special electrical motor(switched reluctance motor)

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This is for those who want to know about switched reluctance moto basics........r

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special electrical motor(switched reluctance motor)

  1. 1. Regency Institute of Technology, Yanam 1 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept Unit-3 Switched Reluctance Motor Introduction Switched Reluctance Motors (SRM) have inherent advantages such as simple structure with non winding construction in rotor side, fail safe because of its characteristic which has a high tolerances, robustness, low cost with no permanent magnet in the structure, and possible operation in high temperatures or in intense temperature variations. The origin of the reluctance motor can be traced back to 1842,but the “reinvention” has been possibly due to the advent of inexpensive, high-power switching devices. The torque production in switched reluctance motor comes from the tendency of the rotor poles to align with the excited stator poles. The operation principle is based on the difference in magnetic reluctance for magnetic field lines between aligned and unaligned rotor position when a stator coil is excited, the rotor experiences a force which will pull the rotor to the aligned position. However, because SRM construction with doubly salient poles and its non-linear magnetic characteristics, the problems of acoustic noise and torque ripple are more severe than these of other traditional motors. The torque ripple is an inherent drawback of switched reluctance motor drives. Construction of a switched reluctance motor (SRM): Switched reluctance motor (SRM) drives are simpler in construction compared to induction and synchronous machines. Their combination with power electronic controllers may yield an economical solution . The structure of the motor is simple with concentrated coils on the stator and no coils or magnets on its rotor. SRM is a type of synchronous machine. It can be seen that both the stator and rotor have salient poles; hence, the machine is a doubly salient, singly excited machine. Stator windings on diametrically opposite poles are connected in series or parallel to form one phase of the motor. Several combinations of stator and rotor poles are possible, such as 6/4 (6 stator poles and 4 rotor poles), 8/4, 10/6 etc. The configurations with higher number of stator/rotor pole combinations have less torque ripple. The Switched Reluctance Motor drives present several advantages as high efficiency, maximum operating speed, good performance of the motor in terms of torque/inertia ratio together with four-quadrant operation, making it an attractive solution for variable speed applications. The very wide size, power and speed range together with the economical aspects of its construction, will give the SRM place in the drives family. The performances of switched reluctance motor strongly depend on the applied control. Fig.1 Switched reluctance motor configurations
  2. 2. Regency Institute of Technology, Yanam 2 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept Operation of a switched reluctance motor (SRM): The reluctance motor is an electric motor in which torque is produced by the tendency of its moveable part to move to a position where the inductance of the excited winding is maximized. Fig.1 shows its typical structure. It can be seen that both the stator and rotor have salient poles; hence, the machine is a doubly salient machine. The rotor is aligned whenever the diametrically opposite stator poles are excited. In a magnetic circuit, the rotating part prefers to come to the minimum reluctance position at the instance of excitation. While two rotor poles are aligned to the two stator poles, another set of rotor poles is out of alignment with respect to a different set of stator poles. Then, this set of stator poles is excited to bring the rotor poles into alignment. This elementary operation can be explained by Fig.2. In the figure, consider that the rotor poles r1 and r1′ and stator poles c and c′ are aligned. Apply a current to phase a with the current direction as shown in Fig.2a. A flux is established through stator poles a and a′ and rotor poles r2 and r2′ which tends to pull the rotor poles r2 and r2′toward the stator poles a and a′, respectively. When they are aligned, the stator current of phase a is turned off and the corresponding situation is shown in Fig.2b. Now the stator winding b is excited, pulling r1 and r1′ toward b and b′, respectively, in a clockwise direction. Likewise, energizing phase c winding results in the alignment of r2 and r2′ with c and c′, respectively. Accordingly, by switching the stator currents in such a sequence, the rotor is rotated. Similarly, the switching of current in the sequence of acb will result in the reversal of rotor rotation. Since the movement of the rotor, 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 motor is referred to as a switched reluctance motor (SRM). Consideration of this basic operation highlights that the operating direction has nothing to do with the current polarity. Direction of rotation is determined by the sequence of the energized phases, and the frequency of this switching sequence dictates the speed of the rotor. Fig.2 Operation of switched reluctance motor (a) Phase c aligned and (b) Phase a aligned Torque: The torque production in the switched reluctance motor can be explained using the elementary principle of electromechanical energy conversion. In the case of a rotating machine, the incremental mechanical energy in terms of the electromagnetic torque and change in rotor position can be written as: ΔWm = TeΔθ --------------(1) where Te is the electromagnetic torque and Δθ the incremental rotor angle. Therefore,
  3. 3. Regency Institute of Technology, Yanam 3 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept the electromagnetic torque can be obtained by: -----------------------(2) For the case of constant excitation (i.e., when the mmf is constant), the incremental mechanical energy is equal to the change of magnetic coenergy, Wf′: --------------------------(3) By the theory of electromagnetic field, if no magnetic saturation exists, the coenergy at any position in the motor can be expressed by: ---------------------------(4) where L(θ, i) is the stator inductance at a particular position, and i the stator phase current. Hence, the electromagnetic torque is: ------------(5) Equation (5) has the following implications: 1. The torque is proportional to the square of the current and hence, the current can be unipolar to produce unidirectional torque. This is a distinct advantage in that only one power switch is required for the control of current in a phase winding and thereby makes the drive economical. 2. Since the torque is proportional to the square of the current, it has a good starting torque. 3. Because the stator inductance of a stator winding is a function of both the rotor position and stator current, thus making it nonlinear, a simple equivalent circuit development for SRM is not possible. 4. A generation action is made possible with unipolar current due to its operation on the negative slope of the inductance profile. As a result, this machine is suitable for four-quadrant operation with a converter. 5. Because of its dependence on a power converter for its operation, this motor is an inherently variable-speed motor drive system. SRM Configurations: Switched reluctance motors can be classified as shown in Fig.3. The initial classification is made on the basis of the nature of the motion (i.e., rotating or linear). Fig.3 Classification of SRM Equivalent Circuit: An elementary equivalent circuit for the SRM can be derived neglecting the mutual inductance between the phases as follows. The applied voltage to a phase is equal to the sum of the resistive voltage drop and the rate of the flux linkages and is given as:
  4. 4. Regency Institute of Technology, Yanam 4 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept Static and Dynamic torque production: Static torque production: Consider the primitive reluctance motor in Fig. 7.6(a). When current is passed through the phase winding the rotor tends to align with the stator poles; that is, it produces a torque that tends to move the rotor to a minimum-reluctance position. FIG. 7.6. Elementary reluctance motor showing principle of torque production: (a) primitive motor; (b) field energy and coenergy.
  5. 5. Regency Institute of Technology, Yanam 5 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept If magnetic saturation is negligible, then the relationship between flux linkage & current at the instantaneous rotor position  is a straight line whose slope is the instantaneous inductance L. Thus Dynamic torque production : Under normal operating conditions at speed, the energy exchanges, both incremental and total, can be determined by integrating the voltage equation and developing the conversion loop in the ѱ,i diagram. The necessary time-stepping procedure was developed by Stephenson and Corda (1979) and only the outline of their method is described here. The voltage equation is integrated in the form through one time-step, giving a new value of ѱ. If the speed is assumed constant, the integration can be done with respect to rotor angle 9. Otherwise the rotor angle must be determined by a simultaneous integration of the mechanical equations of motion, as is normal in such simulations. At the end the time-step, ϴ and ѱ are both known, and the current i can be determined from the magnetization curve for that rotor angle. To minimize this computation Stephenson and Corda used a set of polynomials to represent the magnetization curves at a number of rotor angles between the aligned and unaligned positions, and then applied an interpolation procedure at the end of each time-step to determine the current from the fiux- linkage at the particular rotor position. The instantaneous torque can be determined from the difference-approximation to the partial derivative of co energy at constant current, by a second interpolation procedure that uses stored field energies precalculated at discrete current levels at each of a number of rotor angles.
