UNIVERSITY OF NAIROBI DESIGN OF AN INVERTER DRIVE FOR A DC MOTOR Project Number: PRJ 077 By OWITI EMMANUEL ONDIEGE F17/8281/2004 Supervisor: MR. S.L OGABA Examiner: MR. V.M DHARMADHIKARY Project report submitted in partial fulfillment of the requirement for the award of the degree of Bachelor of Science in Electrical and Electronic Engineering of the University of Nairobi 20 th MAY 2009 Department of Electrical and Information Engineering
ABSTRACT Today in industries the world over, a large part of electric energy consumption is by electric motor drives. This brings out the need for the design of electric motor drive control systems with the main aims being to increase the efficiency, economize power consumption and achieve a fully controllable variable speed drive system. The project entailed the study of the conventional methods of dc motor control hence noting their inefficiencies, then the design and implementation of an inverter drive that can be used to control the speed of a dc motor. ii
Table of Contents ABSTRACT... ii ACKNOWLEDEGMENT... v CHAPTER 1: INTRODUCTION... 1 1.1 Advantages of Electronic Control... 1 1.2 Objectives... 2 CHAPTER 2: ELECTRIC MOTORS... 4 2.1 How a motor works... 4 2.2 Direct Current (DC) motors... 6 2.2.1 Separately excited DC motor... 8 2.2.2 Self excited DC motor: series motor... 8 2.2.3 Self excited DC motor: shunt motor... 9 2.2.4 DC Compound Motor... 10 2.3 AC Motors... 11 2.3.1 Synchronous motor... 12 2.3.2 Induction motor... 13 CHAPTER 3: SPEED CONTROL OF DC MOTORS... 15 3.1 SPEED CONTROL OF DC SHUNT MOTORS.... 16 3.1.1 Flux control method.... 16 3.1.2 Armature control method.... 17 3.1.3 Voltage control method... 18 3.2 SPEED CONTROL OF DC SERIES MOTORS... 19 3.2.1 Flux control method.... 20 3.2.2 Armature resistance control.... 21 3.2.3 Series-Parallel control method.... 22 iii
CHAPTER 4: FINAL DESIGN... 24 4.1 THEORY OF THE CIRCUIT... 24 4.2 COMPONENT DESIGN... 27 4.2.1 Input rectifier... 27 4.2.2 Control circuit... 28 4.2.3 DC AC Converter... 30 4.2.4 Rectifier and filter... 32 4.3 RESULTS AND ANALYSIS... 34 CHAPTER 5: CONCLUSION... 36 5.1 RECOMMENDATIONS AND FURTHER WORK... 36 REFERENCES... 37 APPENDIX : DATA SHEETS... 38 iv
ACKNOWLEDEGMENT I would like to thank Mr. Ogaba for spending his valuable time in supervising my project and assisting me in all ways possible in order for me to meet the project requirements, and to Mr. Dharmadhikary for taking his time to examine my project. I would also like to thank the technical staff; Mr. Muraba and Mr. Wanyoike for their technical assistance. Lastly I would like to sincerely thank my classmates who, not just throughout the project but throughout the five years, have provided immense support and advice. v
CHAPTER 1: INTRODUCTION It is essential to vary the speed of electric drives in different field of applications. Normally in all process industries, it is desired that the system be set at slow speed in the beginning and then gradually increased to meet the maximum production rate. An example of a newspaper printing press may be taken here. Various pages of the newspaper are printed by different stands and then combined, cut and folded. The entire process is first set at a slow pace so that, the wastage of newsprint is minimum. Once the process is set, the speed of the entire system is increased to quicken the production. There can be many processes where variable speed drives are required. One of the major achievements of power electronic technology in the field of control is the control of dc motor drives. Because of their tremendous control capabilities, solid state motor control schemes have almost replaced the conventional electrical control methods. Because of their manifold advantages such as compactness, fast response, higher efficiency, more control capabilities, more reliability and less cost, power electronic controlled schemes have totally dominated the field of control of dc motors. There are several advantages of electronic control systems as compared to the conventional methods. They are as follows. 1.1 Advantages of Electronic Control 1. Electronic control systems are very compact and small in size, and thus require comparatively less space. 2. They require less power. 3. They are fast in response. 4. They are much more accurate and efficient than the conventional methods. 5. System reliability is much more than the usual conventional methods. 6. An electronic control system is economical as compared to other systems as it involves minimum maintenance cost. 7. An electronic system is highly protective and the devices under control are much safe as compared to other systems. 1
