EMT 462 ELECTRICAL SYSTEM TECHNOLOGY Chapter 2: AC Machines By:
En. Muhammad Mahyiddin Ramli Slide 2 Chap 2: AC Machines 2 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 Slide 3 Chap 2: AC
Machines 3 AC Machinery Fundamentals 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. A SIMPLE LOOP IN A
UNIFORM MAGNETIC FIELDS. e ind V e ind = max sin t 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 Slide 4
Chap 2: AC Machines 4 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. THE ROTATING MAGNETIC
FIELD AC Machinery Fundamentals But how the stator rotate? Slide 5
Chap 2: AC Machines 5 The efficiency of an AC machines is defined
as: Four types of losses in AC machines: Electrical or copper
losses (I 2 R losses) Core losses Mechanical losses Stray load
losses AC MACHINE POWER LOSSES AC Machinery Fundamentals Slide 6
Chap 2: AC Machines 6 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. AC Machinery Fundamentals Slide 7 Chap 2: AC Machines
7 INDUCTIONMOTOR Slide 8 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. Slide 9 Chap 2: AC Machines 9 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). INDUCTION MOTOR CONSTRUCTION
Slide 10 Chap 2: AC Machines 10 a) Cage rotor b) Wound rotor
induction motor Two types of rotor which can placed inside the
stator. a)Squirrel-cage induction motor (also called cage motors)
b)Wound-rotor induction motor Wound rotor induction motors more
expensive- maintenance Slide 11 Chap 2: AC Machines 11 Two types of
rotor which can placed inside the stator. a)Squirrel-cage induction
motor (also called cage motors) b)Wound-rotor induction motor a)
Squirrel cage the conductors would look like one of the exercise
wheels that squirrel or hamsters run on. Slide 12 Chap 2: AC
Machines 12 Two types of rotor which can placed inside the stator.
a)Squirrel-cage induction motor (also called cage motors)
b)Wound-rotor induction motor b) Wound rotor have a brushes and
slip ring at the end of rotor. (Y-connected) Slide 13 Chap 2: AC
Machines 13 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. INDUCTION MOTOR CONSTRUCTION Slide 14 Chap 2: AC
Machines 14 Small cage rotor induction motor Large cage rotor
induction motor INDUCTION MOTOR CONSTRUCTION Slide 15 Chap 2: AC
Machines 15 The speed of the magnetic fields rotation in a cage
rotor induction motor (Figure 7.6, Chapman) is given by: Where n
sync = synchronous speed [r/min] f e = 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 B S, rotating counterclockwise direction.
INDUCED TORQUE IN AN INDUCTION MOTOR INDUCTION MOTOR CONCEPT Slide
16 Chap 2: AC Machines 16 Basic Induction Motor Concepts This
rotating magnetic field B S passes over the rotor bars and induces
a voltage, e ind 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, B R. 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 Slide 17 Chap 2: AC Machines
17 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 : THE CONCEPT OF ROTOR SLIP Slip
speed is defined as the differences between synchronous speed and
rotor speed : Where n slip = slip speed of the machines n sync =
speed of the magnetic field n m = mechanical shaft speed of motor
Basic Induction Motor Concepts Slide 18 Chap 2: AC Machines 18 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. Basic Induction Motor Concepts
Slide 19 Chap 2: AC Machines 19 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 n m
=0 r/min & the rotor frequency f r =f e slip, s = 1 n m =n sync
& the rotor frequency f r =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 n sync and the
speed of the rotor n m. Basic Induction Motor Concepts THE
ELECTRICAL FREQUENCY ON THE ROTOR THE ELECTRICAL FREQUENCY ON THE
ROTOR. Slide 20 Chap 2: AC Machines 20 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 n sync = 120f e /P, so Therefore, f r =
frequency rotor; f e = frequency stator Basic Induction Motor
Concepts Slide 21 Chap 2: AC Machines 21 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? Example 2.1: Induction
Motor - Taken from Chapmans Book, Example 7-1, pg. 387. Slide 22
