Abstract- An efficient and reliable operation of the Switched Reluctance Motor (SRM) can be achieved only by proper estimation of the rotor position. The rotor position information in switched reluctance motor (SRM) drives is essential for generating the proper firing sequence and for controlling the speed of the drive. Also SRM drives in recent days are more expensive than their conventional AC and DC drives. This is to a great extent caused by the lack of a standardized power electronic converter for SRM drives. This paper describes a new and simple method for determining the rotor position of an switched reluctance motor (SRM) based on motor design structure and a low cost converter for low and high speed operations. The scheme is successfully implemented with minimum number of switching devices and position sensors to control the speed of the switched reluctance motors in both the direction. This controller executes controller algorithms via micro-controller to generate SRM driving signal.
Keywords: Switched Reluctance motors, position sensors, micro-controllers.
I.INTRODUCTION
The switched reluctance drive system is coming out as a new competitor in the growing market for high performance variable speed drives on account of many advantages. Most highlighted among these are the simplicity, controllability and high efficiency [4]. Besides these unique advantages, the major drawback of this motor is the requirement of rotor position sensors ,rotor position information and the presence of large torque ripple. Therefore rigorous research has been underway in many countries using sensors (sensor) or to eliminate (sensor less) discrete position sensor, while providing the necessary rotor position information for the drive controller. But both sensor and sensor less provide mixed advantages and disadvantages. In both the techniques, we need to carry out huge mathematical calculation and certain experiments to find out performance parameters of the SRM [5][6][1]. It is also very essential, that rotor position should be properly synchronized with stator phase excitation; therefore, the information about the rotor position is essential for proper switching of stator phases [5]. In general encoder [8] or a resolver is generally used to detect the position of the rotor, however the resolution of the position sensor, the more the unit price increase. In addition to this increasing price of the encoder, the proper arrangement of position sensor and huge mathematically calculation, experimental set up involved, increases the complicity in designing the control circuit as a result overall cost of the total drive is becoming more expensive. In this paper, a new way of finding the rotor
Fig. 1 Four Phase Switched Reluctance motor
position is presented, which requires neither high cost resolution encoder, nor a huge mathematical calculations or any extra experimental set up. A micro-controller is used to generate the necessary firing pulses to each phase device based on the information received from the position sensors to have a proper control in either direction.
The scheme is tested for speed control of a 1kW, 4 phases, 15000 rpm, 8/6 pole switched reluctance motor. The proposed converter and control scheme is simple, cost effective and is useful for high speeds and low rated applications. The power and control circuits of the SRM drive are designed, developed and tested in the laboratory.
BASIC PRINCIPLE OF OPERATION [1][5][6]
The switched reluctance motor is a doubly salient, single excited variable reluctance motor .The switched reluctance motor with eight stator poles and six rotor poles is shown in Figure 1. A coil, wound around each stator pole, is connected, usually in series with the coil on the diametrically opposite stator pole to form a phase winding .The reluctance of the flux path between two diametrically opposite stator poles varies as a pair of rotor poles rotates into and out of alignment with them. Since inductance is inversely proportional to reluctance, the inductance of a phase winding is at a maximum when the rotor is in the aligned position (e.g. phase D in Fig. 1) and at a minimum when the rotor is in the unaligned position (e.g. phase B in Fig. 1). A pulse of positive torque is produced if a current flow in a phase winding as the inductance of that phase winding is increasing. A negative torque contribution is
avoided if the current is reduced to zero before the inductance starts to decrease again. The rotor speed can be varied by changing the frequency or the duty cycle of the phase current pulses or by voltage control, i.e. varying the magnitude of the input pulse voltage, while retaining synchronism with the rotor position.
