AUTOMATIC CLOSED LOOP SPEED CONTROL OF DC MOTOR USING IGBT

Transcription

1 International Journal of Recent Innovation in Engineering and Research Scientific Journal Impact Factor by SJIF e- ISSN: AUTOMATIC CLOSED LOOP SPEED CONTROL OF DC MOTOR USING IGBT Ankush A. Kosare 1, Sneha D. Masarkar 2 and Sachin K. Suryvanshee 3 1,2,3 Department of Electrical Engineering, GNIT Nagpur/ RTMNU, India Abstract- This paper presents a systematic development of the analysis, design, and testing of a insulated gate bipolar transistor (IGBT) controlled separately excited dc motor drive system. The controlled (IGBT) rectifier provides a low impedance adjustable 'DC' voltage for the motor armature, thereby providing speed control. Design of both a proportional and proportional integral controller is outlined and simulink experimental results are given in MATLAB. The new design method gives us a simple and powerful way to design speed controller for DC motor. This report also identifies and describes the design choices related to P and PI controller for a DC motor. Keywords- Pulse width modulation (PWM), separately excited DC motor (SEDM), Proportional Integral controller (PI controller),pic microcontroller, Digital signal oscilloscope(dso) I. INTRODUCTION Speed control of DC motor could be achieved using conventional or electrical techniques. In the past, speed controls of dc drives was mostly mechanical, requiring large size hardware to implement. Advances in the area of power electronics have brought a total revolution in the speed control of dc motor drives. These drives have now dominated the area of variable speed because of their low cost, reliability and simple control. Drives are widely used in applications requiring adjustable speed, good speed regulation and frequent starting, braking and reversing. Some important applications are: rolling mills, paper mills mine winders, hoists, machine tools, traction, printing presses, textile mills, excavators and cranes. Closed loop or feedback systems generally have the advantages of greater accuracy, improved dynamic response, and reduced effect of disturbances such as loading [4].With solid-state power controllers, protection can be an important considerations. This paper presents a systematic development, using transfer function techniques, of a closed loop speed control scheme and an outline of the control design. The necessity and significance of the control loops and different parameters are shown. II. MATHEMATICAL MODEL OF DC MOTOR This DC motor system is a separately excited DC motor, which is often used to the speed controlling and the position adjustment. This paper focuses on the study of DC motor linear speed control, therefore, the separately excited DC motor is adopted. Consider the separately excited dc motor with armature voltage control shown in Fig. l (a). The voltage loop equation is e a = R a i a + L a + e g (1) (b) Fig.1. Development of motor transfer function. (a) Separately excited dc motor model. (b) Complete transfer function separately excited DC rights Reserved Page 57

2 Where, e g = k a ø n.. (2), The torque balance equation is, T e = J In the Laplace domain, (1)-(4) can be written as, + Bn +T L.. (3), Where, T e = k a ø i a. (4) E a (s) = E g (s) + R a l a (S) + L a SI a (S). (5), E g (s)= k a øn(s) (6) Thus, T e (s)= JsN(S) + BN(S) +T L (s) (7), T e (s)= k a (s) ø I a (S) (8) I a (s)= = ( ).. (9), Where, N(s)= = ( ). (10), Where, These relationships are shown in block diagram form in Fig. l (b). Note the feedback loop present in the form of the back EMF. This provides the moderate speed regulation inherent in the separately excited dc motor. From Fig. l(b), an expression can be obtained for the change in speed N(s) due to disturbances in applied voltage (E a (s)) and load torque T L (s)[5]. N(s)= (11), Where, = ( k a ø ), k a ø =, ( k a ø ) 2 If we neglect the load torque term for now = If <<<< (which is almost always the case), then can be neglected and the expression simplifies to = = (12), =, < Referring to Fig. l(b): =. (13), Therefore = * =.. (14) Where, =, =, Available Online at : Page 58

