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Basic Electrical Engineering
15ELE15/25
D
...
Machines:
Syllabus:
Working principle of D
...
Machine as a generator and a motor
...
Emf equation of
generator, relation between induced Emf and terminal voltage with an enumeration
of brush contact drop and drop due to armature reaction
...
C
...
Types of D
...
motors,
characteristics and applications
...
C
...
Introduction:
➢ The converters which are used to continuously translate an electrical input to a
mechanical output or vice versa are called as DC machines
...
Working principle of D
...
Machine as a generator and a motor:
Working principle of D
...
Machine as a generator:
➢ Whenever a coil is rotated in a magnetic field an Emf will be induced in this coil and
is given by e=B*l*v*Sinθ volts/coil side where, B=The flux density in Tesla, l=the
active length of the coil side in meters v=the velocity with which the coil is moved in
meters/sec and θ is the angle between the direction of the flux and the direction of
rotation of the coil side
...
Working principle of D
...
Machine as a motor:
➢ Whenever a current coil is placed under a magnetic field the coil experiences a
mechanical force, and is given by F= B*I*l*Sinθ Newton/coil side where, I is the
current through the coil
...
Construction of DC Machine:
Dept of EEE, SVIT
Basic Electrical Engineering
15ELE15/25
Salient parts of a D
...
Machine are:
(i)Field system (poles)
(ii)Coil arrangement (armature)
(iii)Commutator
(iv)Brushes
(v)Yoke
Yoke:
i) It serves the purpose of outermost cover of the D
...
machine
...
ii) It provides mechanical support to the poles
...
It provides a path of low reluctance for magnetic
flux
...
➢ It directs the flux produced through air gap to armature core, to the next pole
...
to achieve this, pole shoe has given a
particular shape
Field winding [F1-F2]:
➢ The field winding is wound on the pole core with a definite direction
...
As it helps in producing the magnetic field
i
...
exciting the pole as electromagnet it is called Field winding or Exciting winding
...
It consists of slots on its
periphery and the air ducts to permit the air flow through armature which serves
cooling purpose
...
➢ This needs rectifications in case of D
...
generator which is possible by device
called commutator
...
➢ Ball bearings are usually used as they are more reliable
...
Types of D
...
Armature Windings
Lap Winding
Wave Winding
In this winding all the coils carrying current in
In this winding all the pole groups of
the same direction are connected in series i
...
,
the coils generating emf in the same
coils carrying current in one direction are
direction
at
any
instant
of
time
connected in one series circuit and coils carrying
are connected in parallel by the brushes
...
2
...
Wave winding is also known as series
windings
...
3
...
The number of parallel paths is always equal
to the number of poles i
...
, A = P
...
e
...
4
...
The number of brushes required by this
winding is always equal to the number
winding is always equal to 2
...
5
...
The machine using wave winding does
requires equalizer rings for obtaining
require dummy coils to provide the mechanical
better commutation
...
6
...
Wave windings are used for high voltage and
voltage and high current machines
...
Dept of EEE, SVIT
Basic Electrical Engineering
15ELE15/25
Emf Equation of DC Generator:
For one revolution of the conductor,
Let, Φ = Flux produced by each pole in weber (Wb) and
P = number of poles in the DC generator
...
Now, according to Faraday’s law of induction, the induced emf of the armature conductor is
denoted by “e” which is equal to rate of cutting the flux
...
Here, Z = total numbers of conductor A = number of parallel paths
Then, Z/A = number of conductors connected in series
We know that induced Emf in each path is same across the line
Therefore, Induced Emf of DC generator E = Emf of one conductor × number of conductor
connected in series
...
e
...
➢ In general the method employed to connect field and armature winding has classify
into two groups
...
That means the field winding is electrically separated from the armature circuit
...
➢ The electromotive force that produced by generator at armature winding is supply to a
field winding (shunt field) instead of DC source from outside of the generator
...
