362 Mechanical Engineering Technician UNIT 7 Electromagnetic Induction Structure 7.1 Introduction 7.2 Faraday s laws of Electromagnetic Induction 7.3. Lenz s law 7.4. Fleming s right and rule 7.5. Self Inductance 7.6. Mutual inductance Learning Objectives Electro Magnetic induction faradays laws of electromagnetic induction of first and second law and explanation. Lenz s law experiment. Fleming s right hand and its sums. Types of induced e.m.f. Dynamically induced e.m.f, statically induced e.m.f, Self induced e.m.f. and its uses, Mutually induced e.m.f and uses model sums. 7.1 Introduction The transfer of electric energy from one circuit to another without any electrical connections is called induction. When electric energy is transferred by means of a magnetic field it is called electromagnetic induction.
Paper - III Electrical Technology 363 Electromagnetic induction occurs whenever there is a relative movement between a conductor and a magnetic field. This phenomenon is discovered by Michael Faraday. He formulated two laws of electromagnetic induction. 7.2 Faraday s Laws of Electromagnetic Induction First law Whenever any conductor is made to rotate in a magnetic field and hence cut the magnetic lines of force or the flux, e.m.f. will be induced in that conductor. Fig. 7.1. Electro Fig. 7.2. Inductance Second law The second law states that the magnitude of the induced e.m.f. is directly proportional to the rate of change of flux linked with the conductor. Explanation Assuming that the coil has N number of turns and the flux linked with the coil changes from the initial value wb to the final value wb in t second, then the flux linked is the product of the number of turns and the flux linked with the coil. Then we have, Then we have, Initial flux linked = N Final flux linked = N Nφ Induced e.m.f. 2 Nφ α 1 volts t or Taking the constant of proportionality as unit, d e N
364 Mechanical Engineering Technician Where N = Number of turns of the coil d = the rate of change of flux. Model Sum : A flux of 0.015 wb lines a coil having 350 turns. The flux through the coil is decreased to zero at a uniform rate in 0.2 sec. Calculate the induced E.M.F. No. of turns (N) = 350 Total flux ( ) Time (t) = 0.015 wb = 0.2 sec Formula d 0.015 e = = 350 26. 25 volts. 0.02 Direction of Induced E.M.F. and Current The direction e.m.f. and hence current in a conductor can be determined by 1. Lenz s law or 2. Flemings right hand rule. 7.3 Lenz s Law When an E.M.F. is induced in a circuit electromagnetically the current set up always opposes the motion of change in current which produces it. Experiment : In the fig. When north pole of bar magnet move towards one coil it results as induced e.m.f. or current which is in opposite direction in the coil. North pole of induced coil and magnetic North pole repel each other. Fig. 7.3. Lenz s Law When magnet move away from the coil the induced E.M.F. or current will be in clock wise direction in the coil.
Paper - III Electrical Technology 365 South pole which is produced in induced coil attract the North pole of the magnet. According to the law of conservation of energy Lenz s law can be explained. Movement of magnet will produce induced current and opposes the repulsion or attraction of the magnet. Energy is transformed due to the law of conservation of Energy. But energy will not increase. 7.4 Fleming s Right hand rule It states that, Place therefore finger middle finger and thumb of the right hand mutually perpendicular to each other such that, for finger points the direction of the magnetic field, thumb points the direction of motion of the conductor, then the middle finger gives the direction of induced current flowing through it. Fig. 7.4. Fleming s Right Hand Rule Problems 1. A coil of 360 turns is linked by a flux of 100 - wb. If the flux is reversed in 0.01 second, find the emf induced in the coil. Solution. N = 360 = 100 x 10-6 wb = - 100 x 10-6 wb = 0.01 sec. d = 100 x 10-6 - (-100 x 10-6 ) = 200 x 10-6 wb
366 Mechanical Engineering Technician = 7.2 V Ans. 2. A dc motor field pole is wound with 400 turns and carries 25 m-wb when excited. The exciting current is then switched off and the flux is reduced to 0.6 mwb in 0.03 sec. Calculate the average emf induced in the coil. Solution: N = 400 t = 0.03 sec. = 25 x 10-3 wb = 0.6 x 10-3 wb d = 25 x 10-3 0.6 x 10-3 = 24.4 x 10-3 wb = 325.333 V Ans. Types of Induced EMF There are two type of induced EMF a. The emf produced by the generator or Dynamo means magnetic line of forces cut by the conductors generate the emf called Dynamically Induced EMF Ex. Generators. b. When we cut the magnetic fixed flux conductor produced by the Alternating current (A.C) which generate EMF is called Statistically induced EMF Ex. Transformer. Types of Induced EMF Dynamically induced emf Statistically induced emf Stationary permanent Stationary conductor Self Induced EMF Mutually induced magnet and moving and moving EMF (Motor) (Generator)
Paper - III Electrical Technology 367 Dynamically Induced EMF Thew emf induced in a conductor due to motion (either conductor electromagnet) is called dynamically induced emf. Consider a single conductor of length I meter moving at right angles to a uniform magnetic field of B wb/m 2 with a velocity of Vm/s. Suppose the conductor moves through a small distance of dx in seconds. The area swept by the conductor da 1dx Flux cut, d = flux density x area swept = B 1 dx. Statically induced E.M.F. The emf induced by variation of flux in a stationary conductor and magnet is known as statically induced emf. It can further divided into a) self induced emf b) Mutually induced emf. 7.5 Self Inductance When Alternating current (A.C) is in one circuit which induced EMF is called self induced EMF. It opposes the Allied EMF given to a circuit. This induced EMF is called back or counter emf. Here L is called proportionality constant of self induction N L = Henry i Induction is measured in Henry Rate of change of current in a cost is unit, in this induced EMF is called self induction of wire. Henry: It is defined as the inductance of a circuit in which a counter emf of 1 volt is generated when the current is changed at the rate of 1 ampere per Fig. 7.5. Self Induction
