TOPIC 3 DC MACHINES
DC Machines 2
Historical Development Direct current (DC) motor is one of the first machines devised to convert electrical power into mechanical power. Its origin can be traced to the disc type machines inverted by Michael Faraday Faraday s primitive design was quickly improved upon and many DC machines were built in the 1880s when DC was principle form of electric power generation 3
Historical Development With the advent of alternating voltage and subsequent invention of induction motor with lower manufacturing costs DC machine became less important. 4
Historical Development In recent years, the use of DC machines has become almost exclusively associated with application where: the unique characteristic of DC motor of high starting torque justify its cost, or portable equipment must run from a DC power supply( or battery). 5
Historical Background The ease with which DC motor lends itself to speed control has long been recognised. Compatibility with power electronic devises like thyristors (SCR) plus better performance due to availability of new improved material in magnetic circuits and brushes has also revitalised interest in DC machines 6
Historical Development Today conventional DC generators are being replaced by the solid state rectifiers where ac supply is available. The same is not true for dc motors because of Constant mechanical power output or constant torque High starting torque Rapid acceleration or deceleration Responsiveness to feedback signals Power rating:1w to 10,000 hp
Application of dc motors steel and aluminum rolling mills, traction motors in electric vehicles, electric trains and overhead cranes, control devices and robots, hand power tools, kitchen gadgets like food blenders etc. 8
Construction of DC Machines 9
Main Parts A D.C. machine consists of the three main parts: Stator Rotor or Armature Commutator and brush gear 10
DC machine Construction 11
Stator DC machine stator with visible poles. 12
Stator Stator is the stationary (fixed) part of the machine. It produces the main magnetic flux. Stator consists of the following parts: Yoke Is the outer frame of the stator which is a hollow cylinder of cast steel or rolled steel. An even number of pole cores are bolted to the yoke 13
Stator Yoke Yoke serves the following two purposes: (i) it supports the pole cores and acts as the protecting cover to the machine (ii) it forms a part of the magnetic circuit Pole cores Are made of sheet steel laminations that are insulated from each other and riveted together 14
Stator Field Windings Each pole core has one or more field windings placed over it to produce a magnetic field. The field windings are connected in series with one another such that when the current flows through them, alternate north and south poles are produced in the direction of rotation. 15
A Two Pole DC Motor dcmotor 16
A Four Pole DC Motor dcmotor 17
Rotor or Armature The armature is the rotating part of the dc machine It consists of a shaft upon which a laminated cylinder called armature core is mounted. Armature core has slots on its outer surface which accommodate the armature windings 18
Purpose of the Armature The purpose of the armature is to provide the energy conversion in a DC machine. armature converts mechanical energy to electrical energy. 19
Armature Winding in a DC Machine 20
Armature The conductors are suitably connected to form armature winding. Two types of winding are used, namely WAVE and LAP. 21
Rotor or Armature Rotor of a dc motor. 22
Commutator and Brush gear The commutator is a mechanical rectifier which produces direct current from alternating current generated in the armature coils. The commutator which rotates with the armature is made up of a series of segments which are conductors separated by thin pieces of an insulator such as mica. 23
DC Motor Construction Details of the commutator of a dc motor. 24
Commutator and brush gear Each commutator segment is connected to the ends of the armature coil. Current is collected from or delivered to the armature by means of two or more carbon brushes mounted on the commutator. Each brush is supported in a metal box called brush holder. Pressure exerted by the brushes on the commutator can be adjusted and maintained at constant value by a spring.