1.2 Objectives The objectives of the project were to: Do a study of dc motor control by conventional methods. Design an inverter drive to control a dc motor. Test the designed inverter drive. A study of the control of dc motors was carried out and was dealt with in chapter 3 of the project report. The three main methods that were discussed were: a. Varying the flux per pole. This is known as flux control method. b. Varying the resistance in the armature circuit. This is known as armature control method. c. Varying the applied voltage. This is known as voltage control. It is seen that these conventional methods, though they formed the background by which the power electronic control devices are used in control, they are found to be wasteful in terms of the amount of power consumed. 2
AC MAINS Control Unit - Rectifier - 555 timer - Voltage Comparator - Optical Coupler Semiconductor Switching Device (MOSFETs) Dc output Dc motor Figure 1.1 Block diagram of the inverter drive In chapter 4, the final design schematic is shown (figure 4.1). Here an in depth explanation of the circuit is given with the circuit being segmented to give the reader better understanding. The circuit could also be represented as shown in the block diagram in figure 1.1. 3
CHAPTER 2: ELECTRIC MOTORS An electric motor is an electromechanical device that converts electrical energy to mechanical energy. This mechanical energy is used, for example, in rotating a pump impeller, fan or blower, driving a compressor, lifting materials etc. Electric motors are used at home (mixer, drill, fan) and in industry. Electric motors are sometimes called the work horses of industry because it is estimated that motors use about 70% of the total electrical load in industry. 2.1 How a motor works The general working mechanism is the same for all motors. (Figure 2.1): An electric current in a magnetic field will experience a force. If the current carrying wire is bent into a loop, then the two sides of the loop, which are at right angle to the magnetic field, will experience forces in opposite directions. The pair of forces creates a turning torque to rotate the coil. Practical motors have several loops on an armature to provide a more uniform torque and the magnetic field is produced by electromagnet arrangement called the field coils. 4
Figure 2.1 Basic Principle of how Electric Motors Work In understanding a motor it is important to understand what a motor load means. Load refers to the torque output and corresponding speed required. Loads can generally be categorized into three groups: Constant torque loads are those for which the output power requirement may vary with the speed of operation but the torque does not vary. Conveyors, rotary kilns, and constant-displacement pumps are typical examples of constant torque loads. Variable torque loads are those for which the torque required varies with the speed of operation. Centrifugal pumps and fans are typical examples of variable torque loads (torque varies as the square of the speed). Constant power loads are those for which the torque requirements typically change inversely with speed. Machine tools are a typical example of a constant power load. Components of electric motors vary between different types of motors and are therefore described for each motor separately. 5
Electric Motors Alternating Current (AC) Motors Direct Current(DC) Motors Synchronous Induction Separately Excited Self Excited Single-Phase Three-Phase Series Compound Shunt Figure 2.2 Classifications of the Main Types of Electric Motors 2.2 Direct Current (DC) motors Direct-current motors, as the name implies, use direct-unidirectional current. DC motors are used in special applications where high torque starting or smooth acceleration over a broad speed range is required. A DC motor and has three main components: Field pole. The interaction of two magnetic fields causes the rotation in a DC motor. The DC motor has field poles that are stationary and an armature that turns on bearings in the space between the field poles. A simple DC motor has two field poles: a north pole and a south pole. The magnetic lines of force extend across the opening between the poles from north to south. For largerr or more complex motors there are one or more electromagnets. These electromagnets receive electricity from an outside power source and serve as the field structure. 6
Armature. When current goes through the armature, it becomes an electromagnet. The armature, cylindrical in shape, is linked to a drive shaft in order to drive the load. For the case of a small DC motor, the armature rotates in the magnetic field established by the poles, until the north and south poles of the magnets change location with respect to the armature. Once this happens, the current is reversed to switch the south and north poles of the armature. Commutator. Its purpose is to overturn the direction of the electric current in the armature. The commutator also aids in the transmission of current between the armature and the power source. The main advantage of DC motors is speed control, which does not affect the quality of power supply. It can be controlled by adjusting: The armature voltage increasing the armature voltage will increase the speed The field current reducing the field current will increase the speed. DC motors are available in a wide range of sizes, but their use is generally restricted to a few low speed, low-to-medium power applications like machine tools and rolling mills because of problems with mechanical commutation at large sizes. Also, they are restricted for use only in clean, non-hazardous areas because of the risk of sparking at the brushes. DC motors are also expensive relative to AC motors. The relationship between speed, field flux and armature voltage is shown in the following equation: Back electromagnetic force: E = K v ΦN (2.1) Torque: T = K t ΦI a (2.2) Where: E = electromagnetic force developed at armature terminal (volt) Φ = field flux which is directly proportional to field current N = speed in RPM (revolutions per minute) 7
