Four Quadrant Converter Operations for a DC Motor from the Personal Computer

Transcription

1 Four Quadrant Converter Operations for a DC Motor from the Personal Computer T Maity, Non-members A Ghosh, Non-members S K Bhunia, Non-members In this paper a software program is developed through which all four quadrant chopper operations are achieved for a dc motor via a personal computer (PC). The method described is an integrated, automated and efficient control, where all the operations are possible interactively. The software program developed here in assembly language in the C environment to generate the controllable switching signals for precise operations. These signals are suitable for the triggering operation of four power electronic switches of the chopper. The program was compiled and run, the switching signals were interfaced, and the chopper drive was run successfully. The minimum hardware requirement for the control circuit leads it to a more reliable system. A user friendly control room method has been adopted using the Keyboard for smooth speed control of the motor and also for the forward-reverse, regenerative braking and emergency stop operations. Keywords : Four quadrant operations; Chopper; Speed control; IGBT INTRODUCTION In today's industry scenario, the process automation is taking an important part to bring out the optimum output as well as to achieve zero downtime of machine. Therefore, real life foolproof system is in demand and the computer is playing a vital role in industrial automation. Today, the continuous development of microprocessor leads to the computing system faster, reliable, versatile, and powerful. The 80486D and Pentium contain a numeric coprocessor that allows them to perform complex calculations using floating-point arithmetic 1. Personal computer is also making its place at the industrial application. A microprocessor with both parallel and serial interfacing has a wide variety of applications. The speed control of motor is a very special application. The basic output interface receives data from the microprocessor and must usually hold it for some external device. It's latches are often built into the I/O device. The latches store the number output by the microprocessor from the data bus. Latches are needed to hold the data because when the microprocessor executes an OUT instruction, the data are not present for sufficient amount of time for visualization. The data are held until the next OUT instruction executes. The chopper fed dc motor is one application where both the speed control operation as well T Maity is with the Department of Electrical Engineering, College of Engineering and Management, Kolaghat, A Ghosh is with Squarem Electra Private Limited while S K Bhunia is with Tata Consultancy Services, Kolkata This paper (modified) was received on June 16, Written discussion on this paper will be entertained upto March 31, as the regenerative braking action can be performed in most efficiently. The speed control of dc motor over a wide range is possible with rectifiers as well as with dc choppers. The latter is preferred for the following reasons. Better control performance with rectifier requires increasing number of phase, which involves increased cost, both for the transformer as well as the rectifier. Moreover, the higher chopper frequency provides a smoother output and also helps in reducing the region of discontinuous conduction in the speed-torque plane. 2 Chopper based dc drives provide improved performance with less complicated circuitry. In the microprocessor used to control a thyristor based two quadrant chopper control dc motor, had a frequency of 400 Hz. But commutation circuit is an additional burden for that and the system is not user friendly and interactive. In 4, the scheme is proposed for a four quadrant operation using processor 89C2051 and MOSFET, but it has also some operating limitations. As most of the plants use PC for different applications today, the following approach of using PC to control dc motor will involve no such additional investment. CONTROL METHOD The control scheme developed and reported in this paper is related with four quadrant chopper dc drive and for that a separately excited dc motor is chosen. Four quadrant chopper is very efficient and versatile mode of drive, as shown in Figure 1, where any of the following four operations is possible, 5 Forward power control During forward power control, a chopper fed drive Vol 87, January