Commutator and Brush gear 26
Brushes The brushes on a d.c. machine are made up of carbon. Carbon has a number of qualities that make it ideal for this purpose:- It is very soft; It is a conductor; It self-lubricates. 27
DC Motor Construction Cutaway view of a dc motor. 28
EXCITATION The magnetic flux in a dc machine is produced by field coils carrying current. The production of magnetic flux in a machine by circulating current in the field winding is called EXCITATION. 29
Types of dc machines There are two methods of excitation, namely separate excitation and self excitation. In separate excitation the field coils are energised by separate dc source. In self excitation the current flowing through the winding is supplied by the machine itself. 30
Types of DC Machines In self excited machine, residual magnetism must be present in the ferromagnetic circuit of the machine in order to start the self-excitation process. Direct current machines are named according to the connection of the field winding with the armature. 31
Types of dc machines The principle types of DC machines are: Separately excited DC machine Shunt-Wound DC machine Series-Wound DC machine Compound machine 32
Types of DC Machines separate excitation series, shunt compound 33
Separately excited Separately excited dc machine The field coils are energised by a separate dc source. 34
Shunt wound Shunt wound dc machine the field coils are connected in parallel with the armature. Since the shunt field receives the full output voltage of the generator or the supply voltage of a motor, it is generally made of large number of turns of fine wire carrying a small field current 35
Series wound Series wound dc machine Field coils are connected in series with the armature. Series winding carries the armature current and consists of few turns of wire of large cross-sectional area. 36
Compound wound Compound wound dc machine machine having both a shunt winding and series coils. The shunt winding has many turns of fine wire and the series winding has few turns of large cross-sectional area. 37
Compound wound Compound machine may be connected in two ways: short shunt compound machine, if the field is connected in parallel with the armature alone long shunt compound machine, if the shunt field is in parallel with both armature and series field. 38
Compound wound If the magnetic flux produced by the series winding assists(aids) the flux produced by the shunt field winding, the machine is called cumulatively compounded. If the series flux opposes the shunt field flux, the machine is said to be differentially compounded. 39
Equivalent Circuit of a DC Machine
Separately excited dc machine 41
Self excited dc machines 42
Self excited dc machine 43
Machine equations V f I f R f V t E a I a R a 44
DC Generator 45
Introduction The generator is based on the principle of electromagnetic induction discovered in 1831 by Michael Faraday. Faraday discovered that if an electric conductor, like a copper wire, is moved through a magnetic field, electric current will flow in the conductor. So the mechanical energy of the moving wire is converted into the electric energy of the current that flows in the wire as alternating current. 46
Introduction To change the Simple Generator into a direct-current generator, the current must be made to move in only one direction. In a dc generator this is achieved by use of mechanical rectifier referred to as COMMUTATOR. The generated dc is the transferred to the generator terminals through carbon brushes. 47
Residual magnetism in self excited DC Generator The soft iron of the electro-magnets retains a small amount of magnetism, known as residual magnetism, even when there is no field current. This residual magnetism is sufficient to induce an EMF in the armature of the generator when it first starts to rotate, which initiates a current flow from the generator. 48
Residual magnetism Residual magnetism may be lost, due to excess heat, shock or reversal of field current flow. The residual magnetism can be restored by briefly passing a current through the field. This is known as field flashing, or flashing the field. 49
Principle of operation of DC Generator 50
Basic DC Generator Convert Mechanical (motion) energy to electrical energy. Demo: Spinning a coil of wire in a magnetic field produces alternating current 51
Simple DC generator Let us consider a simple DC generator with following components: A magnetic field A single conductor, or loop A commutator and Brushes 52
Principle of operation The North and South poles produce a dc magnetic field. As the loop is turned in the field the conductor sides cut the magnetic flux lines, which induce an alternating voltage in the conductors The induced alternating voltage is first changed into unidirectional voltage by the commutator and brushes and then delivered to the load connected across the terminals. 53
Commutation Commutation is the process of producing a unidirectional or direct current from the alternating current generated in the armature coils. 54
Voltage output using slip rings 55
Voltage output using split rings(commutator) 56
Simple generator 57
Armature Reaction When current flows in the armature conductors it produces a magnetic field surrounding the conductors. The armature flux reacts with the main flux. The effect of the armature flux on the` main field flux is called armature reaction. 58
Armature Reaction The armature flux has two effects on the main flux: (i) It distorts the main flux which causes sparking at the commutator (ii)it weakens the main flux which reduces the generated voltage 59
Effect on flux distribution: magnetic neutral plane shift (a)initially the pole flux is uniformly distributed and the magnetic neutral plane is vertical (b) Effect of the air gap on the flux field causes the distribution of the flux to be no longer uniform across the rotor 60
Effect on flux distribution: magnetic neutral plane shift (c) The fields interact to produce a different flux distribution in the rotor. (d) The combined flux in the machine has the effect of strengthening or weakening the flux in the pole. Neutral axis is therefore shifted in the direction of motion. 61
Problems with commutation in real DC machines One of the problems associated with armature reaction is arcing and sparking at the brushes!
Methods of improving commutation SPARKING can be minimised by: Use of high resistance brushes which increase the circuit resistance of the coils undergoing commutation Use of interpoles which are small auxiliary poles placed midway between the main poles. They are connected in series with the armature.
Interpoles or compoles Main poles Interpoles Stator of a dc machine showing interpoles 64
Lap and Wave winding Armature coils are connected to form either LAP or WAVE windings. Lap Winding: Ends of the armature coil are connected to adjacent segments on the commutator so that the total number of parallel paths is equal to total number of the poles ie A=P. Used in low-voltage, high current machine Wave Winding: The ends of each armature coil are connected to armature segments some distance apart, so that only two parallel paths are provided between the positive and negative brushes, ie A=2. Used in high voltage, low current machines. 65