Introduction to AC Machines

EEE352 AC Machines
Dr Albert Awopone

learning Objectives
• The course is designed to equip students with the knowledge and
understanding relating to the construction, operation, testing and control of
synchronous machines.
• Students will then be able to undertake performance analysis of
Asynchronous and synchronous machines both in motoring and generating
modes.

Course Content
• Synchronous Generator: Construction, Excitation system, Equivalent circuit,
Phasor diagram, Power & Torque, Measurement of model parameters,
Effect of load changes on a generator, Parallel operation of generators
• Synchronous Motor: Basic principle, Equivalent circuit, Torque speed
characteristics, Power and torque equation, Phasor diagram, characteristics,
Starting and motor ratings
• Three Induction Motor: Slip and its effect on rotor frequency and rotor
voltage, Equivalent circuit, Power and torque; losses, efficiency and power
factor.
• Single Phase Induction Motor

AC Machines
The principle of operation is govern by (i) the law of electro-magnetic
induction and (ii) law of interaction.
(i) Law of electromagnetic induction (Faraday’s 1st law): An emf is induced in
a conductor whenever it cuts across the magnetic field
(ii) (ii) Law of interaction: whenever a current carrying conductor is placed in
the magnetic field, by the interaction of the magnetic fields produced by
the current carrying conductor and the main field, force is exerted on the
conductor and torque is developed

AC Machines
Synchronous Machines Asynchronous Machines
(Induction Machine)
Synchronous
Generator
Synchronous
Motor
Induction
Generator
Induction
Motor
A primary source of
electrical energy
Used as motors as well as
power factor compensators
(synchronous condensers)
Most widely used
electrical motors in both
domestic and industrial
applications
Due to lack of a separate field
excitation, these machines are
rarely used as generators.

Synchronous Machines
The mechanical power or energy is converted into electrical power or energy
with the help of an AC machine called alternator or synchronous generator.
when the same machine can be used to convert electrical power or energy
into mechanical power or energy, then it is known as a synchronous motor.
Thus, the same machine can be operated as a generator or as a motor and in
general, it is called as a synchronous machine.

General Aspects of Synchronous Machines
When the machine is to work as a generator, it has to run at synchronous
speed (Ns) to generate power at certain frequency ( f ), called power
frequency. In Ghana its value is 50 Hz, whereas in the USA it is kept at 60 Hz.
When the machine works as a motor, it can rotate only at synchronous speed
(Ns) since the magnetic poles are locked with the revolving field.
If the machine fails to rotate at synchronous speed, it is palled out of step and
stops.
Hence, synchronous machine (generator or motor) is a machine which only
runs at synchronous speed and maintains the relation;
𝑁𝑆 =
120 𝑓
𝑃 (𝑟𝑝𝑚)

Relation between Frequency, Speed and Number of Poles
The machine is shown having P number of poles on the
rotor revolving at a speed at Ns rpm
When a conductor passes through a pair of poles one
cycle of emf is induced in it.
∴ 𝑁𝑜. 𝑜𝑓 𝑐𝑦𝑐𝑙𝑒𝑠 𝑝𝑒𝑟 𝑟𝑒𝑣 =
𝑃
2
𝑁𝑜. 𝑜𝑓 𝑟𝑒𝑣 𝑚𝑎𝑑𝑒 𝑝𝑒𝑟 𝑠𝑒𝑐 =
𝑁𝑠
60
𝑓 = 𝑛𝑜. 𝑜𝑓 𝑐𝑦𝑐𝑙𝑒𝑠 𝑝𝑒𝑟 𝑟𝑒𝑣 × 𝑛𝑜. 𝑜𝑓 𝑟𝑒𝑣 𝑝𝑒𝑟 𝑠𝑒𝑐
𝑓 =
𝑃
2
×
𝑁𝑠
60
=
𝑃𝑁𝑠
120
𝑐𝑦𝑐𝑙𝑒𝑠 𝑠 𝑜𝑟𝐻𝑧

The two coils of the 4-pole generator of Fig. 5.8 are seats of identical emfs
and can be connected in series or parallel as shown in Figs 5.10(a) and (b).
The series connection gives double the voltage of one coil and can handle the
same maximum current as any one coil.
The parallel connection has the same voltage as that of each coil and has
twice the maximum current-carrying capacity of one of the coils.
The designer exploits the series-parallel arrangement of coil groups to build a
machine of desired voltage and current rating.

