Speed Control of Dc Motor Using Chopper
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International Journal of Engineering, Management & Sciences (IJEMS) ISSN-2348 –3733, Volume-1, Issue-10, October 2014 Speed Control of Dc Motor Using Chopper Abhishek Soni arrive at a mathematical description that contains the relevant Abstract— Some devices convert electricity into motion but dynamic characteristics of the particular model. do not generate usable mechanical power as primary objective and so are not generally referred to as electric motors. Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force. The reverse process, producing electrical energy from mechanical energy, is done by generators such as an alternator or a dynamo; some electric motors can also be used as generators, for example, a traction motor on a vehicle may perform both tasks. Electric motors and generators are commonly referred to as electric machines. Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives. They may be powered by direct current, e.g., a battery powered portable device or motor vehicle, or by alternating current from a central electrical Figure 1: Electromechanical Energy Conversion distribution grid or inverter. The smallest motors may be found in electric wristwatches. Medium-size motors of highly DC machines may also work as brakes. The brake mode is a standardized dimensions and characteristics provide convenient mechanical power for industrial uses. The very largest electric generator action but with the electrical power either motors are used for propulsion of ships, pipeline compressors, regenerated or dissipated within the machine system, thus and water pumps with ratings in the millions of watts. Electric developing a mechanical braking effect. It also converts some motors may be classified by the source of electric power, by their electrical or mechanical energy to heat, but this is undesired. internal construction, by their application, or by the type of The major advantages of DC machines are easy speed and motion they give. torque regulation. The major parts of any machine are the The physical principle of production of mechanical force by stationary component, the stator, and the rotating component, the interactions of an electric current and a magnetic field was the rotor. known as early as 1821. Electric motors of increasing efficiency were constructed throughout the 19th century, but commercial exploitation of electric motors on a large scale required efficient Types of DC Motor electrical generators and electrical distribution networks. DC Motors are electrically identical to dc generators. In fact, The speed of a DC motor can be varied by controlling the field the same dc machine may be driven mechanically to generate flux, the armature resistance or the terminal voltage applied to a voltage, or it may be driven electrically to move a the armature circuit. The three most common speed control mechanical load. While this is not normally done, it does methods are field resistance control, armature voltage control, point out the similarities between the two machines. and armature resistance control. In this section, modeling There are three types of DC Motors: procedure of these three methods and feedback control method for DC motor drives for dynamic analysis are presented. Series DC Motors. Index Terms— DC (direct current), PMSM (Permanent Magnet Synchronous Motor). Shunt DC Motors. Compound DC Motors. Emf Equation of DC Motor I. INTRODUCTION When the motor armature rotates the conductor also rotates A theory is a general statement of principle abstracted from and hence cut the flux. In accordance with the law of observation. And a model is a representation of a theory that electromagnetic induction emf is induced in them, whose can be used for control and prediction. For a model to be direction found by Flemings right hand rule, is in opposition useful, it must be realistic and yet simple enough to to the applied voltage because of its opposite direction it is understand and manipulate. These requirements are not easily referred to as counter emf or back emf Eb. The rotating fulfilled as realistic models are seldom simple and simple armature generating the back emf Eb is like a battery of emf Eb models are seldom realistic. put across supply mains of V volt. Obviously V has to drive Ia The scope of a model is defined by what is against the opposition of Eb. The power required to overcome considered relevant. Features or behavior that is relevant must this opposition is EbIa. be included in the model and those that are not can be ignored. In DC Motor power is converted into mechanical energy. Modeling refers to the process of analysis and synthesis to Ia = Net Voltage/Resistance, Where Ra is the resistance of armature circuit Eb = volt, where N is in rps. Manuscript received October 20, 2014 Back emf depends upon the other factors of the armature Abhishek Soni, Asst. Prof. Swasthya Kalyan Technical Campus, Jaipur, India speed if speed is high Eb is large, hence armature current Ia is 5 www.alliedjournals.com Speed Control of Dc Motor Using Chopper small. If speed is less then Eb is less hence motor current flows chopper involves one stage conversion, these are more which develop motor torque. efficient Choppers are now being used all over the world for Starting of DC Motor rapid transit systems. These are also used in trolley cars, The counter-emf aids the armature resistance to limit the marine hoist, forklift trucks and mine haulers. The future current through the armature. When power is first applied to a electric automobiles are likely to use choppers for their speed motor, the armature does not rotate. At that instant the control and braking. Chopper systems offer smooth control, counter-emf is zero and the only factor limiting the armature high efficiency, faster response and regeneration facility. The current is the armature resistance. Usually the armature power semiconductor devices used for a chopper circuit can resistance of a motor is less than 1 Ω; therefore the current be force commutated thyristor, power BJT, MOSFET and through the armature would be very large when the power is IGBT.GTO based chopper are also used. These devices are applied. This current can make an excessive voltage drop generally represented by a switch. When the switch is off, no affecting other equipment in the circuit and even trip overload current can flow. Current flows through the load when switch protective devices. is “on”. The power semiconductor devices have on-state Therefore the need arises for an additional resistance in series voltage drop of 0.5V to 2.5V across them. For the sake of with the armature to limit the current until the motor rotation simplicity, this voltage drop across these devices is generally can build up the counter-emf. As the motor rotation builds up, neglected. As mentioned above, a chopper is dc equivalent to the resistance is gradually cut out. an ac transformer, have continuously variable turn’s ratio. Like a transformer, a chopper can be used to step down or step Three point starter up the fixed dc input voltage. The incoming power is indicated as L1 and L2. The Starting Chopper Circuit components within the broken lines form the three-point The second possibility of controlling the armature current is starter. As the name implies there are only three connections to use a step-up converter. The step-up converter is usually to the starter. The connections to the armature are indicated as attributed the name chopper in the literature. A1 and A2. The ends of the field (excitement) coil are The circuitry layout of this mean of control is shown in figure indicated as F1 and F2. In order to control the speed, a field (a). The controlled switch of the chopper circuit is biased by rheostat is connected in series with the shunt field. One side of an hysteresis controller. The hysteresis controller is the line is connected to the arm of the starter (represented by programmed to guaranty an armature current waveform an arrow in the diagram). The arm is spring-loaded so, it will similar to the one shown in figure (b). return to the "Off" position when not held at any other position. Figure 2 Three point Starter of DC Motor On the first step of the arm, full line voltage is applied across the shunt field. Since the field rheostat is normally set to minimum resistance, the speed of the motor will not Figure 3 Using Chopper circuit Mean. a) Circuit be excessive; additionally, the motor will develop a large Topology starting torque. b) Hysteresis Controller Function As it can been seen, the chopper circuit did perform its duty as The starter also connects an electromagnet in series with intended but that was at the expense of delaying the motor the shunt field. It will hold the arm in position when the from reaching its steady state (rated value) in a short time. arm makes contact with the magnet. Themotor reaches its rated speed at time = 8 seconds. The ratio between the maximum and rated values of the armature Meanwhile that voltage is applied to the shunt field, and current is 1.23 but the armature current has a lot of ripples the starting resistance limits the current to the armature. which might be harmful to the armature circuitry. IV. MODELLING AND SIMULATION OF SPEED As the motor picks up speed counter-emf is built up; the CONTROL OF DC MOTOR arm is moved slowly to short. To produce a good design, there needs to be some amount of III. SPEED CONTROL OF DC MOTOR USING CHOPPER modeling or simulations done to avoid aimless trial and error A chopper is a static power electronic device that converts techniques with the actual equipment (the DC motor).