1. EMT 462 ELECTRICAL SYSTEM TECHNOLOGY
Chapter 2: AC Machines
By:
En. Muhammad Mahyiddin Ramli
MMR

2. EMT 462 ELECTRICAL SYSTEM TECHNOLOGY
AC Machines: Introduction
2 major classes:
a) Asynchronous machines / induction machines :–
Motors or generators whose field current is supplied by magnetic induction (transformer action) into their field windings.
b) Synchronous machines :–
Motors or generators whose field current is supplied by a separate dc power source.
Note: 1) Induction motor has the same physical stator as a synchronous machine, with a different rotor construction.
2) The fields circuit of most synchronous and induction machines are located on their rotors.
Motors = ac electrical energy  mechanical energy
Generators = mechanical energy  ac electrical energy
Chap 2: AC Machines
MMR

3. AC Machinery Fundamentals
A SIMPLE LOOP IN A UNIFORM MAGNETIC FIELDS.
A rotating loop of wire within the magnetic field.
Magnetic field produced by a large stationary magnet produce-constant and uniform magnetic field, B.
Rotation of the loop induced a voltage in the wire.
Current flows in the loop, a torque will be induced on the wire loop.
eind V ө
eind = Фmax ω sin ωt
Thus, the voltage generated in loop is a sinusoid whose magnitude is equal to the product of the flux and rotation speed of the machine
Chap 2: AC Machines

4. AC Machinery Fundamentals
THE ROTATING MAGNETIC FIELD
When two magnetic fields are present in a machine, a torque will be created which will tend to line up the two magnetic fields.
Magnetic field is produced by the stator and rotor of an ac machine.
Then a torque will be induced in the rotor cause the rotor to turn and align itself with the stator magnetic field.
The induced torque in the rotor would cause the rotor to constantly “ chase “ the stator magnetic field around in circle the basic principle of all ac motor operation.
But how the stator rotate?
Chap 2: AC Machines

5. AC Machinery Fundamentals
AC MACHINE POWER LOSSES
The efficiency of an AC machines is defined as:
Four types of losses in AC machines:
Electrical or copper losses (I2R losses)
Core losses
Mechanical losses
Stray load losses
Chap 2: AC Machines

6. AC Machinery Fundamentals
EMT 462 ELECTRICAL SYSTEM TECHNOLOGY
AC Machinery Fundamentals
VOLTAGE REGULATION AND SPEED REGULATION
VR is a measure of the ability of a generator to keep a constant voltage at its terminals as load varies. It is defined as follow:
SR is a measure of the ability of a motor to keep a constant shaft speed as load varies.
Chap 2: AC Machines
MMR

7. INDUCTION
MOTOR
Chap 2: AC Machines

8. Induction Motors
Induction motors are the motor frequently encountered in industry.
It simple, rugged, low-priced and easy to maintain.
It run essentially constant speed from zero to full-load.
The speed is frequency-dependent and consequently these motors are not easily adapted to speed control
Induction machines is called induction because the rotor voltage (which produces the rotor current and the rotor magnetic field) is induced in the rotor winding rather than physically connected by wires.
Chap 2: AC Machines

9. INDUCTION MOTOR CONSTRUCTION
A 3-phase induction motor has two main parts :
A stationary stator (stationary part of the machine)
Revolving rotor (rotating part of the machine)
The rotor is separated from the stator by a small air gap (the tolerances is depending on the power of the motor).
Chap 2: AC Machines

10. Two types of rotor which can placed inside the stator.
Squirrel-cage induction motor (also called cage motors)
Wound-rotor induction motor
a) Cage rotor
b) Wound rotor induction motor
Wound rotor induction motors more expensive- maintenance
Chap 2: AC Machines

11. Two types of rotor which can placed inside the stator.
Squirrel-cage induction motor (also called cage motors)
Wound-rotor induction motor
a) Squirrel cage – the conductors would look like one of the exercise wheels that squirrel or hamsters run on.
Chap 2: AC Machines

12. Two types of rotor which can placed inside the stator.
Squirrel-cage induction motor (also called cage motors)
Wound-rotor induction motor
b) Wound rotor – have a brushes and slip ring at the end of rotor. (Y-connected)
Chap 2: AC Machines

