Electromagnetism and Electrical engineering Flashcards
Reluctance
measure of oppostion by a magnetic circuit to the setting of the flux
R=1/μ1/A=1/μrμo1/A
reciprocal of reluctance = permeance (G)
Faradays Electromotive Induction
Lenz law
Flemings left hand rule and right hand rule
First Law - whenever the magnetic field linked with a circuit changes, an emf is induced in it
second law: the magnitude of the induced emf is equal to the rate of change of flux linkage
e ∝ dφ/dt
e=N*dφ/dt
Lenz law: an induced current always flows in a direction such that is opposes the change which produces it
e=-N*dφ/dt
Left hand rule
electric motors
Right hand rule
electric generators
thumb -> force, 2nd-> magnetic field
3rd finger, direction of current
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.
Armature Winding of a DC Machine is wound by one of the two methods
Lap Winding
Wave Winding
The difference between these two is merely due to the end connections and commutator connections of the conductor.
It may be simplex, duplex or multiplex.
DC generators
Working principle
Faraday’s Electromagnetic Induction
Dynamically Induced EMF
simplex, duplex or multiplex lap winding
SIMPLE LAP winding,
the two ends of a coil are connected to adjacent commutator segments
No of parallel paths(A) is equal to the no of poles(P), (A=P).
High current, low voltage
current splits, btw paths as they are parallel, voltage is splits btw each conductor
WAVE WINDING
A conductor under one pole is connected at the back to a conductor which occupies an almost corresponding position under the next pole which is of opposite polarity.
A=2
low current, high voltage
EMF equation of DC circuits
Eg= PφN/60*Z/A=PZφN/60A
φ=flux per pole
Z=Conductors
P=poles
A=No of parallel paths
Eg=EMF in any parallel path
LAP:
φNZ/60 V
WAVE
φPNZ/120 V
Separately Excited DC generators
Shunt Wound DC generators
Series Wound DC generators
Ia = IL
V= Eg-IaRa V
IaRa => armature resistance drop
power devoloped = EgIa (W)
power delivered = VI2
SHUNT WOUND DC
Ia=Ish+IL
V=Eg-IaRa
Power developed :EgIa
power delivered: V*IL
Ish=R/Vsh
Rsh is more!
SERIES WOUND
Rsc (series field winding) is less
Ia=Isc=IL
V=Eg-Ia(Ra+Rsc)
armature will be series with fielding winding
total current and volts in lap and wave winding
lap
Itot=I(per conductor)A
Vtot =V(conductor)Z/A
wave
Itot = I(per conductor)2
Vtot =V(conductor)Z/2, A=2
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.
V-IaRa-Eb=0
Ia=V-Eb/Ra
V and Raare fixed, therefore, armature current 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
back end emf is opposite to source voltage
If load increases:
N, Eb decreases
Ia increases
Load falls
N, Eb increases
Ia decreases
Note: Therefore, that energy conversion in a dc motor is only possible due to the production of back emf.
Mechanical power developed in the armature = EbIa
Torque and losses in electrical machines
Twisting or Turning Force about an axis
Ta=0.159φlaZ(P/A) Nm
Derivation present!!
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
DC generator and motor power and 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. Hence they are called as Induction Motor.
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
Simple, low cost, robust, low maintenance
SQUIRREL CAGE ROTOR
consist of laminated cylindrical core having slots at the outer periphery.
Copper/aluminum bar conductors are placed in the slots and short circuited at each end by copper/aluminum 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.
induction motor synchronous speed
3Φ stator winding is excited by 3Φ supply, which will produce 3Φ flux of constant magnitude in the stator.
wires/coils present at 3 phase, with variations in ac current will produce a rotating magnetic field due to 3 phase input power
This flux will rotate at a speed, called as synchronous speed(Ns).
This flux is called as Rotating Magnetic Field (RMF)
Ns=120f/p
f= frequency of the supply
P= number of poles
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
The speed of the Rotor (N) of the induction motor is always less than synchronous speed (Ns). N<Ns
The difference between the flux speed (Ns) and the rotor speed (N) is called as Slip (s).
Usually it is expressed in percentage as
%S=Ns-N/Ns*100
N=Ns(1-S)
-> S*f=fr