3 III. CLOSED LOOP SPEED CONTROL If a dc tacho-generator is attached to the motor shaft, a speed signal can be fed back and the error en(s) used to control the armature voltage. This is shown in Fig. 2. The applied armature voltage is controlled using a full bridge converter. If the proper firing control scheme is used [7], a linear relationship between the control EC and the armature voltage Ea can be obtained. If the small time delay associated with the converter is neglected, then = = corresponds to 0 firing angle and VLL is the ac line-to line rms voltage. Several types of speed controllers are possible, two more common ones being proportional and proportional-integral (PI). Here, a proportional controller is considered. The total transfer function is =, where = If Also =, where,... (15) >> 1, = * = ( )... (16) The current response to a step change in input is = =, where =, = = [1+ ]... (17) Since >>, can be neglected and normalizing the current with respect to the steady-state change (α) (18) Fig. 2: Speed control loop. An input change in results in a large sudden change in current which decays slowly. This transient over-current is undesirable from the standpoint of converter rating and protection. This is particularly the case for starting or large changes. It would, therefore, seem beneficial to limit the current to some maximum allowable value. This cannot be done in the configuration of Fig. 2 where the motor voltage is controlled by the speed error. Any attempt to clamp this speed error will limit the motor voltage. Neglecting armature losses, Available Online at : Page 59

4 this will limit the speed but not the current. However, a current limit can be implemented if we first construct an inner current control loop using the clamped speed error as the current reference. IV. CURRENT CONTROL LOOP The inner current control loop is shown in Fig. 3. is the gain of the current transducer, which may be simply a sampling resistor in the armature circuit. is the gain of the current controller which here is assumed to be proportional.the transfer function will be = = ( ) =, Since >> 1, and + Also >>, Therefore, A pole zero cancellation is now possible, resulting in no overshoot or time delay. In practice, there will be a delay due to the electrical time constant and the converter delay. Both these are usually sufficiently small to be neglected as done. Fig. 3: Current control loop. Then, =... (19) Because is directly related to, a limit on will effectively limit the current. This inner loop can now be incorporated within the speed control loop, using the clamped speed error as the current reference. This is shown in Fig. 4(a). This can be simplified, by using (19) and neglecting the nonlinear clamping, to the diagram in Fig. 4(b). Now = = =... (20) For >>> 1, = from (15) and (20) we get Also = =... (21) While this is not very different from (16), the expression is only true while is less than the limiting value. If, during acceleration or load changes, the speed error is large such that is clamped at the maximum value, the current will be limited = to a maximum value =. Available Online at : Page 60

5 (a) Fig. 4: Proportional speed control with inner current control loop. (a)complete transfer function representation with tachogenerator (b) Speed control loop with PI control The addition of integral feedback can be used to eliminate the steady-state error and to reduce the forward gain required. To obtain this integral component, the proportional speed controller is replaced by a PI-type controller. The new controller transfer function is (1 + )/ s. The resulting block diagram is presented in Fig. 4 (b). The overall transfer function becomes (b) = For >> 1 = =, =, The poles of this expression occur at S 1, S 2 = (, Designing for the commonly accepted damping ratio of 0.707, = 2. The natural frequency is then w n = 1/ and the time constant. Choosing a particular desired natural frequency, both the gain, can be calculated. For example, for w n = I0 rad/s V. DESIGN PARAMETERS,, The results to be presented were obtained using a 0.5-hp r/min separately excited dc motor and a single-phase full bridge thyristor converter (IGBT). The parameters and constants describing the system are presented in the Appendix. Assuming that the feedback gains are fixed at these values given in the Appendix, only the gains and, are left to be determined. The clamping value of must also be chosen. The gains can be chosen on the basis of steady-state error considerations where = For the current control loop, = and = Thus, = Where is the desired steady-state current error. While the current error is not critical, it must not be too large.the speed controller gain can be chosen in the same manner as. In this case = =,, Thus = Available Online at : Page 61

6 VI. MATLAB SIMULATION Fig. 5. Proportional speed control with inner current control loop and motor speed loop.( with tachogenerator) Fig.6.Proportional-Integral speed control with inner current control loop.( with tachogenerator) Fig.7.Response of P and PI control to step change in speed reference.(speed Response) Fig.8.Response of P and PI control to step change in current reference.(current Response) VII. HARDWARE IMPLEMENTATION Fig.9. Block diagram of proposed circuit. Fig.10. Picture of the designed project. Two quadrant chopper topology includes two controllable semiconductor devices i.e. IGBT 1 and IGBT 2, forming the well known bridge leg structure named half bridge.the way in which the half bridge structure used for a two quadrant chopper is connected to the supply DC voltage obtaining through rectifier circuit and to the motor. Available Online at : Page 62

