The document discusses the fundamentals of single phase induction motors and DC machines, highlighting that single phase induction motors are not self-starting due to the nature of the magnetic field produced. It details the construction and operation of DC machines, including their main parts: magnetic-field system, armature, and commutator. Additionally, it explains the principle of generating electromotive force (emf) in DC machines, providing a derived equation for emf based on various factors.

1. PEE-102A
Fundamentals of Electrical Engineering
Lecture-10
Instructor:
Mohd. Umar Rehman
EES, University Polytechnic, AMU
May 27, 2021
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2. Single Phase Induction Motor
From application point of view, the single phase IM is more common than the
three phase IM because single phase supply is readily available.
There are no constructional differences between a 1-ph IM and 3-ph IM except
that the stator has a single phase winding and is supplied with a single phase
AC voltage, so that single phase AC current flows through the winding.
However, 1-phase AC current is not sufficient to produce a rotating magnetic
field, and that means that the 1-phase IM is NOT self-starting unlike a 3-phase
IM.
This the major disadvantage of the 1-phase IM
The requirement for producing a rotating magnetic field in a 1-phase IM is the
provision of at least two phases supplied with 2-phase currents having some
phase angle difference.
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3. Why 1-Phase IM is not self-starting? I
When single phase AC is supplied to the single phase winding of stator of a
1-ph IM, 1-phase AC current flows through it.
However, the magnetic field produced by this current in pulsating in nature
This means that the magnetic field starts to rise in one direction, goes to zero,
and then builds up in the opposite direction.
Due to inertia, the rotor is unable to rotate. Hence, a 1-phase IM is not self
starting.
However, when an initial push in some direction of rotation is given to the rotor,
it starts rotating in that direction. This is manual method and is not practically
possible.
Special methods are used to drive a 1-phase IM in practice.
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4. Why 1-Phase IM is not self-starting? II
Many theories have been developed to explain this phenomenon in detail. One
of the them is the Double Field Revolving Theory.
According to this theory a a pulsating magnetic field is resolved into two rotating
magnetic fields. They are equal in magnitude but opposite in directions. The
induction motor responds to each of the magnetic fields separately. The net
torque in the motor is equal to the sum of the torque due to each of the two
magnetic fields.
Since, the two torques are in opposite direction, net torque is zero and the
motor does not start.
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5. DC Machines
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6. DC Machines
DC Machines are types of electrical machines that use DC current in the case
of DC motors and generates DC voltages in case of DC generator.
DC motor transforms electrical power into mechanical power and the generator
converts mechanical power into electrical.
Most DC machines are similar to AC machines: i.e. they have AC voltages and
current within them.
DC machines have DC outputs just because they have a mechanism convert-
ing AC voltages to DC voltages at their terminals. This mechanism is called a
commutator; therefore, DC machines are also called commutating machines.
DC generators are not as common as they used to be, however, DC motors
have many applications.
Principle of Working of DC Generator: An electric generator is based on the
principle that whenever flux is cut by a conductor, an EMF is induced which will
cause a current to flow if the conductor circuit is closed.
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7. Construction of DC Machines
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8. Construction of DC Machines: Some Images
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9. Construction of DC Machines: Some Images
Figure: DC Machine Armature (Laminated)
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10. Construction of DC Machines: Some Images
Figure: Pole (Electromagnet Type)
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11. Construction of DC Machines: Some Images
Figure: Cut-away view of DC Machine
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12. Description of DC Machine Parts
A DC machine consists of three main parts
1. Magnetic-field system
2. Armature
3. Commutator and brushgear
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13. 1. Magnetic-field system
The magnetic-field system is the stationary (fixed) part of the machine. It produces
the main magnetic flux. The outer frame or yoke is a hollow cylinder of cast steel or
rolled steel. An even number of pole cores are bolted to the yoke. The yoke serves
the following two purposes:
(a) It supports the pole cores and acts as protecting cover to the machine.
(b) It forms a part of the magnetic circuit.
Since the poles project inwards they are called salient poles. Each pole core has a
pole shoe having a curved surface. The pole shoe serves two purposes:
(i) It supports the field coils.
(ii) It increases the cross-sectional area of the magnetic circuit and reduces its
reluctance.
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14. 1. Magnetic-field system...Contd
The pole cores are made of sheet steel laminations that are insulated from each
other and riveted together. The poles are laminated to reduce eddy-current loss.
Each pole core has one or more field coils (windings) placed over it to produce
a magnetic field. The field coils (or exciting coils) are connected in series with one
another such that when the current flows through the coils, alternate north and south
poles are produced in the direction of rotation.
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15. 2. Armature
The rotating part of the DC machine is called the armature. The armature consists of
a shaft upon which a laminated cylinder, called armature core, is mounted. The ar-
mature core has grooves or slots on its outer surface. The laminations are insulated
from each other and tightly clamped together. In small machines the laminations are
keyed directly to the shaft. In large machines they are mounted on a spider. The
purpose of using laminations is to reduce eddy-current loss.
The insulated conductors are put in the slots of the armature core. The conductors
are wedged bands of steel wire are fastened round the core to prevent them flying
under centrifugal forces. The conductors are suitably connected. This connected
arrangement of conductors is called armature winding. Two types of windings are
used-wave and lap.
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16. 3. Commutator and brushgear
Alternating voltage is produced in a coil rotating in a magnetic field, To obtain di-
rect current in the external circuit a commutator is needed. The commutator, which
rotates with the armature, is made from a number of wedge-shaped hard-drawn cop-
per bars or segments insulated from each other and from the shaft. The segments
form a ring around the shaft of the armature. Each commutator segment is con-
nected to the ends of the armature coils.
Current is collected from the armature winding by means of two or more carbon
brushes mounted on the commutator. Each brush is supported in a metal box called
a brush box or brush holder. The pressure exerted by the brushes on the commu-
tator can be adjusted and is maintained at a constant value by means of springs.
Current produced in the armature winding is passed on commutator and then to the
external circuit by means of brushes.
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17. EMF Equation of DC Machines I
As the armature of a DC machine rotates, EMF is induced in the conductor
coils due to EMI.
In case of a generator, the EMF is known as generated EMF.
In case of a motor, the induced EMF is known as back EMF or counter EMF.
The expression for both cases remains the same.
We shall now derive the EMF equation of a DC machine.
Let,
φ = Useful flux per pole (Wb)
P = Total number of poles
Z = Total number of conductors in the armature
N = Speed in RPM
A = No. of Parallel Paths (A = 2 or A = P)
Z/A = No. of armature conductors in series for each parallel path
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18. EMF Equation of DC Machines II
Now, EMF induced in one conductor is
E =
∆φ
∆t
Also, flux cut by one conductor in one revolution of the armature is
∆φ = Pφ
Time taken to complete one revolution is
∆t =
1
N
min =
60
N
sec
Hence, EMF in one conductor is
E =
∆φ
∆t
=
NPφ
60
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19. EMF Equation of DC Machines III
Total EMF generated
= EMF per conductor × No. of conductors per parallel path
E =
NPφ
60
×
Z
A
E =
NPφZ
60A
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