B - B - 3.11 - Inductance Inductor PDF

Module 3 Electrical Fundamentals II Topic 3.11: Inductance and Inductor  INTRODUCTION On completion of this topic you should be able to: 3.11.1 State Faraday’s Law 3.11.2 Describe the action of inducing a voltage in a conductor...

Module 3 Electrical Fundamentals II Topic 3.11: Inductance and Inductor  INTRODUCTION On completion of this topic you should be able to: 3.11.1 State Faraday’s Law 3.11.2 Describe the action of inducing a voltage in a conductor moving in a magnetic field. 3.11.3 Describe induction principles 3.11.4 Describe the effects of the following on the magnitude of an induced voltage: Magnetic field strength Rate of change of flux Number of conductor turns 3.11.5 Describe mutual induction 3.11.6 Describe the effect the rate of change of primary current and mutual inductance has on induced voltage. Continued... 30-03-2024 Slide No. 2  INTRODUCTION On completion of this topic you should be able to: 3.11.7 Describe how the following factors affect mutual inductance: Number of turns in each coil Physical size of each coil Permeability of each coil Position of the coils with respect to each other 3.11.8 Describe Lenz’s Law and polarity determining rules 3.11.9 Describe back EMF and self induction 3.11.10 Describe Saturation point 3.11.11 Describe the principle uses of inductors 30-03-2024 Slide No. 3  BASIC INDUCTOR It is the production of voltage across a conductor situated in a changing magnetic field or a conductor moving through a stationary magnetic field: When a magnet moves in, an EMF is induced in meter When magnet is held stationary, there is no current When magnet moves out, the induced EMF is in an opposite direction If the loop is moved instead of the magnet, an EMF is also produced 30-03-2024 Slide No. 4  BASIC INDUCTOR OPERATION Michael Faraday found experimentally that: The magnitude of the induced EMF in a circuit is proportional to the rate at which the magnetic flux changed If a circuit contains N (number) of tightly wound loops and the flux through each loop changes by ΔФ during an interval Δt , the average EMF induced is given by Faraday’s Law: Where: ‘Ε’ = Electromotive force (EMF) ‘-‘ = The effect of Lenz’s law ‘N’ = Number of turns ‘ΔФ’ = Rate of magnetic flux change ‘Δt’ = Time interval 30-03-2024 Slide No. 5  VARIATIONS OF FARADAY'S LAW The concept of Faraday's Law is that any change in the magnetic environment of a coil of wire will cause a voltage (EMF) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated: In this example, a variable magnetic field is created when a coil of wire is moving into a constant magnetic field, therefore: A motional emf is induced, and is proportional to the speed of the coil is moved into the magnetic field 30-03-2024 Slide No. 6  LENZ’S LAW Lenz’s Law determines the direction of induced current flow: “The polarity of the induced EMF is such that the induced current itself creates a magnetic flux in the circuit that tends to oppose the change that produced it” In the examples: If B (flux) field is increasing, the induced field acts in opposition to it If B (flux) field is decreasing, the induced field opposes the change to keep it constant 30-03-2024 Slide No. 7  MAGNETIC FIELD OF CURRENT (ELECTROMAGNETISM) Magnetic field exists when electric current is flowing along a wire. Magnetic field strength is proportional to current. 30-03-2024 Slide No. 8  MAGNETIC FIELD OF CURRENT (ELECTROMAGNETISM) Direction of magnetic field dependent on direction of electrical current in the wire. Simple rule for remembering direction of magnetic field around a conductor. Left-Hand Rule: Grasp conductor in left hand with the thumb pointing in the direction of current Fingers will circle conductor in direction of magnetic field This rule is used for Electron Flow ONLY 30-03-2024 Slide No. 9  ELECTROMAGNETIC INDUCTION Note both conductors are stationary. If current to coil is switched on and off, magnetic field will build up and collapse. This will induce an EMF into the secondary conductor. Movement of magnetic field is produced by varying current in coil producing field. 30-03-2024 Slide No. 10  ELECTROMAGNETIC INDUCTION As field expands, lines of force cut Therefore, in accordance with Lenz’s law, across adjacent secondary conductor this voltage will be in a direction which and induces a voltage into this opposes the magnetic field produced by conductor the current in the primary conductor. 