BASICS OF ELECTRICAL MACHINES-1 PRESENTATION BY: Er. Satnam Singh Lecturer(Electrical engg.) GPC Mohali (Khunimajra) Contents – – – – – Overview of Electrical Machines Construction Principle of Operation Types of DC Generator Types of DC Motor LEARNING OBJECTIVES • Upon completion of this chapter the student should be able to: – State the principle by which machines convert mechanical energy to electrical energy. – Discuss the operating differences between different types of generators and motor. – Understand the principle of dc generator and dc motors. Overview of Electrical Machines • A machine is an electromechanical energy conversion device which converts the form of energy i.e. from mechanical to electrical or from electrical to mechanical energy • Depending upon the working of the machine ,the electromechanical energy conversion device may be named as generator or motor. Overview of Electrical Machines Generator • A generator is a machine that converts mechanical energy into electrical energy by using the principle of magnetic induction. Principle of magnetic induction in DC machine GENERATOR ACTION • The figure shows an elementary coil rotating in a stationary magnetic field between a pair of magnetic poles. • Let it rotate in anticlockwise direction at an angular velocity of ω radian per sec, by some prime-mover giving the generator a mechanical torque Tm. WORKING PRINCIPLE OF A DC GENERATOR • The coil side or conductors cut the magnetic lines of force and hence an emf is induced in the coil.(Faraday’s law) •The coil is further connected to an external resistance R , therefore current ‘i’ flow through the coil side as well as through the load resistance R. •The direction of the current is marked on the coil sides. •Now in the second action start ,i.e. a current carrying coil or conductor is placed in the magnetic field and hence the conductor will experience a force or torque Te. The direction of the torque will be opposing to the driving torque Tm Tm = Te + Tf ωTm = ωTe + ωTf where ωTm = mechanical input power ωTe = mechanical power available for conversion into electrical power ωTf = mechanical power lost due to friction FARADAY’S LAW OF ELECTRO-MAGNETIC INDUCTION • • • • • When a conductor cuts across the magnetic field, an e.m.f. is induced in the conductor When the magnetic flux linking with any circuit or coil changes an e.m.f. is induced in the circuit The deflection in the galvanometer needle shows that e.m.f. is induced in the coil. The direction of the induced e.m.f. in the coil depends upon the direction of the magnetic field and the direction of motion of the coil. E.m.f. induced in the coil only when flux linking with the circuit changes i.e. either magnet or coil is in motion. Motor • A motor is a machine that converts electrical energy into mechanical energy by supplying an electrical power (voltage and current). MOTOR ACTION • The figure shows a coil placed in a constant stationary magnetic field. The resistance R is removed and in place of this a battery is connected across the coil. •Now the current ‘i’ will start flowing through the coil. The current carrying conductor produces a magnetic field Fr •The field Fr tries to come in line with the main field Fm. •Thus an electromagnetic torque Te is developed in the coil in anticlockwise direction. Therefore the coil starts rotating in an anticlockwise direction say at an angular speed ‘ω’ rad/sec. •Now again the second action starts, when the coil moves in the magnetic field, flux is cut by the conductors and hence an emf is induced in them. •The direction of this induced emf will be opposing the cause due to which way the emf is induced. •Thus the induced emf will be opposing the supply voltage v and the current in the coil will flow due to the difference between v and e. If r is the resistance of the coil. •Then applied voltage V = e + ir. •Multiplying both side by ‘i’ in equation we get •Vi = e.i + i2r •Here Vi = electrical power input to the machine •e.i. = electrical power available for conversion into mechanical power •I2r = power lost due to resistance of the coil. CONSTRUCTION Main parts of dc machine are: 1. Field magnet frame or yoke 2. Pole cores and pole shoes 3. Pole coil or field coils 4. Armature core 5. Armature winding 6. Commutator 7. Brushes 8. Brush holder 9. Bearing 10.Shaft FIELD MAGNET FRAME OR YOKE The yoke or outer frame is the covering provided to dc generator and it serves the following purpose 1. It provides a mechanical support for the poles. 2. It act as a protective cover against mechanical damage 3. It provide a passage for the magnetic flux produced by the poles. POLE CORE AND POLE SHOES •The pole core itself may be made of solid piece of cast iron or cast steel, but pole shoe is laminated and is screwed to the pole face by means of counter sunk screw. •The pole cores may be made of thin laminations of steel, riveted together.This type of pole is held in position with the frame by means of bolts. •The pole shoe serves the two purpose as under. 1. It support the pole coils. 