Electrical Engineering - Electric Circuits, Study notes of Electrical and Electronics Engineering

Download Electrical Engineering - Electric Circuits and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! Magnetic Circuits Electrical Engineering         (ELE-232) What we need to run an electrical machine? 2 Material used in Machine Manufacturing • Steel to conduct Magnetic Flux • Copper to conduct Electric Current • Insulation material for isolation purpose 5 Electric Machine Terminology • The two main parts of a generator or motor can be described in either mechanical or electrical terms. • Mechanical Parts: – Rotor: The rotating part of an electrical machine – Stator: The stationary part of an electrical machine 6 Electric Machine Terminology Electrical Parts: Armature: The power-producing component of an electrical machine. The armature windings generate the electrical current. The armature can be on either the rotor or the stator. Field: The magnetic field component of an electrical machine. It is used to produce magnetic field in machine The magnetic field can be provided by either electromagnets or permanent magnets mounted on either the rotor or the stator. 7 MZS FKEE, UMP 10 Iron core Insulation between segments Coil commutator interconnection Insulation between segments Commutator Coil insulation C°PPer segment 11  Commutator 12 Hans Oersted • Discovered the relationship between magnetism and electric current • A magnetic field is produced by current running through the wire MAGNETIC FIELDS • What is magnetic Field…… • How we represent magnetic field….. 16 MAGNETIC FIELDS • In the region surrounding a permanent magnet there exists a magnetic field, which can be represented by magnetic flux lines similar to electric flux lines. • Magnetic flux lines, however, do not have origins or terminating points as do electric flux lines but exist in continuous loops, as shown in Fig. • The symbol for magnetic flux is the Greek letter  (phi). 17 • The magnetic field strength at a is twice that at b since twice as many magnetic flux lines are associated with the perpendicular plane at a than at b. 20 Ww OQ —l uu LL. o _ Lu = O < = a ro f-\ fo-™ / ell fa \ (fat pp lilt y poy EEE Ey poy fF (EEE ey roy it Phy ity Ly i i! i | | | Wey | | | | Pret | 3 badiilly) 44 2 bit foot oO iui Lyllyl} y Oo ee en ee Cc , 4 1 Lyllql ly a vid flatly | 2 \ yy Ee i 5 \ VRITTTTLA yy D> wef IN = x! \ « YX ~~ — — s e 21 22 If unlike poles of two permanent magnets are brought together, the magnets will attract. Ww OQ —l uu LL. o _ Lu = O < = ¢ What is figure indicates.... Flux lines MAGNETIC FIELDS • If a nonmagnetic material, such as glass or copper, is placed in the flux paths surrounding a permanent magnet, there will be an almost unnoticeable change in the flux distribution. 26 • However, if a magnetic material, such as soft iron, is placed in the flux path, the flux lines will pass through the soft iron rather than the surrounding air because flux lines pass with greater ease through magnetic materials than through air. MZS FKEE, UMP 27 • To understand how an electrical machines works, the key is to understand how the electromagnet works. • The principles of magnetism play an important role in the operation of an electrical machines. Introduction Review of Electromagnetism • The basic idea behind an electromagnet is extremely simple: a magnetic field around the conductor can be produced when current flows through a conductor. • In other word, the magnetic field only exists when electric current is flowing • By using this simple principle, you can create all sorts of things, including motors, solenoids, read/write heads for hard disks and tape drives, speakers, and so on. MAGNETIC FIELDS ¢ Electromagnet —_oeT oe oe eo ee oy, “ a“ ~— “ (oY) sO , s~ % 4 4 4 Ly iy “ N] Am fo fp fo fe S wf Y ™ —— = \ TX SS - Steel oP “Te —--<-— _-* 32 • It simply shows how a current-carrying wire generates a magnetic field. • If you point your thumb in the direction of the current, as shown, and let your fingers assume a curved position, the magnetic field circling around those wires flows in the direction in which your four fingers point. 35 MAGNETIC FIELDS • A magnetic field is present around every wire that carries an electric current. • The direction of the magnetic flux lines can be found simply by placing the thumb of the right hand in the direction of conventional current flow and noting the direction of the fingers. • (This method is commonly called the right-hand rule.) 36 MAGNETIC FIELDS . . Conductor Magnetic flux lines \ ces ons, pe ~ \ i f ‘ | ~~ i i ‘, \ Magnetic flux lines around a current-carrying conductor. 37 • Thumb is the direction that the conductor is moving • Index point to the direction of magnetic lines • Middle finger=current flow Flemings right hand generator rule: Why right hand rule is important? 41 Working of electrical generator is based on right hand rule. 42 • As you can see, if your index finger points in the direction of a magnetic field, and your middle finger, at a 90 degree angle to your index, points in the direction of the charged particle (as in an electrical current), then your extended thumb (forming an L with your index) points in the direction of the force exerted upon that particle. 45 Left Hand Rule • This can also be remembered using "FBI" and moving from thumb to second finger. • The thumb is the force F • The first finger is the magnetic field B • The second finger is the of current I 46 Why left hand rule is important? 47 Working Principle of Electric Motor • An electric motor converts electrical energy into mechanical energy. • It works on the principle of the magnetic effect of current. • A current-carrying coil rotates in a magnetic field. DC Motor Rule • If an Electric current flows through the loop of copper wires that are between the poles of a magnet, an upward force will move one wire side up and a downward force will move the other wire side down. 51 DC Motor • It is based on the principle that when a current- carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming's Left-hand rule and whose magnitude is given by •  Force, F = B i l newton  – Where B is the magnetic field in weber/m2. – i is the current in amperes and  – l is the length of the coil in meter.              