13. INDUCTION MOTOR CONSTRUCTION
Cage induction Motor rotor consists of a series of conducting bars laid into slot carved in the face of rotor and shorted at either end by large shorting ring
A wound rotor has a complete set of three-phase winding that are mirror images of the winding on the stator.
The three phases of the rotor windings are usually Y-connected, the end of the three rotor wires are tied to slip ring on the rotor shaft.
Rotor windings are shorted through brushes riding on the slip rings.
Wound-rotor induction motors are more expensive than the cage induction motors, they required much more maintenance because the wear associated with their brushes and slip rings.
Chap 2: AC Machines

14. INDUCTION MOTOR CONSTRUCTION
Small cage rotor induction motor
Large cage rotor induction motor
Chap 2: AC Machines

15. INDUCTION MOTOR CONCEPT
INDUCED TORQUE IN AN INDUCTION MOTOR
The speed of the magnetic field’s rotation in a cage rotor induction motor (Figure 7.6, Chapman) is given by:
Where nsync = synchronous speed [r/min] fe = System frequency [Hz] p = number of poles
This equation shows that the synchronous speed increases with frequency and decrease with the number of poles.
The three-phase of voltages has been applied to the stator, and three-phase set of stator current is flowing . These currents produce a magnetic field BS , rotating counterclockwise direction.
Chap 2: AC Machines

16. Basic Induction Motor Concepts
This rotating magnetic field BS passes over the rotor bars and induces a voltage, eind in them:
where, v = velocity of the bar relative to the magnetic field B = magnetic flux density vector l = length of conductor in the magnetic field
It is the relative motion of the rotor compared to the stator magnetic field that produces induced voltage in a rotor bar. The rotor current flow produces a rotor magnetic field, BR.
The induce torque in the machine is given by:
The voltage induced in a rotor bar depends on the speed of the rotor relative to the magnetic fields
Chap 2: AC Machines

17. THE CONCEPT OF ROTOR SLIP
Basic Induction Motor Concepts
THE CONCEPT OF ROTOR SLIP
Slip speed is defined as the differences between synchronous speed and rotor speed:
Where nslip = slip speed of the machines
nsync = speed of the magnetic field
nm = mechanical shaft speed of motor
The other term used to describe the relative motion is slip, which is relative speed expressed on a per unit or a percentage basis. The slip is defined as :
Chap 2: AC Machines

18. Basic Induction Motor Concepts
The previous equation also can be expressed in term of angular velocity  (radians per second) as :
If the rotor turns at synchronous speed, s=0 ; if the rotor is stationary (locked or stop) , s=1. All normal motor speeds fall somewhere between those limits.
As for mechanical speed
These equation are useful in the derivation of induction motor torque and power relationship.
Chap 2: AC Machines

19. Basic Induction Motor Concepts
THE ELECTRICAL FREQUENCY ON THE ROTOR.
The induction motor works by inducing voltages and current in the rotor of the machine-called a rotating transformer.
Like a transformer; primary (stator) induced a voltage in the secondary (rotor)
Unlike a transformer, the secondary frequency not necessarily the same as primary.
If the rotor of a motor is locked so that it cannot move, the rotor will have the same frequency as the stator.
If the rotor turns at synchronous speed, the frequency on the rotor will be zero.
For nm=0 r/min & the rotor frequency fr=fe  slip, s = 1
nm=nsync & the rotor frequency fr=0  slip, s = 0
- For any speed in between, the rotor frequency is directly proportional to the difference between the speed of the magnetic field nsync and the speed of the rotor nm.
Chap 2: AC Machines

20. THE ELECTRICAL FREQUENCY ON THE ROTOR
Basic Induction Motor Concepts
THE ELECTRICAL FREQUENCY ON THE ROTOR
Since the slip of the rotor is defined as :
Then the rotor frequency can be expressed as :
Substituting between these two equation become :
But nsync = 120fe/P, so
Therefore,
fr = frequency rotor; fe = frequency stator
Chap 2: AC Machines

21. Example 2.1: Induction Motor
A 208-V, 10-hp, four-pole, 60-Hz, Y- connected induction motor has a full-load slip of 5%.
a) What is the synchronous speed of this motor?
b) What is the rotor speed of this motor at the rated load?
c) What is the rotor frequency of this motor at the rated load?
d) What is the shaft torque of this motor at the rated load?
- Taken from Chapman’s Book, Example 7-1, pg. 387.
Chap 2: AC Machines