7 VIII. HARDWARE RESULTS Fig.12.Waveform of Tacho-generator at 750 rpm (Proportional Integral loop). Fig.13.Waveform of Tacho-generator at 750 rpm (Proportional Integral loop). In fig 13, it shows the waveform across the tacho-generator when micro-controller acts as proportional integral (PI) controller in the DC motor speed control system. The addition of integral feedback can be used to eliminate the steady-state error. To obtain this integral component, the proportional controller is replaced by a PI-type controller. The performance of dc motor drive is more better in case of PI controller approach than P controller. Now if a load is applied on the motor, the speed of the motor will suddenly decrease as shown in fig 14. And with the decrement of the speed output voltage will also decrease. This output voltage is fed to the ADC input of the micro-controller by using a potentiometer. As earlier a range of voltages are set for the fixed duty cycle, so when the new value of voltage will be sensed by the micro-controller it will also sense the decrement of the speed by comparing two values. Now the controller unit will tend to improve the speed of the system, so that the output voltage remains same of the tacho-generator. Fig.14.Waveform of Tacho-generator at 750 rpm on 2Kg load (Proportional Integral loop). IX. CONCLUSION Fig. 15. PWM Waveform at 750 rpm (Proportional Integral loop). This paper has presented the development of a insulated gate bipolar transistor DC motor drive using a low order linear model to approximate the actual system. The experimental results have shown that this simplification is valid from an engineering point of view. A complete layout of DC drive system is obtained. A DC motor specification is taken and corresponding parameters are found out from derived design approach. Then designing of current and speed controller is done. MATLAB simulation for speed control of separately excited DC motor which will implement in hardware to Available Online at : Page 63

8 observe actual feasibility of the approach applied in this project. An embedded system is designed and implemented in this project. Controlling a separately excited DC motor with speed feedback through a tacho-generator is successfully implemented using PIC16 microcontroller.lcd (Liquid crystal display) is used to show the reference speed and feedback speed. In different case study either takes the conventional P controller or PI controller, the performance of dc motor drive is more better in case of PI controller approach. APPENDIX The motor used to obtain the experimental results was a 0.5-hp 220-V 1450-r/min dc machine with the following parameters: R a = Armature resistance 22 Ώ, L a = Armature inductance H., J = Inertia constant (for two machines) kg-m 2, B m =Friction coefficient (for two machines) N m/rad/s., k a ø Back EMF and torque constant = = 1.16 V/rad/s. An external inductance and a sampling resistor were added to the armature circuit giving: L ext = H, and R ext = 0.4 Ώ. The effective electrical time constant was then = 0.046/1.0 = 46 ms. The following constants can be obtained: = J/B m = 23.68s, = = = 72 rad/s/a. = 6.2s, = = A/V, The tacho-generator and current transducer gains were, = 0.57 V/rad/s, = 0.5 V/A. The tachogenerator filter time constant was = 0.1 S. The converter gain was = = REFERENCES [1] Geza Joos and Thomas H Barton DEC 1975,"Four-Quadrant DC Variable Speed Drives Design Considerations",IEEE Transactions of Industrial Electronics and Control Instrumentation (VOL-63)NO.12 [2] Paresh C SEN and Murray L. Macdonald NOV. 1978,"Thyristorized DC Drives with Regenerative Braking and Speed Reversal",IEEE Transactions of Industrial Electronics and Control Instrumentation (IECI : NO.4, [3] Payal P.Raval, Prof.C.R.mehta March 2012, "MODELLING, SIMULATION AND IMPLE- MENTATION OF SPEED CONTROL OF DC MOTOR USING PIC 16F877A", International Journal of Emerging Technology and Advanced Engineering.ISSN , Volume 2, Issue [4] Y. S. Ettomi, S. B. M. Noor, S. M.'Bashi and M. I. Hassan DEC 2003,"Micro Controller Based Adjustable Closed- Loop DC Motor Speed Controller",IEEE Transactions of Industrial Electronics and Control Instrumentation /03, [5] Zhijun Liu and Lianzhi Jiang 2011,"PWM Speed Control System of DC motor Based on AT89S51",IEEE Transactions of International Conference on Electronic and Mechanical Engineering and Information Technology /11/26.00 [6] Nazanin Afrasiabi and Mohammdreja Hairi Yazdi 2013,"Dc Motor Control Using Chop- per",global Journal of Science, Engineering and Technology (ISSN : )Issue 8, [7] Dubey, G.K 2004, "Fundamentals of Electrical Drives". New Delhi, Narosa Publishing House,2009 [8] Liangzhong Jiang, P.S.," Design of the Closed Loop Speed Control System for DC Motor", China, SIMULINK User's Guide, The Math Works Inc Available Online at : Page 64