They may be further classified as: a) DC Shunt generator
b) DC Series generator
c) DC Compound generator
...
Dept of EEE, SVIT
Basic Electrical Engineering
15ELE15/25
b) Series generator:
➢ The field winding and armature winding is connected in series
...
➢ Therefore, field winding conductor must be sized enough to carry the load current
consumption and the basic circuit
c) Compound generator :
➢
➢
The compound generator has provided with magnetic field in combine with excitation
of shunt and series field winding, the shunt field has many turns of fine wire and
caries of a small current, while the series field winding provided with a few turns of
heavy wire since it is in series with an armature winding and caries the load current
...
These conductors lie in the magnetic field of the air gap
...
The conductors lie near the surface of the rotor at a common radius from its centre
Hence, a torque is produced around the circumference of the rotor, and the rotor starts
rotating
...
C
...
➢ The induced emf acts in opposite direction to the applied voltage V(Lenz’s law) and is
known as back emf
...
It
does so by generating a voltage which opposes the supply voltage, thus reducing the
current
...
makes the d
...
motor a self-regulating machine i
...
, it
makes the motor to draw as much armature current as is just sufficient to develop the
torque required by the load
...
C
...
e
...
Types of DC Motors:
Motors are classified into 3 types: a) DC Shunt motor
...
c) DC Compound motor
...
➢ The current through the shunt field winding is not the same as the armature current
...
➢ Therefore, series field winding carries the armature current
...
➢ There are two types of compound motor connections :
1) Short-shunt connection Compound Motor
Dept of EEE, SVIT
Basic Electrical Engineering
15ELE15/25
➢ When the shunt field winding is directly connected across the armature
terminals it is called short-shunt connection
...
Torque equation of a DC Motor:
Multiplying the equation (1) by Ia we get
Where,
VIa is the electrical power input to the armature
...
We know that,Total electrical power supplied to the armature = Mechanical power
developed by the armature + losses due to armature resistance
Now, the mechanical power developed by the armature is Pm
...
Where n is in revolution per seconds (rps) and T is in Newton-Meter
...
Therefore,
So, the torque equation is given as
Dept of EEE, SVIT
Basic Electrical Engineering
15ELE15/25
Characteristics of DC Motors:
The three important characteristic curves are
1
...
Speed Vs Armature current characteristic (N/Ia)
3
...
Torque Vs Armature current characteristic (Ta/Ia):
We know that in a DC Motor Ta α ΦIa
...
➢ Hence the armature current differs and the speed of a DC Shunt motor will
continue constant which is shown in the fig (dotted Line AB)
...
➢ On the other hand, back EMF reduces marginally more than that the speed of
the shunt motor decreases to some extent with load
...
DC Series Motor:
Dept of EEE, SVIT
Basic Electrical Engineering
15ELE15/25
Torque Vs Armature current characteristic (Ta/Ia):
We know that
Ta ∝ ΦIa
➢ Up to Magnetic Saturation Φ ∝ Ia, the armature torque Ta is directly proportional to
square of the armature current (i
...
Ta ∝ I2a)
...
armature current curve up to magnetic saturation is a
parabola, which is shown in the characteristic curve OA
...
➢ As a result the armature torque vs
...
Speed Vs Armature current characteristic (N/Ia):
The Series Motor Speed N is known as
We know that back EMF
N α (Eb/Φ)
...
As soon as the Ia increases, the back EMF Eb reduced due to drop in Ia (Ra+Rse) even
though the flux increased
...
Therefore,
N α (1/ Φ)
...
armature current characteristic follows the hyperbolic curve up to magnetic
saturation {α (1/ Φ)}
...
The armature current will increases whenever the mechanical load of the motor increase
...
Speed Vs Torque characteristic (N/Ta):
➢ From this curve it is evident that the series motor runs at low speed when it develops
high torque and vice versa
...
➢ As a result flux gets strengthened and drops the speed of the motor
...
Characteristic of Compound motor:
Necessity of a Starter:
➢ In a D
...