Despite its simple structure, the real-time control of SRM is quite challenging. Chappell et a1 [2] and Oza et al have reported the use of microprocessor, and Siemens 80535 microcontroller [3] for control of SRM. Various techniques describing the instantaneous torque control and sensor less operation have used analog and digital I C s, 8096 micro-controller and digital signal processors TMS320CS0, TMS320C240 for control of SRM
II.DESCRIPTION OF THE HARDWARE
The prototype drive comprises of four phase, IGBT inverter[5] and position sensors, voltage control, gate drive circuit for IGBT'S and power supply unit as shown in figure 2(a). Position sensors are used for rotor position sensing. The details of power and control circuits are given below
Fig 2(a). Configuration of SRM drive
A Power Circuit [5]
The inverter consists one switching transistor per phase. IGBT's are used in all four phases. Furthermore the supply unit consists of six sealed maintenance free (SMF) Lead-acid batteries each rated for 12V, 18Ah and a potential divider to get a variable voltage. Fig 2. (b) shows a converter configuration with one IGBT and one diode. A set of two capacitors in series provides three wire dc supply Equal value resistances are connected across each capacitor to balance the voltage .Each phase windings is energized by turning on the respective phase device. If phase A is to be energized, the transistor Q1 has to be turned on, then the current circulates through Q1, phase A and the capacitor C1 and when Q1 is turned off, the current continues to flow through winding, D2 and C2 .The energy stored in the phase A winding during on period is dumped to C2. The phase A during off period is subjected to negative supply through C2, which helps in quick demagnetization of the Motor phase A and reducing the effect of negative torque. Similar process is followed for other phases.
Fig. 2(b) Inverter schematic circuit for SRM.
B. Position Sensor Conditioning Circuit
Fig. 2(c) Position sensor circuit of SRM
Two photo diodes and two infra -red (IR) diodes are used as sensors. Two number of IR diodes are directly connected in series to the in built dc supply of regulated 5V through a current limiting resistor in order to ensure constant current. The photo diodes are individually reversed biased from 5 volts dc regulated source. The photo diodes and the IR diodes are placed facing each other mounted on the stator in specific position so that, the infra red light emitted by the IR diode directly fall on the photo diode. A slotted disc is mounted on rear end of the rotor with similar number of slots to that of the rotor so that the slots appear in between the space of the IR diode and the photo diode. Thus while the rotor makes a rotation the IR light falling on the photo diode is interrupted through the slots of the disc. While the light is interrupted the photo diode resistance falls sharply to a low value. This fall of resistance forms a difference in potential to a comparator made out of Op-Amp, thus the output of the comparator is either 0 or 1 .Two comparators are used to get states such as 01, 11, 10, 00, which fall in synchronization with rotor position.
For the starting purpose the corresponding state decides triggering of appropriate stator pole and while running it follows the sequence from there onwards.
C. Location of position sensors and Exciting sequence.
A pair of IR transmitting and receiving sensors are used to sense the rotor position which are placed on the stator and an external rotor disc respectively. Based on the over-lapping of the rotor pole face with stator pole face, sensors generate four states. When the sensor signals were both low there is no overlapping of rotor pole face with stator pole face. If both signals were high, rotor pole face completely overlaps with the stator pole face.
These states are shown in the truth tables. Based on this the respective phases are being excited to rotate the motor in clock-wise direction as well as in anti-clock wise directions.
