1. INTRODUCTION OF ELECTRICAL MACHINES
UNIT -1
INTRODUCTION OF ELECTRICAL MACHINES

2. 1. DIFFERENCE BETWEEN MOTOR AND GENERATOR
Motor converts electrical energy into mechanical energy.
• A generator converts
mechanical energy into
electrical energy
DC motor uses Fleming left hand rule .
• Generator uses Fleming
right hand rule.
Efficiency of motor is ratio of mechanical power to Electrical power
• Efficiency of generator is
ratio of Electrical power to mechanical power.

3. 2. Fleming Left Hand Rule
It is used to determine the direction of force acting on a current carrying conductor placed in a magnetic field . The middle finger , the fore finger and thumb of the left hand are kept at right angles to one another . The middle finger represent the direction of current The fore finger represent the direction of magnetic field The thumb will indicate the direction of force acting on the conductor This rule is used in motors

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5. 3. Fleming Right Hand Rule
It is used to determine the direction of emf induced in a Conductor. The middle finger , the fore finger and thumb of the left hand are kept at right angles to one another. The fore finger represent the direction of magnetic field The thumb represent the direction of motion of the conductor The middle finger will indicate the direction of the inducted emf . This rule is used in DC Generators

6. 4. Working principle of Motor
A motor is an electrical machine which converts electrical energy into mechanical energy. The principle of working of a DC motor is that "whenever a current carrying conductor is placed in a magnetic field, it experiences a mechanical force". The direction of this force is given by Fleming's left hand rule and it's magnitude is given by F = BIL. Where, B = magnetic flux density, I = current L = length of the conductor within the magnetic field.

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8. 5. Working principle of Generator
When a conductor moves in a magnetic field, an emf is induced across the conductor. According to Faraday's law of Electroctromagnetic induction, when a conductor links with a changing flux, it will have an induced emf across it. The value of induced emf across the conductor depends on the rate of change of flux linkage with the conductor. The direction of the induced emf in the conductor can be determined by Fleming's Right Hand Rule. This rule says that on your right hand if you stretch your thumb, first finger and second finger perpendicular to each other, and if you align your right-hand thumb along the direction of motion of the conductor in the magnetic field, and first finger along the direction of magnetic field, then you second finger indicates the direction of emf in the conductor.

9. 6. LENZ’S LAW
The direction of induced emf is given by Lenz’s law. According to this law, the induced emf will be acting in such a way so as to oppose the very cause of production of it . e = -N (dØ/dt) volts

10. 7. Faraday law of electromagnetic induction
Faraday's first Law Any change in the magnetic field of a coil of wire will cause an emf to be induced in the coil. This emf induced is called induced emf and if the conductor circuit is closed, the current will also circulate through the circuit and this current is called induced current. Faraday's Second Law It states that the magnitude of emf induced in the coil is equal to the rate of change of flux that linkages with the coil. The flux linkage of the coil is the product of the number of turns in the coil and flux associated with the coil. According to Faraday's law of electromagnetic induction, the rate of change of flux linkage is equal to induced emf.

11. UNIT-2
DC MACHINE

12. 1. CONSTRUCTION AND WORKING PRINCIPLE OF DC MOTOR
Principle :- A DC motor like we all know is a device that deals in the conversion of electrical energy to mechanical energy and this is essentially brought about by two major parts required for the construction of DC motor, namely.
Stator – The static part that houses the field windings and receives the supply and,
Rotor – The rotating part that brings about the mechanical rotations
Other than that there are several subsidiary parts namely the
Yoke of DC motor
Poles of DC motor
Field winding of DC motor
Armature winding of DC motor
Commutator of DC motor
Brushes of DC motor

13. 1.1 DC Machine constructional view

14. 1.2 Yoke of DC Motor
The magnetic frame or the yoke of DC motor made up of cast iron or steel and forms an integral part of the stator or the static part of the motor. Its main function is to form a protective covering over the sophisticated inner parts of the motor and provide support to the armature. It also supports the field system by housing the magnetic poles and field winding of the DC motor.