Types of Synchronous Machine
According to the arrangement of the field and armature windings,
synchronous machines may be classified as
1. Stationary Armature - Rotating Field (Above 5 kVA)
2. Stationary Field – Rotating Armature (Below 5 kVA

Advantages of Rotating Field System
1. The armature winding is more complex than the field winding. Therefore,
it is easy to place armature winding on stationary structure.
2. In the modern alternators, high voltage is generated, therefore, heavy
insulation is provided and it is easy to insulate the high voltage winding
when it is placed on stationary structure.
3. The size of the armature conductors is much more to carry heavy current,
therefore, high centrifugal stresses are developed. Thus, it is preferred to
place them on stationary structure.
4. The size of slip rings depends upon the magnitude of flow of current,
therefore, it is easy to deliver small current for excitation, through slip
rings of smaller size when rotating field system is used.

Advantages of Rotating Field System
5. It is easier to build and properly balance high speed rotors when they
carry the lighter field system.
6. The weight of rotor is small when field system is provided on rotor and as
such friction losses are produced.
7. Better cooling system can be provided when the armature is kept
stationary.

Constructional Features of Synchronous Machines
The important parts of a synchronous machine are given below:
1. Stator
2. Rotor
3. Miscellaneous

1. Stator:
The outer stationary part of the machine; it has the following important parts:
i. Stator frame: It is the outer body of the machine made of cast iron and it
protects the inner parts of the machine. It can be also made of any other
strong material since it is not to carry the magnetic field.
ii. Stator Core: made of silicon steel material. It is made from number of
stamping which are insulated from each other. Its function is to provide
an easy path for the magnetic lines of force and accommodate the stator
winding.
iii. Stator Winding: Slots are cut on the inner periphery of the stator core in
which three-phase or one-phase winding is placed. Enamelled copper is
used as winding material.

2. Rotor:
The rotating part of the machine is called rotor. From construction point of
view, there are two types of rotors named as
(i) Salient pole type rotor;
(ii) Non-salient pole type rotor.

Salient Pole type {Projected Poles}
Projected Poles. Poles are mounted on the larger
circular frame.
Field Winding are connected in series.
Ends of the field winding are connected to the DC
Supply through Slip Rings
Poles are Large Diameter and short Axial Length.
Laminated to reduced Eddy Current Losses
Employed for Low and Medium Speed 120 to 500RPM
(Diesel & Hydraulic Turbines)
This cannot be used for Large speed

Parts of rotor of salient pole alternator
a. Spider: is made of cast iron to provide an easy path for the
magnetic flux. It is keyed to the shaft and at the outer surface,
pole core and pole-shoe are keyed to it
b. Pole core and pole shoe: It is made of laminated sheet
material. Pole core provides least reluctance path for the
magnetic field and pole shoe distributes the field over the
whole periphery uniformly to produce sinusoidal wave form of
the generated emf.
c. Field winding or Exciting winding: Field winding is wound on
the former and then placed around the pole core. DC supply is
given to it through slip rings. When direct current flows
through the field winding, it produces the required magnetic
field.
• Damper winding: At the outermost periphery, holes are provided
which copper bars are inserted and short-circuited at both the
sides by rings forming damper winding. Damp rotor oscillations
during transient-conditions and to facilitate smooth operation
under unbalanced load conditions.

NON SALIENT POLE TYPE
• Smooth cylindrical rotor or TURBO ALTERNATOR
field winding used in high speed alternators driven
by steam turbines .
• Features
• Smaller diameter and larger axial length compared to
salient pole type machines, of the same rating.
• Less Windage loss.
• Speed 1200 RPM to 3000 RPM. Better Balancing..
Noiseless Operation
Flux distribution nearly sine wave
Frequency 50 Hz
Ns = 120 F / P
Poles 2 4 6
Speed 3000 1500 1000

Parts of rotor of non-salient pole alternator
a. Rotor core: made of silicon steel stampings.
It is keyed to the shaft. At the outer
periphery slots are cut in which exciting
coils are placed. It provides an easy path to
the magnetic flux.
b. Rotor winding or Exciting winding: It is
placed in rotor slots and current is passed
through the winding in such a way that
poles are formed according to the
requirement

3. Miscellaneous Parts: The following are few important miscellaneous
parts;
i. Brushes: are made of carbon and these just slip over the slip rings. DC
supply is given to the brushes. From brushes current flows to the slip rings
and then to the exciting winding.
ii. Bearings: are provided between the shaft and outer stationary body to
reduce the friction. The material used for their construction is high carbon
steel.
iii.Shaft: is made of mild steel. Mechanical power is taken or given to the
machine through shaft.

