Question

Q1. Explain the basic construction of DC machines.

Q2. Explain the basic operating principle of DC generators.

Q3. In a DC machine, if the armature winding is replaced by permanent magnets, can the machine still work? Give your explanation

Answer 1

1.CONSTRUCTION OF DC MACHINES.

A DC Machine is the an electro-mechanical energy conversion device.A DC machine consists of two basic parts;     stator and rotor.The below figure shows the basic construction diagram of a dc machine.

a) YOKE : The outer frame of a dc machine is called yoke. It is made of cast iron or steel. It provides mechanical strength to the whole assembly. Also it provide a path for the magnetic flux produced by the field winding.

b) POLES AND POLE SHOES : Poles are joined to the yoke with the help of bolts. They carry the field winding and pole shoes are fastened to them. Pole shoes serve two purposes; i) They support field coils. ii) Spread out the flux in air gap uniformly.

c) FIELD WINDING :They are made of copper. Field coils are former wound and placed on each pole and are connected in series. When energized, they form alternate North and South poles.

ARMATURE CORE : Armature core is the rotor of a dc machine. It is cylindrical in shape with slots carry armature winding. The armature is built of thin laminated circular disks for reducing eddy current loss.

ARMATURE WINDING : It is usually former wound copper coil which rests in armature slots. The armature conductors are insulated from each other and also form armature core. Armature winding can be wound by either lap or wave winding methods.

COMMUTATOR AND BRUSHES : Physical connection to the armature winding is made through commutator - brush arrangement. The function of a commutator, in a dc generator is to collect current. Where as in a dc motor is to provide current to the armature conductors. A commutator consists of set of copper segments which are insulated from each other. Brushes are usually made of carbon or graphite. They rest on commutator segment and slide on the segments when the commutator rotates keeping the physical contact to collect or supply current.

2) WORKING PRINCIPLE OF DC GENERATOR

In a DC generator, Field coils produce an electromagnetic field and the armature conductors are rotated into the field. Thus according to Faraday's laws of electromagnetic induction, Which states that whenever a conductor is placed in a varying magnetic field, an emf gets induced in the conductor, so here also when the armature rotates it cuts the flux so emf gets induced in the conductors and when a closed path is provided current will circulate within the path.

3) As explained in the generator working principle when the armature conductors rotates it cuts the flux and emf is produced. But here the current produced will be AC. It is with the help of split ring commutator we are converting the AC to DC in case of DC machines.

As shown in figure when the armature conductors rotates the split rings also move from one brush to another so that the current in the output will be DC only. This is how a DC machines operates. But for this to happens rotating part always need to be armature.Otherwise commutation won't be possible.so if the armature winding is replaced by permanent magnets the machine can't work as a dc machine.