Chap 2: AC Machines 22 a)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. The
Equivalent Circuit of An Induction Motor Slide 23 Chap 2: AC
Machines 23 The Equivalent Circuit of An Induction Motor
SymbolDescription a eff Effective turn ratio ratio of the
conductors per phase on the stator to the conductors per phase on
the rotor R1R1 Stator Resistance X1X1 Stator Leakage Reactance RcRc
Magnetizing reactance ROTOR IDEAL TRANSFORMER XmXm Resistance
losses (correspond to iron losses, windage and friction losses)
E1E1 Primary internal stator voltage ERER Secondary internal rotor
voltage R Rotor Resistance XRXR Rotor Reactance A) TRANSFORMER
MODEL STATOR Slide 24 Chap 2: AC Machines 24 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. A) TRANSFORMER
MODEL The Equivalent Circuit of An Induction Motor Slide 25 Chap 2:
AC Machines 25 B) THE ROTOR CIRCUIT MODEL Suppose the motor run at
a slip s, meaning that the rotor speed is n s (1-s), where n s 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 f r = sf e Assuming E R0
is the magnitude of the induced rotor voltage at LOCKED ROTOR
condition the actual voltage induced because of slip (s) is, E R =
sE R0 The resistor is not frequency sensitive, the value of R R
remain the same. The rotor inductance is frequency sensitive (X=
L=2 fL) then X R = sX R0 Figure 6 shows the equivalent circuit when
motor is running at a slip (s). The Equivalent Circuit of An
Induction Motor Slide 26 Chap 2: AC Machines 26 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. The
Equivalent Circuit of An Induction Motor B) THE ROTOR CIRCUIT MODEL
Slide 27 Chap 2: AC Machines 27 Then, resulting rotor equivalent
circuit as below. The rotor current flow can be found as : Z Req E
R = sE R0 jX R =jsX R0 R The rotor circuit model of an induction
motor The Equivalent Circuit of An Induction Motor B) THE ROTOR
CIRCUIT MODEL Slide 28 Chap 2: AC Machines 28 Then the rotor
equivalent circuit become: Z Req The Equivalent Circuit of An
Induction Motor B) THE ROTOR CIRCUIT MODEL The rotor circuit model
with all the frequency (slip) effects concentrated in resistor R R
E R0 jsX R0 Slide 29 Chap 2: AC Machines 29 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 motors rotor circuit by using effective turn ratio a eff
: The Equivalent Circuit of An Induction Motor C) THE FINAL
EQUIVALENT CIRCUIT Slide 30 Chap 2: AC Machines 30 The rotor
circuit model that will be referred to the stator side as shown
below The per-phase equivalent circuit of an induction motor. The
Equivalent Circuit of An Induction Motor C) THE FINAL EQUIVALENT
CIRCUIT Slide 31 Chap 2: AC Machines 31 Power and Torque in
Induction Motors Output is mechanical. Input is 3 phase system of
voltages and currents. P RCL =I 2 R Electrical to mechanical power
conversion The power flow diagram of an induction motor shows the
relationship between the input electric power and output mechanical
power. P SCL Losses In Stator Windings / I 2 R P core Hysteresis
& Eddy Current Slide 32 Chap 2: AC Machines 32 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: (a) The air-gap power,
(b) The power converted, P conv (c) The output power, (d) The
efficiency of the motor Solution: (a)The air-gap power, Example
2.2: Power-in in Induction Motors P AG = P in P SCL - P CORE P IN =
3 V T I L cos = 3 (480V)(60A)(0.85) = 42.4 kW P AG = P in P SCL P
CORE = 42.4 kW 2 kW 1.8 kW = 38.6 kW Slide 33 Chap 2: AC Machines
33 (b)The power converter, P conv (c) The output power. (d) The
efficiency, (contd) P CONV = P AG P RCL = 38.6 kW 700 W = 37.9kW P
OUT = P CONV P F&W P MISC = 37.9 kW 600 W 0 W = 37.3 kW 37.3 kW
42.4 kW x 100% = 88% = %100X P P in out Slide 34 Chap 2: AC
Machines 34 Power and Torque in Induction Motors The per-phase
equivalent circuit of an induction motor Input currentWhere Slide
35 Chap 2: AC Machines 35 Torque-speed characteristics a) A typical
induction motor torque-speed characteristic curve. b) Showing the
extended operating ranges (braking region and generator region) (a)
(b) Slide 36 Chap 2: AC Machines 36 Torque-speed characteristic 1.
