It is also a basis for mri technology, therefore teslas name was honored with the tesla unit, used to measure the capacity of mri machines. Rotating magnetic field is magnetic field which rotates in space about some point or axis. The rotating magnetic field the basic idea of an electric motor is to generate two magnetic fields. Magnets have two poles, a north pole and a south pole. Rotating magnetic field in an ac machine about circuit.
When a 3phase winding is energized from a 3phase supply, a rotating magnetic field is produced. Thus revolving field is a key to the operation of the induction motor. This is a key principle in the operation of the alternatingcurrent motor rotating magnetic fields are often utilized for electromechanical applications such as induction motors and. Synchronous and asynchronous developing a rotating magnetic field in stator coil the stator of an ac motor is a hollow cylinder in which coils of insulated wire are inserted. To understand ac motor operation, its important to look into the development of rotating magnetic fields. Working of three phase induction motor production of rotating magnetic field. Induction motor seminar ppt, pdf report study mafia. For that, we will first imagine one stator of an electric motor where threephase winding is physically distributed in the stator core in such a manner that winding of each phase is separated from other by 120 o in space. In an induction motor induction refers the field in the rotor is induced by the stator currents and asynchronous refers that the rotor speed is not. Lamme has given an excellent description of singlephase induction motor operation based on a conception of two oppositely rotating magnetic. Elemental principles an examination of the rudimentary interaction between inductance coils and field magnets will provide some insight into the principles behind the rotary magnetic field. Before we look at how the rotating field is produced, we should be clear what it actually is. This post will explain you in brief about production of rotating magnetic flux for 2 phase as well as 3 phase supply.
A rotating magnetic field rmf is produced in a 3ph induction motor when a 3ph supply displaced from each other by 120 deg in time is applied to a 3ph winding displaced from each other by 120 deg in space, naturally. Only in this way will there be a relative movement between the winding and rotating magnetic field to cut magnetic lines. In this way, due to a balanced threephase supply to the mechanically balanced threephase distributed winding in the stator of an ac machine produces a continuously rotating magnetic field. What is rotating magnetic field the engineering knowledge. S l r the flux in the singleturn coil on the rotor surface is. If rotor runs at the synchronous speed, which is the same speed of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating. The induction motor stators magnetic field is therefore changing or rotating relative to the rotor. The reason why the rotor of an induction motor is set into rotation is as follow. This video provides basic construction and working principle of three phase induction motor. It will be created in three phase induction motors stator. Rotating magnetic field in ac machines 1 introduction in a dc machine, the stator winding is excited by dc current and hence the eld produced by this winding is time invariant in nature. These coils are designed to produce a rotating magnetic field, which turns the rotor the. The production of rotating magnetic field in 3 phase supply is very interesting. Dc motors depend either on mechanical or electronic commutation to create rotating magnetic fields.
Eddy currents are induced in the rotor conductors by the. Because both the rotor and stator iron surfaces are smooth apart from the regular slotting, and are separated by a small air gap, the flux produced by the stator windings crosses the air gap radially. Consider a simple electromechanical device consisting of a piece of iron with a copper loop. When the 3 phase stator windings are fed by 3 phase supply, a magnetic flux of constant magnitude which is rotating at synchronous speed is set up. Synchronous speeds for commonly used pole numbers are given in the table below. In this animation you get the principle of the rotating magnetic. Generally, it is known as the asynchronous motor because its running speed is less than its synchronous speed. The speed of the rotating magnetic field of the stator in the 3phase induction motor is the same as the speed of the alternator that is supplying power to the motor if the two machines motor and alternator have the same number of poles.
All poly phase electrical machines are associated with rotating magnetic field in the air gap. A rotating magnetic field is a magnetic field that has moving polarities in which its opposite poles rotate about a central point or axis. The pole number must be an even integer, since for every n pole there must be a s pole. Rt and he demonstration model sees a magnetic field the system consists of an electromechanical structure representing the stator of an elementary twophase, twopole induction motor and a microcontroller based low frequency twophase a. The north and south poles continuously rotates with a specific speed, called synchronous speed. Rotating magnetic fields the principle of rotating magnetic fields is the key to the operation of most ac motors. Up to now we have considered the rotating field in the stator iron of a threephase machine. How rotating magnetic field works in ac machines eep. The rotor will constantly be turning to align its magnetic field with that of the stator field.
Working of three phase induction motor production of rotating magnetic field the stator of the motor consists of overlapping winding offset by an electrical angle of 120 o. Concept of rotating magnetic field electrical concepts. However, instead of the rotor being constructed from permanent magnet material, the induction motor has a rotor constructed only from conductive material. Magnetic fields are an invisible force that attracts ferromagnetic metals such as iron, steel, nickel and cobalt. In a motor, on the other hand, the rotational motional voltage generated in the armature opposes the applied voltage. The operating principle for all threephase motors is the rotating magnetic field. Elleec cttrriicaall immaacchhiinneess 7ivv t ccooddee. The rotor body is the rotating part of the motor and is made of a steel core with aluminium windings. Production of rotating magnetic field, slip and rotor frequency is also discussed.
