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

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

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

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

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

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

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

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

Electro Mechanical System The Rotating Field Lecture 19 Electro Mechanical System

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

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