  6. 6. Regency Institute of Technology, Yanam 6 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept Current Regulation: For motoring operation the pulses of phase current must coincide with a period of increasing inductance, i.e. when a pair of rotor poles is approaching alignment with the stator poles of the excited phase. The timing and dwell of the current pulse determine the torque, the efficiency, and other parameters. In d.c. and brushless d.c. motors the torque per ampere is more or less constant, but in the SR motor no such simple relationship emerges naturally. With fixed firing angles, there is a monotonic relationship between average torque and r.m.s. phase current, but in general it is not very linear. This may present some complications in feedback- controlled systems although it does not prevent the SR motor from achieving 'near-servo quality' dynamic performance, particularly in respect of speed range, torque/inertia, and reversing capability. The general structure of a simple control scheme is much the same as that of the brushless d.c. drive (Fig. 7.14). More complex controls are required for higher-power drives, particularly where a wide speed range is required at constant power, and microprocessor controls have been developed and used very effectively, (Chappell et al. 1984; Bose et al. 1986). Because the characteristics of the SR drive are essentially controlled by the phasing of switching instants relative to the rotor position, digital control is not only very natural but can be implemented extremely effectively with flexibility or 'programmability' of the characteristics and with reliable, repeatable results. It is characteristic of good operating conditions that the conversion loop fits snugly in the space between the unaligned and aligned magnetization curves, as in Figs 7.15 and 7.19. Figure 7.15(b) corresponds to high-speed operation where the peak current is limited by the self-e.m.f. of the phase winding. A smooth current waveform is obtained with a peak/r.m.s. ratio similar to that of a half sine wave. At low speeds the self-e.m.f. of the winding is small and the current must be limited by chopping or p.w.m. of the applied voltage. The regulating strategy employed has a marked effect on the performance and the operating characteristics. Figure 7.17 shows a current waveform controlled by a 'hysteresis-type' current-regulator that maintains a more or less constant current throughout the conduction period in each phase. Figure 7.20(a) shows schematically the method of control. As the current reference increases, the torque increases. At low currents the torque is roughly proportional to current squared, but at higher currents it becomes more nearly linear. At very high currents saturation decreases the torque per ampere again. This type of control produces a constant-torque type of characteristic as indicated in Fig. 7.21. With loads whose torque increases monotonically with speed, such as fans and blowers, speed adjustment is possible without tachometer feedback, but in general feedback is needed to provide accurate speed control. In some cases the pulse train from the shaft position sensor may be used for speed feedback, but only at relatively high speeds. At low speeds a larger number of pulses per revolution is necessary, and this can be generated by an optical encoder or resolver, or alternatively by phase-locking a high-frequency oscillator to the pulses of the commutation sensor (Bose 1986). Systems with resolver-feedback or high-resolution optical encoders can work right down to zero speed. The 'hysteresis-type' current regulator may require current transducers of wide bandwidth, but the SR drive has the advantage that they can be grounded at one end, with the other connected to the negative terminal of the lower phaseleg switch. Shunts or Hall-effect sensors can be used, or alternatively, 'Sensefets' with in-built current sensing. Much of the published literature on SR drives describes this form of control. Figure 7.20(b) shows an alternative regulator using fixed-frequency p.w.m. of the voltage with variable duty-cycle. The current waveform is similar to that shown in Fig. 7.13, except that after commutation the current decays through the diodes somewhat more rapidly because the reverse voltage applied is effectively 1/d times the forward voltage applied before commutation. (d = duty cycle). The torque and energy-conversion loop are similar to Figs 7.14 and 7.15. The duty-cycle (or 'off-time') of the p.w.m. can be varied by a simple monostable circuit. This form of control is similar to armature-voltage control in a d.c. motor.
  7. 7. Regency Institute of Technology, Yanam 7 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept FIG. 7.20. Schematic of current-regulator for one phase, (a) Hysteresis-type, (b) Voltage-p.w.m. type (duty-cycle control). FIG. 7.21. Constant-torque characteristic obtained with regulator of Fig. 7.20(a). Current feedback can be added to the circuit of Fig. 7.20(b) to provide a signal which, when subtracted from the voltage reference, modulates the duty cycle of the p.w.m. and 'compounds' the torque-speed characteristic. It is possible in this way to achieve under-compounding, over- compounding, or flat compounding just as in a d.c. motor with a wound field. For many applications the speed regulation obtained by this simple scheme will be adequate. For precision speed control, normal speed feedback can be added. The current feedback can also be used for thermal over current sensing. A desirable feature of both the 'hysteresis-type' current-regulator and the voltage p.w.m. regulator is that the current waveform tends to retain much the same shape over a wide speed range. When the p.w.m. duty cycle reaches 100 per cent the motor speed can be increased by increasing the dwell (the conduction period), the advance of the current-pulse relative to the rotor position, or both. These increases eventually reach maximum practical values, after which the torque becomes inversely proportional to speed squared, but they can typically double the speed range at constant torque. The speed range over which constant power can be maintained is also quite wide, and very high maximum speeds can be obtained, as in the synchronous reluctance motor and induction motor, because there is not the limitation imposed by fixed excitation as in PM motors.
