1. Review of Electromagnetism
FKEE, KUKTEM
Review of Electromagnetism
BEE2123 ELECTRICAL MACHINES
Muhamad Zahim
Ext: 2312
MZS FKEE, UMP
Muhamad Zahim Sujod

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. Faraday’s Law and Lenz’s Law a b
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

12. 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

13. Voltage Induced from a time changing magnetic field
MZS FKEE, UMP

14. 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

15. 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

16. 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

17. 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

18. 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

19. Production of a Magnetic Field
Total flux:
MZS FKEE, UMP

20. Magnetic Circuit Electric circuit equation: Magnetic circuit equation:
Analogy: Electric circuit & Magnetic circuit
Electric circuit equation:
Magnetic circuit equation:
MZS FKEE, UMP

21. 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

22. Magnetic saturation & hysteresis in ac magnetic field
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
unmagnetized Material
MZS FKEE, UMP

23. 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

24. 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

25. 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