T = electromagnetic torque I a = armature current K v = voltage constant K t = torque constant 2.2.1 Separately excited DC motor In many traction applications where both armature voltage and stator current are needed to control the speed and torque of the motor from no load to full load, the separately excited DC motor is used for it s high torque capability at low speed achieved by separately generating a high stator field current and enough armature voltage to produce the required rotor torque current. As torque decreases and speed increases, the stator field current requirement decreases and the armature voltage increases. Without any load the speed of the separately excited motor is limited by the armature voltage and stator field current. 2.2.2 Self excited DC motor: series motor In the series motor, the field windings, consisting of a relatively few turns of heavy wire, are connected in series with the armature winding. The same current flows through the field winding and the armature winding. Any increase in current, therefore, strengthens the magnetism of both the field and the armature. Because of the low resistance in the winding, the series motor is able to draw a large current during starting. This high starting current is what produces a high starting torque, which is the series motor s principal advantage. The speed of a series motor is dependent upon the load. Any change in load is accompanied by a substantial change in speed. A series motor will run at high speed when it has light load and at low speed with a heavy load. If the load is removed, the motor may operate at such high speed that the armature will fly apart. Series motors are suited for applications requiring a high starting torque, such as cranes and hoists. 8
Figure 2.3 Characteristics of a DC Series Motor 2.2.3 Self excited DC motor: shunt motor In a shunt motor, the field winding (shunt field) is connected in parallel with the armature winding (A) as shown in figure 2.4. The total line current is therefore the sum of field current and armature current. The resistance in the field winding is high. Since the field winding is connected directly across the power supply, the current through the field is constant. The field current does not vary with motor speed, as in the series motor and, therefore, the torque of the shunt motor will vary only with the current through the armature. The torque developed at starting is less than that developed by a series motor of equal size. The speed of the shunt motor varies very little with changes in load. When all load is removed, it assumes a speed slightly higher than the loaded speed. This motor is particularly suitable for use when constant speed is desired and when high starting torque is not required. 9
Figure 2..4 Characteristics of a DC Shunt Motor 2.2.4 DC Compound Motor This motor is a combination of the series and shunt motors. There are two windings in the field: a shunt winding connected in parallel and a series winding which is in series with the armature as shown in figure 2.5. The shunt winding is composed of many turns of fine wire and is connected in parallel with the armature winding. The series winding consists of a few turns of large wire and is connected in series with the armature winding. The higher the percentage of compounding (i.e. percentage of field winding connected in series), the higher the starting torque this motor can handle. For example, compounding of 40-50% makes the motor suitable for hoistss and cranes, but standard compound motors (12%) are not. The starting torque is higher than in the shunt motor but lower than in the series motor. Variation of speed with load is less than in a series wound motor but greaterr than in a shunt motor. The compound motor is used whenever the combined characteristics of the series and shunt motors are desired. 10
Figure 2.5 Characteristics of a DC Compound Motor 2.3 AC Motors Alternating current (AC) motors use an electrical current, which reverses its direction at regular intervals. An AC motor has two basic electrical parts: a "stator" and a "rotor". The stator is in the stationary electrical component. The rotor is the rotating electrical component, which in turn rotates the motor shaft. The main advantage of DC motors over AC motors is that speed is more difficult to control for AC motors. To compensate for this, AC motors can be equipped with variable frequency drives but the improved speed control comes together with a reduced power quality. Induction motors are the most popular motors in industry because of their ruggedness and lower maintenance requirements. AC induction motors are inexpensive (half or less of the cost of a DC motor) and also provide a high power to weight ratio (about twice that of a DC motor). 11
