Introduction to Electrical Machines

1
Introduction to Electrical Machines
2
Contents

• Basic construction and principles
• DC machines
• Synchronous machines
• Induction machines

3
Objectives

• When you have studied this chapter, you should
• have an understanding of electrical machines
  construction.
• understand the principles of DC machines.
• understand the principles and application of
  synchronous machines.
• understand the principles and application of
  induction machines.

4

• 1. Basic Construction

                                                 

5
Introduction
One of energy can be obtained from the other form
with the help of converters. Converters that are
used to continuously translate electrical input
to mechanical output or vice versa are called
electric machines. The process of translation
is known as electromechanical energy conversion.
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Electrical system
Mechanical system
Electric Machine
e, i
T, n
Motor
Energy flow
Generator

• An electrical machine is link between an
  electrical system and a mechanical system.
• Conversion from mechanical to electrical
  generator
• Conversion from electrical to mechanical motor

7
Electrical Machines
DC machine
AC machine
Induction machine
Synchronous machine

• Machines are called AC machines (generators or
  motors) if the electrical system is AC.
• DC machines (generators or motors) if the
  electrical system is DC.

8
Coupling magnetic fields
Electrical system
Mechanical system
e, i
T, n

• Two electromagnetic phenomena in the electric
  machines
• When a conductor moves in a magnetic field,
  voltage is induced in the conductor.
• When a current-carrying conductor is placed in a
  magnetic field, the conductor experiences a
  mechanical force.

9
Electric Machines Basic Structure

• The structure of an electric machine has two
  major components, stator and rotor, separated by
  the air gap.
• Stator
• Does not move and normally is the outer frame of
  the machine.
• Rotor
• Is free to move and normally is the inner part of
  the machine.

• Both rotor and stator are made of ferromagnetic
  materials.

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DC Machines Construction
11
Induction Machines Construction
12
Synchronous Machines Construction
13
2. DC Machines
14

• 3. Synchronous Machines

15
Classification of AC Rotating Machines

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

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Synchronous Machine

• Unlike induction machines, the rotating air gap
  field and the rotor rotate at the same speed,
  called the synchronous speed.
• Synchronous machines are used primarily as
  generators of electrical power, called
  synchronous generators or alternators.
• They are usually large machines generating
  electrical power at hydro, nuclear, or thermal
  power stations.
• Application as a motor pumps in generating
  stations, electric clocks, timers, and so forth
  where constant speed is desired.

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Synchronous Machines
View of a two-pole round rotor generator and
exciter
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Synchronous Machine

• Round Rotor Machine
• The stator is a ring shaped laminated iron-core
  with slots.
• Three phase windings are placed in the slots.
• Round solid iron rotor with slots.
• A single winding is placed in the slots. Dc
  current is supplied through slip rings.

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Round Rotor Machine
20
Synchronous Machine

• Salient Rotor Machine
• The stator has a laminated iron-core with slots
  and three phase windings placed in the slots.
• The rotor has salient poles excited by dc
  current.
• DC current is supplied to the rotor through
  slip-rings and brushes.

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Salient Rotor Machine
22
Synchronous Generator

• Principle of Operation
• 1) From an external source, the field winding is
  supplied with a DC current -gt excitation.
• 2) Rotor (field) winding is mechanically turned
  (rotated) at synchronous speed.

3) The rotating magnetic field produced by the
field current induces voltages in the outer
stator (armature) winding. The frequency of these
voltages is in synchronism with the rotor speed.
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Parallel Operation of Synchronous Generator

• Generators are rarely used in isolated
  situations. More commonly, generators are used in
  parallel, often massively in parallel, such as in
  the power grid. The following steps must be
  adhered to
• when adding a generator to an existing power
  grid
• 1) RMS line voltages of the two generators must
  be the same.
• 2) Phase sequence must be the same.
• 3) Phase angles of the corresponding phases must
  be the same.
• 4) Frequency must be the same.

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• 4. Induction Machines

25
Induction Machine

• The induction machine is the most rugged and the
  most widely used machine in industry.
• Both stator and rotor winding carry alternating
  currents.
• The alternating current (ac) is supplied to the
  stator winding directly and to the rotor winding
  by induction hence the name induction machine.
• Application (1f) washing machines,
  refrigerators, blenders, juice mixers, stereo
  turntables, etc.
• 2f induction motors are used primarily as
  servomotors in a control system.
• Application 3f pumps, fans, compressors, paper
  mills, textile mills, etc.

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Induction Motors

• The single-phase induction motor is the most
  frequently used motor in the world
• Most appliances, such as washing machines and
  refrigerators, use a single-phase induction
  machine
• Highly reliable and economical

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Induction Motors

• For industrial applications, the three-phase
  induction motor is used to drive machines
• Large three-phase induction motor. (Courtesy
  Siemens).

