Speed Torque Characteristic Of Dc Motor Fed By H Bridge Converter

Transcription

1 Speed Torque Characteristic Of Dc Motor Fed By H Bridge Converter Manjunath B. Ranadev *1, R. L. Chakrasali 2 EEE Department, KLE Institute of Technology, Hubli, Karnataka, India EEE Department, SDM College of Engg., & Tech., Dharwad, Karnataka, India *1 mranadev@gmail.com; 2 pratisatu@yahoo.co.in Abstract Direct current (DC) motors have already become an important drive configuration in many applications where wide range of power and speed is required. One of the ways to achieve this is by deploying H Bridge converter. The H Bridge circuit provide Speed Torque characteristics in forward and backward directions for a Direct current motor. The H Bridge circuit is designed to provide protection and isolation. Pulse width modulation (PWM) control technique is conveniently used for DC drive system. An attempt is made in this paper, to obtain Speed Torque characteristics of a variable speed DC motor drive system using PWM control in two quadrant operation mode. This concept of speed and power control employing H Bridge converter is proved through experimentation. Keywords H Bridge Converter; PWM; DC drive; Speed Torque; Chopper. Introduction Most of the drives used in the industries today are electrical. A reliable drive system for every application in an industry is the most important requirement in the present days. Electric drives are widely used in the industry because of high reliability and versatility. The most common expectation from any industrial drive is precise control of torque and speed, with longterm stability and higher efficiency. In industrial drive environment DC drives provides excellent dynamic as well as steady state performance. In a DC machine decoupled control of field and armature current will provide an accurate speed control under all dynamic load conditions because of having two separate physical windings viz armature and field. The advent of power electronics transformed the fundamental ability of the control circuits. H Bridge converter with PWM control can be used to obtain the speed torque characteristics of DC motor in either direction. However, such converters have rather complex commutation circuits that are reflected in the cost and reliability of the system. There are a few number of models available for performance evaluation of dc drives. In this paper, a model is proposed, developed and presented in the generalized form. The proposed converter topology is described in detail. An experimentation is carried out under varied loading conditions and the results are tabulated to draw the inferences. Proposed Work In general, H bridge circuit in its simplest form, involving four switching elements and the load connected in the centre in a H like configuration is shown in Fig. 1. The MOSFET s (Metal Oxide Semiconductor Field effect Transistor) are used as power switching devices, which are indicated as S1, S2, S3 and S4 in the circuit. Four Schottky diodes D1, D2, D3 and D4 are also used. Alternatively, other switching devices such as GTO (Gate Turn Off Thyristor), IGBT (Insulated Gate Bipolar Transistor) can be used for the same purpose. While designing the circuit, depending on the type of the devices used, care should be taken to select the ratings. The selection of ratings also depends on the specifications of the load and the required chopping frequency. FIG.1. H BRIDGE CONFIGURATION. The basic operating mode of H bridge is fairly simple. 4