2.3.1 Synchronous motor A synchronous motor is an AC motor, which runs at constant speed fixed by frequency of the system. It requires direct current (DC) for excitation and has low starting torque, and synchronous motors are therefore suited for applications that start with a low load, such as air compressors, frequency changes and motor generators. Synchronous motors are able to improve the power factor of a system, which is why they are often used in systems that use a lot of electricity. The main components of a synchronous motor are: Rotor. The main difference between the synchronous motor and the induction motor is that the rotor of the synchronous motor travels at the same speed as the rotating magnetic field. This is possible because the magnetic field of the rotor is no longer induced. The rotor either has permanent magnets or DC-excited currents, which are forced to lock into a certain position when confronted with another magnetic field. Stator. The stator produces a rotating magnetic field that is proportional to the frequency supplied. This motor rotates at a synchronous speed (N s ), which is given by equation (2.3): Where: N s = 120 f / P (2.3) f = frequency of the supply P= number of poles 12
2.3.2 Induction motor Induction motors are the most common motors used for various equipments in industry. Their popularity is due to their simple design, they are inexpensive and easy to maintain, and can be directly connected to an AC power source. An induction motor has two main electrical components (Figure 2.6): 1. Rotor : Induction motors use two types of rotors: A squirrel-cage rotor consists of thick conducting bars embedded in parallel slots. These bars are short-circuited at both ends by means of short-circuiting rings. A wound rotor has a three-phase, double-layer, distributed winding. It is wound for as many poles as the stator. The three phases are wired internally and the other ends are connected to sliprings mounted on a shaft with brushes resting on them. 2. Stator. The stator is made up of a number of stampings with slots to carry three-phase windings. It is wound for a definite number of poles. The windings are geometrically spaced 120 degrees apart. Figure 2.6 An Induction Motor 13
Induction motors can be classified into two main groups: Single-phase induction motors. These only have one stator winding, operate with a single-phase power supply, have a squirrel cage rotor, and require a device to get the motor started. This is by far the most common type of motor used in household appliances, such as fans, washing machines and clothes dryers, and for applications for up to 3 to 4 horsepower. Three-phase induction motors. The rotating magnetic field is produced by the balanced threephase supply. These motors have high power capabilities, can have squirrel cage or wound rotors (although most have a squirrel cage rotor), and are self-starting. It is estimated that about 70% of motors in industry are of this type, are used in, for example, pumps, compressors, conveyor belts, heavy-duty electrical networks, and grinders. They are available in 1/3 to hundreds of horsepower ratings. Induction motors work as follows. Electricity is supplied to the stator, which generates a magnetic field. This magnetic field moves at synchronous speed around the rotor, which in turn induces a current in the rotor. The rotor current produces a second magnetic field, which tries to oppose the stator magnetic field, and this causes the rotor to rotate. In practice however, the motor never runs at synchronous speed but at a lower base speed. The difference between these two speeds is the slip, which increases with higher loads. Slip only occurs in all induction motors. To avoid slip, a slip ring can be installed, and these motors are called slip ring motors. The following equation can be used to calculate the percentage slip: % Slip = 100% (2.4) Where: N s = synchronous speed in RPM N b = base speed in RPM 14
CHAPTER 3: SPEED CONTROL OF DC MOTORS Although a far greater percentage of electric motors in service are ac motors, the dc motor is of considerable industrial importance. The principal advantage of a dc motor is that its speed can be changed over a wide range by a variety of simple methods. Such fine speed control is not possible with ac motors. In fact fine speed control is one of the reasons for the strong competitive position of dc motors in modern industrial applications. In this chapter the various methods of speed control of dc motors are discussed. The speed (N) of a dc motor is given by: N α N = rpm (3.1) Where R = R a shunt motor R = R a + R se series motor I a armature current E b back emf From equation (3.1), it is clear that there are three main methods of controlling the speed of a dc motor, namely: i. By varying the flux per pole (. This is known as flux control method. ii. By varying the resistance in the armature (R a ) circuit. This is known as armature control method. iii. By varying the applied voltage, V. This is known as voltage control method. 15
