All about ac machines

1. Prof. Sidhant S. Kulkarni 1
Sanjivani College of Engineering, Kopargaon
Department of MechatronicsEngineering
(An Autonomous Institute)
Affiliated to Savitribai Phule Pune University
Accredited ‘A’ Grade by NAAC
________________________________________________________________________________________
Subject: Basic Electrical and Electronics Engineering (BEEE-BE104)
UNIT-3: PRINCIPLES OF ALTERNATING CURRENT (AC)
MACHINERY
Topic: Introduction

2. Unit-III PRINCIPLES OF ALTERNATING CURRENT (AC) MACHINERY Hr CO
s
• Introduction to inductance, capacitance, and impedance.
• Introduction to single phase and poly phase supply.
• Principle of operation and construction of single phase
transformers (core and shell type only). Losses, efficiency and
voltage regulation,
• Working of auto-transformer (Only equation based
numerical).
• Construction and working principle of three phase induction
motors, types: torque slip characteristics; power stages and
industrial applications (Only equation based numerical).
06
Hrs
.
CO
1
CO
3
Contents of Unit III
Prof. Pritish M. Vibhute 2

3. Fundamental entities
• Amplitude Frequency Phase

4. Inductors & capacitors
- Energy storage devices
- Passive components
- Electrostatic &
electromagnetic

7. Introduction to single phase and poly phase
supply
• Types of ALTERNATING Currents (AC)
• Single phase:-
–Domestic applications- low power appliances
–Only two wires [phase (230V-50Hz )and a neutral]
• Poly Phase:-
–In industrial applications- high power appliances
–Four wires[three phase wires(RYB- each 120degree phase
difference) and one neutral]
–1.732 (the square root of 3)times more power with the same
current than single phase
–Reduces sizes of electrical construction eg wire size.

9. Principle of
Working of
transformer :-
Mutual
Inductance

10. Single phase transformers
Magnetic
and electric
circuits
Limbs
Yoke

11. Step up & Step Down transformer
11

12. • Shell Type:- Efficient (2 cores)
• Difficult to repair as
sandwiched winding
• Poor natural cooling
• Used in LV Power Supply
• Core type:-
• Easy to repair as separate windings
• Used in high voltage supply like DP

14. Losses in DC machines
Primary Winding copper loss
Secondary winding copper loss

15. Losses in transformer and types

16. Transformer core Lamination

17. Transformer efficiency
• Efficiency do not have any unit as it’s a ratio
• Its factor(<1) in above formula
• If multiplied by 100, then unit is %

18. Regulation of transformer
• What is constant parameter stands for?
• What is regulation?
• Definition:- Change in terminal voltage from no load to full load
expressed as percentage fraction of no load voltage is known as
regulation of transformer.
• Note :- Total power = Active power + reactive power

19. Auto transformer
• An autotransformer is a electrical
transformer with only one winding (one
winding transformer)
• Where primary & secondary shares
same common single winding
• Also known as
– Dimmerstat
– Dimmer
– Variac
– VaribaleAC supply
• Applications

20. Types & advantages of Autotransformer
Advantages:-
•Lower losses hence higher
efficiency
•Low cost & size as less
copper used
•Can have variable output
•Better voltage regulation
BUT No isolation

21. It is desired to have 4.13 mwb maximum flux in the core
of a transformer operating at 110v and 50Hz. Determine
Number of turns in the primary

22. If primary voltage is 230V, 50Hz, N1=500 and Bm=0.4144T
then the crossectional area of the core will be

23. A step down transformer operates at 50 Hz when primary
voltage is 230v with 500turns the maximum flux will be

24. A 3300/200V, 100KVA transformer has N2=80. find rated
primary and secondary currents I1 andI2

25. A 3300/200V, 100KVA transformer has N2=80. find rated
primary and secondary currents I1 and I2

26. 3 Phase Induction motor
• 80% of motors
world wide are IM
• 50% power consumption
• No starter required
• Cheapest, efficient, reliable
• Robust motor and no maintenance
• Designed by Nikola Tesla (inventor of AC)
• Constant speed motor and good speed regulation
• High starting torque and overload capacity
• Synchronous motor

28. Working principle

29. Construction of 3 φ IM

30. • Stator Parts
1. Stator Frame
2. Stator Core
3. Stator Windings:- The 3-phase winding are
arranged in such a manner that they produce
one Rotating Magnetic Field(RMF)
– Delta or star connections

31. Rotor design:- Type 1. Squirrel cage
Two types
1. Squirrel cage
2. Phase wound
Rotor Parts:-
1. Laminated Core
2. Conductor/windings
1. Parallel
2. skewed
3. End rings
Very
Common
type of
motor. 95%
IM are of
this type

32. 2. Sleep ring/ phase wound/ wound rotor
Uses of external
resistors :-
1. To increase
starting torque
2. To decrease start
current
3. To control speed
This motor has limited
application. Only 5%
IM in the world are of
this type

34. Working of 3 Phase IM
1. Overlapping windings of stator creates rotating magnetic field at
synchronous speed Ns(speed at which rotating magnetic field rotates)
2. Which in turn induces emf in rotor bars as bars are initially stationary
[Faraday’s law] (that’s why the name Induction motor)
3. Due to end rings a closed path found to the induced current, which
produced magnetic flux.
4. Hence as per Fleming's rule rotor starts rotating at
speed Nr.
1. As Nr tends to Ns, Nr < Ns and Ns α f(freq of I/P)

35. Concept of Synchronous speed and slip
• Speed increases as Potential Difference is
higher and reduced with Potential Difference
• Induction motor can not run at Ns as at Ns
relative moment becomes zero and hence no
emf will be induced in rotor.
• Hence rotor decelerate, as rotor speed
reduces relative moment reestablished and
emf induced again hence speed again increase.
Hence IM can not run at synchronous speed.
• Slip at rest=1
• 1 to 5% max
𝒔𝒍𝒊𝒑 𝒔 =
𝑵𝒔−𝑵𝒓
𝑵𝒔
𝒔𝒍𝒊𝒑 𝒔% =
𝑵𝒔−𝑵𝒓
𝑵𝒔
∗𝟏𝟎𝟎%

36. Torque-slip characteristics
• Three region
– Low slip(S~=0)
• T α S
– Medium slip
• T α 1/S
– High slip(s=1)
• T = 0

37. Torque-slip characteristics

38. Power Stages of IM
P2:Pc:Pm
1:s: (1-s)
(iron & copper losses)

39. Efficiency of Induction motor
• Exactly same as that
of transformer
• Maximum when
losses are at its
minimum

40. Applications of IM
• Squirrel cage IM has
– Constant speed and lower initial torque
• Hence applications are same as that of DC shunt motor
• Slip ring IM has
– Variable and controllable speed and
– Higher initial torque
• Hence Applications are same as that of DC series motor

41. Single phase induction motor
• Advantages:- simple in construction, cheap in cost, reliable and
easy to repair and maintain
• Applications:- vacuum cleaners, fans, washing machines,
centrifugal pumps, blowers, washing machines.
• Note:-
– Needs Auxiliary winding
– Starter is most

42. Numerical treatment
• Calculate the percentage slip for an 6 pole 50 Hz, 3 phase
induction motor of rotates at 960 RPM.
• The power input to a 3 phase induction motor is 60kW. The net
stator losses are 1kW. Find the total mechanical power developed
and rotor copper loss per phase if motor is running with a slip of
3%.

43. Prof.N.I.Bhopale ,Department of E&Tc Engineering 46
Thank You!