15. 1.3 Poles of DC Motor
The magnetic poles of DC motor are structures fitted onto the inner wall of the yoke with screws. The construction of magnetic poles basically comprises of two parts. Namely, the pole core and the pole shoe stacked together under hydraulic pressure and then attached to the yoke. These two structures are assigned for different purposes, the pole core is of small cross-sectional area and its function is to just hold the pole shoe over the yoke, whereas the pole shoe having a relatively larger cross-sectional area spreads the flux produced over the air gap between the stator and rotor to reduce the loss due to reluctance. The pole shoe also carries slots for the field windings that produce the field flux.

16. 1.4 Field Winding of DC Motor
The field winding of DC motor are made with field coils (copper wire) wound over the slots of the pole shoes in such a manner that when field current flows through it, then adjacent poles have opposite polarity are produced. The field winding basically form an electromagnet, that produces field flux within which the rotor armature of the DC motor rotates, and results in the effective flux cutting.

17. 1.5 Armature Winding of DC Motor
The armature winding of DC motor is attached to the rotor, or the rotating part of the machine, and as a result is subjected to altering magnetic field in the path of its rotation which directly results in magnetic losses. For this reason the rotor is made of armature core, that’s made with several low-hysteresis silicon steel lamination, to reduce the magnetic losses like hysteresis and eddy current loss respectively. These laminated steel sheets are stacked together to form the cylindrical structure of the armature core.

18. 2. ARMATURE WINDING
Armature windings are two types 1. LAP WINDING • A = P • The armature windings are divided into no. of sections equal to the no of poles. • It is connected in series 2. WAVE WINDING • A = 2 • It is used in low current output and high voltage. • It is connected in parallel.

19. 3. EMF EQUATION OF GENERATOR
Let,
Ø= flux per pole in weber
Z = Total number of conductor
P = Number of poles
A = Number of parallel paths
N =armature speed in rpm
Eg = emf generated in any on of the parallel path
Eg = PφNZ/60A

20. 4. Types of DC Generators
The types of DC motor can be listed as follows- DC motor
Permanent Magnet DC Motor
Separately Excited DC Motor
Self Excited DC Motor

21. 5. Armature reaction
Armature flux superimposes with the main field flux and, hence, disturbs the main field flux. This effect is called as armature reaction in DC machines. perpendicular to the lines of force.
Effects of Armature Reaction
It decreases the efficiency of the machine.
It produces sparking at the brushes.
It produces a demagnetizing effect on the
main poles.
It reduces the emf induced.
Self excited generators some times fail to build up emf.

22. 5.1 Armature reaction remedies
Extra turns in the field winding
Slots are made on the tips to increase the reluctance
The laminated cores of the shoe are staggered
In big machines the compensating winding at pole shoes produces a flux which just opposes the armature mmf flux automatically

23. 6. Commutation
Reversal of current in the armature coil by means of brushes and commutator segments.

24. 6.1 Methods of Improving Commutation
There are three methods of sparkles commutation:
Resistance Commutation
Voltage Commutation
Compensating Windings

25. 7. CharacteristicS of DC motors
T/ Ia characteristic as Ta ∝ ɸ.Ia
N/ Ia characteristic as N ∝ Eb/ɸ
N/T characteristic
DC Series motor Characteristics

26. Continue… DC Shunt motor Characteristics
Dc shunt motor is called constant speed motor

27. 8. Speed control of DC motors
According to the speed equation of a dc motor
N ∞ Eb/φ
∞ V- Ia Ra/ φ
Thus speed can be controlled by-
Flux control method: By Changing the flux by controlling the current through the field winding.
Armature control method: By Changing the armature resistance which in turn changes the voltage applied across the armature

28. 8.1 Armature Resistance Control Method

29. 8.2 Field Control of DC Series Motor

30. 9. STARTER OF DC MOTOR
A large current flows through the armature during starting . This current is large enough to damage the armature circuit . So, we need starter.
Starter Limit the starting current.
Two types of Starter
Three point starter
Four point starter.

31. 9.1 Three point starter