Some Special Features of Salient and Non-salient Structures
Usually the salient pole field structure has the following special features:
1. These are of larger diameter and shorter length.
2. Usually, 2/3rd of the pole pitch is covered by the pole shoes
3. To reduce eddy current losses, the poles are laminated.
4. The machine having such structure are employed with hydraulic turbines
or with diesel engines which are usually operated at low speeds (100 to
375 rpm)

Some Special Features of Salient and Non-salient Structures
The non-salient field structure has the following special features.
1. They are of smaller diameter and of very long axial length.
2. Robust construction and noiseless operation.
3. Less windage (air-resistance) loss.
4. Better in dynamic balancing.
5. High operating speed (3000 rpm).
6. Nearly sinusoidal flux distribution around the periphery, and therefore,
gives a better emf waveform than that obtainable with salient poles field
structure.
7. There is no need of providing damper windings (except in special cases to
assist in synchronising) because the solid field poles themselves act as
efficient dampers.

Excitation Systems
• Since the field winding is on rotor, a special arrangement is necessary to
connect DC source to the field. In small size synchronous machines,
generally the field winding is excited from a separate DC source through slip
rings and brushes.
• Slip rings are metal rings completely encircling the shaft of the machine, but
insulated from it. A brush rides and slips over each slipping. The positive end
of a DC voltage source is connected to one brush and negative end is
connected to another brush.
• In large machines, various schemes are employed to supply DC excitation to
the field winding. Some of the most important excitation systems are given
below:

Excitation Systems
DC Exciters
conventional method of exciting the field windings of synchronous generators.
In this method, three machines namely pilot exciter, main exciter and the main 3-phase alternator
are mechanically coupled and are therefore, driven by the same shaft.
The pilot exciter is a DC shunt generator feeding the field winding of a main exciter.
The main exciter is a separately-excited DC generator which provides the necessary current to the
field winding of the main alternator through brushes and slip rings.
suffers from cooling and maintenance problems associated with slip rings, brushes and commutator
with the higher rating alternators

Excitation Systems
The advantages of static excitation
1. Its response time is very small about 20 m sec.
2. It eliminates the exciter windage loss and commutator, bearing and winding maintenance.
3. As the excitation energy is taken from alternator terminals itself, the excitation voltage is direct
proportional to alternator’s speed. This improves the overall system performance.
Static Excitation System
In this method, the excitation power for the main alternator
field is drawn from output terminals of the main 3-phase
alternator itself. For this purpose, a three-phase transformer
T steps down the alternator voltage to the desired value.
This three-phase voltage is fed to a three-phase full wave
bridge rectifier using thyristors. The firing angle of these
thyristors is controlled by means of a regulator which picks
up the signal from alternator terminals through potential
transformer PT and current transformer
The controlled DC power output from thyristor unit is
delivered to the field winding of main alternator through
brushes and slip rings.

Excitation Systems
The main exciter’s field is fed from a shaft driven PMG, having rotating permanent
magnets attached to the shaft and a stationary 3-phase armature.
The AC output of PMG is rectified by three-phase full-wave phase controlled thyristor
bridges.
The thyristor assembly is usually housed in removable drawers, which can be taken out
easily for repair.
The excitation system consists of an alternator
rectifier main exciter and a pilot exciter
(permanent magnet generator PMG).
Both the main exciter and pilot exciter are driven
directly from the main shaft.
The main exciter has a stationary field and a
rotating armature, which is directly connected,
through silicon rectifiers S1 to the main alternator
field. Thus the slip rings and brushes are
eliminated.
Brushless Excitation System

Excitation Systems
The regulator controls excitation by supplying a buck-boost control signal, which
adds algebraically to the base setting.
The regulator elements also comprise of solid state circuits. This excitation
system has a short time constant and a response time of less than 0.1 second.
The thyristor bridges are controlled by a set of
dual firing circuits operating in parallel.
The base excitation is controlled by an input
setting to the thyristor gating circuits.
This control signal is derived from the PMG via
a regulated DC supply, which also serves the
regulator logic circuitry.
Brushless Excitation System

Thanks
• Next lecture: Armature winding

Reference
Main Textbook
1. Sahdev, S. K. (2017). Electrical machines. Cambridge University Press.
References
2. Theraja, B. L., Theraja, A. K., Patel, U., Uppal, S., Panchal, J., Oza, B., ... & Patel, R.
(2005). A textbook of electrical technology vol ii. S. Chand publishers,.
3. Gonen, T. (2011). Electrical Machines with MATLAB®. CRC press.

Introduction to AC Machines

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