52 Working Principle of Electric Motor • When a current is allowed to flow through the coil MNST by closing the switch, the coil starts rotating anti-clockwise. • This happens because a downward force acts on length MN and at the same time, an upward force acts on length ST. • As a result, the coil rotates anti-clockwise. Torque Working Principle of Electric Motor • Current in the length MN flows from M to N and the magnetic field acts from left to right, normal to length MN. • Therefore, according to Fleming’s left hand rule, a downward force acts on the length MN. Working Principle of Electric Motor • Similarly, current in the length ST flows from S to T and the magnetic field acts from left to right, normal to the flow of current. Therefore, an upward force acts on the length ST. • These two forces cause the coil to rotate anti- clockwise. Left Hand Rule DC Motor of Force Direction of Current Axis of rotation cotation • A force, F, acts on the loop due to the interaction of the magnetic field of the permanent magnets and the magnetic field created by the current flowing in the loop. • This force is proportional to the flux density, B, the current flowing, I, and the effective length of the conductor, L, i.e. F = BIL 61 • The force is made up of two parts, one acting vertically downwards due to the current flowing from C to D and the other acting vertically upwards due to the current flowing from E to F (from Fleming’s left hand rule). • If the loop is free to rotate, then when it has rotated through 180°, the conductors are as shown in Figure (b). 62 With reference to Figure (a), when a direct voltage is applied at A and B, then as the single-loop conductor rotates, current flow will always be away from the commutator for the part of the conductor adjacent to the N-pole and towards the commutator for the part of the conductor adjacent to the S- pole. Thus the forces act to give continuous rotation in an anti-clockwise direction. 65 • The arrangement shown in Figure (a) is called a ‘two- segment’ commutator and the voltage is applied to the rotating segments by stationary brushes, (usually carbon blocks), which slide on the commutator material, (usually copper), when rotation takes place. • In practice, there are many conductors on the rotating part of a d.c. machine and these are attached to many commutator segments. • A schematic diagram of a multi-segment commutator is shown in Figure (b). Snapshots of working of DC Motor MZS FKEE, UMP 67 Electric current supplied externally through a commutator When electric current passes through a coil in a magnetic field, the magnetic force produces a torque which turns the DC Motors. • AC motor is the motor that converts AC electrical energy at its input into mechanical energy. • AC motor is classified into several types: –Asynchronous motor or induction AC motor –Synchronous motor AC Motor Difference between DC and AC Motor Question • List three sources of magnetic fields. Generator Parts • Prime mover: mechanical work which turns the rotor, may be a steam turbine, gas turbine, diesel engine... • Armature windings: the conductor in which the output voltage is induced • Field windings: the conductors used to produce the electromagnetic field (needs a DC power supply) • Stator: stationary housing of the generator • Rotor: rotates inside the stator, moved by a prime mover (steam turbine, gas turbine, internal combustion engine, etc…) • Sliding contacts (slip-rings and brushes): used to conduct the field or armature current to and from the rotor • AC generator is the generator that converts mechanical energy at its prime mover into AC electricity. • AC generator is classified into several types: – Asynchronous AC generator or induction AC generator, – Synchronous AC generator AC Generator AC Generator • A.C. generators or alternators (as they are usually called) operate on the same fundamental principles of electromagnetic induction as D.C. generators. • Alternating voltage may be generated by rotating a coil in the magnetic field or by rotating a magnetic field within a stationary coil. • The value of the voltage generated depends on- –  The number of turns in the coil. –   Strength of the field. –   The speed at which the coil or magnetic field rotates.  77 Working Principle of Electric Generator • The principle of working of an electric generator is that when a loop is moved in a magnetic field, an electric current is induced in the coil. • It generates electricity by rotating a coil in a magnetic field. • The following figure shows a simple AC generator. Working Principle of Electric Generator • MNST → Rectangular coil • A and B → Brushes • C and D → Two slip rings • X → Axle • G → Galvanometer Working Principle of Electric Generator • If axle X is rotated clockwise, then the length MN moves upwards while length ST moves downwards. • Since the lengths MN and ST are moving in a magnetic field, a current will be induced in both of them due to electromagnetic induction. Working Principle of Electric Generator • The direction of the induced current in the coil gets reversed as TSNM. • As the direction of current gets reversed after each half rotation, the produced current is called an alternating current (AC). Working Principle of Electric Generator • To get a unidirectional current, instead of two slip rings, two split rings are used, as shown in the following figure. Working Principle of Electric Generator • In this arrangement, brush A always remains in contact with the length of the coil that is moving up whereas brush B always remains in contact with the length that is moving down. • The split rings C and D act as a commutator. Answer • What is the difference between a split-ring and a slip-ring commutator? • A split-ring commutator makes the current change direction every half-rotation, whereas a slip-ring commutator simply maintains a connection between the moving rotor and the stationary stator. Two Types of AC Generators • Revolving armature – rotor is an armature which is rotating inside a stationary electromagnetic field – seldom used since output power must be transmitted through slip-rings and brushes • Revolving field – DC current is supplied to the rotor which makes a rotating electromagnetic field inside the stator – more practical since the current required to supply a field is much smaller than the output current of the armature Revolving Armature FIELO WINDINGS OUTPUT ONE REVOLUTION • DC generator is the generator that produces DC power i.e., constant power P=V*I by taking mechanical energy as input. • Example of a DC generator is dynamo. DC Generator DC Generator • It is based on the principle of production of dynamically (or motionally) induced e.m.f (Electromotive Force). Whenever a conductor cuts magnetic flux, dynamically induced e.m.f. is produced in it according to Faraday's Laws of Electromagnetic Induction. This e.m.f. causes a current to flow if the conductor circuit is closed. Hence, the basic essential parts of an electric generator are : • A magnetic field and • A conductor or conductors which can so move as to cut the flux. 96 DC Generator 97