presentation on 3 phase motor

7/27/2019 Presentation on 3 Phase Motor

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PRESENTATION

ON3 PHASE - INDUCTION MOTOR



1Introduction

2 History

3 Principle of operation4 Starting of Motor

5 Speed control

6 Types of 3 Phase Induction Motors

7 Construction8 Production of Rotating magnetic Field

Contents



An induction motor (IM) is a type of

asynchronous AC

motor where power

is supplied to the

rotating device by

means of

electromagneticinduction

Three-phase induction motors

INTRODUCTION



History

The induction motor with a wrapped rotor was invented by Nikola Tesla in

1888 in the United States. In his scientific work, Tesla laid the foundations

for understanding the way the motor operates.

The induction motor with a cage was invented by Mikhail Dolivo-

Dobrovolsky about a year later in Europe. Technological developmentin the field has improved to where a 100 hp (73.6 kW) engine from 1976

takes the same volume as a 7.5 hp (5.5 kW) engine did in 1897.

Currently, the most common induction motor, is the cage rotor motor.



Principle of operationThe operation of a 3 Phase Induction Motor is based

upon the application of Faraday’s Law ,Len’s Law

and Lorentz force on a conductor when kept in a

magnetic field whose direction can be found by

Fleming’s Left Hand Thumb Rule.

In a 3 Phase Induction motor ,current is supplied onto the

rotor. This then creates a magnetic field which, through

magnetic interaction, links to the rotating magnetic field in

the stator which in turn causes the rotor to turn. The

rotating Magnetic flux induces currents in the windings of

the rotor in a manner similar to currents induced in

A transformer's secondary winding.



The currents in the rotor windings in turn create magnetic

fields in the rotor that react against the stator field. Due to

Lenz’s Law, the direction of the magnetic field created will be

such as to oppose the change in current through the rotor windings. The cause of induced current in the rotor windings is

the rotating stator magnetic field, so to oppose the change in

rotor-winding currents the rotor will start to rotate in the

direction of the rotating stator magnetic field. The rotor

accelerates until the magnitude of induced rotor current and

torque balances the applied load..

http://en.wikipedia.org/wiki/Image:Rotatingfield.png



In a three phase induction motor, the induced emf in the rotor circuit depends

on the slip of the induction motor and the magnitude of the rotor current

depends upon this induced emf (electromotive force). When the motor is

started, the slip is equal to 1 as the rotor speed is zero, so the induced emf inthe rotor is large. As a result, a very high current flows through the rotor. This is

similar to a transformer with the secondary coil short circuited, which causes

the primary coil to draw a high current from the mains. Similarly, when an

induction motor starts, a very high current is drawn by the stator, on the order of

5 to 9 times the full load current. This high current can damage the motor windings and because it causes heavy line voltage drop, other appliances

connected to the same line may be affected by the voltage fluctuation. To avoid

such effects, the starting current should be limited ……

STARTING OF MOTOR



The rotational speed of the rotor is controlled by the number of pole pairs (number of windings in the stator) and by the frequencyof the supply voltage. Before the development of cheap power electronics, it was difficult to vary the frequency to the motor andtherefore the uses for the induction motor were limited. There are

various techniques to produce a desired speed. The mostcommonly used technique is PWM(Pulse Width Modulation), inwhich a DC signal is switched on and off very rapidly, producing asequence of electrical pulses to the inductor windings

SPEED CONTROL



TYPES OF 3 PHASE

MOTORS SQUIRREL CAGE MOTOR : Motor consisting of Squirrel

Cage rotor .

SLIP – RING MOTOR : Motor consisting of

slip – ring rotor.



The stator consists of wound 'poles' that carry the supply current that induces

a magnetic field in the conductor. The number of 'poles' can vary betweenmotor types but the poles are always in (i.e. 2,4,6 etc) pairs. There are two

types of rotor

Construction

1. Squirrel-cage rotor

2. Slip ring rotor

The most common rotor is a squirrel-cage rotor. It is made up of bars of either

solid copper (most common) or aluminum that span the length of the rotor, and

are connected through a ring at each end



Formulas

The relationship between the supply frequency, f , the number of pole pairs, p,

and the synchronous speed, n, is given by f = p*n.

From this relationship:

Speed of rotating field (n) = f/P (revs.s-1)

Speed of rotor = n(1-S) (rev.s-1)where S is the slip.

Slip is calculated using:

% slip = (n - r) / n * 100

where r is the rotor speed

In contrast, a synchronous motor always runs at either a constant speedN=(120f)/P or zero.



Production of Rotating

Magnetic Field

It is important to understand the principle of rotating

magnetic field in order to understand the operation of anInduction motor. When a three phase voltage is applied to

the stator winding , a rotating magnetic field of constant

magnitude is produced. This rotating field is produced by the

Contributions of phase windings carrying appropriate time

displaced currents. These currents which are time displacedby 120 electrical degrees.



We will now consider a stator structure depicted along with

three phase windings.For convenience, each phase is represented

by a single coil (though the winding is distributed in practice).

The coil a-a’ represents the entire phase winding for phase a.

Similarly b-b’ and c-c’ represent the coils for phases b and c.

Each phase winding produces flux along its own flux axis and

these axes are separated by 120 electrical degrees.



Thank You

This presentation has been made in an attempt to explain the concept behind an

induction by the group comprising members:

Pushkar A Pandit (Roll No- 46)

Nikki Paliwal (Roll No- 74)

Ashwin Koche (Roll No-29)

Pankaj Sachdev (Roll No-61)

Ayush (Roll No-30)

Arpit Jaiswal(Roll No-28)

Kunal Rewatkar(Roll No-76)

presentation on 3 phase motor

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