2 Figure 1 Four quadrant chopper for separately excited DC motor operates in the first quadrant, where armature voltage and current are positive. Switches 3 and 4 remain off. When Switch 1 and Switch 2 are turned on together, the supply voltage appears across the motor terminals and the armature current rises. When Switch 2 is turned off, the armature current is forced to decay through diode D3 and Switch 1. Switch 1 remains on during this process. Forward Regeneration Switches 1,2 and 3 are turned off. Switch 4 acts as chopper switch. When Switch 4 is turned on, the armature current, which rises, flows through Switch 4 and D2. When Switch 4 is turned off, the motor, acting as a generator, returns energy to the supply through diodes D1 and D2. Reverse Power Control Switches 1 and 2 are off. When Switch 3 and Switch 4 are turned on together, the armature current rises and flows in reverse direction. When Switch 4 is turned off, the armature current decays through D1 and Switch 3. Switch 3 remains on during this process. Reverse Regeneration Here Switches 1,3 and 4 are off. Switch 2 acts as chopper. When Switch 2 is turned on, the armature current rises through Switch 2 and D4. When Switch 2 is turned off, the armature current falls and the motor returns energy to supply through D3 and D4. All the four quadrant operations are tabulated in Table 1. For this experimental work, the switches selected are IGBTs. Table 1 Four quadrants Sw1, Sw2, Sw3? Off Sw4? Chops Sw1, Sw2? Off Sw3? On Sw4? Chops Va Sw3, Sw4? Off Sw1? On Sw2? Chops Sw1, Sw3, Sw4? Off Sw2? Chops Ia DESCRIPTION OF SOFTWARE Scheme The control software developed here is in the C environment with the help of assembly language programming. C supports assembly level programming. The mnemonic must follow the keyword _asm. The asm statements had become very important during the development of the software, because it uses delay loops for real time operations. Now C language also has the provision for a delay function delay(n), where n is in ms. The function comes under the header file dos.h. The minimum delay obtained is 1 ms, ie, n = 1 and here lies the real problem. The software developed here requires delays of even less than 1 ms. So, the help of assembly level programming is taken for the work. The software developed replaces the total hardware required to generate the switching signals. The user of the program can easily control the armature voltage and hence the speed of the dc machine. The user is first asked for the desired speed as a percentage of the rated speed of the machine. Thereafter, the user controls the direction of rotation by specifying the option. On the basis of the above mentioned data, the switching signals are generated. Description For the above mentioned control, the switching and control signals are generated at the parallel port of the PC by configuring the AL register and then OUT-ing its contents. The Figure 2 explains the process of configuration of the AL register. All the bits are configured in accordance to the mode of operation. The program first checks whether the direction mentioned by the user is forward or reverse for running the motor. In case of forward mode operation, the bit b 2 is kept high continuously and the bit b 1 is kept high for the T_on period and low for the T_off period. The T_on period is calculated by the subroutine ton_delay which has the user's speed input as the parameter. The T_off and the T_on are selected such that (T_on + T_off) is constant throughout the operation. However, the time period T can be changed from the software program (if required for the variable frequency mode operation). The armature current sensed, as shown in Figure 1 is fed through an A/ D converter to the same port (EPP-enhanced parallel port). EPP generates and controls all the transfers to and from the peripheral, assigning both the EPP address register and the EPP data register 6. Data can be read through the same data bus by simply generating control signal under EPP mode to the port by the software program. The whole conversion process is controlled by the control software through control pins of the ADC as well as the parallel port control bits 6. After the T period, the keyboard is scanned for any keyboard interrupt. Interrupt 21h (DOS function, service number 06h) is used to scan the 4 IE(I) Journal-ET

3 Forward regenrative braking bit to switch 4 0, when brake is not applied toggle between 0 and 1 when brake is applied Forward mode speed control bit to Switch1, 1 in the motoring mode, 0 in the regenerative braking mode Reverse regenerative braking bit to Switch 2 0, when brake is not applied toggle between 0 and 1 when brake is applied Reverse mode speed control bit to Switch 3, 1 in the motoring mode, 0 in the regenerative braking mode Forward mode speed control bit to Switch 2, 1 in the t_on period, 0 in the t_off period and in the regenerative mode Reverse mode speed control bit to Switch 4, 1 in the t_on period, 0 in the t_off period and in the regenerative mode Start bit 0 for start, 1 for halt b 0 b 1 b 2 b 3 b 4 b 5 b 6 b 7 Current sense THRO' ADC in EPP mode SW 1 SW 2 SW 3 SW 4 Figure 2 PC parallel port configuration keyboard 7. The program continues to generate the waveform unless interrupted. As soon as there is an interrupt, the ASCII value of the key pressed get stored in the AL register. The program compares the value stored in AL with h, i, d and b for stopping the machine, increasing the armature current, decreasing the armature voltage and braking respectively. When h key on the keyboard is pressed, all the bits are reset. At the output, no signal is received (halt condition). When b key is pressed, the armature voltage that existed before the application of the brakes is stored and T_on is made zero. Bits b 1 and b 2 (for forward operation) or bits b 5 and b 6 (for reverse operation) are then reset. To activate the regenerative braking operation, the bit b 3 or b 4 is kept high and low alternately. Later to restart if necessary, the speed can be slowly increased by pressing the i key. The special feature of the program is that pressing the b key once again can also retrieve the voltage before the application of the brakes. Sequence of Operation At starting operation of the motor, the program takes the percentage of the input voltage from the user as an input parameter. The user cannot input beyond 100% speed. In the next step after pressing enter, the user will be asked the direction of operation. Depending upon the speed percentage and forward/reverse input, the computer generates the square wave output at its parallel printer port. Figure 3 explains the sequence of operation for a particular direction of rotation. For operations of keys i or d, the t_on and t_off periods are readjusted and the speed percentage is displayed accordingly. During starting, a soft starting technique is adopted through program, which is based on the armature current feedback of motor sensed by the PC. In this scheme, the t_on time of switching signals are slowly increased to the set level, to control the inrush of starting current. One waveform of starting switching signals of duty cycle is the 0.5 is shown in the Figure 4. An upper limit is also set in the program for the current and if the current cross that limit, the t_on period of the output is reduced by itself and after certain period this is again increased to its set value. Through this repetitive process, finally the motor reaches to its steady state. This is a protection from any overloading situation. For braking operation, one IGBT is in operation and the armature current is monitored continuously to limit the braking current as well as to finish the braking operation. HARDWARE ARE Corresponding data from a 25 pin printer port is transferred using a 25 pin D connector and a FRC cable through four opto-couplers (for isolating PC from the external circuit). The bits b 0, b 1, b 2, b 4, b 5, b 6 are connected to the pin 2, 3, 4, 6, 7 and 8 of the D connector respectively. These control signals are fed to the IGBTs of four quadrant chopper through the driver circuits, as shown in Figure 5. A driver circuit consists of a comparator (high frequency op amp LM318) followed by a push-pull amplifier. A npn and a pnp transistor are used Vol 87, January

