LOW SPEED OPERATION OF DSP BASED PWM CONTROL OF 4 PHASE 8 / 6 SWITCHED RELUCTANCE MOTOR DRIVE

This paper presents the speed control of 4 phase 8/6 Switched Reluctance Motor drive with PWM controller. The motor is fed from an Asymmetrical bridge converter, because of its fault tolerant capability and all the phases are independently excited. The Hall sensors provided in the motor provide signals corresponding to the position of the rotor. The speed is controlled by varying the duty ratio of the PWM controller. The pulses to the IGBT switches are generated using a Digital Signal Processor. The SRM drive with PWM controller is simulated in MATLAB/SIMULINK and also experimentally implemented at no load & partial load. The waveforms of the PWM signals and phase currents are captured by means of Digital storage oscilloscope and compared with the simulation results.


INTRODUCTION
The Switched Reluctance Motor (SRM) is a good competitor to conventional AC/DC drives in variable speed applications because of its advantages like high speed, high robustness, high reliability, low cost, and high rotor temperature handling capability [1], [2].The phase winding is to be excited during the positive increasing region of the phase inductance to get the motoring action.This is done through an Asymmetrical converter,asit has advantages that both the hard chopping and soft chopping are possible.The control strategies of SRM are mainly the Hysteresis Current Control, PWM Control and the Single Pulse Voltage Control.Hysteresis Current Control and PWM Control are used for low and medium speed operations.The Single Pulse Voltage Control controls the speed and torque by regulating the turn-on and turn-off angles.This is suitable for high speed operation, but not to low speed operation because of high current peaks [3], [4], [5].The speed control of Asymmetrical converter fed 4 phase SRM using DSP is investigated experimentally in [6].
This paper presents the speed control of 4 phase 8/6 SRM using DSP TMS320F2407A.Simulation and experimental results are presented to verify the PWM Controller.

PRINCIPLE OF PWM
Two significantly different methods namely Hysteresis current control and PWM control are used below the base speed operation of the motor.The basic difference between the two is that Hysteresis type controllers aim to control the current during the conduction period whereas PWM type controllers control the average phase voltage during conduction period [7], [8].In this control, the average phase voltage during the conduction period is controlled by varying the duty ratio of the top switch.At the end of the conduction interval both switches are opened, in which case the phase voltage is equal to -Vdcfor the time interval in which the phase current is greater than zero.This type of control has a constant PWM switching frequency, which can be advantageous in terms of reduction in acoustic noise.

MATHEMATICAL ANALYSIS
Where Vdc is the dc link voltage, D is the PWM duty ratio and Tpwm is the PWM period Where n is the PWM period number of each conduction time, Di and Tpwmi present the duty ratio and period of each PWM cycle respectively.For SRM average terminal voltage can be expressed as Where i is the phase current, L is the phase inductance and ω is the speed.
Where n is the number of phases and W is the coenergy which could be calculated from

EXPERIMENTAL RESULTS
The PWM control of SRM drive was operated from no-load and partial-load at 1400 rpm in closed -loop mode.The PWM frequency is set at 20 kHz.The phase current waveforms of four phases under no load condition at 1400 rpm are shown in Fig. 5 ISSN 1335-8243 (print) © 2014 FEI TUKE ISSN 1338-3957(online), www.aei.tuke.skIt is clear that by changing the duty ratio, output torque and speed can be controlled 4. BLOCK DIAGRAM The whole setup consists of following important units viz TMS320F2407A DSP Controller, IGBT Intelligent Power Module (IPM), SRM, Signal Conditioner and Mechanical loading arrangement.The interconnections of these units are shown in the block schematic diagram of Fig 1.The IPM consists of a single phase bridge rectifier with capacitors at the output.This rectifier provides the rectified DC voltage to the IGBT based Asymmetrical bridge converter.The IPM is rated for 25A, 1200V.Hall Effect transducers are used to sense the dc link current, dc link voltage and the output currents of the converter.The heart of the closed loop control scheme is the DSP controller.The inputs to the DSP are provided through 26 pin connector.The various analog inputs to the DSP are dc link current IDC, dc link voltage VDC, the four phase currents of the IGBT converter I1, I2, I3 and I4 and isolated fault signal.These are given through signal conditioner block.The digital inputs are Hall sensor signals.

Fig. 1
Fig. 1 Block Schematic Diagram of Control Scheme

Fig. 2 Fig. 3 Fig. 4
Fig. 2 Flow chart of closed loop operation of the drive

Fig. 5 Fig. 6
Fig. 5 Experimental waveforms of the drive at No load, 1400 RPM (a) Phase current waveforms of four phases (b) PWM signal and current in one phase