30-03-2024 Slide No. 11  SELF-INDUCTANCE Even a perfectly straight length of conductor has some inductance. Current changes thru conductor produce magnetic field changes – EMF induced. This EMF is ‘self-induced EMF’ because it is induced in current carrying conductor. Also referred to as ‘counter electromotive force’ (CEMF) or ‘back EMF’. Polarity of ‘back EMF’ is in opposite direction to applied voltage of conductor. Overall effect will be to oppose a change in current magnitude. 30-03-2024 Slide No. 12  SELF-INDUCTANCE For explanation – conductor looped so two portions of conductor lie next to each other. Field around each conductor cuts across other portion of same conductor. With increasing current, flux field expands cutting across a portion of conductor. This results in an induced EMF in the opposite direction. 30-03-2024 Slide No. 13  SELF-INDUCTANCE Direction of this induced voltage – determined by ‘left-hand rule for generators’. Figure A – expanding electromagnetic field. Figure B – collapsing field – back EMF has reversed direction. The induced voltage is now in the same direction as the battery voltage. Most important thing to note is that ‘back EMF’ opposes BOTH changes in current. 30-03-2024 Slide No. 14  MUTUAL INDUCTANCE Magnetic flux from 1 conductor induces EMF in another electrically isolated conductor. Circuits are electrically separated but magnetically coupled together. Transformers uses the principle of mutual inductance. 30-03-2024 Slide No. 15  MUTUAL INDUCTANCE Primary Current Affecting Induced Voltage Large wire powered, smaller wire secondary We have seen that a changing magnetic field from one coil can induce a voltage in the second, and the fact that Faraday found the rate of cutting magnetic flux affects the induced voltage can be put together with the magnetic force (MMF) is the number of turns multiplied by the current then any increase in the MMF will increase the voltage generated in the second conductor. To increase the MMF we can increase the Number of turns, however by doing this it would also increase the Back EMF(BEMF) reducing the current through the coil to an even lower level than it was to start with, this reduces the MMF to a lower level reducing the induced voltage in the second coil. 30-03-2024 Slide No. 16  MUTUAL INDUCTANCE To increase the current flow in the primary wire Apply a larger voltage to overcome the BEMF thus current rises, BEMF would rise so we would have to increase the voltage more to overcome the new BEMF. In other words to double the voltage in the primary wire the applied voltage may have to triple. Each rise in primary current will increase the voltage out or the second conductor. 30-03-2024 Slide No. 17  MUTUAL INDUCTANCE In the figure at top right the output is 29 volts but it is not connected to a circuit so no current is flowing. If a load is added then the situation changes now current flows in the secondary circuit, just as current changing in the Primary circuit induced a voltage in the secondary the current in the secondary will now have its own magnetic field but it opposes the primary magnetic field weakening it. 30-03-2024 Slide No. 18  MUTUAL INDUCTANCE The reduced total flux in the primary means a lower BEMF in the primary therefor the current will increase and flux increased until the BEMF and EMF are once again in balance with a voltage and current in the secondary. 30-03-2024 Slide No. 19  POSITIONING OF COILS The amount of mutual inductance depends on the relative positions of the two coils. Close together – large common amount of flux – high mutual inductance. 30-03-2024 Slide No. 20  POSITIONING OF COILS Considerable distance – small common amount of flux – low mutual inductance. 30-03-2024 Slide No. 21  POSITIONING OF COILS Perpendicular – NO mutual inductance. Mutual inductance can be increased greatly by mounting coils on a common iron core. 30-03-2024 Slide No. 22  MUTUAL INDUCTANCE Mutual inductance is dependent upon: Number of turns in each coil Physical size of each coil Permeability of each coil Position of coils with respect to each other 30-03-2024 Slide No. 23  MUTUAL INDUCTION Increasing the following will increase the EMF induced into the secondary: Magnetic field strength Number of conductor turns Rate of change of flux (increasing frequency) Permeability of core/s 30-03-2024 Slide No. 24  MUTUAL INDUCTION The ‘coefficient of coupling’ between two coils is equal to the ratio of the flux cutting one coil to the flux originated in the other coil. If the two coils are positioned so that all of the flux of one coil cuts all turns of other, coils are said to have a unity (1) coefficient of coupling. It is never exactly equal to unity (1), but it approaches this value in certain devices. 