MZS FKEE, UMP

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Review of Electromagnetism

Muhamad Zahim Muhamad Zahim Ext: 2312Ext: 2312

BEE2123 BEE2123 ELECTRICAL MACHINESELECTRICAL MACHINES

BEE2123 BEE2123 ELECTRICAL MACHINESELECTRICAL MACHINES

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Learning Outcomes

At the end of the chapter, students should be able to:Understand the fundamental lawsfundamental laws in the

dynamic magnetic systemsdynamic magnetic systems and their relation to the electrical machinesrelation to the electrical machines.

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Introduction to Electrical Machines

An electric machine is a device which converts electrical powerconverts electrical power (voltages and currents) into mechanical powermechanical power (torque and rotational speed), and/or vice versaand/or vice versa.

A motormotor describes a machine which converts electrical power to mechanical powerelectrical power to mechanical power; a generator (or alternator) generator (or alternator) converts mechanical mechanical power to electrical powerpower to electrical power.

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Introduction to Electrical Machine

Many electric machines are capable of performing both as motors and performing both as motors and generatorsgenerators;

The capability of a machine performing as one or the other is often through the action of a magnetic field, to perform such conversions.

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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.

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Review of Electromagnetism

The basic idea behind an electromagnet is extremely simple: a magnetic field around a magnetic field around the conductor can be produced when the conductor can be produced when current flows through a conductor.current flows through a conductor.

In other word, the magnetic field only exists magnetic field only exists when electric current is flowingwhen 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

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Magnetic Field

Unlike electric fields (which start on +q and end on –q), magnetic field encircle their current source.

The field weakens as you move away from the wire Ampere’s circuital law - the integration path

length is longer idH

.

A circular magnetic field develops around the wire follows right-hand rulesright-hand rules

field is perpendicular to the wire and that the field's direction depends on which direction the current is flowing in the wire

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Example of Electromagnetic

An electromagnet can be madecan be made by winding the winding the conductor into a coil and applying a DC voltageconductor 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 airair.

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Adding an Iron Core

IronIron is a is a better conductor of fluxbetter conductor of flux than than airair. 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 the magnetic field is strengthened.strengthened.

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Strength of Magnetic Field (Cont)

Because the magnetic field around a wire is magnetic field around a wire is circular and perpendicular to the wirecircular and perpendicular to the wire, an easy way to amplifyamplify the wire's magnetic field is to coilcoil the wire

The strength of the magnetic field in the DC electromagnet can be increased by increasing the by increasing the number of turns in the coil.number of turns in the coil. The greater the The greater the number of turns the stronger the magnetic number of turns the stronger the magnetic field will be.field will be.

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Faraday’s Law and Lenz’s Law

Faraday’s Law :Faraday’s Law : If a magnetic flux, , in a coil is changing in changing in time (n turns),time (n turns), hence a voltage, Vab is induced

Lenz’s Law :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)

ab

tNV

V = induced voltageN = no of turns in coil = change of flux in coilt = time interval

Vt

If no turns :

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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

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Voltage Induced from a time changing magnetic field

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Voltage Induced in a conductor moving in a magnetic field

Faraday’s Law for moving conductors : 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 forceElectromotive force, or emfemf.

The value of the induced voltage is given by

E = Blvwhere

EE = induced voltage (V)BB = flux density (T)ll = active length of the conductor in the magnetic field (m)vv = relative speedspeed of the conductor (m/s)

The polarity of induced voltage is given by the right-hand rule.

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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 barForce is exerted on the bar due to interaction between current and interaction between current and magnetic fluxmagnetic flux to the right and made the bar move with certain velocitythe bar move with certain velocity. The mechanical power out of the bar.

FF = ilB

Force induced on the conductor:

Unit: (N)

The direction of force is given by the right-hand rule.

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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

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Production of a Magnetic Field

The production of a magnetic field by a current is determine by Ampere’s law:

netIdlH H = magnetic field intensitydl = differential element of length along the path of integration

cl

NiH lc = mean path length

Magnetic field intensity:

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Production of a Magnetic Field

HB u = magnetic permeability of material

r0 u0 = permeability of free spaceur = relative permeability of material

m/H104 70

The strength of the magnetic field flux produced in the core also depends on the material of the core.

Magnetic flux density:

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Production of a Magnetic Field

cl

NiHB

BA

cl

NiA

Total flux:

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Magnetic Circuit

iRV

Ni

Electric circuit equation:

Magnetic circuit equation:

Analogy: Electric circuit & Magnetic circuit

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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?

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Magnetic saturation & hysteresis in ac magnetic field

unmagnetized Material

Iron becomes magnetically saturated

Magnetism increase as magnetic field magnetized

unmagnetized iron

a

b

c

d

Applied field is reduced; the magnetism reduced thru diff. curve since iron tends to retains magnetized state - hence produced permanent magnet, Residual Flux, res

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)

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

Hm

Magnetic field density

Bm

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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.

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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.

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Eddy Current Loss

• Eddy current can be reduced in 2 ways:

1. Adding a few percent of silicon to iron to increase the resistivity.

2. Laminate core with thin laminations and insulated from each other.

Hysteresis loss + eddy current loss = Core lossHysteresis loss + eddy current loss = Core loss