  8. 8. Regency Institute of Technology, Yanam 8 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept Torque /speed characteristic: The generic form of the torque/speed capability curve is shown in Fig. 7.22. For speeds below ωb the torque is limited by the motor current (or the controller current, whichever is less). Up to the 'base speed' wb it is possible, by means of the regulators in Fig. 7.20, to get any value of current into the motor, up to the maximum. The precise value of current at a given operating point depends on the load characteristics, the speed, and the regulator and control strategy. In the speed range below ωb the firing angles can be chosen to optimize efficiency or minimize torque ripple. If the load never needs to operate at high speeds above a>b, it will usually be possible to design the pole geometry to optimize these parameters without regard to the efficiency at high speeds, and this provides considerable design freedom to obtain smooth torque and simplify the control. The 'corner point' or base speed ωb is the highest speed at which maximum current can be supplied at rated voltage, with fixed firing angles. If these angles are still kept fixed, the maximum torque at rated voltage decreases with speed squared. However, if the conduction angle is increased (mainly by advancing the turn-on angle) there is a considerable speed range over which maximum current can still be forced into the motor, and this sustains the torque at a level high enough to maintain a constant-power characteristic, even though the core losses and windage losses increase quite rapidly with speed. This is shown in Fig. 7.22 between points B and P. The angle ӨD is the 'dwell' or conduction angle of the main switching device in each phase. It should generally be possible to maintain constant power up to 2-3 times base speed. The increase in conduction angle may be limited by the need to avoid continuous conduction, which occurs when the conduction angle exceeds half the rotor pole-pitch. It may be limited to lower values by the core loss or other factors. At P the increase in ӨD is halted and higher speeds can now only be achieved with the natural characteristic, i.e. torque decreasing with speed squared. At very low speeds the torque/speed capability curve may deviate from the flat-torque characteristic. If the chopping frequency is limited (as with GTO thyristors, for example), or if the bandwidth of the current regulator is limited, it may be difficult to limit the peak current without the help of the self-e.m.f. of the motor, and the current reference may have to be reduced. This is shown in curve (i) in Fig. 7.22. On the other hand, if this is not a problem, the very low windage and core losses may permit the copper losses to be increased, so that with
  9. 9. Regency Institute of Technology, Yanam 9 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept higher current a higher torque is obtained, as shown in curve (ii). Under intermittent conditions, of course, very much higher torques can be obtained in any part of the speed range up to base speed. In Fig. 7.23 this can be seen by extrapolating the constant-duty-cycle curves above the maximum current locus. It is important to note that the current which limits the torque below base speed is the motor current (or converter output current). The d.c. supply current increases from a small value near zero speed to a maximum value at base speed. Basically this is because the power increases in proportion to the speed as long as the torque is constant. With fixed d.c. supply voltage at the input to the converter, the d.c. supply current is then proportional to the speed. If the torque is less than maximum, of course the d.c. supply current is also smaller. Rotor Position 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. The commonly used position sensors are phototransistors and photodiodes, Hall elements, magnetic sensors, pulse encoders and variable differential transformers. Phototransistor Sensors The phototransistor sensor is based on the photoelectric principle. Fig.14 shows the basic structure of the phototransistor sensor. As shown in the figure, a revolving shutter with a 120° electric angle gap is installed on the rotor shaft, rotating with the rotor of the SRM. Phototransistors of the same number as the motor phases (three phases in the figure) are fixed on the stator. When the gap is aligned with the phototransistor PT1, the phototransistor will generate a current due to the light, while phototransistor PT2 and PT3 have only a very small leakage currents because the light is blocked by the revolving shutter. In this case, the stator phase associate with PT1 should be turned on. Similar situation will occur when the gap of revolving shutter is aligned with PT2 or PT3. Hall Position Sensors The function of a Hall sensor is based on the physical principle of the Hall effect named after its discoverer E. H. Hall: It means that a voltage is generated transversely to the current flow direction in an electric conductor (the Hall voltage), if a magneticfield is applied perpendicularly to the conductor. A typical structure of Hall position sensor for three phase motor is illustrated in Fig.15. It is made up of three Hall components and a rotating plate with permanent magnet fixed on the rotor shaft. Similar to the gap of the phototransistor sensors, the permanent magnet on the rotating plate is installed suitably so that the output of the Hall components can indicate the proper rotor position for the phase current control.
