Question

Describe the construction of direct current dynamo drawing labelled diagram. Explain its working. What is the difference between direct current and alternating current? Explain it.

Answer 1

A dc generator is an electrical machine which converts mechanical energy into direct current electricity.

Construction of direct current dynamo

It mainly consists of three main parts, i.e. Magnetic field system, Armature and Commutator and Brush gear. The other parts of a DC Generator are Magnetic frame and Yoke, Pole Core and Pole Shoes, Field or Exciting coils, Armature Core and Windings, Brushes, End housings, Bearings and Shafts.

1. Magnetic Field System of DC Generator :

The Magnetic Field System is the stationary or fixed part of the machine. It produces the main magnetic flux.

2. Magnetic Frame and Yoke:

The outer hollow cylindrical frame to which main poles and inter-poles are fixed is known as Yoke. It is made of cast steel or rolled steel for the large machines and for the smaller size machine the yoke is generally made of cast iron.

It provides mechanical protection to the inner parts of the machines.

3. Pole Core and Pole Shoes :

The Pole Core and Pole Shoes are fixed to the magnetic frame or yoke by bolts. Usually, the pole core and shoes are made of thin cast steel or wrought iron laminations which are riveted together under hydraulic pressure. The poles are laminated to reduce the Eddy Current loss.

4. Field or Exciting Coils :

Each pole core has one or more field coils (windings) placed over it to produce a magnetic field. The enamelled copper wire is used for the construction of field or exciting coils.

5. Armature of DC Generator :

The rotating part of the DC machine or a DC Generator is called the Armature.The armature consists of a shaft upon which a laminated cylinder, called Amature Core is placed.

6. End Housings :

End housings are attached to the ends of the Mainframe and provide support to the bearings.

7. Bearings :

The ball or roller bearings are fitted in the end housings. The function of the bearings is to reduce friction between the rotating and stationary parts of the machine.

8. Shaft :

The shaft is made of mild steel with a maximum breaking strength. The shaft is used to transfer mechanical power from or to the machine.

Working :

Suppose initially the plane of the coil is horizontal. The split rings S1 touches the brush B1 and S2 touches B2.

When coil ABCD is rotated clockwise, magnetic field lines passing through it change. The arm AB moves upwards and arm CD moves downwards.

According to Fleming’s right-hand rule, the induced current in the armature coil flows in the direction ABCD. In the external circuit, current flows from B2 to B1.

As the coil crosses its vertical position, the slip ring S1 come in contact with B2 and S2 with B1. The arm AB now moves downwards and arm CD moves upwards. The induced current in the armature flows along DCBA.

But in the external circuit, the current still flows from B2 to B1.

Hence, a unidirectional current flows in the external circuit.

Difference between direct current and alternating current

1. Alternating current is safe to transfer over longer distances and can provide more power but Direct Current can't be transferred over longer distances.

2. The frequency of alternating current is 50Hz or 60Hz depending upon the country while the frequency of direct current is zero.

3. AC current reverses its direction while flowing in a circuit while DC current flows in one direction in the circuit.

4. The magnitude of AC current varies with time while the magnitude of DC current remains constant.

5. AC current can be obtained from A.C Generator while DC current can be obtained from Cell or Battery.