The induced torque of the motor is zero at synchronous speed. 2.
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. 3.
There is maximum possible torque that cannot be exceeded (called
pullout torque or breakdown torque ) is 2-3 times the rated
full-load torque. 4. Starting torque on motor is slightly larger
than full-load. 5. The torque on the motor for a given slip varies
as the square of the applied voltage. 6. 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. 7. 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 ). Slide 37 Chap 2: AC Machines 37 Speed Control
of Induction Motors 1. By pole changing 2. By line frequency
control 3. By line voltage control 4. 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 Slide 38 Chap 2: AC Machines 38 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 Slide 39 Chap 2: AC Machines 39 SYNCHRONOUSMACHINES
Slide 40 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.
Slide 41 Chap 2: AC Machines 41 INTRODUCTION (Review) 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 Faradays Law induces
voltages in the coils of wire. (generator operation) 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. Slide 42 Chap 2: AC
Machines 42 SYNCHRONOUS MACHINES CONSTRUCTION Origin of name: syn =
equal, chronos = time Synchronous machines are called synchronous
because their mechanical shaft speed is directly related to the
power systems line frequency. 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 Slide 43 Chap 2: AC Machines 43
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 machines 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. SYNCHRONOUS MACHINES
CONSTRUCTION Slide 44 Chap 2: AC Machines 44 2 types of rotors -
cylindrical (or round) rotors - salient pole rotors. Salient pole
rotor less expensive than round rotors and rotate at lower speeds
ROTOR 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. SYNCHRONOUS MACHINES
CONSTRUCTION Slide 45 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. Slide 46 Chap 2: AC Machines 46 Equivalent
Circuit of a synchronous Generator The per phase equivalent circuit
Slide 47 Chap 2: AC Machines 47 If the generator operates at a
terminal voltage V T 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; Synchronous Generator voltage regulation Slide 48 Chap 2: AC
Machines 48 The phasor diagram is showing the relationship among
the voltages within a phase (E ,V , jX S I A and R A I A ) and the
current I A in the phase. Unity P.F (1.0) Phasor diagram of a
synchronous generator Slide 49 Chap 2: AC Machines 49 Leading P.F.
Lagging P.F Phasor diagram of a synchronous generator Slide 50 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. P conv
Slide 51 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 Slide 52 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: Slide 53 Chap 2: AC
Machines 53 Synchronous Generator In real synchronous machines of
any size, the armature resistance R A is more than 10 times smaller
than the synchronous reactance X S (X s >> R A ). Therefore,
R A can be ignored Simplified phasor diagram with armature
resistance ignored. Slide 54 Chap 2: AC Machines 54 SYNCHRONOUS
MACHINES 2) MOTOR Equivalent circuit Equivalent circuit of
synchronous motor: Slide 55 Chap 2: AC Machines 55 Power and Torque
in Synchronous Motor The power-flow diagram of a synchronous
machines Slide 56 Chap 2: AC Machines 56 A three-phase,
wye-connected 2500 kVA and 6.6 kV generator operates at full- load.
The per-phase armature resistance R a and the synchronous
reactance, X d, are (0.07+j10.4) . Calculate the percent voltage
regulation at: (a) 0.8 power-factor lagging, and (b) 0.8
power-factor leading. Example 3.3 : Synchronous Generator. Slide 57
Chap 2: AC Machines 57 Solution: Slide 58 Chap 2: AC Machines58
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