22. The Equivalent Circuit of An Induction Motor
Transformer model
- model of the transformer action-induction of voltages and currents in the rotor circuit of an IM is essentially a transformer operation.
- as in transformer model – certain resistance, self inductance in primary (stator) windings; magnetization curve and etc.
b)Rotor circuit model
The greater the relative motion between the rotor and the stator magnetic fields, the greater the resulting rotor voltage and frequency.
Locked-rotor or blocked-rotor –the largest relative motion when the rotor is stationary.
c) Final Equivalent circuit
- Refer the rotor part of the model over the stator side.
Chap 2: AC Machines

23. The Equivalent Circuit of An Induction Motor
A) TRANSFORMER MODEL
STATOR
IDEAL TRANSFORMER
ROTOR
Symbol
Description
aeff
Effective turn ratio – ratio of the conductors per phase on the stator to the conductors per phase on the rotor R1
Stator Resistance X1
Stator Leakage Reactance Rc
Magnetizing reactance Xm
Resistance losses (correspond to iron losses, windage and friction losses) E1
Primary internal stator voltage ER
Secondary internal rotor voltage RR
Rotor Resistance XR
Rotor Reactance
Chap 2: AC Machines

24. The Equivalent Circuit of An Induction Motor
A) TRANSFORMER MODEL
Induction motor operates on the induction of voltage and current in its rotor circuit from the stator circuit (transformer action).
An induction motor is called a singly excited machine, since power is supply to only the stator circuit.
The flux in the machine is related to the integral of the applied voltage E1.
The curve of magnetomotive force versus flux (magnetization curve) for this machine is compared to a similar curve for a power transformer.
Chap 2: AC Machines

25. B) THE ROTOR CIRCUIT MODEL
The Equivalent Circuit of An Induction Motor
B) THE ROTOR CIRCUIT MODEL
Suppose the motor run at a slip s, meaning that the rotor speed is ns (1-s), where ns is the synchronous speed, then this modify the values of VOLTAGE and CURRENT on the primary and secondary side.
The frequency of the induced voltage at any slip will be given
fr = sfe
Assuming ER0 is the magnitude of the induced rotor voltage at LOCKED ROTOR condition the actual voltage induced because of slip (s) is,
ER = sER0
The resistor is not frequency sensitive, the value of RR remain the same.
The rotor inductance is frequency sensitive (X=L=2fL) then
XR = sXR0
Figure 6 shows the equivalent circuit when motor is running at a slip (s).
Chap 2: AC Machines

26. The Equivalent Circuit of An Induction Motor
B) THE ROTOR CIRCUIT MODEL
Equivalent circuit of a wound-rotor when it at locked or blocked condition
The frequency of the voltages and currents in the stator is f, but the frequency of the voltages and currents in the rotor is sf.
Chap 2: AC Machines

27. The Equivalent Circuit of An Induction Motor
B) THE ROTOR CIRCUIT MODEL
ER = sER0
jXR=jsXR0 RR
Then, resulting rotor equivalent circuit as below.
The rotor current flow can be found as :
ZReq
The rotor circuit model of an induction motor
Chap 2: AC Machines

28. The Equivalent Circuit of An Induction Motor
B) THE ROTOR CIRCUIT MODEL
Then the rotor equivalent circuit become:
The rotor circuit model with all the frequency (slip) effects concentrated in resistor RR
ER0
jsXR0
ZReq
Chap 2: AC Machines

29. The Equivalent Circuit of An Induction Motor
C) THE FINAL EQUIVALENT CIRCUIT
Remember, in transformer, the voltages, currents and impedances on the secondary side of the device can be referred to PRIMARY side by turn ratio of the transformer :
The same transformation can be used for the induction motor’s rotor circuit by using effective turn ratio aeff :
Chap 2: AC Machines

30. The per-phase equivalent circuit of an induction motor.
The Equivalent Circuit of An Induction Motor
C) THE FINAL EQUIVALENT CIRCUIT
The rotor circuit model that will be referred to the stator side as shown below
The per-phase equivalent circuit of an induction motor.
Chap 2: AC Machines

31. Power and Torque in Induction Motors
Electrical to mechanical power conversion
Input is 3 phase system of voltages and currents.
Output is mechanical.
PSCL – Losses In Stator Windings / I2R
Pcore – Hysteresis & Eddy Current
PRCL=I2R
The power flow diagram of an induction motor – shows the relationship between the input electric power and output mechanical power.
Chap 2: AC Machines

