Constructional Details of DC machine:
The above figure shows constructional details of a simple 4-pole DC machine. A DC
machine consists of two basic parts; stator and rotor. Basic constructional parts of a DC
machine are described below.
1. Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast iron or
steel. It not only provides mechanical strength to the whole assembly but also
carries the magnetic flux produced by the field winding.
2. Poles and pole shoes: Poles are joined to the yoke with the help of bolts or welding.
They carry field winding and pole shoes are fastened to them. Pole shoes serve two
purposes; (i) they support field coils and (ii) spread out the flux in air gap uniformly.
3. Field winding: They are usually made of copper. Field coils are former wound and
placed on each pole and are connected in series. They are wound in such a way that,
when energized, they form alternate North and South poles.
4. Armature core: Armature core is the rotor of a dc machine. It is cylindrical in shape
with slots to carry armature winding. The armature is built up of thin laminated
circular steel disks for reducing eddy current losses. It may be provided with air
ducts for the axial air flow for cooling purposes. Armature is keyed to the shaft.
5. Armature winding: It is usually a former wound copper coil which rests in
armature slots. The armature conductors are insulated from each other and also
from the armature core. Armature winding can be wound by one of the two
methods; lap winding or wave winding. Double layer lap or wave windings are
generally used. A double layer winding means that each armature slot will carry two
different coils.
6. Commutator and brushes: Physical connection to the armature winding is made
through a commutator-brush arrangement. The function of a commutator, in a dc
generator, is to collect the current generated in armature conductors. Whereas, in
case of a dc motor, commutator helps in providing current to the armature
conductors. A commutator consists of a set of copper segments which are insulated
from each other. The number of segments is equal to the number of armature coils.
Each segment is connected to an armature coil and the commutator is keyed to the
shaft. Brushes are usually made from carbon or graphite. They rest on commutator
segments and slide on the segments when the commutator rotates keeping the
physical contact to collect or supply the current.
WORKING PRINCIPLE OF A DC MOTOR :
The DC motor is the device which converts the direct current into the mechanical work.
It works on the principle of Lorentz Law, which states that “the current carrying
conductor placed in a magnetic and electric field experience a force”. And that force is
called the Lorentz force. The Fleming left-hand rule gives the direction of the force.
Fleming Left Hand Rule:
If the thumb, middle finger and the index finger of the left hand are displaced from each
other by an angle of 90°, the middle finger represents the direction of the magnetic field.
The index finger represents the direction of the current, and the thumb shows the
direction of forces acting on the conductor.
For simplicity, consider that the armature has only one coil which is placed between the
magnetic field shown below in the figure A. When the DC supply is given to the armature
coil the current starts flowing through it. This current develops their own field around
the coil. Figure B shows the field induces around the coil.
By the interaction of the fields (produces by the coil and the magnet), resultant field
develops across the conductor. The resultant field tends to regain its original position,
i.e. in the axis of the main field. The field exerts the force at the ends of the conductor,
and thus the coil starts rotating.
Let the field produces by the main field be F m, and this field rotates in the clockwise
direction. When the current flows in the coil, they produce their own magnetic field says
Fr. The field Fr tries to come in the direction of the main field. Thereby, the torque act
on the armature coil.
Back EMF in a DC Motor:
When a Dc source is used to excite the armature conductor, which is placed in the main
magnetic field, an electromagnetic torque is developed and hence, the armature of the
DC motor starts rotating. Due to the rotation of armature, the armature conductors cut
the magnetic flux of the main magnetic field and hence an induced emf is developed in
the armature conductors. Since this induced emf developed in the armature conductors
opposes the cause that produces it, this induced emf is known as back emf E b.
∅𝑍𝑁𝑃
Eb= 60𝐴
The significance of back emf is to regulate the armature current according to the load
connected to the motor.