➢ Thus, additional resistance must be added to the armature circuit at the instant of
closing the switch to start the motor
...
Back emf is
generated as the motor armature start to rotate in presence of magnetic field and
it is counter to the supply voltage
...
➢ We know that general emf equation E=Eb+laRa
...
Hence it is necessary to use
starter to limit the starting current to allowable lower value
...
Measuring Instruments:
Syllabus: Construction and Principle of operation of dynamometer type wattmeter and single
phase induction type energy meter
...
Hence moving coil
is connected across the voltage and thus the current flowing through this coil is always
proportional to the voltage
...
Now the reason is very
obvious of using two fixed coils instead of one, so that it can be constructed to carry
considerable amount of electric current
...
Earlier these fixed coils are designed to carry the current
of about 100 amperes but now the modern wattmeter are designed to carry current of about
20 amperes in order to save power
...
e
...
Gravity controlled system
cannot be employed because they will contain appreciable amount of errors
...
Scale - There is uniform scale is used in these types of instrument as moving coil moves
linearly over a range of 40 degrees to 50 degrees on either sides
...
For any circuit with fluctuating torque, the instantaneous torque is proportional to
instantaneous power
...
For sinusoidal alternating quantities the average power is
V
...
cosϕ, where
V= RMS value of voltage I = RMS value of current, and ϕ= phase angle between V and I
Hence an electrodynamometer instrument, when connected as shown in figure, indicates the
power, irrespective of the fact it is connected in an AC or DC circuit
...
➢ A phase difference of 900 is set between current coil and pressure coil with the help of
copper shaded rings
...
Inducing currents in the disc
...
➢ The disc rotates in a particular direction and the number and speed of rotations
depends on the energy consumed by the load
...
A 4 pole d
...
The armature & the field
2
...
4
...
6
...
5Ω & 100Ω respectively
...
If the flux/pole is 20mWb, calculate the speed & gross torque
...
c series motor is running with a speed of 1000rpm, while taking current of 22A
from the supply
...
The armature & series
winding resistances are 0
...
4Ω
...
c generator has a lap wound armature having 24 slots with 10
conductors/slot
...
04Wb, calculate the Emf generated in the
armature
...
c shunt motor is 4A on no load
...
2 ohms respectively
...
A 200V, 4 pole, lap wound, d
...
The
resistance of armature winding is 0
...
The motor takes a current of 21A, the flux/pole is 30mWb
...
Determine the speed at which it is running if the flux/pole is
50mWb
...
An 8 pole generator has 500 armature conductors and has a useful flux/pole of 0,065
Wb
...
A 4 pole generator with wave wound armature has 51 slots each having 24
conductors
...
01 Wb
...
9
...
c shunt motor takes an armature current of 110a at 480V
...
2 ohm
...
The flux/pole is 0
...
Calculate the speed, the torque developed in
armature
...
A 4 pole DC shunt motor takes 22
...
5ohms,
Rsh=125ohms, the armature is wave wound with 300 conductors
...
02Wb, calculate speed, torque and power developed
...
200V lap wound DC shunt motor has 800 conductors on its armature, the resistance of the
armature winding is 0
...
Find the speed and torque developed by the
motors
...
200V lap wound DC shunt motor has 800 conductors on its armature, the resistance of the
armature winding is 0
...
Find the speed and torque developed by the
motors
...
A 30 KW, 300 V DC shunt generator has armature and field resistance of 0
...
Calculate the total power developed by armature when it delivers
full output power
...
A DC shunt motor takes an armature current of 110A at 480V
...
2 ohms, The machines has 6 poles, and armature is lap connected with 864 conductors
...
05 Wb, calculate speed and torque developed by the armature
...
The emf generated in the armature of a shunt generator is 625 V, when delivering its full
load current of 400A to the external circuit
...
06 ohms
...
220 V series motor is taking a current of 40A, resistance of armature 0
...
25 ohms
...
Dept of EEE, SVIT