32. Example 2.2: Power-in in Induction Motors
An induction motor draws 60A from a 480 V, 60 Hz, 50-hp, three-phase line at a power of 0.85 lagging. The stator copper losses are 2000W and the rotor copper losses are 700W. The rotational losses include 600W of friction and wind-age, 1800W of core and negligible of stray load losses. Calculate the following quantities:
The air-gap power,
The power converted, Pconv
The output power,
The efficiency of the motor
Solution:
(a)The air-gap power,
PAG = Pin – PSCL - PCORE
PIN = √3 VTIL cos θ
= √3 (480V)(60A)(0.85)
= 42.4 kW
PAG = Pin – PSCL – PCORE
= 42.4 kW – 2 kW – 1.8 kW
= 38.6 kW
Chap 2: AC Machines

33. (cont’d) % 100 X P h (b)The power converter, Pconv
(c) The output power.
(d) The efficiency,
PCONV = PAG – PRCL
= 38.6 kW – 700 W = 37.9kW
POUT = PCONV – PF&W – PMISC
= 37.9 kW – 600 W – 0 W = 37.3 kW
37.3 kW kW
x 100% = 88% = %
100 X P in
out h
Chap 2: AC Machines

34. Power and Torque in Induction Motors
The per-phase equivalent circuit of an induction motor
Input current
Where
Chap 2: AC Machines

35. Torque-speed characteristics
(b)
a) A typical induction motor torque-speed characteristic curve.
b) Showing the extended operating ranges (braking region and generator region)
Chap 2: AC Machines

36. Torque-speed characteristic
The induced torque of the motor is zero at synchronous speed.
The torque speed curve is nearly linear between no-load and full-load. In this range, the rotor resistance is much larger than the rotor reactance. So, the rotor current increasing linearly.
There is maximum possible torque that cannot be exceeded (called pullout torque or breakdown torque) is 2-3 times the rated full-load torque.
Starting torque on motor is slightly larger than full-load.
The torque on the motor for a given slip varies as the square of the applied voltage.
If the rotor of the induction motor is driven faster than synchronous speed, then the direction of the induced torque in the machine reverse and become generator.
If motor turning backward, relative to the direction of the magnetic field, the induced torque will stop the machine very rapidly and will try to rotate it in the other direction (called plugging).
Chap 2: AC Machines

37. Speed Control of Induction Motors
By pole changing
By line frequency control
By line voltage control
By changing the rotor resistance
To vary the synchronous speed which is the speed of the stator and rotor magnetic field
To vary the slip of the motor for a given load
Chap 2: AC Machines

38. Induction Motor Ratings
EMT 462 ELECTRICAL SYSTEM TECHNOLOGY
Induction Motor Ratings
The most important ratings:
Output power
Voltage
Current
Power factor
Speed
Nominal frequency
*Refer Chapman pg. 465
*Note: 1 h.p = 746 Watts
Chap 2: AC Machines
MMR

39. SYNCHRONOUS
MACHINES
Chap 2: AC Machines

40. INTRODUCTION (Review)
Transformer – energy transfer device (transfer energy from primary to secondary) form of energy remain unchanged. (Electrical)
(DC/AC) Machines – electrical energy is converted to mechanical or vice versa.
Motor operation
The field induced voltage, E permits the motor to draw power from the line to be converted into mechanical power. This time, the mechanical output torque is also developing. The induced voltage is in opposition to the current flow-called counter emf.
Chap 2: AC Machines

41. INTRODUCTION (Review)
Generator operation The field induced voltage, E is in the same direction as the current and is called the “generated voltage”. The machine torque opposes the input mechanical torque that is trying to drive the generator, and it is called the counter torque.
Generally, the magnetic field in a machine forms the energy link between the electrical and mechanical systems.
The magnetic field performs two functions:
Magnetic attraction and repulsion produces mechanical torque (motor operation)
The magnetic field by Faraday’s Law induces voltages in the coils of wire. (generator operation)
Chap 2: AC Machines

42. SYNCHRONOUS MACHINES CONSTRUCTION
Have an outside stationary part, (stator) The inner rotating part (rotor)
The rotor is centered within the stator.
Air gap - the space between the outside of
the rotor and the inside of the stator
Origin of name: syn = equal, chronos = time
Synchronous machines are called ‘synchronous’ because their mechanical shaft speed is directly related to the power system’s line frequency.
Chap 2: AC Machines

