Electromagnetism and Electrical engineering Flashcards
DC motor
When kept in a magnetic field, a current-carrying conductor gains torque and develops a tendency to move. In short, when electric fields and magnetic fields interact, a mechanical force arises. This is the principle on which the DC motors work.
armature: rotating part of a DC motor that interacts with the magnetic field to produce motion. (the metal conductor)
Comutator
A mechanical switch that reverses the direction of current flow in the armature windings.
ensures that the armature continues to rotate in the same direction by changing the current’s polarity at the right moments during each half-turn.
Brushes:
components that maintain electrical contact with the commutator.
provide a path for current to flow from the external circuit to the armature windings via the commutator, allowing the motor to operate.
armature rotor
includes the armature core, which is made of laminated iron, and the windings (typically copper) that carry the current.
It rotates within the motor, interacting with the magnetic field created by the stator to produce motion.
Back EMF
. A motor has coils turning inside magnetic fields, and a coil turning inside a magnetic field induces an emf.
This emf, known as the back emf, acts against the applied voltage that’s causing the motor to spin in the first place, and reduces the current flowing through the coils of the motor.
Iadepends on back emf, which in turn depends on speed of the motor.
The presence ofBack EMFmakes the D.C. motor a self-regulating machine.
i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load.
As it spins faster back end EMF Increases, net voltages decreases, current is lower
Note: Therefore, that energy conversion in a dc motor is only possible due to the production of back emf.
DC generators
electrical machines
DC generator and motor power and loss
Dynamically Induced EMF
Losses in electrical machines
Losses in DC Machines:
Copper loss -> Armature, Series field, Shunt filed copper loss
Iron loss-> Eddy current loss, Hysteresis loss
Mechanical loss-> Friction loss, Windage loss
only Armature, Series field are variable loss, rest are constant loss
- Input from turbine
- mechanical loss
- Electromagnetic Power= EgIa
- Armature Copper+Brush contact Loss
- Armature Terminal Power =VIa
- Series Field loss IL^2Ra
- Shunt Field Loss If^2Ra
-Ouput power VIL
DC motor
- Input from Mains VIL
- Series Field loss IL^2Ra
- Shunt Field Loss If^2Ra
- Armature Terminal Power =VIa
- Armature Copper+Brush contact Loss
- Electromagnetic Power= EbIa
induction motor
DC: Power is conducted directly to the armature through brushes and commutator. Hence they are Conduction Motor.
Induction motor: Rotor will not get the electric power by conduction, instead by induction.
two types of AC motors:
Asynchronous (induction) motor & synchronous motor
ESA!!!!
They are simple and rugged.
Its cost is low and it is reliable.
It has high efficiency.
Maintenance cost is less.
It is self-starting motor.
It can be manufactured with characteristics to suit most industrial requirements.
They are the most widely used electric motors in industry.
Induction motor has 2 main parts :
Rotating part (Rotor)
Stationary part (Stator)
Stator
It consist of laminated cylindrical core having slots at the inner periphery.
Insulated stator conductors are placed inside the slots
The conductors are either in star or delta to form 3Φ winding. It is been excited by 3Φ supply
2 TYPES OF INDUCTION MOTOR
Squirrel Cage Rotor
Rotor winding is composed of copper bars
embedded in the slots and shorted at both the ends by end rings or A bolted on either side
Simple, low cost, robust, low maintenance
SQUIRREL CAGE ROTOR
consist of laminated cylindrical core having slots at the outer periphery.
Cu/Al bar conductors are placed in the slots and short circuited at each end by Cu/Al rings called as short circuiting rings
The rotor windings are permanently short circuited & its not possible to add any external resistance
The rotor slots are not parallel to the shaft but skewed to
Reduce humming
Reduce magnetic locking of stator and rotor.
Phase wound Rotor/Slip Ring Rotor
Rotor windings are wound by wires. The winding terminals can be connected to external circuits through the slip rings and brushes.
More expensive
It consist of laminated cylindrical core having slots at the outer periphery & carries 3Φ insulated windings.
The 3 finish terminals are connected together forming a star point & the 3 star terminals are connected to 3 slip rings fixed on the shaft.
PRINCIPLE OF OPERATION:
when a three-phase stator winding supply is is connected to three-phase balanced supply, a rotating magnetic field is created. past rotor conductors, due to induction, EMF is As this field moves EMF induced in these conductors which in turn circulates currents in rotor conductors The current carrying rotos conductors in presence of magnetic field experience force which rotates the rotor in same direction field in order to as rotating magnetic satisfy Lenz’s law
induction motor synchronous speed
A RMF is set up in the stator, when 3Φ supply is given.
The stationary rotor conductors cuts the revolving field and due to electromagnetic induction an EMF is induced in rotor conductors.
As the rotor conductors are short circuited, current flows through them.
Hence it becomes a current carrying conductor in the magnetic field, which experience the force and start rotating
