ELECTRICAL MACHINES HOW DO THEY WORK??
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