𝑀 = 𝐾 𝐿1 𝑥𝐿2 where: M = Mutual inductance in henrys K = Coefficient of coupling L1L2 = Inductance in henrys 30-03-2024 Slide No. 25  MUTUAL INDUCTION If all flux produced by one coil cuts only half of other coil – coefficient of coupling is 0.5. Coefficient of coupling is designated by the letter K. Mutual inductance between two coils, L1 and L2, is expressed in terms of the inductance of each coil and the coefficient of coupling K. 𝑀 = 𝐾 𝐿1 𝑥𝐿2 where: M = Mutual inductance in henrys K = Coefficient of coupling L1L2 = Inductance in henrys 30-03-2024 Slide No. 26  MAGNETIC SATURATION When a material is magnetically saturated, NO additional amount of external magnetisation force will cause an increase in its internal level of magnetisation. Therefore, after saturation, NO increase in primary current will increase amount of induced EMF in the secondary. 30-03-2024 Slide No. 27  MAGNETIC SATURATION 30-03-2024 Slide No. 28  MAGNETIC SATURATION Characteristic of an electrical circuit that opposes the starting, stopping, or a change in value of current. Inductance opposes change in current Symbol for inductance is L. Basic unit of inductance – HENRY (H) e.g. L = 5mH. One Henry is equal to the inductance required to induce one volt in an inductor by a change of current of one ampere per second. 30-03-2024 Slide No. 29  MAGNETIC SATURATION Physical analogy of inductance – pushing a wheelbarrow. Takes more work to start the load moving than it does to keep it moving. Once load is moving, easier to keep load moving than to stop it again – INERTIA. Inertia is the characteristic of mass which opposes a CHANGE in velocity. Inductance has same effect on current in an electrical circuit. It requires more energy to start or stop current than it does to keep it flowing. 30-03-2024 Slide No. 30  INDUCTANCE Characteristic of an electrical conductor that OPPOSES CHANGE in CURRENT. Symbol for inductance is L. Basic unit of inductance is the HENRY (H). One Henry is equal to inductance required to induce one volt in an inductor by a change of current of one ampere per second. 30-03-2024 Slide No. 31  INDUCTORS Inductors are also known as chokes, reactors, and coils These three names are descriptive of inductor characteristics: ‘chokes off’ and restricts sudden changes in current ‘reacts’ against (resists) changes, either increases or decreases, in current Inductors are usually ‘coils’ of wire Inductance is the result of a voltage being induced in a conductor. Magnetic field that induces the voltage in conductor is produced by conductor itself. 30-03-2024 Slide No. 32  EXAMPLE USES OF INDUCTANCE Radio Antenna Radio waves are electromagnetic. Oscillating magnetic field of electromagnetic waves (EM) induce an EMF in coil. Induction Stove Changing flux through bottom of metal pot generates eddy currents. Heat is dissipated in metal pot, but not in glass pot or glass stove top. Toroid Choke A sudden surge in current is partially choked off by ‘back EMF’ induced when magnetic flux through loop suddenly changes. Flux change is multiplied by presence of soft-iron cylinder surrounding the wire 30-03-2024 Slide No. 33  INDUCTANCE Property of an electrical circuit which opposes any change in current in that circuit. If current increases, the induced current tries to stop or delay that increase. If current decreases, induced current delays decrease – tries to keep current flowing. 30-03-2024 Slide No. 34  INDUCTANCE To increase inductance, conductor can be formed into a loop or coil. Current through coil produces magnetic field that encircles loop as shown in A. As current increases, magnetic field expands and cuts all loops as shown in B. Current in each loop affects all other loops. Field cutting other loops has effect of increasing opposition to current change. 30-03-2024 Slide No. 35  INDUCTOR TYPES Made by forming wire around a core – classified according to core type. The core material is normally one of two basic types: soft-iron or air. An iron-core inductor and its schematic symbol shown in figure A. Air-core inductor may be nothing more than a coil of wire, but is usually a coil formed around a hollow form of some non- magnetic material. This material serves no purpose other than to hold the shape of the coil. An air-core inductor and its schematic symbol are shown in figure B. 30-03-2024 Slide No. 36  FACTORS AFFECTING COIL INDUCTANCE Several physical factors which affect the inductance of a coil: Number of turns in the coil Diameter of the coil Coil length Type of material used in the core Number of layers of winding in the coils Inductance depends entirely upon the physical construction of the circuit. Can only be measured with special laboratory instruments. 