Rotor Position Estimation And Design Of Control Circuit Biology Essay

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An efficient and reliable operation of the Switched Reluctance Motor can be achieved only by proper estimation of the rotor position. The rotor position information in Switched Reluctance Motor drives is essential for determining the switching instants for proper control of speed, torque and torque pulsations. This paper describes a new method of estimation rotor position for switched reluctance motor (SRM) drives based on design structure considerations. The proposed scheme is successfully implemented to control the speed of the switched reluctance motors in both the direction using a micro-controller and high resolution position sensors.

The switched reluctance drive system which includes the motor and the electronic drive circuit is emerging as a competitor in the growing market for high performance variable speed drives on account of their several advantages. Most prominent among these are the simple structure, rugged construction, reduced component count of the converter because of uni-directional current requirements .In the past, the basic problem to rapid commercialization of switched reluctance motors was due to the fact that few engineers were trained to perform the exacting calculations and specialized design that this technology required because cost effective electronic components were not available. Today low cost microcontroller and memory chips as well as high power semiconductor devices are readily available. Switched reluctance motors maintain higher torque and efficiency over a broader speed range than other advanced variable-speed systems.

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These systems are extremely versatile. They can be software programmed to operate over a wide range of speeds, and also to run forwards or backwards as either a motor or generator, with torque and speed independently matched precisely to the requirements of the load They are also highly tolerant to heat, vibration, overloads, and hostile environments. Presently, SR drives are commercially available from fractional HP to 500HP. However, higher ratings of several thousand horsepower are practically possible.

BASIC PRINCIPLE OF OPERATION

The switched reluctance motor is a doubly salient variable reluctance motor. A typical four-phase switched reluctance motor with eight stator poles and six rotor poles is shown in

Fig. 1 Four Phase Switched Reluctance motor

Fig. 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 the 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 1 in Fig. 1) and at a minimum when the rotor is in the unaligned position (e.g. phase 3 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 while retaining synchronism with the rotor position.

Reluctance motors can operate with any number of phase windings, although there are some guidelines governing the choice of stator and rotor pole numbers. Usually there is one more stator pole pair than rotor pole pair, although many other combinations are possible. To ensure starting torque in either direction, there must be at least three stator phases. Both the stator and the rotor are made of laminated CRGO to reduce the iron losses in the motor. The absence of permanent magnets or coils on the rotor means that torque is produced purely by the saliency of the rotor laminations. The direction of the torque produced is independent of the direction of the flux through the rotor and, hence, the direction of current flow in the stator phase windings is not important. A uni polar phase current in the reluctance motor results in simpler and more reliable power convertor circuits.

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.

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In this paper new approach in estimating the rotor position and micro-controlled based scheme for speed control of a 1kW, 4 phases, 2000 rpm, 8/6 pole switched reluctance motor is developed. The proposed scheme is simple, cost effective and is useful for low speeds and low rated applications. The power and control circuits of the SRM drive are designed, developed and tested in the laboratory.

I I. DESCRIPTION OF THE HARDWARE

The prototype drive comprises of four phase R-dump MOSFET inverter and position sensors, PWM current control, gate drive circuit for MOSFET and power supply unit. Position sensors are used for rotor position sensing. The details of power and control circuits are given below.

A Power Circuit [5]

The inverter consists of a PWM inverter with one switching transistor per phase. MOSFET's are used in all four circuits. Furthermore the supply unit consists of three Sealed Maintenance Free (SMF) Lead-Acid batteries each rated for 12V, 18Ah and an inverter. The common point of phase windings is supplied from the DC Source positive , while the phase windings are connected through the switching transistors in an appropriate order to the negative of the DC source.

Fig. 2(a) Inverter schematic circuit for SRM

Fig 2.(a) shows a converter configuration with one transistor, one diode and with dissipating resistor per phase of the SRM. ,When T1 is turned off, the current freewheels through D1, charging C, and later flows through the external resistor R. This resistor partially dissipates the energy stored in phase A. This has small disadvantage that the current in phase A will take longer to extinguish compared to recharging the source, how ever the loss of energy is very minimum.

