3ph i.m introduction

8/3/2019 3Ph I.M Introduction

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Instructional Objectives

Construction

Types of rotor

Operating PrincipleEffect of slip in rotor

Torque equation- Starting & Running

Torque-Slip Characteristics

By

P.Anitha,Lecturer

EEE Dept

Three-phase induction motor


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Type of Electric Motors

Electrical current reverses direction

Two parts: stator and rotor Stator: stationary electrical component

Rotor: rotates the motor shaft

Speed difficult to control

Two types

Synchronous motor

Induction motor

AC Motors


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AC motors

Alternating current (AC) motors use an

electrical current, which reverses its direction

at regular intervals.

An AC motor has two basic electrical parts: a

"stator" and a "rotor". The stator is in the

stationary electrical component. The rotor is

the rotating electrical component, which inturn rotates the motor shaft.


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The main advantage of DC motors over AC

motors is that speed is more difficult to control

for AC motors. To compensate for this, AC motors can be

equipped with variable frequency drives but

the improved speed control comes together

with a reduced power quality.

Disadvantages of AC motors


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Type of AC Motors

There are two types of AC motors:

synchronous and induction.

The main difference between the synchronous

motor and the induction motor is that the rotor

of the synchronous motor travels at the same

speed as the rotating magnetic field.


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Constant speed fixed by system frequency

DC for excitation and low starting torque:

suited for low load applications

Can improve power factor: suited for high

electricity use systems

Synchronous speed (Ns):

AC MotorsSynchronous motor

Ns = 120 f / P

F = supply frequency

P = number of poles


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A synchronous motor is an AC motor, whichruns at constant speed fixed by frequency ofthe system.

It requires direct current (DC) for excitationand has low starting torque, and synchronousmotors are therefore suited for applicationsthat start with a low load, such as aircompressors, frequency changes and motorgenerators.


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Synchronous motors are able to improve thepower factor of a system, which is why theyare often used in systems that use a lot of

electricity. This motor rotates at a synchronous speed,

which is given by the following equation Ns = 120 f / P

Where:

f = frequency of the supply frequency

P= number of poles


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Induction motors are the most common motors used

for various equipments in industry.

Their popularity is due to their simple design, they are inexpensive (half or less of the cost of a DC motor)

High power to weight ratio (about twice that of a DC motor)

are simple, rugged, low-cost and easy to maintain.

They run at essentially constant speed from zero-to-full load.

can be directly connected to an AC power source

AC MotorsInduction motor


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An induction motor has two main electricalcomponents as shown in the figure


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Construction

Frame

- Outer part of the body To support the stator core & windings

To protect the inner parts

Serve as a ventilating housing

Stator core


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Construction

The iron core has cylindrical

shape and is laminated with

slots.

The iron core on the figure haspaper liner insulation placed in

some of the slots.

In a three-phase motor, the

three phase windings are placed

in the slots.


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Construction

Stator or primary or field winding

- Stator winding is 3 ph winding supplied

from 3 ph source.- 3 ph can be connected in either star or

delta depending upon the method of

starting


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Stator and rotor magnetic circuit


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Rotor-Induction motors use two types of rotors:

A squirrel-cage rotor consists of thick conducting

bars embedded in parallel slots. These bars are short-

circuited at both ends by means of short-circuitingrings.

A wound rotor has a three-phase, double-layer,

distributed winding. It is wound for as many poles as

the stator. The three phases are wired internally andthe other ends are connected to slip-rings mounted on

a shaft with brushes resting on them.


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Squirrel cage rotor

Squirrel cage rotor consists of copper bars,slightly longer than the rotor, which are pushedinto the slots.

The ends are welded to copper end rings, sothat all the bars are short circuited.

In small motors, the bars and

end-rings are die-cast in

aluminum to form an

integral block.


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Squirrel cage rotor.

This rotor has a laminated iron core with slots, and is mountedon a shaft.

Aluminum bars are molded in the slots and the bars are shortcircuited with two end rings.

The bars are slanted on a small rotor to reduce audible noise.

Fins are placed on the ring that shorts the bars. These finswork as a fan and improve cooling.


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Squirrel cage rotor


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wound rotor Most motors use the squirrel-cage rotor because of the

robust and maintenance-free construction.

However, large, older motors use a wound rotor with

three phase windings placed in the rotor slots.

The windings are connected in a three-wire wye.

The ends of the windings are connected to three slip rings.

Resistors or power supplies are connected to the slip ringsthrough brushes for reduction of starting current and speed

control


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wound rotor

The external resistors are mainly usedduring start-up under normal runningconditions the windings short-circuited

externally.


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Stator.The stator is made up of a numberof stampings with slots to carry three-phase windings. It is wound for a definite

number of poles. The windings aregeometrically spaced 120 degrees apart


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Operating Principle


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Operation Principle

The three-phase stator is supplied by balanced threephasevoltage that drives an ac magnetizing current through eachphase winding.

The magnetizing current in each phase generates a

pulsating ac flux.

The flux amplitude varies sinusoidally and the

direction of the flux is perpendicular to the phase

winding.

The total flux in the machine is the sum of the three fluxes.


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The summation of the three ac fluxes results in a rotating flux,which turns with constant speed and has constant amplitude.

The rotating flux induces a voltage in the short-circuitedbars of the rotor. This voltage drives current through the bars.

The induced voltage is proportional with the difference ofmotor and synchronous speed.

Consequently the motor speed is less than the

synchronous speed

The interaction of the rotating flux and the rotor current

generates a force that drives the motor. The force is proportional with the flux density and the rotor

bar current


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The figure shows the three components

of the magnetic field at a phase angle of

60. Each phase generates a magnetic field

vector.

The vector sum of the component

vectors a

, b

, c

gives theresulting rotating field vector

rot,

The amplitude is 1.5 times the

individual phase vectoramplitudes,and rot rotates

with constant speed.


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Induction motor-Points to be remembered

The speed of flux cutting is the difference

between the magnetic field speed and the rotor

speed.

The two speeds can be calculated by using the

radius at the rotor bar location and the

rotational speed.


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Induction motor-Points to be remembered

The voltage and current generation in the rotor bar require a

speed difference between the rotating field and the rotor.

Consequently, the rotor speed is always less than the

magnetic field speed.

The relative speed difference is the slip, which is calculated

using

P

fN

N

NNs

S

S

S

120


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Induction Motors: Active PowerFlow


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Induction Motors: Active PowerFlow


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Torque/Speed Curve


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Complete Torque-SpeedCharacteristics


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THANK YOU

3ph i.m introduction

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