FUNDAMENTALS OF ELECTRICAL ENGINEERING ENT 163 LECTURE 6

FUNDAMENTALS OF ELECTRICAL ENGINEERING [ ENT 163 ] LECTURE #6 b MAGNETISM AND ELECTROMAGNETISM HASIMAH ALI Programme of Mechatronics, School of Mechatronics Engineering, Uni. MAP. Email: hashimah@unimap. edu. my

CONTENTS • INTRODUCTION • THE MAGNETIC FIELD • ELECTROMAGNETISM • ELECTROMAGNETIC DEVICES • MAGNETIC HYSTERESIS • ELECTROMAGNETIC INDUCTION

ELECTROMAGNETISM Electromagnetism is the production of a magnetic field by current in a conductor. Magnetic field around a currentcarrying conductor • The right hand rule is used to determine the direction of the lines of force.

ELECTROMAGNETISM Conductor is a material which contains movable electric charges in which an electric current can be placed. • Principle – when an electric potential difference is impressed across separate points on a conductor, the mobile charges within the conductor are forced to move, and electric current between those points appears in accordance with Ohm’s law; metallic and non-metallic. .

ELECTROMAGNETISM Magnetic lines of force are continuous along wire Magnetic field around a current-carrying conductor • Insulators refer to non-conducting materials. • Current – produces an electromagnetic field around a conductor. • Although magnetic field cannot be seen, but its capable of producing visible effects.

ELECTROMAGNETISM • When current passes through a conductor, an electromagnetic field is created around the around. A current-carrying wire – inserted through a sheet of paper in perpendicular direction, iron filings placed on the surface of the paper arrange themselves along the magnetic lines of force in concentric rings. The north pole of a compass placed in the electromagnetic field will point in the direction of the line of force.

ELECTROMAGNETISM • The field is stronger closer to the conductor and becomes weaker with increasing distance from the conductor. • Direction of the lines of force can be determine from the left-hand rule. • For parallel conductors: • The conductors repel with each other when the currents are in opposite direction. • The conductors attract with each other when the currents are in the same direction.

ELECTROMAGNETISM Several important electromagnetic properties: • Permeability, µ i. Mechanical property of a material ii. Higher the permeability, the more easily a magnetic field can be established. iii. Permeability of a vacuum, iv. Relative permeability of material,

ELECTROMAGNETISM • Reluctance, in a material. Opposition to the establishment of a magnetic field l=length of magnetic path µ=permeability A=cross-sectional area of the material • Electromagnetic coil: q The magnetic field is produced by a straight wire but not very strong. q Stronger field is produced by coiling wires around a piece of soft iron q Also known as solenoid. q Shape of magnetic field is same as bar magnet. q The soft iron inside the coil makes the magnetic field stronger because it becomes a magnet itself when the current is flowing.

ELECTROMAGNETISM q Soft iron is used because it loses its magnetism as soon as the current stops flowing (temporary magnet). q In this way, the electromagnet can be switched on and off by turning the electricity on and off. q The strength of the magnetic field around the coil can be increased by: • Using a soft iron core. • Using more turns of wire on the coil. • Using a bigger current. q Reversing the direction of the current will reverse the magnetic field direction.

ELECTROMAGNETISM Magnetomotive force (mmf) is a force that produces the magnetic field. Unit as ampere-turn(At) Equation: • Fm= magnetomotive force N=number of turns of wire I= current in amperes Ohm’s law for magnetic circuits: Where flux (ø), is analogous to current, the mmf (Fm) is analogous to voltage and the reluctance, ( ) is analogous to

ELECTROMAGNETIC DEVICES q Electromagnets are used in devices such as magnetic disk, electric motors, speakers, solenoids and relays. q Read/write function on a magnetic surface.

ELECTROMAGNETIC DEVICES Solenoid. q The solenoid is used for applications such as opening and closing valves and automobile door locks. q Solenoid is a type of electromagnetic device that has a movable iron core called plunger. q The movement of this iron core depends on both an electromagnetic field and a mechanical spring force. q Basic structure – consists of a cylindrical coil of wire wound around a nonmagnetic hollow form; a stationary iron core is fixed in position at the end of the shaft, and a sliding iron core is attached to the stationary core with a spring. Sliding core (plunger) Stationary core Spring Coil

ELECTROMAGNETIC DEVICES q Basic solenoid operation: § At rest (unenergized) state – plunger – extended. § Solenoid is energized by the current through the coil. § The current sets up an electromagnetic field that magnetizes both iron cores. § The south pole of the stationary core attracts the north pole of the movable core, which causes it to slid inward, thus retracting the plunger and compressing the spring.

ELECTROMAGNETIC DEVICES § As long as there is coil current, the plunger remains retracted by the attractive force of the magnetic fields. § When the current is cut off, the magnetic field collapse; and the force of the compressed spring pushes the plunger back out.

ELECTROMAGNETIC DEVICES Relay: q Relays differ from solenoids in that the electromagnetic action used to open/ close electrical contacts rather than to provide mechanical movement. q Basic structure of a relay:

ELECTROMAGNETIC DEVICES • Relay – consists of two circuits • Circuit 1 is a simple electromagnet which requires only a small current. • When the switch is closed, current flows and the iron rocker arm is attracted to the electromagnet. • The arm rotates about the central pivot and pushes the contacts together. Circuit 2 is now switched on. • Circuit 2 may have a large current flowing through it, to operate powerful motor or very bright lights. • When the switched is opened the electromagnet release the rocker arm and the spring moves the contacts apart. Circuit 2 is now switched off.