3.1 SPEED CONTROL OF DC SHUNT MOTORS. The speed of a shunt motor can be changed by: i. Flux control method. ii. iii. Armature control method. Voltage control method. The first method i.e. flux control, is frequently used because it is simple and inexpensive. 3.1.1 Flux control method. It is based on the fact that by varying the flux, the motor speed (N α 1 ) can be changed and hence flux control method. In this method, a variable resistance (known as shunt field rheostat) is placed in series with shunt field winding R sh. Figure 3.1 Flux control method The shunt field rheostat reduces the shunt field current I sh and hence the flux. Therefore, we can only raise the speed of the motor above the normal speed. 16
Advantages i. This is an easy and convenient method. ii. It is an inexpensive method since very little power is wasted in the shunt field resistor due to relatively small value of shunt field current I sh. Disadvantages i. Only speeds higher than normal can be obtained since the total field circuit resistance cannot be reduced below the shunt field winding R sh. ii. There is a limit to the maximum speed obtainable by this method. It is because if the flux is too much weakened, the commutation becomes poorer. 3.1.2 Armature control method. This is based on the fact that by varying the voltage available across the armature, the back emf and hence the speed of the motor can be changed. This is done by inserting a variable resistance i.e. controller resistance R c,in series with the armature. Figure 3.2 Armature control method Due to the voltage drop in the controller resistance, the back emf E b is decreased. Since N α E b, the speed of the motor is reduced. The highest speed obtainable is that corresponding to R c = 0 i.e. normal speed. Hence this method can only provide speeds below the normal speed. 17
Disadvantages. i. A large amount of power is wasted in the controller resistance since it carries full armature current I a. ii. The speed varies with load since the speed depends upon the voltage drop in the controller resistance and hence on the armature current demanded by the load. iii. The output and efficiency of the motor are reduced. iv. This method results in poor speed regulation. 3.1.3 Voltage control method. In this method, the voltage source supplying the field current is different from that which supplies the armature. This method avoids the disadvantages of poor speed regulation and low efficiency as in armature control method. However, it is quite expensive. Therefore this method of speed control is employed for large size motors where efficiency is of great importance. Multiple voltage control. In this method, the shunt field of the motor is connected permanently across a fixed voltage source. The armature can be connected across several different voltages through a suitable switchgear. In this way, voltage applied across the armature can be changed. The speed will be approximately proportional to the voltage applied across the armature. Ward-Leonard System. In this method, the adjustable voltage for the armature is obtained from an adjustable voltage generator while the field circuit is supplied from a separate source. The armature of the shunt motor (whose speed is to be controlled) is connected directly to a dc generator driven by a constant speed ac motor. The field of the shunt motor is supplied from a constant-voltage exciter. The field of the generator is also supplied from the exciter. The voltage of the generator can be varied by means of its field regulator. 18
By reversing the field current of the generator by a controller, the voltage applied to the motor is reversed. Sometimes a field regulator is included in the field circuit of the shunt motor for additional speed adjustment. With this method, the motor may be operated at any speed up to its maximum speed. Advantages. i. The speed of the motor can be adjusted through a wide range without resistance losses which results in high efficiency. ii. The motor can be brought to a standstill quickly, simply by rapidly reducing the voltage of generator. When the generator voltage is reduced below the back emf of the motor, this back emf sends current through the generator armature, establishing dynamic braking. While this takes place, the generator operates as a motor driving the ac motor which returns power to the line. iii. This method is used for the speed control of large motors when a dc supply is not available. The disadvantage of this method is that a special motor-generator set is required for each motor and the losses in this set are high if the motor is operating under light loads for long periods. 3.2 SPEED CONTROL OF DC SERIES MOTORS The speed of dc series motors can be obtained by: i. Flux control method. ii. Armature resistance control method. iii. Series-Parallel control method 19