The magnetic field, revolving in the stator, will then produces the torque which will pull on the field in the rotor and establish rotor rotation. Induction motor speed at what speed will the im run. A magnetic field in a stator can be made to rotate electrically, around and around. Induction motor equations engn1931f spring 2017 2 let.
It will now be shown that when threephase windings displaced in space by 120 0, are fed by threephase current displaced in time by 120 0, they produce a resultant magnetic flux which rotates in space as if actual magnetic poles were being rotated mechanically. Therefore, the direction of rotation of a 3phase induction motor can be reversed by interchanging any two of the three motor supply lines. The magnetic rotating field rmf is the basis for the threephase motors whether synchronous or asynchronous induction motors. Free download induction motor seminar ppt, pdf report.
As we shall see, the rotor in a 3phase induction motor runs in the same direction as the rotating magnetic field. Three phase induction motor advantages disadvantages. Rotating field concept before discussing the theory of operation for the induction motor a very basic concept, that of a rotating field, must be understood. An everyday ac induction motor with its case and rotor removed, showing the copper windings of the coils that make up the stator the static, nonmoving part of the motor. A rotating magnetic field, as in the synchronous motor, also forms the basic operating principle of an asynchronous induction motor. How an induction motor works by equations and physics. Rotating magnetic field speed n 1 is also known as synchronous speed. There are 3 elements that trigger the magnetic field to turn.
In todays tutorial, we are gonna have a look at what is rotating magnetic field. Whenever the ac supply is connected to the stator windings, line currents i r, i y, and i b start flowing. Production of rotating magnetic fields in polyphase a. Why is a capacitor used in a singlephase motor and not in. Rotating magnetic field an overview sciencedirect topics. This field is such that its poles do no remain in a fixed position o. A rotating and constant resultant magnetic field rotating at a constant speed may be produced by any threephase. L r s and be the angular velocities of the magnetic field line frequency, rotor, and slip respectively. The stator has a three phase stationary winding which can be either star connected or delta connected. In an induction motor, when ac supply is given to the the stator, magnetic flux is produced which is revolving at synchronous speed. By reciting the faust in the park in budapest, he discovered the rotating magnetic field which is the heart of his induction motor and alternating current electricity. Thus, the description of the rotating magnetic field given thus far applies to both motor and generator action in ac machines.
In this machine the conversion of energy from electrical to mechanical form or vice versa is possible by one of the following ways. These magnetic fields follow the fundamentals of electromagnetism to rotate the shaft of an ac motor lets take a closer look at an electric motor stator. This induces an opposing current in the induction motors rotor, in effect the motors secondary winding, when the latter is shortcircuited or closed through an external impedance. With the use of the rotating magnetic field from the stator, the rotor is able to produce torque.
Ideally, the rotation changes direction at a constant angular rate. Threephase induction motor rotor speed n will not be accelerated to the rotating magnetic field speed n 1. Hence the magnetic flux is said to rotate at synchronous speed. Rotating magnetic field in 3 phase induction motor your. The general expression for the speed of the field which is known as the synchronous speed ns, in revmin is. Both synchronous and induction types of motors rely on rotating magnetic fields in their stators to cause their rotors to turn. Production of rotating magnetic field in polyphase stator. The field which is continuously rotating with a particular axis is known as rotating magnetic field. The winding wound on the stator of three phase induction motor is also called field winding and when this winding is excited by three phase ac supply it produces a rotating magnetic. The singlephase motor stator has a laminated iron core with two windings arranged perpendicularly. Rotating magnetic field in ac machines 1 introduction 2.
When we connect the primary winding, or the stator to a 3 phase ac source, it establishes rotating magnetic field which rotates at the synchronous speed. An induction motor can be defined as the ac electric motor device in which a power source fed by the alternating current with the help of a primary winding. When the 3phase stator windings, are fed by a 3phase supply then, as seen from above, a magnetic flux of constant magnitude, but rotating at synchronous speed, is set up. In the design of the induction motor, operational characteristics can be determined through a. Rotating magnetic field 3 phase, operating principles. The speed of the rotating magnetic field is the same as the speed of the alternator that is supplying power to the motor if the two have the same number of poles. How is a rotating magnetic field is produced in a three. The induction motor rotates due to the rotating magnetic field in 3 phase induction motor, which is produced by the stator winding in the air gap between in the stator and the rotor. The stator of the motor consists of overlapping winding offset by an electrical angle of 120 o. Ac induction motors how ac motors work explain that stuff. The theory is given of a rotating probe for measuring the inhomogeneity of a magnetic field. A singlephase ac induction motor depends on extra electrical components to produce this rotating magnetic field. In this article, we will try to understand the theory behind the production of rotating magnetic field.
1530 697 1557 880 887 338 885 973 643 831 892 805 173 1340 1144 274 1329 1097 1017 406 1362 250 1191 493 13 16 181 147 1220 1196 90 98 222 1331 1494 319 1204 1347