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## Review of Electromagnetism

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**Review of Electromagnetism**BEE2123 ELECTRICAL MACHINES Muhamad Zahim Ext: 2312 MZS FKEE, UMP**Learning Outcomes**• At the end of the chapter, students should be able to: • Understand the fundamental laws in the dynamic magnetic systems and their relation to the electrical machines. MZS FKEE, UMP**Introduction to Electrical Machines**• An electric machine is a device which converts electrical power (voltages and currents) into mechanical power (torque and rotational speed), and/or vice versa. • A motor describes a machine which converts electrical power to mechanical power; a generator (or alternator) converts mechanical power to electrical power. MZS FKEE, UMP**Introduction to Electrical Machine**• Many electric machines are capable of performing both as motors and generators; • The capability of a machine performing as one or the other is often through the action of a magnetic field, to perform such conversions. MZS FKEE, UMP**Introduction to Electrical Machine**• 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. MZS FKEE, UMP**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 MZS FKEE, UMP**Magnetic Field**• Unlike electric fields (which start on +q and end on –q), magnetic field encircle their current source. field is perpendicular to the wire and that the field's direction depends on which direction the current is flowing in the wire A circular magnetic field develops around the wire follows right-hand rules • The field weakens as you move away from the wire • Ampere’s circuital law - the integration path length is longer MZS FKEE, UMP**Example of Electromagnetic**• An electromagnet can be made by winding the conductor into a coil and applying a DC voltage. • The lines of flux, formed by current flow through the conductor, combine to produce a larger and stronger magnetic field. • The center of the coil is known as the core. In this simple electromagnet the core is air. MZS FKEE, UMP**Adding an Iron Core**• Iron is a better conductor of flux than air. The air core of an electromagnet can be replaced by a piece of soft iron. • When a piece of iron is placed in the center of the coil more lines of flux can flow and the magnetic field is strengthened. MZS FKEE, UMP**Strength of Magnetic Field (Cont)**• Because the magnetic field around a wire is circular and perpendicular to the wire, an easy way to amplify the wire's magnetic field is to coil the wire • The strength of the magnetic field in the DC electromagnet can be increased by increasing the number of turns in the coil.The greater the number of turns the stronger the magnetic field will be. MZS FKEE, UMP****a b Faraday’s Law and Lenz’s Law • Faraday’s Law : If a magnetic flux, , in a coil is changing in time (n turns), hence a voltage, Vab is induced • Lenz’s Law :if the loop is closed, a connected to b, the current would flow in the direction to produce the flux inside the coil opposing the original flux change.(in other words, Lenz’s Law will determine the polarity of the induced voltage) V = induced voltage N = no of turns in coil = change of flux in coil t = time interval If no turns : MZS FKEE, UMP**Faraday’s Law**• The effect of magnetic field: • Induced Voltage from a Time Changing Magnetic Field • Production of Induced Force on a Wire • Induced Voltage on a Conductor Moving in a Magnetic Field MZS FKEE, UMP**Voltage Induced from a time changing magnetic field**MZS FKEE, UMP**Voltage Induced in a conductor moving in a magnetic field**• Faraday’s Law for moving conductors : For coils in which wire (conductor) is moving thru the magnetic flux, an alternate approach is to separate the voltage induced by time-varying flux from the voltage induced in a moving conductor. • This situation is indicates the presence of an electromagnetic field in a wire (conductor). This voltage described by Faraday’s Law is called as the flux cutting or Electromotive force, or emf. • The value of the induced voltage is given by E =Blv where E = induced voltage (V) B = flux density (T) l= active length of the conductor in the magnetic field (m) v = relative speed of the conductor (m/s) The polarity of induced voltage is given by the right-hand rule. MZS FKEE, UMP**Induced Force**The electrical circuit consists of battery, resistor, two stationary rails, and movable bar that can roll or slide along the rails with electrical contact. • When switch is closed: • Current will not start immediately as inductance of the circuit. (However time constant L/R is very small). Hence, current quickly reach V/R. • Force is exerted on the bar due to interaction between current and magnetic flux to the right and made the bar move with certain velocity. The mechanical power out of the bar. Force induced on the conductor: F = ilB Unit: (N) The direction of force is given by the right-hand rule. MZS FKEE, UMP**Induced Force (Cont)**• The motion of the bar produces an electromagnetic force. The polarity of the emf is +ve where the current enters the moving bars. The moving bar generates a ‘back’ emf that opposes the current. • The instantaneous electrical power into the bar = mechanical output power MZS FKEE, UMP**Production of a Magnetic Field**• The production of a magnetic field by a current is determine by Ampere’s law: H = magnetic field intensity dl = differential element of length along the path of integration Magnetic field intensity: lc = mean path length MZS FKEE, UMP**Production of a Magnetic Field**• The strength of the magnetic field flux produced in the core also depends on the material of the core. Magnetic flux density: u = magnetic permeability of material u0 = permeability of free space ur = relative permeability of material MZS FKEE, UMP**Production of a Magnetic Field**Total flux: MZS FKEE, UMP**Magnetic Circuit**Analogy: Electric circuit & Magnetic circuit Electric circuit equation: Magnetic circuit equation: MZS FKEE, UMP**Example**• A ferromagnetic core is shown in Figure. Three sides of this core are of uniform width, while the fourth side is somewhat thinner. The depth of the core (into the page) is 10cm, and the other dimensions are shown in the figure. There is a 200 turn coil wrapped around the left side of the core. Assuming relative permeability is 2500, how much flux will be produced by a 1 A input current? MZS FKEE, UMP**Magnetism increase as magnetic field magnetized unmagnetized**iron b Bm Iron becomes magnetically saturated Applied field is reduced; the magnetism reduced thru diff. curve since iron tends to retains magnetized state - hence produced permanent magnet, Residual Flux, res c Hm a Magnetic field density AC increased in negative direction, magnetic field reversed , the iron reversely magnetized until saturated again If continue apply ac current, curve continue to follow S-shaped curve (hysteresis curve) d The area enclosed by hysteresis curve is energy loss per unit volume per cycle – heats the iron and is one reason why electric machines become hot Therefore, it is required to select magnetic materials that have a narrow hysteresis loop Magnetic saturation & hysteresis in ac magnetic field unmagnetized Material MZS FKEE, UMP**Hysteresis Loss**• During a cycle of variation of i (hence H), there is a net energy flow from the source to the coil-core assembly and return to the source. • Energy flowing is greater than energy returned. • This energy loss goes to heat the core. • The loss of power in the core due to the hysteresis effect is called hysteresis loss. MZS FKEE, UMP**Eddy Current Loss**• Voltage will be induced in the path of magnetic core because of time variation of flux enclosed by the path. • A current, known as an eddy current will flow around the path. • Because core has resistance, a power loss will be cause by the eddy current and appear as heat in the core. MZS FKEE, UMP**Eddy Current Loss**• Eddy current can be reduced in 2 ways: • Adding a few percent of silicon to iron to increase the resistivity. • Laminate core with thin laminations and insulated from each other. • Hysteresis loss + eddy current loss = Core loss MZS FKEE, UMP