A DC Machine can be used as DC Motor as well as DC Generator. Construction wise both are same but they have different working.
CONSTRUCTION OF DC MACHINE
Figure given below shows the Construction of a 4-pole DC machine.
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| 4-POLE DC MACHINE |
Essential Parts of the DC Machine are discussed below:
YOKE
Also called as Frame. Main function of the Yoke is to protect the internal parts of machine from any mechanical injury, dust or moisture. It provides mechanical support to the machine. It also provides the passage for magnetic flux produced by the poles.
For small machines, yoke is made up of Cast Iron and for large machines it is made up of fabricated steel.
POLE AND POLE SHOE
Pole of a DC machine is like an Electromagnet. Field winding are placed on it and produces the magnetic flux. Poles are made up of thin cast iron lamination riveted together.
Purpose of the pole shoe is to enlarge the cross section area so that the reluctance of the magnetic path is reduced. It spreads the magnetic flux in the air gap more uniformly.
FIELD WINDING
Purpose of the field winding is to produce the magnetic flux when an electric current is passed through it. It is placed on pole and a small DC source is connected to it. Material used for Field Winding is Enamelled Copper Wire.
ARMATURE
Armature is the rotating part of the machine and is cylindrical in shape. It is made up of thin silicon steel lamination, which are circular in shape and are riveted together. Thin lamination are used to reduce Eddy Current Loss.
On the outer periphery/circumference of the armature, slots are provided to accomodate the Armature winding.
ARMATURE WINDING
They are placed in the slots provided on Armature. Made up of enamelled copper wire and have multi-turns. When the Armature rotates, it will also rotate and an emf is induced in these winding.
COMMUTATOR
It is mounted on the same shaft as the Armature. Its function is to connect the rotating Armature winding to the stationary external circuit using brushes.It also provide uni-directional torque in case of DC motor.
Commutator is cylindrical in shape and is made up of hard drawn copper segments. These segments are insulated from each other by a thin sheet of mica.
BRUSHES
Brushes are placed and pressed upon the commutator and makes a connecting link between the armature winding and the external circuit. Brushes are made up of high grade carbon and it is placed in a particular position around commutator by brush holder.
WORKING OF A DC MACHINE
As we said earlier, Construction wise both DC motor and DC Generator are same but working principle of both are very different.
WORKING OF DC MOTOR
DC Motor converts the Electrical energy (in DC) into Mechanical Energy. Working of DC Motor is based upon the fact that, when a current carrying conductor is placed in a magnetic field, then it experiences a mechanical force whose direction is given by "Fleming's Left Hand Rule" and magnitude of that force is given by:
F = B * I *L
where, F = Force Experienced by the conductor.
B = Flux Density
I = Magnitude of Current carried by conductor
L = Length of Conductor
WORKING OF DC GENERATOR
DC Generator converts the Mechanical Energy into Electrical Energy (in DC). Its working principle is Faraday's Law of Electromagnetic Induction which states that, whenever a conductor is placed in a varying magnetic field (or conductor is moved in magnetic field), then an emf is induced in it. If the circuit of the conductor is closed, then current also starts flowing through it. Direction of the Induced Current can be determined by Fleming's Right Hand Rule.
Magnitude of the EMF induced is given the EMF Equation of DC Machine (read this article).
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