B. Position Sensor Conditioning Circuit

Fig. 2(b) 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

D

0

1

2

C

0

0

3

B

Table II: Truth Table for forward direction (Anti Clock-Wise direction)

Position Sensor 1

Position Sensor 2

State

Phase to be excited

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1

0

0

C

1

1

1

B

0

1

2

A

0

0

3

D

Fig 3(a) depicts State: 1. 3(b) depicts State: 0

3 (c) depicts Sate: 4 .3(d) depicts State: 2

In Figure 4 (a), assume that at that moment only phase 1 is excited (switched on) State 1. Since at present the rotor is at the misaligned position, the rotor will turn clockwise to the aligned position in Figure 4 (b) because this is the nearest aligned position; If phase 1 is switched off so that the current in phase 1 is Is1 = 0 and phase 4 is switched on (excited) when the rotor reaches the aligned position in Figure 4 (b), the rotor will keep its clockwise rotation from its current misaligned position to the nearest aligned position Figure 4 (c) with respect to phase 4; Now the rotor turns to be at the misaligned

position in Figure 4 (c) with respect to phase 3, if phase 3 is switched on (excited) instead of phase 4, the rotor will keep clockwise rotation to Figure 4 (d), etc

Therefore, the stator exciting sequence D, C, B, A D ... generates clockwise rotation. For the same reason, the sequence A, B, C, D, A... yields counterclockwise rotation.

Figure. 4. Exciting Sequence A, D, C, B, A ... generates Clockwise Rotation

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.

Fig. 5. Exciting Sequence A, B, C, D, A, ... generates Anti clock-wise Rotation.

D. Gate Drive circuit.

Fig. 6 Basic Gate Drive Circuit of SRM

A quad operational amplifiers is used as four comparators as shown in the fig. 6, the output of which are used for driving the power MOSFETS A , B, C and D .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 MOSFETs.

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 prior to using SRM. After successful testing with linear load, first the SRM position sensor circuit is tested for getting appropriate logic four different states .There after the unit was tested with DC supply of 36V 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, speed reversal and variable speed response. 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 and anti-clock wise direction are shown in fig 8 (a) and 8 (b)

Fig 7. Inverter Circuit of Phase A and Phase B

Fig 8(a) Firing pulses for phase A and phase C in Clock -Wise direction

Fig 8(b) Firing pulses for phase A and phase C in Anit-Clock Wise - direction

IV.TEST RESULTS AND DISCUSSIONS

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 MOSFET 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 and anti-clock wise at low speeds and also at high speed are shown in fig 9(a) ,9(b) and 9(c) respectively, and the line current in clock wise direction is shown in fig 9 (d)

Fig 9 (a) Phase Currents of Phase A and Phase C in Clock wise direction at 1800 RPM

Fig 9 (b) Phase Currents of Phase A and Phase C in Clock wise direction at 1100RPM

A Variable Speed Response:

The motor is started initially with a pulse width of 200us on period and 100 us off period. The motor attained the initial speed of 1700 RPM in 2.8 sec. The speed of the motor can be controlled by controlling the pulse width of on time. Different speeds can be obtained with different pulses and speed of the motor can be varied either directions.

Fig 9 (c) Phase Currents of Phase A and Phase C in Anti-Clock wise direction at 1100RPM

Fig 9 (d) Line Current in Clock wise direction

10 (a) Current through phase A capacitor in Single pulse mode

In clock -wise direction

10 (b) Current through phase A capacitor in Single pulse mode

In Anti clock -wise direction

10 (c) Volage across phase A capacitor in Single pulse mode

clock -wise direction

10 (c) Volage across phase C capacitor in Single pulse mode

Anti -clock -wise direction

The experimental results for current through capacitor and voltage across capacitor in both clock wise and anti clock wise direction are shown in fig 10 (a), 10(b), 10 (c) and 10 (d) respectively. It is observed that, the capacitor is charged during the freewheeling period and is discharged during on period. through the switching device. For the experimental results it is observed that for a input voltage of 36V, the voltage across the capacitor is not exceeding more than 40Volt.

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 R-dump inverter is found to be more attractive when compared to C-dump using split DC source with twice the DC voltage and two capacitors continuously under charge ,discharge cycle which reduces life cycle of the controller, which is not suitable for continuous operation in industrial 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-magnetreluctance motor drives. Oxford University Press, 1989.