DC Machines:
The machine which produce direct quantity (voltage or current) or operated by direct voltage is called
so.
Types of DC machines:
These are of two types: 1. DC Generator
2. DC Motor
1. DC Generator
Def: It is an electromechanical device that convert mechanical energy to electrical
energy.
Principle of operation: It work’s on Faradays’s law of electromagnetic induction.
Which states that when a conductor links with rate of change of flux then emf is induced
on it.
Construction:
Main parts of DC machines:
a) Yoke
b) Pole shoe, field winding, field poles
c) Armature, armature winding
d) Commutator
e) Brushes
(i) Yoke:
It is a outer frame of machine.
It protect the machine
It provides mechanical supports to the machine.
It supports pole shoe.
It provides path for magnetic flux.
It protects internal parts of the machines.
(ii) Pole shoe, fielding, field poles
It supports the field winding.
Power is supplied to the field winding from the external DC source.
When current flows through the field winding the pole shoe behaves like a magnetic
pole (field pole).
Another important function of field pole is it distributes the magnetic flux in wider
area.
(iii) Armature: (Armature core, armature slot, armature winding)
It is a cylindrical structure.
It is a rotating parts of machine. Hence, it is also called rotor.
On armature, slots are there.
Armature conductors (windings) are placed on the slot.
Two ends of armature conductors are connected to commutator.
Armature cores are laminated to reduce eddy current loss.
(iv) Commutator:
It is also circular (cylindrical) types.
These are the copper bars separated by the insulations.
It is connected to armature. Hence, it also rotates when armature rotates.
It collects the alternating current from armature conductors and converted it to direct
current.
As it converts the alternating quantity to direct quantity and rotating in nature, so it is
called mechanical rectifier.
(v) Brushes:
It is made by carbon or graphite.
It is fixed in nature.
Commutator bars are touched with it.
Its main function is to collect the current from the commutator and sends to the load.
2. Working
Flux is flowing from North Pole to South Pole.
Armature having armature conductors are placed inside the magnetic field
Now armature is rotated.
When armature rotates, at different position the flux linkage are different. And hence,
according to the Faradays’s law of electromagnetic induction principle, the emf is
induced on it. It is called generated emf, which is given by
ZN P
Eg Volt.
60 A
Where, = Flux per pole.
Z= Number of conductors.
N= Circular speed of armature.
P= Number of poles.
A= Number of parallel path
Eg =Generated emf in the armature conductor.
As the external circuit is closed (load is connected on it), an induced current flows
through it. The direction of induced current is determined by Fleming’s right hand rule.
DC Motor
Def: It is an electromechanical device that convert electrical energy to mechanical
energy.
Principle of operation: This DC or direct current motor works on the principal, when
a current carrying conductor is placed in a magnetic field, it experiences a torque and
has a tendency to move. This is known as motoring action.
Construction: (Main parts)
(Same as DC Generator)
Working:
I I
Flux is flowing from North Pole to South Pole.
Supply is given from a voltage source V.
Hence, current I flows through the armature conductors.
And hence, according to the motor action that is when a current carrying conductor is
placed in a magnetic field, it experiences a torque.
As it experiences the torque, hence it start to rotate. The direction of rotation can be
determined by Fleming’s left hand rule.
As, it rotates then it cuts the magnetic field ( i.e. flux linkages changes), Hence emf is
induced on it. This induced emf opposes the supply voltage (V).
As the generated emf in motor opposes the supply emf, it is called back emf ( Eb ).
And
ZN P
Eb
60 A
