m.e project

ELECTRICAL MACHINES

HOW DO THEY WORK??

The components of the motor effect

• a current carrying conductor +

• a permanent magnetic field

results in a force being exerted on the conductor which causes the conductor to

move

Flemings Left Hand rule - used to predict the direction of the force actingon the conductor in a magnetic field

Direction of the magnetic field: from N – S

Current direction is the direction of flow of the conventional current

Basic Electric MachinesOutside part of machine does not move, is stationary

Is called stator

Held stationary through vibration damping attachments

Central part of machine rotates

Is called Rotor

Shaft will rotate within some form of bearing.

Lorentz force on a conductor:

Open right hand rule

BilF

Source: T. Wildi, Electrical Machines, Drives and Power Systems, 5th Edition, Prentice-Hall, 2002

Basic Electric Machines

• A pole is a magnetic pole, that is north or south.

• Must always have a north and a matching south pole so poles always are in pairs.

• On next slide is a two pole or one pole-pair DC machine.

• Commutators are mechanical switches that change direction of current.

Basic Electric Machines

South pole North pole

Magnetic flux north to south

X

South pole North pole

elevation

plan

Rotation path

Conductor with current out of page

Conductor with current into page

Apply Lenz’s Law open right hand rule

Rotates about axis

Force

Force

Basic DC Machines

Southpole X

Force

Northpole

X

X

Angular velocity

Zero Force

X

Basic DC Machines

Southpole X North

pole

X

X

Zero Force

X

+V

Brush and gap sized to avoid short circuit.

X

Commutator

• The power flows in electric machines are reversible.

• To operate machine as motor supply electric power to get mechanical power.

• To operate as generator supply mechanical power to generate electrical power.

• To operate DC machine as generator remove DC voltage supply and externally rotate shaft

• Conductor moving through (cutting lines of) magnetic flux induces voltage and/or current.

Commutator

DC Machines• The rotor has a ring-shaped

laminated iron core with slots.

• The commutator consists of insulated copper segments mounted on an insulated tube.

• Two brushes are pressed to the commutator to permit current flow.

• The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing.

DC Machine Construction

|

Shaft

Brush

Coppersegment

InsulationRotor

Winding

N S

Ir_dcIr_dc/2

RotationIr_dc/2

Ir_dc

12

3

45

6

7

8

Polewinding

Figure 8.2 Commutator with the rotor coils connections.

Rotating electrical machines

• These can be divided into:Generators – which convert mechanical energy into electrical energy.Motors – which convert electrical energy into mechanical energy.

• Both types operate through the interaction between a magnetic field and a set of windings.

What is a Motor?

• Definition:A rotating machine that converts electrical power (either alternating current or direct current) into mechanical power.

MMootortor

Encoder

Brush CoverBrush

Iron less winding

Housing Motor pinionBall bearing

Ball bearing

Motor plange

Output shaft

Magnet shaftCommutatorCommutator

Gear head flange

PlanetsPlanet carrier plate

Internal gear

Different parts of a Different parts of a MotorMotor

How Does A Motor Do Work?How Does A Motor Do Work?

If the current carrying wire is bent into a loop, then the two sides of the loop which are at right angles to the magnetic field will experience forces In opposite directions.

An electric current in a magnetic field will experience a force.

Contd.Contd.

How Does A Motor Do Work?How Does A Motor Do Work?The pair of forces creates a turning influence or torque to rotate the coil.

Practical motors have several loops on an armature to provide a more uniform torque and the magnetic field is produced by an electro magnet arrangement called the field coils.

DC Motors• When current flows in a conductor it produces a

magnetic field about it - as shown in (a) below.• When the current-carrying conductor is within an

externally generated magnetic field, the fields interact and a force is exerted on the conductor - as in (b).

• DC motor characteristics:– Many forms – each with slightly different

characteristics.– Again can be permanent magnet, or series-

wound, shunt-wound or compound wound.– Figure below shows a shunt-wound DC motor.

Different Types of DC Motor

Induction Motor. Universal Motor. Conventional DC Motor. Brush-less DC Motor.

Induction motors: These are perhaps the most

important for DC motor. Rather than use slip rings to

pass current to the field coils in the rotor, current is induced in the rotor by transformer action.

The stator is similar to that in a synchronous motor.

The rotor is simply a set of parallel conductors shorted together at either end by two conducting rings.

Induction motor

Contd.Contd.

Advantages:• Cheap.• Quiet. • Long lasting. • Creates no interference .

Disadvantages: a. Wants to turn at constant speed. b. Cannot turn faster than 1500rpm (4-polemotor). c. Kind of big and bulky for the power it develops.

Induction motors

Universal Motors:While most motors operate from either AC or DC, some can operate from either.These are universal motors and resemble series-wound DC motors, but are designed for both AC and DC operation. Typically operate at high speed (usually > 10,000 rpm). Offer high power-to-weight ratio. Ideal for portable equipment such as hand drills and

vacuum cleaners.

Contd.Contd.

Universal

Motor

Contd.Contd.

Advantages:• Will turn at any speed you want it to, including really fast.• A lot of power in a small package.Disadvantages:• Horrible.• Arcing brushes create radio interference, ozone, noise.

Conventional DC motor

There are plenty of these in the average household, lurking inside battery powered toys, the cassette player, cordless drill.

Advantages: • Reasonably inexpensive.

• Easy to control. • Adaptable.

Disadvantages: • Brushes eventually wear out.

• Brushes create electrical interference. • Brushes are bad.

Contd.Contd.

Magnetic

poles

Brushless DC motor

DC GeneratorsDC Generators

Self Self excitedexcited

Separately-Separately-excitedexcited

ShuShuntnt

SeriSerieses

CompoCompoundund

Long shunt

Short shunt

CONSTRUCTION OF A DC GENERATOR

The mains parts of a d. c generator are:1. Yoke2. Field System3. Conductor system4. Commutator5. Brushes6. Shaft & bearings

Contd.Contd.

1.Yoke: Yoke is the outer most part of the machine All parts of the machine are enclosed with in

this, hence they are fully protected.Functions:• It provides mechanical support to the poles.• It provides a low reluctance path to the

magnetic flux.

2. Field System: It contains three main components,i. Pole core.ii. Pole shoe.iii. Field coil.Pole core: It is the Pole body & it is fitted to the Yoke

by means of bolts & nuts.Functions:a. To support for the Field coil.b. To establish Magnetic flux.

Pole shoe: It is a projection of the pole core and integral

with it. It extends a large area of the air gap under the pole.

Functions:a. It supports the field coil.b. It enables the lines of magnetic flux to cross

the air gap radially.c. It spreads out the magnetic flux over a large

area of the air gap.

3. Conductor system:a) Armature winding: It has two types,

Lap & Wave.

b) Armature core: It is to support the armature winding. It is minimise power loss due to production of eddy currents.

4. Commutator: The main function of commutator is to collect the current induced in the armature winding, and rectify it i.e. convert the alternating current induced in the armature winding into unidirectional current.

5. Brushes : It’s main function is to collect the rectified current from the commutator and lead this current to the external load circuit.

YokeYoke

Eye boltEye bolt

CommutatorCommutator

ArmatureArmature

Armature Armature windingswindings

LegsLegs

FieldField coilscoils

BrushesBrushes

DC Generator:

•DC generator characteristics:– Vary slightly between forms.– Examples shown here are for a shunt-wound

generator.

contdcontd..

3 Point dc motor Starter

• A squirrel-cage induction motor