DC Generators

Principle

DC generator or Direct Current generator are machines that converts mechanical energy to electrical energy. This conversion of energy is based on the principle of the production of dynamically induced emf.

Working

In a Direct current generator the field is on the stator, whereas the armature is on the rotor. The field coil is excited by a DC supply. The rotor is rotated with the help of a prime mover, thereby cutting the flux produced by the field coils. The armature cuts the rotor flux and an alternating emf is induced in it, which is converted into a unidirectional emf with the help of the commutator.

DC Generator Construction

A DC motor can be used as a DC generator without any constructional changes and vice versa.

Yoke

The outer frame of a dc machine is called as yoke. Yokes are made of cast iron in case of small generators. But for large generators usually cast steel or rolled steel is employed. The purpose of the yoke in electrical machine is to provide mechanical strength and to carries the magnetic flux produced by the poles.

Commutator

Commutator is of cylinderical structure and is built up of wedge-shaped segments of high-conductivity hard-drawn or drop forged copper. Each segments in the commutator are insulated from each other by thin layers of mica. The number of segments in the commutator is equal to the number of armature coils. The purpose of commutator is to convert alernating current induced in the armature conductors into unidirectional current in the external load circuit.

Field winding

The field winding consist of copper wire or copper strip, are wounded on the poles of the electric machine. When current is passed through these coils, they electromagnetise the poles which produced the necessary flux that is cutted by revolving armature conductors.

Armature winding

The armature windings, which are the first wound in the form of flat rectangular coils, are usually former wound. Finally, the armature coils are pulled into their appropriate shape in a coil puller. The various conductors in the coils are insulated from each other. In electrical machine, the armature conductors are placed in the armature slots. The armature conductors are held in the slots by wedges made of fibre, driven into the top of the slots to avoid the flying of conductors under the centrifugal forces.

Brushes

Usually, the brushes are housed in a brush holder. The function of brushes is to collect current from the commutator and to supply the same to the external circuit. The brushes are placed in such a way that they can rest on the commutator by placing them in the brush holder against the action of a spring.

Bearings

The brushes are usually made of carbon or graphite and are in the shape of a rectanglar block. The ball bearings are habitually employed for their reliability. For heavy duties purpose, roller bearings are widely prefered.

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