DC Machines Chapter 3 PDF

ELECTRICAL MACHINES-1(CAS) (ENEL3109) CHAPTER-3: DC MACHINES Dr Parmal Singh Solanki College of Engineering & Technology, University of Technology and Applied Sciences, Suhar  Acknoledgements Charles I. Hubert, Electric Machines: Theory, Operating Applica...

ELECTRICAL MACHINES-1(CAS) (ENEL3109) CHAPTER-3: DC MACHINES Dr Parmal Singh Solanki College of Engineering & Technology, University of Technology and Applied Sciences, Suhar  Acknoledgements Charles I. Hubert, Electric Machines: Theory, Operating Applications, Adjustment and Controls, 2nd Edition, Prentice Hall, 2002. ELEC3261-Electrical Machines 1 2  IMPORTANT NOTE These power point slides are not the substitute of text book. Please read the text book to under stand the topics and subtopics covered in these slides. Chapter-10 to 12 of TB-1 will be useful to understand the concepts of course contents of this chapter  Outlines Construction and theory of operation Classification of DC Machines Armature voltage and developed torque Armature reaction, inter-poles and compensating windings DC Generators DC Motors Speed Control Power Flow and Efficiency Applications  Electric Machines Mechanical Electrical Electrical Motor Mechanical Generator Input Output Input Output Electromechanical Energy Conversion i + w Electrical System (v) Ideal Electric Machine Mechanical System (T) _ Motor Energy Flow v i=T w Generator Concepts from Electromagnetism Induced e.m.f (pp 23) e N e v B v e=Blv S B Right hand screw rule Electromagnetic Force (ppi 19) i N F F F=Bli B S B Right hand screw rule Construction of DC Machines Construction of DC Machines Construction of DC Machines 4 Pole DC Machine Construction of DC Machines 2 Pole DC Machine Shaft Armature Commutator Stator pole Field coil Internal View of DC Machine Classification of DC Machines Classification based on connection Series Classification based on excitation Shunt self Compound separately Field Armature Separately excited Field Field Armature Armature Self excited 1- Shunt Self excited 2- Series Classification of DC Machines Self Excited 3- Compound ff A1 fs ff A1 fs F1 D1 D2 F1 D1 D2 F2 F2 A2 A2 i- Short-shunt Cumulative ii-Long-shunt Cumulative ff A1 fs ff A1 fs D1 D2 F1 D1 D2 F1 F2 F2 A2 A2 iii- Short-shunt Differential iv-Long-shunt Differential Armature Winding (in DC Motor, armature is rotor) The turn, coil, and the winding are shown schematically as: End connection Conductors Rotor or armature Turn Coil Winding A turn consists of two conductors connected to one end by an end connector. A coil is formed by connecting several turns in series. A winding is formed by connecting several coils in series. Types of Windings Terminology in Armature Winding Pole pitch: It is defined as number of armature slots per pole. For example, if there are 36 conductors and 4 poles, then the pole pitch is 36/4=9. Coil span or coil pitch : It is the distance between the two sides of a coil measured in terms of armature slots Front pitch (YF): It is the distance, in terms of armature conductors, between the second conductor of one coil and the first conductor of the next coil. OR it is the distance between two coil sides that are connected to the same commutator segment. Back pitch (YB): The distance by which a coil advances on the back of the armature is called as back pitch of the coil. It is measured in terms of armature conductors. Resultant pitch (Yr): The distance, in terms of armature conductor, between the beginning of one coil and the beginning of the next coil is called as resultant pitch of the coil. Application of Armature Windings The armature winding is the main current-carrying winding in which the electromotive force or counter-emf of rotation is induced. The current in the armature winding is known as the armature current. The location of the winding depends upon the type of machine. Generally there are two types of armature winding in the DC machines. They are classified as follow: (i) Lap winding : It is of two types (a) Simplex lap winding (b) Duplex lap winding (ii) Wave winding. The difference between these two is merely due to the end connections and commutator connections of the conductor. Sr No Lap Winding Wave winding 1 No of parallel paths = No of Poles Number of parallel paths = Always 2 2 No of brush sets required = No of poles No of brush sets required = Always 2 3 Preferable for high current, low voltage Preferable for high voltage, low current capacity generators capacity generators 4 It gives less emf compared to wave This winding gives sparkles winding. This winding requires more commutation. The reason behind is no. of conductors for giving the same that it has two parallel paths emf, it results high winding cost. irrespective of no. of poles of the Generally used for machines of machine. ratings above 500 kW Generally used for machines of ratings below 560 kW Voltage induced in Armature 𝑛𝑃𝑁𝑎 𝜙𝑝 Pn 𝐸𝑎 = f = 30 120 Ea = Voltageinduced in armature (V ) f = frequency in Hz P = number of field poles n = rotational speed ( rpm)  p = pole flux (Wb) N a = Number of turns of conductor in armature Za Na = Where Z a = total number of armature conductor 2a 𝑛𝑃𝑍𝑎 𝜙𝑝 𝐸𝑎 = 𝑊ℎ𝑒𝑟𝑒 𝑎 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑎𝑟𝑎𝑙𝑙𝑒𝑙 𝑝𝑎𝑡ℎ𝑠 60𝑎 Voltage Induced in Armature Average value of induced emf in terms of total armature conductors is 𝐸𝑎 = 𝑛𝑃𝑍𝑎 𝜙𝑝 60𝑎 The number of parallel paths and the number of series-connected conductors required for a given kW rating determined by the system voltage. Low-voltage, high-current machines have more parallel paths, Example-1 A 4 pole 100 kW DC machine operating at 1500 rpm has a generated emf of 220 V. If speed is reduced to half of its rated value and the pole flux is doubled, determine: (i) induced emf (ii) frequency of voltage waveform in armature winding Part (i) We know by formula that nPZ a  p PZ a Ea = If kG = , then Ea = n p kG or 𝐸𝑎 𝛼 𝑛∅ 60a 60a There are two case given in problem for speed and magnetic flux Solution Cont.. E1  n p 1  n p  =  E2 = E1  2 E2  n p   n p  2 1 0.5n  2 p E2 = 220  = 220V np (ii ) Frequency of the voltage waveform Pn 4  1500  0.5 f = = = 25 Hz 120 120 Analysis: How you will get frequency = 50 Hz ? Will it be possible to induced voltage more than 220 V.. How Problem-1 Definition of Terms used in DC Machines Commutator: The rectangular-shaped voltage wave generated within a DC armature coil is changed to a unidirectional voltage in the load circuit by means of a mechanical rectifier, called Commutator mounted on the armature shaft. Commutation: The rotating Commutator and stationary brushes constitute a rotary switch that provides a switching action, called commutation, that switches the internal alternating voltage and current of an AC generator to direct voltage and direct current in the external circuit. Interpoles: These are small poles, called interpoles or commutating poles, are located between the main field poles. The interpoles are used to minimize sparking between the graphite or metal-graphite Brush and the Commutator. Brushes provide connection between Commutator and external load.  Inter-poles and Compensating winding In order to neutralize the cross magnetizing effect of armature reaction, a compensating winding is used. The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature. Basic DC Generator The basic DC generator, called a shunt generator, has its field winding connected either in parallel with the armature or to a separate source of excitation such as a battery or another generator (called an exciter). Complete equivalent circuit of DC Gen Voltage and Current Equations The relationship between field current and induced armature voltage is called the magnetization curve or open-circuit characteristic Problem-2 Magnetization Curve 290 V 290V 8.9A 8.9A Voltage Regulation It is the % change in internal voltage from no load to rated load with respect to rated voltage. % Problem-3: A 100 kW, 1800 rpm generator operating at rated load has a terminal voltage of 240V. If the voltage regulation is 2.3%, determine the no-load voltage. Solution: Circuit Diagram of Basic DC Motor Complete equivalent circuit of DC Sunt Motor Problem-4 ?? The general speed equation for a DC motor is (10-26)p419 Dynamic Behaviour of DC Motor Speed Regulation: The speed regulation of a DC motor is the % change in speed from no load to rated load with respect to rated speed when operating at rated voltage and rated temperature from a constant-voltage source. Armature Reaction, Interpoles and CW When a generator or motor is loaded, the current in the armature coils develops a magneto-motive force of its own that interacts with the magneto-motive force of the field poles, disturbing the uniform flux distribution in the air gap. This behaviour is called armature reaction To eliminate sparking caused by the emf of self-induction, narrow poles called interpoles or commutating poles are installed in the neutral plane of DC machines. The interpoles windings are connected in series with the armature and forms part of the armature circuit and should not be disconnected or reversed. The compensating winding is connected in series with the armature, and essentially eliminates armature reaction by setting up a mmf that is always equal and opposite to the armature mmf. Armature Reaction Effect on DC M/Cs Armature reaction has two adverse effects on the performance of a DC generator: (i) Armature reaction mmf causes the neutral plane to shift its axis in the direction of rotation. (ii) Flux in the inter-polar region causes a voltage to be generated in the coil undergoing commutation, causing arcing at the brushes. (iii)The net reduction in total pole flux results in an undesirable reduction in generated voltage. Armature Reaction has two adverse effects on the performance of a DC motor: (i) Armature reaction mmf causes the neutral plane to shift its axis in the direction opposite to the direction of rotation (ii) Flux in the inter-polar region causes a voltage to be generated in the coil undergoing commutation, causing arcing at the brushes. (iii) The net reduction in total pole flux results in an undesirable increase in motor speed. Problem-5 P=Vx I Problem-6 Problem-7 Problem-7 Solution Cont.… Problem-8 (Home Work: Similar to Exp-7) Solution-8 Cont.…. Loss and Efficiency of DC Machines Loss and Efficiency of DC Machines Problem-9 Solution-9 cont.… Thank You  Armature Reaction Effect Definition: The armature reaction simply shows the effect of armature field on the main field. In other words, the armature reaction represents the impact of the armature flux on the main field flux. The armature field is produced by the armature conductors when current flows through them. And the main field is produced by the magnetic poles. The armature flux causes two effects on the main field flux. The armature reaction distorted the main field flux It reduces the magnitude of the main field flux Effect of Armature Reaction The effects of Armature Reaction are as follows:- Because of the armature reaction the flux density of over one-half of the pole increases and over the other half decreases. The total flux produces by each pole is slightly less due to which the magnitude of the terminal voltage reduces. The effect due to which the armature reaction reduces the total flux is known as the de-magnetizing effect. The resultant flux is distorted. The direction of the magnetic neutral axis is shifted with the direction of resultant flux in case of the generator, and it is opposite to the direction of the resultant flux in case of the motor. The armature reaction induces flux in the neutral zone, and this flux generates the voltage that causes the commutation problem.