368 Mechanical Engineering Technician second. emf induced in a coil: di E L di where L = inductance of the coil and = rate of change of current Negative sign is for opposing nature of induced emf. Model Sum : The field circuit of a generator has in inductance of 6 H. If the field current 5 is Changing is the average E.M.F. L = 6H; I = 5A; t = 0.2 sec. Li e Volts t Value substitute 6 5 e 150 Volts 0.2 Model Sum : A lighting magnet produces a flux of 0.5 wb. and its existing coil has 100 turns. If the current is suddenly opened, it takes 1/10 sec to fall to zero. Calculate the EMF due to self induction. = 0.5 wb; N=100; t = 1/10 sec. or 0 t1 sec. 100 0.5 e 500 Volts 0.1 Example : A current falls in 0.2 seconds from 200 Amps to 50 amps. If inductance of a coil is 30 mh. Find the emf induced in the coil. Current which falls in 0.2 second = 200 50 = 150 Amps. di di 150 10 E L, Here 750 2 Amps/sec. Inductance L=30 mh = 30 0. 03 1000 H E 0.03 750 22.5 V Ans. - Sing shows that the induced voltage opposes the applied voltage. Explanation of Inductance Take the coil of wires as shown in fig. Connected through a battery and a rheostat. It will be seen that whenever the current is increased, the flux surrounding the coils also increases. The flow of current is always opposed by the instantaneous production of the counter e.m.f. The energy required to over-
Paper - III Electrical Technology 369 come this opposition is supplied by the external source. Uses Of Self-inductance 1. In the Fluorescent tubes for starting purpose and to reduce the voltage (choke). 2. In regulators, to give reduced voltage to the fans. 3. In lightning arrester. 4. In auto-transformer. 5. In smooth chock which is used in welding plant. 6. In rectifiers to keep arc stationary. 7.6 Mutual Induction When to coils of wire placed side by side in one coil current changes and in second coil current (i) (or) induce (V) this is called mutual Induction. Explanation : In fig Arrange two coils A and B. Connect coil A to switch S through battery B is attached to Galvanometer. When we on and off switch S current changes in coil A and produces flux in which magnetic flux changes so in coil B current or EMF induce and Galvanometer pointer will deflect. Current flows in A primary coil is (I) then magnetic flux is According to Faraday s Laws - induced EMF in secondary coil B is then d di E2 ; m ; m = constant this process is called mutual inductance. Note: Values of m. 1. No.of turns of wire in the coil 2. Distance between coils Fig. 7.6. Mutual Induction 3. Depends on geometrical shape of the coil According to Lenz s Law; remember that in coil B induced current or EMF opposes A
370 Mechanical Engineering Technician Uses of Mutual Inductance 1. It is used in ignition coil which is used in motor car. 2. It is also used in inductance furnace. 3. It is the principle of transformer. Model Sum: If two coils having mutual inductions of 0.2 H are placed adjacent to each other. Calculate the induced EMF in the second coil. When the current of 2.5 in the first coil is reduced to zero in 1/100 sec. di e M M = 0.2 H; i = (0.25); t=1/100 sec. 0 2.5 Values substitute 0.2 5 100 500 V 1 100 Key Concepts 1. Electro - magnetic induction is find by Michael Faraday scientist d e N volt 2. Faraday s Laws of Electromagnet induction First Law: When ever any conductor is made to rotate in a magnetic field and hence cut the magnetic lines of force or the flux, e.m.f. will be induced in that conductor. Second Law: The second law states that the magnitude of the induced e.m.f. is directly proportional to the rate of changing of flux linked with the conductor. 3. Lenz s Law used : To find out the direction of the counter - current produced in the discs or armature etc rotating in the magnetic field. 4. Fleming R. hand used: To find out the direction of the current in this con ductor of generator. 5. Types of Inductance: 1. Dynamically induced EMF a. Stationary Permanent b. Stationary conductor 2. Statistically induced EMF a. Self Induced b. Mutually educed EMF
Paper - III Electrical Technology 371 Uses: In the flourecent tube for starting purpose and to reduce the voltage (choke), Regulators, Lighting arrester, Auto transformer, Smooth choke (welding plant), rectifiers b. Mutually Induced Uses: Ignition coil, Furnace, Transformer. Activities - I State the Laws of Faraday s laws of electromagnetic induction First Law Second Law Lenz s Law Activities - II 1. wirte the types of induced EMF 2. Use of Self Inductance 1. 2. 3.
372 Mechanical Engineering Technician 4. 5. 6. 3. Uses of Mutual inductance 1. 2. 3. Model Questions Short Answer Type Questions 1. State Faraday s first Law of electromagnetic Induction. 2. State Faraday s second Law of electromagnetic Induction. 3. State Lenz s Law. 4. State Flemings Right Hand Rule. 5.What are the uses of self inductance. 6. What are the uses of mutual inductance.
Paper - III Electrical Technology 373 Long Answer Type Questions 1. State and explain Faraday s Law of electromagnetic induction. 2. Write short notes on i) Self Induction ii) Mutual Induction. OJT/Project Work: 1. How do you prepare the tube light choke. 2. How do you prepare the fan regulator 3. How do you prepare the ignition coil for using of vehicle starting.