DC Machine
A DC machine may be a DC generator or a DC motor. The DC generator converts mechanical energy into electrical energy, while a DC motor converts electrical energy into mechanical energy.
A DC generator works on the principle that when a conductor moves in a magnetic field causes an emf induced in the conductor. A DC motor works on the principle that when a current carrying conductor is placed in a magnetic field, a mechanical force acts on conductor.
A DC machine has a coil winding wound on the poles, which gives a field stationary in the air gap due to DC supply in the field winding. The armature has commutator type winding with stationary brushes for connection to external circuit.
Construction of D.C. Machines
Basically DC machine consists of two main parts. One of them is the armature and other is field magnets. The amature is rotating part of the machine and field magnets is stationary part. In construction, there is no difference between DC generator and a DC motor.
When DC machine is working as a generator, its armature in rotated by external means and its field is connected directly to low DC supply voltage. Due to the rotation of armature conductors in the magnetic field, an emf is induced in the armature conductors according to the Faraday 's laws of electro-magnetic induction.
When DC machine is working as a motor its armature as well as magnetic field are fed direct with DC supply. The armature starts rotating because when a current carrying conductor is placed in a magnetic field, mechanical force is experienced on the conductor. They have the same parts.
A DC machine consists of the following main parts :
1. Yoke : It is the outer cylindrical frame of the machine. Cast iron yokes are made for small machines because of cheapness but for large machines yoke is made of fabricated steel due to huge permeability. Yoke serves the following purpose :
(i) Poles are fixed on it
(ii) It provides mechanical protection to the inner parts of the machine.
(iii) It provides a passage for magnetic flux produced by poles.
2. Poles : Pole consists of two main parts :
(i) Pole Core : The Pole Core is made of cast iron of solid piece or cast steel. The pole core may be made of thin cast steel laminations riveted together. This type of pole is held in position with frame by means of bolt. The pole core is usually circular in section and field coil is wound over it to carry field current.
(ii) Pole Shoe : The Pole Shoe is laminated and is screwed to the pole face by means of counter sunk screws. The pole serves following purposes :
(a) It supports the field coils.
(b) It spreads out the magnetic flux in the air gap more uniformally.
(c) The Pole shoes have larger cross-section, hence reluctance or magnetic path is reduced.
Armature : The armature is made up of laminations and the armature winding is done on the armature core. The armature should be of magnetic material such as in order to provide a path of low reluctance to the magnetic flux.
Commutator : It is made up of hard drawn copper segments insulated from each other by insulation. The ends of the armature winding are connected to the commutator segments. The function of the commutator is to convert AC into DC.
Brushes : The brushes are made of carbon. The brushes are used to give or get supply to the armature winding. The brushes are placed on the commutator segments.
Bearings : Bearings are fitted at the ends of the shaft and are supported in end plates.
Fan : Fans are fitted on the shaft in big machines to give air for cooling purpose.
Generator Action : In case of generator the armature and commutator are rotated in the magnetic field. The alternating emf is induced in the armature conductors, but with the help of commutator we get unidirectional emf and current in the external circuit.
Motor Action : In case of motor the DC supply is given to the armature conductors through commutator and DC supply is given to the field winding. With the help of commutator, we can change the direction of current in the armature conductors as the armature conductors pass from one pole to the next pole. So we get unidirectional electromagnetic torque produced.
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