2. Being of larger cross section, it spread the flux and also reduces the reluctance of the magnetic path. ARMATURE • The armature core is cylindrical in shape. It is rotating part of the machine •Its body is made up of soft iron stamping or laminations to reduce the eddy current losses. •The lamination are keyed to the shaft. These are insulated from each other by varnish. •At the outer periphery slots are cut. The armature conductors (winding)are placed in these slots •The armature core serves the following purpose 1. It provides a path of low reluctance to the magnetic flux. 2. It house armature conductors DC machine armature ARMATURE WINDINGS •The armature coil are usually former wound. The conductor are placed in the armature slots which are lined with tough insulating material. •The slot insulation is folded over the armature conductors placed in the slots and is secured firmly by bumboo or fibre wedges. •The armature winding are usually of conductors covered with single cotton cover, double cotton cover or enamalled wire •On the basis of connection these are of two types: 1. Lap winding 2. Wave winding ARMATURE WINDINGS • Lap Wound Armatures – are used in machines designed for low voltage and high current – armatures are constructed with large wire because of high current – Eg: - are used is in the starter motor of almost all automobiles – The windings of a lap wound armature are connected in parallel. This permits the current capacity of each winding to be added and provides a higher operating current – No. of current path, C=2p ; p=no. of poles Lap wound armatures ARMATURE WINDINGS (Cont) • Wave Wound Armatures – are used in machines designed for high voltage and low current – their windings connected in series – When the windings are connected in series, the voltage of each winding adds, but the current capacity remains the same – are used is in the small generator – No. of current path, C=2 Wave wound armatures ARMATURE WINDINGS (Cont) FIELD WINDINGS • Most dc machines use wound electromagnets to provide the magnetic field. • When current passed through these coils, they electromagnetise the poles which produce the necessary flux which is cut by the armature conductors when in motion. • Two types of field windings are used : – series field – shunt field FIELD WINDINGS (Cont) • Series field windings – are so named because they are connected in series with the armature – are made with relatively few windings turns of very large wire and have a very low resistance – usually found in large horsepower machines wound with square or rectangular wire. The use of square wire permits the windings to be laid closer together, which increases the number of turns that can be wound in a particular space – Square and rectangular wire can also be made physically smaller than round wire and still contain the same surface area Square wire permits more turns than round wire in the same area Square wire contains more surface than round wire FIELD WINDINGS (Cont) • Shunt field windings – is constructed with relatively many turns of small wire, thus, it has a much higher resistance than the series field. – is intended to be connected in parallel with, or shunt, the armature. – high resistance is used to limit current flow through the field. FIELD WINDINGS (Cont) • When a DC machine uses both series and shunt fields, each pole piece will contain both windings. • The windings are wound on the pole pieces in such a manner that when current flows through the winding it will produce alternate magnetic polarities. Both series and shunt field windings are contained in each pole piece S – series field F – shunt field COMMUTATOR •The commutator is cylindrical in structure and is built up of wedge shaped hard drawn copper segments. •The segment are insulated from each other by a thin sheet of high quality mica. •To prevent them from flying out under the action of centrifugal forces, the segments are provided with “v”-grooves, which are insulated by conical mica-nite ring. •The function of the commutator is to facilitate the collection of current from the armature and to rectify the A.C. induced in the armature into D.C. BRUSH HOLDER AND BRUSHES •The function of brushes is to collect current from the commutator and supply it to the external load circuit. •These are usually made of carbon and are rectangular in shapes. •These brushes are housed in brush holders. •These are held in position under spring tension , the pressure of the spring can be adjusted by altering the position of lever in the notches. •Copper brushes are only used for machine delivering large current at low voltages. BEARING •These are supported in end cover, because of reliability, ball bearing are usually employed . •Though for heavy duty, roller bearing are employed. •These are used to reduce friction and have less wear and tear. SHAFT •The material of the shaft is mild steel . It is used to transfer mechanical power from or to the machine. •The rotating parts e.g. Armature, commutator are mounted to the shaft.