3.2.1 Flux control method. In this method, the flux produced by the series motor is varied and hence the speed. The variation of flux is achieved in the following ways: Field diverter. In this method, a variable resistance (field diverter) is connected in parallel with series field winding. Its effect is to shunt some portion of the line current from the series field winding, thus weakening the field and increasing the speed. (N α 1 ). The lowest speed obtainable is that corresponding to zero current in the diverter (i.e. diverter is open). The lowest speed is hence the normal speed of the motor. Consequently, this method can only provide speeds above the normal speed. The series field diverter method is often employed in traction work. Figure 3.3 Field diverter Armature diverter. In order to obtain speeds below the normal speed available, a resistance (called armature diverter) is connected in parallel with the armature. The diverter shunts some of the line current, thus reducing the armature current. Now for a given load, if I a is decreased, the flux Φ must increase. Since N α 1, the motor speed is decreased. By adjusting the armature diverter, any speed lower than the normal speed can be obtained. 20
Figure 3.4 Armature diverter Tapped field control. In this method, the flux is reduced (and hence speed is increased) by decreasing the number of turns of the series field winding. The switch S can short circuit any part of the field winding, thus decreasing the flux and raising the speed. With full turns of the field winding, the motor runs at normal speed and as the field turns are cut out; speeds higher than normal speed are achieved. Figure 3.5 Tapped field control 3.2.2 Armature resistance control. In this method, a variable resistance is directly connected in series with the supply to the complete motor. This reduces the voltage available across the armature and hence speed falls. By changing the value of variable resistance, any speed below the normal speed can be obtained. This is the most common method employed to control the speed of dc series motors. 21
Figure 3.6 Armature resistance control 3.2.3 Series Parallel control method. Another method used in the speed control of DC series motors is the series- parallel method. In this system which is widely used in traction system, two (or more) similar DC series motors are mechanically coupled to the same load. Figure 3.7a Motors connected in series Figure 3.7b Motors Connected in parallel 22
When the motors are connected in series (figure 3.7a), each motor armature will receive half the normal voltage. Therefore the speed will be low. When the motors are connected in parallel (figure 3.7b), each motor armature receives the normal voltage and the speed is high. Thus two speeds can be obtained. It can be noted that for the same load on the pair of motors, the system would run approximately four times the speed when the machines are in parallel as when they are in series. This is shown in equation (3.2) and (3.3). When in parallel: N α α α (3.2) When in series: N α α α (3.3) 23
CHAPTER 4: FINAL DESIGN The circuit was finally designed to meet the required objectives and specifications. 4.1 THEORY OF THE CIRCUIT The purpose of the design was to come up with an inverter drive to control a dc motor by variation of armature voltage. The motor to be controlled having a voltage rating of 110V DC yet the supply available from the mains being 240V AC. The inverter drive consists of an astable multi-vibrator (555 timer), that produces a 2 KHz square wave pulse, a triac optical coupler that allows the pulse from the 555 timer to reach the switching devices (MOSFETs) only when required. The configuration of the MOSFETs is such that they switch in an anti-phase design such that when Q 4 is ON, Q 5 is OFF and vice-versa (see figure 4.1). The MOSFETs are connected to a high frequency centre-tapped transformer that has its centre tap supplied by a dc voltage that has been rectified from the input ac mains. This produces an ac voltage at the secondary of the transformer (Tx 2 ). The output is then rectified and supplied to the dc motor. This output voltage is sampled and fed-back to the positive terminal of the voltage comparator that also is connected to resistors R 5, R 4 and variable resistor R 6 (figure 4.1). The negative terminal of the voltage comparator is kept at a reference voltage (V ref ) of 12V that is provided for by the zener diode (D 5 ) that is rated 12V. The voltage required at the output (V out ) is set by the variable resistor R 6. This is shown in the equation (4.1), since R 4 and R 5 are fixed then the variable resistor R 6 sets the required voltage. V out = 12V 4 5 6 5 6 (4.1) Should the voltage at the positive terminal of the comparator be less than the voltage at the negative terminal (V ref ) then the output of the comparator is taken to negative hence reverse biasing the diode that is within the triac optical coupler IC. This turns ON the triac which allows the pulse from the 555 timer to pass through. 24