  10. 10. Regency Institute of Technology, Yanam 10 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept Speed and Torque Control : Overall closed-loop speed control is obtained in the conventional way with the speed error acting as a torque demand to the torque control system described below. Figure : Block diagram of Classic converter for SRM drives List out the Advantages and Disadvanatges Of SRM : Advantages The SRM possess a few unique features that makes it a vigorous competitor to existing AC and DC motors in various adjustable-speed drive and servo applications. The advantages of an SRM can be summarized as follows:
  11. 11. Regency Institute of Technology, Yanam 11 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept • Machine construction is simple and low-cost because of the absence of rotor winding and permanent magnets. • There are no shoot-through faults between the DC buses in the SRM drive converter because each rotor winding is connected in series with converter switching elements. • Bidirectional currents are not necessary, which facilitates the reduction of the number of power switches in certain applications. • The bulk of the losses appear in the stator, which is relatively easier to cool. • The torque-speed characteristics of the motor can be modified to the application requirement more easily during the design stage than in the case of induction and PM machines. • The starting torque can be very high without the problem of excessive in-rush current due to its higher self-inductance. • The open-circuit voltage and short-circuit current at faults are zero or very small. • The maximum permissible rotor temperature is higher, since there are no permanent magnets. • There is low rotor inertia and a high torque/inertia ratio. • Extremely high speeds with a wide constant power region are possible. • There are independent stator phases, which do not prevent drive operation in the case of loss of one or more phases. Drawbacks of SRM The SRM also comes with a few disadvantages among which torque ripple and acoustic noise are the most critical. The higher torque ripple also causes the ripple current in the DC supply to be quite large, necessitating a large filter capacitor. The doubly salient structure of the SRM also causes higher acoustic noise compared with other machines. The absence of permanent magnets imposes the burden of excitation on the stator windings and converter, which increases the converter KVA requirement. Compared with PM brushless machines, the per unit stator copper losses will be higher, reducing the efficiency and torque per ampere. However, the maximum speed at constant power is not limited by the fixed magnet flux as in the PM machine, and, hence, an extended constant power region of operation is possible in SRMs. Applications Of SRM The simple motor structure and inexpensive power electronic requirement have made the SRM an attractive alternative to both AC and DC machines in adjustable-speed drives. Few of such applications are listed below. a) General purpose industrial drives; b) Application-specific drives: compressors, fans, pumps, centrifuges; c) Domestic drives: food processors, washing machines, vacuum cleaners; d) Electric vehicle application; e) Aircraft applications; f) Servo-drive. Power Converters for SRMs Since the torque in SRM drives is independent of the excitation current polarity, the SRM drives require only one switch per phase winding. Moreover, unlike the ac motor drives, the SRM drives always have a phase winding in series with a switch. Thus, in case of a shoot- through fault, the inductance of the winding limits the rate of rise in current and provides time to initiate the protection. Furthermore, the phases of SRM are independent and, in case of one winding failure, uninterrupted operation is possible. Following are some configurations of converters used in SRM drives. Asymmetric Bridge Converter Fig.10a 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 the commanded value, the switches are turned off. The energy stored in the motor phase winding will keep the
  12. 12. Regency Institute of Technology, Yanam 12 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept current in the same direct until it is depleted. The waveforms are shown in Fig.10b and c with different switching strategies. (n+1) switches and diode configuration Utilization of the power devices is poor in the asymmetric bridge converter. A more efficient converter topology is shown in Fig.11, which is called (n+1) switch and diode configuration. When 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. Similarly, phase B can be energized by T2 and T3. The merit of this converter is higher utilization of power devices due to the shared switch operation. Nevertheless, the circuit provides restricted current control during overlapping phase currents. Bifilar Type Drive Topology Fig.12a shows a converter configuration with one power switch and one diode per phase but regenerating the stored magnetic energy to the source. This is achieved by having a bifilar winding with the polarity as shown in the figure. The various timing waveforms of the circuit
  13. 13. Regency Institute of Technology, Yanam 13 Prepared byP.Srihari M.Tech, M.I.S.T.E @Asst.Prof. Of EEE Dept are shown in Fig.12b. It is shown that the voltage across the power switch can be very much higher than the source voltage. A disadvantage of this drive is that the SRM needs a bifilar winding, which increases the complexity of the motor. C-Dump Converter The C-dump converter is shown in Fig.13a with an energy recovery circuit. The stored magnetic energy is partially diverted to the capacitor Cd and recovered from it by the single quadrant chopper comprising of Tr, Lr and Dr and sent to the DC source. This configuration has the advantage of minimum power switches allowing independent phase current control. The main disadvantage is that the current commutation is limited by the difference between voltage across Cd, vo and the DC link voltage. Furthermore, the energy circulating between Cd and the DC link results in additional losses in the machine.

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