2 International Journal of Energy and Power (IJEP) Volume 3 Issue 4, November 2014 When S1 and S4 are turned on, the right lead of the motor will be connected to ground, while the left lead is connected to the power supply energizing the motor in the forward direction (Forward Motoring). When S2 and S3 are turned on, the converse will energize the motor to run in the reverse direction (Reverse Motoring). The voltage appearing across the terminals of the motor is varied by changing the On Off time of the pulse width modulated signal. The motor operation for different switch conditions is shown in Table 1. TABLE 1 MOTOR OPERATION FOR DIFFERENT SWITCH CONDITIONS. Switches Modes Of Operation Forward Motoring Reverse Motoring Implementation and Testing S1 S2 S3 S4 on off off on off on on off The testing at different loading conditions is carried out to draw the speed torque characteristic of a DC motor in either direction using H Bridge. Isolated gate drive circuits are used and proper care is taken to ensure that two MOSFET s on the same totem pole are never turned ON during the same time to avoid a catastrophic short circuit. The Schottky type diodes connected across drain and source terminals of the power MOSFET s will protect against power flow in reverse direction as shown in Fig.2. used in the configuration. The selected MOSFET is capable of delivering maximum drain current with good factor of safety. The power MOSFET s are selected looking to the requirements of DC motor. In the present work, the current requirement is approximately 3A to 4A. Considering the above requirements, IRF 40 is found suitable. The ratings of the MOSFET are 1. N channel MOSFET. 2. Maximum voltage rating 0V. 3. Maximum current rating at 25ºC A and 100ºC 6.3A. 4. Gate threshold voltage 2V to 4V. The reverse breakdown voltage of the selected Schottky diode is capable of sustaining maximum voltage between the drain and source terminal of the power MOSFET with gate open. In the present work, diode 1N522 is selected based on the drain to source breakdown voltage of 0V. The motor will rotate in clockwise direction when PWM pulses are given to S1 and S4 and the speed in forward direction is controlled by varying the duty ratio. The duty ratio is given by = ton / T ton Duty interval. T Chopper period. The terminal voltage of dc motor is given by, Va = V. In the above expression as increases Va and speed increases. When pulses are given to S2 and S3, then motor will run in counter clockwise direction as shown in Fig.3. FIG.2. CIRCUIT DIAGRAM OF THE COMPLETE DRIVE. The H Bridge technique can be used for both shunt and separately excited DC motors. In the present work, separately excited DC Motor is used. The reliability of converter bridge circuit mainly depends upon the performance characteristics of the MOSFET s FIG.3. PWM PULSES. Observations, Results and Discussion The arrangement of H Bridge converter module and

3 separately excited DC motor is shown in Fig.4. The relation between the armature current v/s torque characteristics when the motor is run in the Forward and Reverse direction is shown in Fig. and Fig. respectively. It is evident that the increase in the load draw more input power by the way of increased armature current to develop the matching torque as governed by the equation given below. FIG 4. EXPERIMENTAL SETUP. The duty cycle of H bridge converter is varied using PWM technique and the corresponding armature current and speed are recorded. Further, for the same duty cycle the torque developed by the motor is determined. A set of readings are recorded during the experimentation. The above step is repeated for the operation in other quadrant thus, running the motor in reverse direction and the values of armature current and speed are recorded. The relation between the percentage duty cycle and the speed shows that as the duty cycle increases the average voltage across the armature increases and hence the speed which is depicted in Fig.5 & Fig.6 for forward and reverse rotation of the motor respectively. Speed in rpm Where Φ is the flux and Ia is the armature current. In separately excited motor flux is constant, hence the torque increases linearly with armature current FIG..ARMATURE CURRENT V/S TORQUE CHARACTERISTICS ( FORWARD ROTATION) Duty Cycle in % FIG.5. DUTY CYCLE V/S SPEED ( FORWARD ROTATION) Speed in rpm Duty Cycle in % FIG.6. DUTY CYCLE V/S SPEED ( FIG..ARMATURE CURRENT V/S TORQUE CHARACTERISTICS ( The characteristics for Armature current v/s speed for forward and reverse direction is shown in Fig. and Fig.10. There is slight decrease in the speed as the armature current increases till the rated value and further will have sudden drooping showing the inability to develop the power which is beyond the rating. The characteristics obtained here is similar to the typical characteristics obtained by other methods. The speed torque characteristics of a DC motor for forward and reverse rotation is as shown in Fig.11 and Fig.12 respectively. It can be clearly seen from the 51