Table I: Truth Table for forward direction (Clock-Wise direction)
Position Sensor 1
Position Sensor 2
State
Phase to be excited
1
0
0
A
1
1
1
B
0
1
2
C
0
0
3
D
Table II: Truth Table for forward direction (Anti Clock-Wise direction)
Position Sensor 1
Position Sensor 2
State
Phase to be excited
1
0
0
C
1
1
1
D
0
1
2
A
0
0
3
B
Fig 3(a) depicts State: 3 3(b) depicts State: 0
3 (c) depicts State: 1 3(d) depicts State: 2
In Figure 5 (i), assume that at that moment only phase a is excited (switched on) State 0. Since at present the rotor is at the misaligned position, the rotor will turn clockwise to the
aligned position in Figure 5 (ii) because this is the nearest
Fig 4. Inductance profile and arrangement of Position sensors
aligned position; If phase a is switched off so that the current in phase a is Ia = 0 and phase b is switched on (excited) when the rotor reaches the aligned position in Figure 5 (ii), the rotor will keep its clockwise rotation from its current misaligned position to the nearest aligned position Figure 4 (iii) with respect to phase c; Now the rotor turns to be at the misaligned position in Figure 5 (iii) with respect to phase d, if phase d is switched on (excited) instead of phase c, the rotor will keep clockwise rotation to Figure 5 (iv), etc
Therefore, the stator exciting sequence a, b, c, d, a... generates clockwise rotation. For the same reason, the sequence c, d, a, b, c…. yields counterclockwise rotation
The truth table for the clock wise direction is shown in the table I. The truth table for the anti-clock wise direction is shown in the table II. The speed of the SRM drive can be changed by varying the stator phase excitation sequence frequency
Figure. 5. Exciting Sequence A, D, C, B, A generates Clockwise Rotation
The micro controller receives two separate input signals from the position sensor circuits, which will be read, stored, and processed to determine the pulse-width modulated output signals that will excite and control the motor. The output of the micro-controller is fed to the input buffer circuit .A quad operational amplifiers is used as four comparators and, the output of which are used for driving the IGBT's through gate driving circuits. .The inverting output of each comparator is given to a reference voltage through a potential divider from inbuilt regulated 5Volts D.C. The non inverting inputs of the four comparators are driven from the micro controller output.
As per program written in C, output generated from the micro-controller drives respective comparators to output trigger pulses to respective gates of the IGBT's.
Fig. 6. Exciting Sequence A, B, C, D, A, generates Anti clock-wise Rotation.
D. Basic Controller and Gate Drive circuit.
Fig. 7 Basic Controller and Gate Drive Circuit of SRM
III. TESTING OF SRM DRIVE SYSTEM
The hardware consisting of position sensing circuit, control circuit and gate drive circuit are tested individually using linear lamp load and inductive prior to using SRM. After successful testing with linear load and inductive loads, first the SRM position sensor circuit is tested for getting appropriate logic four different states .There after the unit was tested with different DC voltages through potential divider from 12 to 72V with SRM as load and the response is experimentally recorded evaluating the performance of the drive. The testing is repeated for different operating conditions such as starting, reverse rotation and speed reversal. Experimental results are recorded to evaluate the performance of the drive. These are discussed in the next section. Before explaining the drive operation for different operating conditions, the generation of firing pulses for phase A and Phase C in both Clock wise for different voltages in fig 8 (a) 8 (b) and 8(c)
Fig 8(a) Firing pulses for phase A and phase C in Clock -Wise direction V=24 Volts
Fig 8(b) Firing pulses for phase A and phase C in Clock -Wise direction V=48 Volts
Fig 8(b) Firing pulses for phase A and phase C in Clock -Wise direction V=68 Volts
IV.EXPERIMENTAL TEST RESULTS
The excitation pulse for a particular phase is decided from the information received from position sensors. The IR position sensor continuously monitors the rotor position with respect to the arrangement of IR receiving sensors on the stator pole surface and gives the signal micro-controller I/O port which
generate necessary gate drive as per the program written in some other I/O ports to feed to the driver circuit comprising of OP-Amp to excite the correct phase accordingly through IGBT'S gate switching .The steady state current recording of two phase currents (Phase A and Phase C) while the motor is running in clock wise direction at high speed are shown in fig 9(a) ,9(b) and that of (Phase B and Phase D) are shown in 9(c) and 9(d) respectively, and the line current in clock wise direction at high speeds is shown in fig 9 (e) and the voltage and current in phase C is shown in Fig 9(f).