30-03-2024 Slide No. 37  NUMBER OF TURNS Coil A has two turns and coil B has four turns. In coil A, flux field set up by one loop cuts one other loop. In coil B, flux field set up by one loop cuts three other loops. Doubling the number of turns will produce a field twice as strong (if same current). A field twice as strong, cutting twice the number of turns, will induce four times the EMF. Therefore, inductance varies as the ‘square of the number of turns.’ 30-03-2024 Slide No. 38  DIAMETER OF COIL Coil B has twice the diameter of coil A. In coil B, more lines of force exist to induce a back EMF in coil. Inductance increases directly as the ‘cross-sectional area of core increases’. Recall the formula for the area of a circle: A = π r2 Doubling the radius of a coil increases the inductance by a factor of 4. 30-03-2024 Slide No. 39  COIL LENGTH Coil A and B both have 3 turns and same diameter core. Coil A is twice as long as B and coils are widely spaced. Coil A has few flux linkages – relatively low inductance. Coil B closely spaced turns – increased flux linkage, higher L. Doubling the length of a coil while keeping same number of turns halves the value of inductance. 30-03-2024 Slide No. 40  CORE MATERIAL Magnetic core of coil B is a better path Inductance of a coil increases directly as for magnetic lines of force than is the permeability of core material increases. nonmagnetic core of coil A. Soft-iron magnetic core's high permeability has less reluctance to magnetic flux, resulting in more magnetic lines of force – this increases the inductance of coil. 30-03-2024 Slide No. 41  NUMBER OF COIL LAYERS Another way of increasing the inductance is to wind the coil in layers. Coil A is a poor inductor compared to others – widely spaced turns and no layering. Coil B is a more inductive coil – turns are closely spaced and wound in two layers. Two layers link each other with a greater number of flux loops during flux movements. Coil C still more inductive with three layers. Increased number of layers (cross-sectional area) improves flux linkage even more. Note that some turns, such as Y, lie directly next to six other turns (shaded). The inductance of the coil increases with each layer added. 30-03-2024 Slide No. 42  SERIES / PARALLEL INDUCTORS Inductors can be arranged in series or Formulae are (basically) same as that for parallel to achieve desired results. calculating resistance. 30-03-2024 Slide No. 43  VOLTAGE / CURRENT RELATIONSHIP In a purely inductive circuit, current lags voltage by 90 degrees. 30-03-2024 Slide No. 44  L/R TIME CONSTANT Used in delaying the build up/decay Maximum current flows in the inductor after of current in a circuit. 5 LR time constants. Similar to RC circuits (voltage Assume that maximum current in an LR delay) – 1 TC = 63.2% - 5 TC circuit is 10 amperes. 100%. 1 L/R TC is time required for When circuit is energised, 1 TC to 6.32 current in inductor to increase to amps and 5 TC to 10 amps (maximum). 63.2% of maximum. Each TC is 63.2% of difference. 30-03-2024 Slide No. 45  CONCLUSION Now that you have completed this topic, you should be able to: 3.11.1 State Faraday’s Law 3.11.2 Describe the action of inducing a voltage in a conductor moving in a magnetic field. 3.11.3 Describe induction principles 3.11.4 Describe the effects of the following on the magnitude of an induced voltage: Magnetic field strength Rate of change of flux Number of conductor turns 3.11.5 Describe mutual induction 3.11.6 Describe the effect the rate of change of primary current and mutual inductance has on induced voltage. Continued... 30-03-2024 Slide No. 46  CONCLUSION Now that you have completed this topic, you should be able to: 3.11.7 Describe how the following factors affect mutual inductance: Number of turns in each coil Physical size of each coil Permeability of each coil Position of the coils with respect to each other 3.11.8 Describe Lenz’s Law and polarity determining rules 3.11.9 Describe back EMF and self induction 3.11.10 Describe Saturation point 3.11.11 Describe the principle uses of inductors 30-03-2024 Slide No. 47  This concludes: Module 3 Electrical Fundamentals II Topic 3.11: Inductance and Inductor