4 Start A User entry : % Speed Direction of Motion = 0 Key No Z es Display Speed Calculation of t_on and t_off time. manipulation of direction & brake bits. Current Sensing < upper limit? Z Halt key Speed inc. key restart? Stop V=V+1, k=0 Out through port keeping 'speed control bit' high. > 100? Speed dec. key V=V-1,key=0 call subroutine t_on delay V = 100 B V = 0 V < 0? Out through port keeping 'speed control bit' low. Call subroutine t_off delay Sense key pressing A B Brake key Set speed control bit low Calculation of t_on and t_off time for braking switch Out through port bit Sense current = 0? Stop Figure 3 Flowchart of operation 6 IE(I) Journal-ET

5 V Starting switching signals Time, ms s Figure 4 Switching signals during start up Signals_from_PC thr' opto coupler + 12 V 12 V 12 V Figure 5 Driver circuit for IGBT 100 ohm to IGBT_gate for that purpose. Finally, the outputs of these amplifiers go to the IGBTs as switching signals. For the experiment, the Mitsubishi make IGBTs CT60AM-18F have been used. The IGBTs turn on for a positive voltage (+25V max) but turn off for a negative voltage (-25V min). Hence the push-pull circuit is used here. EPERIMENTAL AL RESULTS TS AND CONCLUSION The work has been completed and tested with a motor with following specifications. Laboratory type 1 kw separately excited dc motor, 1400rpm, 200V dc input, constant field voltage of 200V. The switching frequency for IGBTs selected is 1 khz. The Table 2 shows the some of the measurements of speed corresponding to the armature voltage. The advantages observed in this type of control can be summarized as given below. (i) The use of PC in the place of conventional drive is cost effective as we consider various operations can be performed at a time. Table 2 Experimental results Armature Armature Speed, rpm voltage, % voltage (ii) The program interface in PC is more user friendly compared to a conventional drive, enabling flexible and simple operation right from the control room. (iii)the motor speed control can be smooth, practically from zero up to the rated value within an acceptable time period. (iv)minimum hardware is used and so making it more reliable. (v) The four quadrant operations including the regenerative braking action makes it more versatile. Moreover, the control method described in this paper can be extended easily for a closed loop system where the actual speed is sensed, converted to a digital signal and fed to the same parallel port of the PC. In this way the speed regulation is possible. Even the method can be applied for the control of ac motor. REFERENCES 1. B B Brey. 'The Intel Microprocessor 8086/8088, 80186/80188, 80286, 80386, 80486, Pentium and Pentium Co-proessor Architecture, Programming, and Interfacing.' PHI, 3rd edition, 1995, p V R Moorthi. 'Power Electronics'. Oxford University Press, 1st edition, 2005, pp W G Dunford, S G Dewan. 'The Design of a Control Circuit for a Two Quadrant Chopper Based on the Motorola 6800 Microprocessor'. IEEE Trans.Ind.Appl, 1980, p M Lawrence. 'A Microcontroller Based Motor Speed Control.' 5. M H Rashid. 'Power Electronics Circuits, Devices and Application, PHI, 2nd edition, 1998, p P Norton. 'Inside the PC'. PHI, 6th edition, 1997, p K Richard. 'The MS-DOS handbook'. BPB Publication, 1986, p 297, p 108. Vol 87, January