On the positive pulse period, the signal passes through resistor R 10 through to the N-P-N transistor Q 1. This then turns ON the P-N-P transistor Q 3 allowing the MOSFET Q 5 to be turned ON. The rectified voltage from rectifier consisting of diodes D 6, D 7, D 8 and D 9 to the centre tap of transformer (Tx 2 ) hence now passes through the MOSFET Q 5. On the negative pulse period, the P-N-P transistor Q 2 is turned ON this provides a gate signal to the MOSFET Q 4 which turns it ON. This action of switching between Q 4 and Q 5 produces an ac voltage at the secondary of the high frequency transformer. Should there be a change in load or some cause of voltage change such that the voltage at the positive terminal of the comparator is higher than that at the negative terminal (V ref ), then the comparator swings to +Vcc hence forward biasing the diode that is within the triac optical coupler. This turns OFF the triac, hence blocks the pulse from the 555 timer. The dc-ac configuration that consists of the BJTs and MOSFETs is then turned OFF. This causes the voltage at the output to decrease to the required voltage. Hence once the required voltage has been set from the variable resistor R 6 then the circuit is able to control itself due to the feedback path provided. 25
Figure 4.1 Final Circuit Design 26
4.2 COMPONENT DESIGN 4.2.1 Input rectifier The input rectifier consists of diodes D 1, D 2, D 3, D 4 and shunt capacitor C 1 which converts the ac voltage from the transformer, Tx 1 (240V-15V) to dc voltage. Figure 4.2 Input rectifier The ICs (555 timer and comparator) supplied by Tx 1 are rated 100mA each so the total current required = 2 100mA = 200mA. Tx 1 power = Output Voltage * Output Current (4.2) = 15V 200mA = 3VA Diodes D 1, D 2, D 3, and D4 form a full-wave bridge rectifier. The diodes used are determined by the maximum voltage across each diode. V P = Vrms 2 (4.3) = 15 2 = 21.2V The diodes used should be rated more than 200mA and 21.2V, diodes IN4001 rated 50V and 1A were used. 27
Capacitor C 1 is a shunt capacitor filter. The minimum capacitor that could be used was determined from the equation (4.4): Vdc = (4.4) With V P = 21.2, I dc = 200mA, f = 50Hz. The value of C can be found by substitution which is 114µf. The value used was 470µf with a voltage rating of 35V. 4.2.2 Control circuit Figure 4.3 Control circuit The control circuit comprises of an astable multi-vibrator (555 timer) which generates a high frequency (2 KHz) pulsating signal, a voltage comparator (LM393N) and a triac optical coupler (MOC3020). The 555 timer is rated 15V maximum. The values of R 1 and R 2 were found from the equation (4.5a) f =. (4.5a) with f = 2 KHz C 2 (R 1 +2R 2 ) = 28
Setting C 2 to be 10nf, then R 1 + 2R 2 = 72.5KΩ (4.5b) To obtain a square wave signal, R 2 should be greater than R 1. Hence setting R 1 to 6.8 KΩ, substitution in equation (4.5b) gives R 2 = 33K Ω. The data sheet recommends that voltage control terminal be connected to ground via a 10nf decoupling capacitor C 3 = 10nf. R 3 and D 5 set a reference voltage. A reference voltage of 12V is set by the zener diode D 5. D 5 is BZX79C12 rated V z = 12V and I z = 5mA. R 3 = = Hence R 3 design is 680Ω. = 600Ω R 4, R 5 and R 6 sample the output voltage to the motor. The voltage drop across R 6 and R 5 at 110V (this is the rated voltage of the dc motor), should be equal to the reference voltage (12V). 110V 12V (4.6a) 0.11 (4.6b) To get the maximum voltage output the variable resistor (R 6 ) is set at a minimum (zero) hence equation (4.6b) becomes: 0.11 R 5 = 0.11(R 4 + R 5 ) Setting R 4 = 470KΩ then R 5 = 56 KΩ. To attain a lower voltage, the value of the variable resistor (R 6 ) is increased. At a lower scale, to obtain a voltage of 50V: 29
50V = 12V (4.7a) = 0.24 (4.7b) By substitution R 6 = 87KΩ, Potentiometer available = 100KΩ The triac optical coupler is used to allow the pulse from the 555 timer to the inverter when the voltage drop across resistors R 5 and R 6 is less than the reference voltage. The optical coupler MOC3020 rated I f 15mA. Therefore the resistor R 7 has a value of 1KΩ. R 7 = (4.8) = = 1 KΩ 4.2.3 DC AC Converter This employs a push-pull configuration having a high frequency coupling transformer Tx 2. Figure 4.4 DC- AC Converter 30
The transformer which is centre tapped at the primary is fed from the input rectifier made up of diodes D 6, D 7, D 8 and D 9. The diodes used were determined by the maximum voltage across each diode. V P = 240 2 =339V. The output current = 9A. Output power = Output Voltage Output Current Supply current = = 110V 9A = 990VA = = 4.125A C 4 is the shunt capacitor filter. Its value was determined by substitution in equation (4.4). A capacitor of 330µf and voltage rating of 470V was used. The MOSFETs Q 4 and Q 5 are IRF740 rated 10A and 400V V DS max. Transistors Q 2 and Q 3 should be rated more than 100mA. Transistor 2N2907 rated 600mA was used. Resistors R 12 and R 16 were used to ground all leakage currents flowing through the transistors. R 12, R 16 = = = 150Ω R 9, R 14 = I B = = = 1mA Therefore; R 9, R 14 = = 15KΩ 31