Fig 9 (a) Steady State Currents of Phase A and Phase C in Clock wise direction V=68Volts
Fig 9 (b)Steady State Currents of Phase A and Phase C in Clock wise direction V=24Volts
A Variable Speed Response:
The motor is started initially with a voltage of 24 Volts. The motor attained nearly an initial speed of 8000 RPM in 3 sec. The no load current under steady state condition is observed to be 0.5 A (average) .The speed of the motor can be controlled by controlling the magnitude of input voltage. Different speeds can be obtained with different input voltages. Figure 10(a), 10(b) and 10(c) shows the performance curves in anti-clock wise direction at low speed operation.. From the experimental results it is observed that for a input voltage of 72V the voltage across the capacitor is not exceeding more than 40Volts. Figure 11 shows the input voltage and the speed response.
Fig 9 (c)Steady State Currents of Phase B and Phase D in Clock wise direction V=24Volts
Fig 9 (d) Steady State Currents of Phase B and Phase D in Clock wise direction V=68Volts.
Fig 9 (e) Line Current in Clock wise direction V=68 Volt
The results confirm the ability of the low cost controller for satisfactory starting performance in either direction. The raising characteristics of the speed curve show that, the drive can be applicable for automobile applications.
9 (f) Steady State Voltage & Current through phase C at V=68Volts in clock wise direction
Fig 10 (a) Phase Currents of Phase A and Phase C in Anti-Clock wise direction, V=48Volts
Fig 10 (b) Phase Currents of Phase B and Phase D in Anti-Clock wise direction, V=48Volts
Fig 10(c) Steady State Voltage & Current through phase C in Anti - clock wise direction ,V=48Volts.
Figure 11 .Input Voltage Versus Speed
V. CONCLUSlONS
This paper presents a new approach of finding the rotor position of an SRM based on motor construction symmetry with minimum number of sensors and a simple micro-controlled based controller for control of switched reluctance motor in both the directions. The controller is capable of implementing voltage-fed and current fed operation during high speed and low speed region respectively. The hardware developed for speed control of SRM is successfully tested in the laboratory for real time implementation. The results demonstrate that the motor works satisfactorily over a wide range of speed and operating conditions though the excitation pulses to each phase are not equal, as the excitation of each phase winding is decided by the rotor position and the sensors locations. Experimental results validate the practical design of controller and prove the attractive, features of such a control for industrial applications. The split dc inverter is found to be more attractive when compared to R-dump or any other inverters. Based on the experience, it can be concluded that the Laboratory model is best suitable for low cost applications
VI.REFERENCES
[1]T.J.E .Miller, "Electronic Control of switched reluctance Machines" Newnes Power Engineering series 2001
[2] P.H. Chappell, W.F. Ray, and R.J. Blake, "Microprocessor control of a variable reluctance Motor", Proceedings IEE, 131, Pt. B, pp. 117-24, 1984
[3] R. Krishnan, X. Mang, and A.S. Bhardwaj, "Design and performance of a microcontroller based switched reluctance motor drive system", Journal of Electric Machines and Power Systems, vol. 18, 1990, pp.359-373.
[4] P.J. Lawerenson, J.M. Stephenson, P.T. Blenkinsop, J. Corda, and N.N. Fulton, "Variable-speed switched reluctance motors", Proc. IEE, Pt. B, Vol. 127, No. 4, pp. 253-265, July 1980.
[5] Krishnan, R., "switched reluctance motor drives modeling, simulation, analysis and application" CRC Press 2003
[6] B.K. Bose, T.J.E. Miller, P.M. Szczesny, and W.H. Bicknell, "Microcomputer control of switched reluctance motor", IEEE Trans. on Industry Applications", Vol. IA-22, No. 4, July/August 1986, Pp 708-715
[7] Miller, T.J.E.: Brushless permanent-magnet reluctance motor drives. Oxford University Press, 1989.
[8] Jin-Woo Ahn , Sung-Jun Park and Dong-Hee Lee "Novel Encoder for Switched Angle Control of SRM "IEEE Trans. On Industrial Electronics, Vol 53, No. 3 pp 848-854 June 2006