43. SYNCHRONOUS MACHINES CONSTRUCTION
STATOR
The stator of a synchronous machine carries the armature or load winding which is a three-phase winding.
The armature winding is formed by interconnecting various conductors in slots spread over the periphery of the machine’s stator.
When current flows in the winding,
each group produces a magnetic pole having a polarity dependent on the current direction, and a magnetomotive
force (mmf) proportional to the current magnitude.
Chap 2: AC Machines

44. SYNCHRONOUS MACHINES CONSTRUCTION
ROTOR
2 types of rotors
- cylindrical (or round) rotors
- salient pole rotors.
Salient pole rotor less expensive than round rotors
and rotate at lower speeds
The rotor carries the field winding. The field current or the excitation current is provided by an external dc source.
Synchronous machine rotors are simply rotating electromagnets built to have as many poles as are produced by the stator windings.
Dc currents flowing in the field coils surrounding each pole magnetize the rotor poles. The magnetic field produced by the rotor poles locks in with a rotating stator field, so that the shaft and the stator field rotate in synchronism.
Chap 2: AC Machines

45. SYNCHRONOUS MACHINES 1) Generator
The rate of rotation of the magnetic fields in the machine is related to the stator electrical frequency, given as:
The internal generated voltage of a synchronous generator is given as,
This equation shows the magnitude of the voltage induced in a given stator phase.
Chap 2: AC Machines

46. Equivalent Circuit of a synchronous Generator
The per phase equivalent circuit
Chap 2: AC Machines

47. Synchronous Generator – voltage regulation
If the generator operates at a terminal voltage VT while supplying a load corresponding to an armature current Ia, then;
In an actual synchronous machine, the reactance is much greater than the armature resistance, in which case;
Among the steady-state characteristics of a synchronous generator, its voltage regulation and power-angle characteristics are the most important ones. As for transformers, the voltage regulation of a synchronous generator is defined at a given load as;
Chap 2: AC Machines

48. Phasor diagram of a synchronous generator
The phasor diagram is showing the relationship among the voltages within a phase (Eφ,Vφ, jXSIA and RAIA) and the current IA in the phase.
Unity P.F (1.0)
Chap 2: AC Machines

49. Phasor diagram of a synchronous generator
Lagging P.F
Leading P.F.
Chap 2: AC Machines

50. Power and Torque in Synchronous Generator
In generators, not all the mechanical power going into a synchronous generator becomes electric power out of the machine
The power losses in generator are represented by difference between output power and input power shown in power flow diagram below.
Pconv
Chap 2: AC Machines

51. Losses in Synchronous Generator
Rotor
- resistance; iron parts moving in a magnetic field causing
currents to be generated in the rotor body
- resistance of connections to the rotor (slip rings)
Stator
- resistance; magnetic losses (e.g., hysteresis)
Mechanical
- friction at bearings, friction at slip rings
Stray load losses
- due to non-uniform current distribution
Chap 2: AC Machines

52. Synchronous Generator
The input mechanical power is the shaft power in the generator given by equation:
The power converted from mechanical to electrical form internally is given by:
The real electric output power of the synchronous generator can be expressed
in line and phase quantities as:
and reactive output power:
Chap 2: AC Machines

53. Synchronous Generator
In real synchronous machines of any size, the armature resistance RA is more than 10 times smaller than the synchronous reactance XS (Xs >> RA). Therefore, RA can be ignored
Simplified phasor diagram with armature resistance ignored.
Chap 2: AC Machines

54. SYNCHRONOUS MACHINES 2) MOTOR
Equivalent circuit of synchronous motor:
Equivalent circuit
Chap 2: AC Machines

55. Power and Torque in Synchronous Motor
The power-flow diagram of a synchronous machines
Chap 2: AC Machines

56. Example 3.3 : Synchronous Generator.
A three-phase, wye-connected 2500 kVA and 6.6 kV generator operates at full-load. The per-phase armature resistance Ra and the synchronous reactance, Xd, are (0.07+j10.4). Calculate the percent voltage regulation at:
(a) 0.8 power-factor lagging, and
(b) 0.8 power-factor leading.
Chap 2: AC Machines

57. Solution:
Chap 2: AC Machines

58. Success is the sum of small efforts, repeated day in and day out…
- Robert Collier
Chap 2: AC Machines