ELECTROMAGNETIC DEVICES • The advantage of using a relay is that a small current (circuit 1) can be used to switch on and off a circuit with a large current(circuit 2). • This is useful for two reasons: • Circuit 1 may contain a component which only uses small currents, • Only the high current circuit needs to be made from thick wire. Application of relays is to operate the starter motor in cars and the heating circuit in diesel engines.

MAGNETIC HYSTERESIS Magnetizing force, H in a material is a magnetomotive force per unit length of the material. Unit : ampere – turns permeter(At/m) Equation: Fm= magnetomotive force H= magnetizing force I= length of material H is depends on the number of turns of the coil of wire , current through coil, length of material.

MAGNETIC HYSTERESIS q Since, q Recall that, as increase, the flux increases; therefore H increases , therefore B is also proportional to H. q The B-H relationship- showed by a B-H curve, also known as the hysteresis curve.

MAGNETIC HYSTERESIS Hysteresis is a characteristic of a magnetic material whereby a change in magnetization lags the application of a magnetizing force. q Figure below illustrates the development of hysteresis curve: q Start by assuming a magnetic core is unmagnetized (B=0). As the magnetizing force (H) is increased from zero, the flux density (B) increase proportionally. When H reached a certain value, the value of B begins to level off.

MAGNETIC HYSTERESIS q At H continuous to increase, B reaches a saturation value (Bsat) when H reaches a value (Hsat). Once saturation is reached, a further increase in H will not increase B. (Bsat) saturation (Hsat).

MAGNETIC HYSTERESIS q If H is decreased to zero, B will fall back along a different path to a residual value (BR). This indicates that the material continues to be magnetized even with the magnetizing force removed (H=0). The ability of the material, once magnetized, to maintain a magnetized state without the presence of a magnetizing force is called retentivity. (BR). (H=0).

MAGNETIC HYSTERESIS q Reversal of the magnetizing force is represented b negative values of H on the curve and is achieved by reversing the current in the coil of wire. An increase in H in negative direction causes saturation to occur at a value (-Hsat) where the flux density is at its maximum negative value. (-Hsat) saturation (-Bsat)

MAGNETIC HYSTERESIS q When the magnetizing force is removed (H=0), the flux density goes to its negative residual values (-BR) (H=0), (-BR)

MAGNETIC HYSTERESIS q From the (-BR) value, the flux density follows the curve back to its maximum positive value when the magnetizing force equals Hsat in the positive direction. Bsat +Hc Hsat

MAGNETIC HYSTERESIS q The complete B-H curve. called as the hysteresis curve. The magnetizing force required to make the flux density zero is called the coercive force, Hc.

ELECTROMAGNETIC INDUCTION q When a magnetic field is moved past a stationary conductor, there is a relative motion, known as the induced voltage, vind , which across the conductor. q Amount of induced voltage depends on the at which the conductor and the magnetic field move with respect to each other (the faster the relative motion, the greater the induced voltage).

ELECTROMAGNETIC INDUCTION • If the conductor is moved first one way and then another in the magnetic field, a reversal of the polarity of the induced voltage will be observed. • When the relative motion of the conductor is downward, a voltage is induced with the polarity indicated in Figure below. When the relative motion of the conductor id upward, the polarity is as indicated in part (b) of the figure.

ELECTROMAGNETIC INDUCTION • When a load resistor is connected to the conductor, the voltage induced by the relative motion between the conductor and the magnetic field will cause a current on the load, called the induced current (iind). Induced current (iind) in a load as the conductor moves through the magnetic field.

ELECTROMAGNETIC INDUCTION • Figure below shows a current inward through a wire in a magnetic field:

ELECTROMAGNETIC INDUCTION q The electromagnetic field set up by the current interacts with the permanent magnetic field, as a result, the permanent lines of force above the wire tend to be reflected down under the wore because they are opposite in direction to the electromagnetic lines of force. q Therefore the flux density above is reduced and the magnetic field is weakened. The flux density below the conductor is increased and the magnetic field is strengthened. An upward force on the conductor results, and the conductor tends to move toward the weaker magnetic field. q Figure (b) shows the current outward, resulting in a force on the conductor in the downward direction.

ELECTROMAGNETIC INDUCTION q Michael Faraday – discovered the principle of electromagnetic induction in 1831 (moving a magnet through a coil of wire induced a voltage across the coil). q Faraday’s observation: v The amount of voltage induced in a coil is directly proportional to the rate of change of the magnetic field with respect to the coil. v The amount of the voltage induced in a coil is directly proportional to the numbers of turns of wire in the coil.

ELECTROMAGNETIC INDUCTION Faraday’s Law The voltage induced across a coil of wire equals the number of turns in the coil times the rate of change of the magnetic flux. As the magnet moves slowly to the right, its magnetic field is changing w. r. t coil, and a voltage is induced. As the magnet moves rapidly to the right, its magnetic field is changing more rapidly w. r. t coil, and a greater voltage is induced.

ELECTROMAGNETIC INDUCTION Heinrich F. E. Lenz – defines the polarity or direction of the induced voltage Lenz’s Law: When the current through a coil changes, the polarity of the induced voltage created by the changing magnetic field is such that it always oppose the change in current that caused it.

FURTHER READING… Electric Circuit Fundamentals. (7 th Edition), Floyd, Prentice Hall. (chapter 7).