1. Chapter 13 Three-Phase Induction Motors
Lecture 19
Electro Mechanical System

2. Electro Mechanical System
Introduction
Induction motors are probably the most common and frequently encountered machine to be found anywhere in the world, involving domestic, commercial and industrial sector.
Simple design, rugged, low-price, easy maintenance
Run essentially at constant speed from 0 to full load
Speed is frequency dependent.
Not easy to have variable speed control.
Requires a variable-frequency electronic drive for optimal speed control.
Two basic design types
Squirrel-cage
Wound-rotor
Lecture 19
Electro Mechanical System

3. Principal Machine Components
An induction motor has two main parts:
A stationary stator
Steel frame that supports a hollow, cylindrical core
Core; stacked laminations, having a number of evenly spaced slots, providing the space for the stator winding.
A revolving rotor
One of two types of rotor windings
Conventional 3-phase insulated wire (wound-rotor).
similar to the winding on the stator
Aluminum bus bars shorted together at the ends by two aluminum rings, forming a squirrel-cage shaped circuit.
Lecture 19
Electro Mechanical System

4. Electro Mechanical System
Operating Principles
The induction motor operation is based on Faraday’s law and the Lorentz force on a conductor
Consider a series of conductors of length l, whose ends are short circuited by two bars, A and B.
A permanent magnet is place just above this conducting ladder structure.
Magnet moves rapidly to the right at a speed v such that the magnetic flux cuts across the conductors.
Lecture 19
Electro Mechanical System

5. Electro Mechanical System
Operating Principles
As a result following series of events occur:
Voltage E = B l v is induced in each conductor as flux cuts
The induced voltage produces a current I, which flows down the conductor underneath the magnetic pole, through the end-bars and back through the other conductors.
Current-carrying conductors that lie in the magnetic field of the permanent magnet experience a mechanical force, F = I B
Mechanical force always acts in a direction to drag the conductor along with the magnetic field movement.
Lecture 19
Electro Mechanical System

6. Electro Mechanical System
Operating Principles
With freedom to move, conducting ladder accelerates to the right
As the ladder structure picks up speed, the conductors will be cut less rapidly by the moving magnet.
Magnitudes of the inducted voltage E and the driven current I diminish. As a result, the mechanical force will also decrease.
In an induction motor
Ladder is closed upon itself to form a squirrel-case.
Moving magnet is replaced by a rotating magnetic field.
Rotating field is produced by the three-phase ac current that flows in the stator windings.
Lecture 19
Electro Mechanical System

7. Electro Mechanical System
The Rotating Field
Simple stator with 6 salient poles, each with a coil
Coils on diametrically opposite sides connected in series
Two coils are connected to produce mmf in the same direction
Creates three identical sets of windings, labeled AN, BN, CN.
Mechanically spaced at 120 degrees to each other.
Lecture 19
Electro Mechanical System

8. Electro Mechanical System
The Rotating Field
The three windings are wye-connected, with common neutral.
Line-to-neutral impedances are equal, for a balanced load.
The line currents are displaced in time by 120 degrees
Assume a positive rotation: phases sequence = ABC
The magneto-motive force is in-phase with the line currents
The mmf’s are displaced in time by 120 degrees
The mmf’s are displaced around the stator by 120 mechanical degrees
Lecture 19
Electro Mechanical System

9. Electro Mechanical System
The Rotating Field
Lecture 19
Electro Mechanical System

10. Electro Mechanical System
The Rotating Field
At instant 1:
The current Ia has value of 10A Ib and Ic both have –5A
mmf of phase A is 10A x 10 turns = 100 Ampere turns
Similarly mmf of B & C is 50 Ampere turns
Direction of flux can be defined using right hand rule.
Six salient poles together produce a magnetic field having essentially one broad north pole and one broad south pole.
6-pole stator produces 2-pole field, combined field points upward
Lecture 19
Electro Mechanical System

11. The Rotating Field……ctd.
At instant 2, 1/6 cycle later, current Ic attains a peak of – 10 A, while Ia and Ib both have a value of +5 A.
New field has the same shape, but move clockwise by an angle of 60°
In other words, the flux makes 1/6 of a turn between instants 1 and 2.
Each of successive instants 3, 4, 5, 6, and 7, separated by intervals of 1 /6 cycle, magnetic field makes one complete turn during one cycle.
Rotational speed of the field depends upon the duration of one cycle, which in turn depends on frequency of the source. If the frequency is 60 Hz, field makes one turn in 1/60 s, or 3600 revolutions per minute.
If the frequency is 5 Hz, the field makes one turn in 1/5 s, that is, 300 rpm.
Speed of rotating field is synchronized with frequency of the source, it is called synchronous speed.
Lecture 19
Electro Mechanical System