The current flowing through the base of Q 3 is the collector current flowing through Q 1. The transistor C1815 rated 100mA and h fe 100 was used for Q 1. R 13 = R 10 = = = 15KΩ I B = = = 10µA R 10 = = 1.5MΩ 4.2.4 Rectifier and filter This consists of full-wave bridge rectifier and L-C filter. Figure 4.5 Output rectifier 32
Diodes D 10, D 11, D 12 and D 13 form a full-wave bridge rectifier. The diodes used are determined by the maximum voltage across each diode. The diodes used were rated 800Vmax. The inductor L 1 and capacitor C 5 form the L-C filter. The filter should be such that the ripple factor is 1%. r = 1.19 LC (4.9) 0.01 = 1.19 LC LC = 0.0084 Selecting an inductance of 22µH for L, C =. = 382µf Therefore a capacitance of 470µf with a voltage rating of 250V was chosen. The figure 4.6 shows the constructional details of the inverter drive. Figure 4.6 inverter drive designed 33
4.3 RESULTS AND ANALYSIS The inverter drive designed was able to produce dc voltage at the output that could be varied, to be supplied to the dc motor armature. Hence the control was by armature voltage control. The different values of dc voltage were provided for by the variable resistor R 6 (see figure 4.1). On the final packaged device the resistor knob was evenly divided in five divisions, i.e. from dial setting 0 to dial setting 5. Dial setting 0 represented the minimum resistance i.e. zero hence highest value of voltage at the output. Dial setting 5 represented the highest value of resistance i.e. 100KΩ hence the lowest value of voltage available at the output. The results expected (theoretical) and measured were tabulated in table 4.1. Table 4.1 Output Voltages Theoretical and Measured Dial Setting Output Voltage, V dc (Theoretical) Output Voltage, V dc (Measured) 0 110.9 125 1 86.2 100 2 70.8 78 3 60.6 66 4 53.5 56 5 48.2 50 The measured voltages were slightly higher than the expected (theoretical) values. This could be explained due to the fact that on design from calculation, the components such as resistors calculated were not always the standard values of resistors available. The values of such components implemented were chosen to the nearest value. 34
The results (table 4.1) show that the circuit implemented could be used as an inverter drive for a dc motor. This could be used in situations where varying values of dc voltage are required but what is available from the supply is 240 V ac mains supply. The tabulated results also show that the inverter drive is capable of providing voltage of between 50V dc to 125V dc. 35
CHAPTER 5: CONCLUSION From the results obtained (table 4.1), the circuit designed was capable of providing speed control of a dc motor by producing varying dc voltage of between 50V and 125V. This dc voltage being supplied to the armature winding of the dc motor. A survey of the control of dc motors by conventional methods was carried out and the three main methods of control i.e. Varying the flux per pole Varying the resistance of the armature circuit. Varying the applied voltage were studied as required in the objectives of the project. The implementation of the project provided valuable knowledge of dc motors in general and how their control is useful in the industrial field. It also gave a good understanding of the conventional control concepts used in the control of dc motors. 5.1 RECOMMENDATIONS AND FURTHER WORK 1. Diodes should be added to the existing diodes (D 10, D 11, D 12 and D 13 ) in parallel hence enable the device handle more current and a motor with a bigger power rating. 2. The circuit could be further improved by implementing one that controls a specific dc motor, such that the output voltage of the inverter drive and the corresponding speed of the dc motor monitored. A digital display can hence be provided for showing the voltage and exact speed of the dc motor. 36
REFERENCES 1. Rudolf F. Graf, Williams Sheets, The Encyclopedia of Electronic Circuits Vol.4, 1992 McGraw-Hill Professional 2. V.K Mehta, Rohit Mehta, Principles of Electrical Machines, 2002 S.Chand & Company Ltd. 7361, Ram Nagar, New Delhi-110 055, Printed by Rajenda Ravindra Printers (Pvt.) Ltd., 7361, Ram Nagar New Delhi-100 055 3. B. L. Theraja, A Textbook of Electrical Technology, 2005 S. Chand & Company Ltd. 7361, Ram Nagar, New Delhi-110 055, Printed by Rajenda Ravindra Printers (Pvt.) Ltd., 7361, Ram Nagar New Delhi-100 055 4. S K Bhattacharya, S Chatterjee, Industrial Electronics & Control, 1995 Tata McGraw- Hill 5. Hughes Austin, Electric Motors and Drives: Fundamentals, types and applications, 1990 Newnes, Printed by Biddles Ltd 37
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