISTORS Two different implementations of resistor-based current measurement are possible. Only one resistor could be used in the path from all the bottom switches to the negative dc bus rail (say, for an asymmetric converter), as shown in Figure1. The phase-current measurement is not available with this implementation during freewheeling of a phase. More or less an average current can be obtained with this setup. If two phases are simultaneously conducting, then it is not possible to separate each phase current in this method. Therefore, it is useful when no overlapping conduction occurs in the machine

12 phases and average current control is acceptable. In the second arrangement, each phase current is sensed independently with one resistor, as shown in below Figure. Including the resistor in the freewheeling path allows the monitoring of phase currents continuously except when the current is being turned off (commutated) once and for all at the end 8.Describe the construction and working of rotary and linear switched reluctance motors.(june 13,Dec 14, June 2016) Since the torque in SRM drives is independent of the excitation current polarity, the SRM drives require only one switch per phase winding. This is contrary to the ac motor drives where at least two switches per phase are required for current control. Moreover, the windings are not in series with the switches in ac motor drives, leading to irreparable damage in shoot-through faults. The SRM drives always have a phase winding in series with a switch. In case of a shoot-through fault, the inductance of the winding limits the rate of rise in current and provides time to initiate protective relaying to isolate the faults. The phases of the SRM are independent and, in case of one winding failure, uninterrupted operation of the motor drive operation is possible, although with reduced power output.

13 LINEAR SWITCHED RELUCTANCE MACHINES INTRODUCTION Linear motor drives are being increasingly considered for machine tool drives because they reduce the need for mechanical subsystems of gears and rotary-to-linear motion converters, such as lead screws. Positioning accuracy is improved by the absence of gears that contribute to the backlashes in the linear motor drives. Linear machine drives combined with electromagnetic levitation are strong candidates for conveyor applications in semiconductor fabrication plants and possibly in low- and high speed transit applications because of their ability to produce propulsion force on the rotating part, known as the translator, without mechanical contact and friction. Linear switched reluctance machines (LSRMs) are the counterparts of the rotating switched reluctance machines. In fact, the linear switched reluctance machine is obtained from its rotary counterpart by cutting, along the shaft over its radius, both the stator and rotor and then rolling them out. In this section, various linear switched reluctance machine configurations are introduced. Further, the ideal inductance profile is related to the stator and translator lamination dimensions. A similar relationship for the rotary switched reluctance machine that has been derived earlier is worth noting.

14 9.Discuss methods of rotor position sensing and sensorless operation. (June 13, Dec 14) CURRENT SENSING METHODS Current control could be inexpensively achieved both with sensors and without sensors. A number of options emerge as indicated. Direct measurement with galvanically isolated current transducer is still an expensive solution. Noninductive, low-ohmic resistors can be used to monitor the current by measuring the voltage drop across them. They are not galvanically isolated, as in many applications galvanic isolation is not a strict requirement. This method provides the most accurate current measurement with the least amount of investment in materials and processing circuitry, but it does introduce an external resistor component in the circuit that consumes power on the order of a watt or less. The indirect method, such as the one utilizing the resistive nature of the drain-to-source conduction path of a MOSFET for monitoring the current, is an attractive alternative. It also does not require any additional components to serve as a transducer, as the phase power switches could be used for monitoring indirectly 28 the currents. Special MOSFETs, called sense FETs, are available with processed current signals that minimize additional control electronic circuitry to transform the drain source conduction voltage drop to the control level and keep it isolated during its off state from the control circuit. Otherwise, the drain voltage at turn-off will be at least equal to the dc source voltage, which is multiple times greater than the control supply voltage and will damage the control circuitry. SENSORLESS ROTOR POSITION ESTIMATION Incremental Inductance Measurement

15 This method uses the fact that the current rise and fall times are proportional to the incremental inductance, and under certain assumptions the rise and fall times reflect the incremental inductance and hence the rotor position itself. Both these techniques are described in detail and their merits and demerits are discussed. The only advantage of this method is that it only needs to measure the current and its rise or fall time to predict the rotor position and can use the active phase itself. Therefore, it can be implemented in real time and online with very little or no extra cost other than some software codes required in the controller. 10.Compare and contrast the performance of SR motor and VR stepper motor.(dec 13) Switched Reluctance motor Variable reluctance stepper motor Conduction angle for phase current is Stepper motor is usually fed with a controlled and synchronized with the rotor square wave of phase current without position, usually by means of a shaft rotor position feedback. position sensor The SRM is designed for efficient power conversion at high speed 11.Derive the expression for static torque in SRM(Dec 2016) It is usually designed as a torque motor with a limited speed.

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