4 International Journal of Energy and Power (IJEP) Volume 3 Issue 4, November 2014 curve that the increase in the torque reduces the speed the losses, accurate & precise speed control with wide range of variation. The scheme when implemented in large scale will be cost effective, however, care must be taken not to affect the quality of the power FIG..ARMATURE CURRENT V/S SPEED CHARACTERISTICS ( FORWARD ROTATION ) FIG.10.ARMATURE CURRENT V/S SPEED CHARACTERISTICS ( FIG.11.SPEED TORQUE CHARACTERISTICS OF THE MOTOR ( FORWARD ROTATION) The various characteristics obtained in this experiment with the implementation of H bridge converter to control the DC motor by running in either direction are having the same trend and matching with the typical characteristics obtained from the conventional control strategies. The advantage of this method includes reduction in FIG.12. SPEED TORQUE CHARACTERISTICS OF THE MOTOR ( Conclusions The experimental results prove the greater ability of H Bridge circuit to run DC motor in both forward and reverse direction with variable speed over wide range. Further, the use of H Bridge will eliminate the constraints of the source specifications. H Bridge design is really simple for prototyping or really extravagant for added protection and isolation. The characteristics of the motor both in forward and reverse direction are obtained keeping the supply voltage constant. The control is obtained by varying the duty cycle of the timer and the gate pulse provided. The characteristics obtained when run in either direction are similar to the characteristics obtained using the conventional methods. Further, without changing the field supply manually, characteristics are obtained in reverse direction. With proper design of the circuit it is possible that it can be used for higher ratings. A test bench can be set up for testing DC drives using H Bridge will simplify performance evaluation process. REFERENCES Norris, R.N. Sectional Electric Drive for Paper Machines American Institute of Electrical Engineers, Transactions, Volume: XLV, Jan S. A. Hamed and B. J. Chalmers, Performance evaluation of variable speed DC drive with sinusoidal PWM control, in Proc. Sixth Int.Conf. Electrical Machines and Drive, Oxford, U.K., Sept. 13, pp

5 S. A. Hamed, Pulse width modulation control of threephase AC to DC converter feeding DC motor drive systems, Mansoura Engineering Journal (MEJ), vol. 20, no. 3, pp. E.25 E.3, Sept. 15. ANSI/lEEE Std , IEEE Guide: Test Procedures for Direct Current Machines. Mohd. Amir Fikri Bin Awang, Journal, November Sadeq A. Hamed, Performance Evaluation of Three Phase Variable Speed DC Drive Systems with Uniform PWM Control IEEE transactions on power electronics, vol. 12, no. 2 march 1. G.K.Dubey, Fundamentals of Electrical Drives Narosa Publishing House Pvt. Ltd., Dr. P. S. Bimbhra, Electrical Machinery, Khanna publishers, Mr. Manjunath B. Ranadev born in Dharwad district (India) on and obtained Under Graduate (BE) degree in Electrical and Electronics Engineering from S.D.M College of Engineering and Technology, Dharwad affiliated to V.T.U Belgaum, Karnataka (India) during the year 200. He also obtained Post Graduate (M.Tech) degree in Industrial Electronics from SJCE Mysore affiliated to V.T.U Belgaum, Karnataka (India) during the year 200. He has teaching experience of 05 years and is presently working as Assistant Professor in K.L.E. Institute of Technology Hubli, Karnataka, India. His research interests are in the field of Power Electronics and Industrial Drives. Dr. Ramesh L.Chakrasali born in Sholapur district Maharashtra, India. He obtained Under Graduate (B.E) degree in Electrical Engineering from S.D.M College of Engineering and Technology, Dharwad affiliated to Karnataka University Dharwad, Karnataka (India) during the year 1. He also obtained Post Graduate (M.E) degree in Digital Electronics from the above mentioned Institution and University during the year 1. He has completed Ph. D in the field of Hybrid Grids from VTU Belgaum in the year His field of interests are Power Electronics, Power System SCADA, Hybrid Grids and Digital Electronics. He is having 24 years of teaching experience at various positions and presently working as Professor in Electrical and Electronics Engineering Department, S.D.M College of Engineering and Technology, Dharwad. He has published 4 papers in international journals and one in national journal. He has written text books on Basic Electrical Engineering, Testing & Commissioning of Electrical Equipments and Electrical Power Utilization. His current research interest includes Smart Grids. Dr. Ramesh is fellow of Institution of Engineers, IE (India) and Institution of Electronics and Telecommunication Engineers (IETE). He is life member of Indian Society for Technical Education (ISTE). He was member of Editorial Board of monthly technical bulletin published by IE (I) Dharwad local centre. 53