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Induction Machine
Construction of Induction Motor A
typical motor consists of two parts 1-An
outside stationary stator having coils supplied
with AC current to produce a rotating magnetic
field, 2-An inside rotor attached to the output
shaft that is given a torque by the rotating
field.

• General
• The induction machine is used as the most common
  motors in different applications.
• It has a stator and a rotor like other type of
  motors.
• 2 different type of rotors
• 1-squirrel-cage winding,
• 2-Wound-rotor
• Both three-phase and single-phase motors are
  widely used.
• Majority of the motors used by industry are
  squirrel-cage induction motors

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Induction Motor

• Basic principles
• An AC current is applied in the stator armature
  which generates a flux in the stator magnetic
  circuit.
• This flux induces an emf in the conducting bars
  of rotor as they are cut by the flux while the
  magnet is being moved (E BVL (Faradays Law))
• A current flows in the rotor circuit due to the
  induced emf, which in term produces a force, (F
  BIL ) can be changed to the torque as the output.

Induction motor components.
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Induction Motor
Single-phase stator with windings.

• Stator construction
• The stator of an induction motor is laminated
  iron core with slots similar to a stator of a
  synchronous machine
• Coils are placed in the slots to form a three or
  single phase winding.

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Induction Motors Magnetic Circuit
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Squirrel-cage Rotor

• Rotor is from laminated iron core with slots.
• Metal (Aluminum) bars are molded in the slots
  instead of a winding.
• Two rings short circuits the bars.Most of single
  phase induction motors have Squirrel-Cage rotor.
• One or 2 fans are attached to the shaft in the
  sides of rotor to cool the circuit.

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Induction Motor
Compared to squirrel cage rotors, wound rotor
motors are expensive and require maintenance of
the slip rings and brushes, so it is not so
common in industry applications Wound rotor
induction motor was the standard form for
variable speed control before the advent of motor

• It is usually for large 3 phase induction motors.
• Rotor has a winding the same as stator and the
  end of each phase is connected to a slip ring.
• Three brushes contact the three slip-rings to
  three connected resistances (3-phase Y) for
  reduction of starting current and speed control.

Rotor of a large induction motor. (Courtesy
Siemens).
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Synchronous Speed
120f
ns
p
ns synchronous speed r/min f frequency of
supply hertz/Hz P total of magnetic pole
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Slip and Slip Speed
The slip s of an induction motor is the
difference between the synchronous speed and the
rotor speed, expressed as a Percent (per unit)
of synchronous speed
The per-unit slip is given by the equation
S slip ns synchronous speed r/min nr
rotor speed r/min
ns - nr
S
ns
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Voltage and frequency induced in the rotor
The voltage and frequency induced in the rotor
both depend on the slip. They are given by the
following equation
f2 s f
E2 s Eoc (approx.)
f2 frequency of the voltage and current in
the rotor Hz f frequency of the source
connected to the stator Hz s slip E2
voltage induced in the rotor at the slip s Eoc
open-circuit voltae induced in the rotor when at
rest V
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Active Power in a Induction Motor
Poutput
Efficiency (?)
Pinput
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Motor Torque
9.55 Pm
Tm
n
9.55 (1 s) Pr

ns (1 s)
9.55 Pr / ns
Tm 9.55 Pr / ns
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I2R losses in the rotor
s Pr
Pjr
Pjr rotor I2R losses W
s slip
Pr power transmitted to the rotor W
Mechanical Power
Pr - Pjr
Pm
Pr - s Pr
(1 s) Pr
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Example 1
Calculate the synchronous speed of a 3-phase
induction motor having 20 poles when it is
connected to a 50 Hz source.
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Knowing quantities Source frequency 50 Hz,
number of poles 20

120 f
Synchronous speed ns
p

120 x 50

20
ns
300 r/min
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Example 2
A 0.5 hp, 6-pole induction motor is excited by a
3-phase, 60 Hz source. If the full-load is 1140
r/min, calculate the slip.
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Knowing quantities Source frequency 60 Hz,
number of poles 6 Full load/rotor speed
1140 r/min
120 f
Synchronous speed ns
p
120 x 60

6
1200 r/min
ns
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Slip speed ns n 1200 1140 60 r/min
Slip s (ns - n) / ns
60/1200 0.05 or 5

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Example 3
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Student Assignment 2
A single phase, 4 poles induction motor gives the
following data Output 373 W 230 V Frequency
50 Hz., Input current 2.9 A Power factor 0.71
Speed 1410 r.p.m.
a) Calculate the efficiency of the motor
b) Determine the slip of the motor when
delivering the rated output