A DC generator converts the mechanical input into an electrical output. The mechanical energy input is given to the dc generator by means of a prime mover. The prime mover can be a dc motor, water turbine, steam turbines, hand cranks, etc. The electrical energy output is DC power.
This energy conversion process of a dc generator is based on the principle of Faraday’s Law of Electromagnetic induction. In simple words, the working of a dc generator can be defined. When a conductor is rotated in magnetic flux, it cuts the flux, and hence a dynamically induced emf is produced.
When a load is connected, the emf thus induced will cause the current to flow through the conductor. To perform this operation, a DC generator requires two parameters: magnetic lines of force or magnetic flux and a conductor in motion.
To understand the working principle of DC Generator, a single loop coil or winding is considered. The operation is explained with both slip rings and split rings.
How does a DC Generator Work?
Working of DC Generator with Slip rings
To understand the working of a dc generator, let us consider a single turn coil ABCD rotated on a shaft in a clockwise direction. The coil is placed in a uniform magnetic field, between a north pole(N) and a south pole(S).
The following figure shows the diagram of a single turn dc generator. The blue color lines in the below figure represent the magnetic flux lines.
The coil sides AB and CD are connected to the slip rings a and b respectively. From the slip rings, the current is taken out and given to the load through brushes 1 and 2.
When the coil rotates through successive positions in the magnetic field, the flux linked with it changes. According to Faraday’s Law of Electromagnetic induction, the emf induced is proportional to the rate of change of flux linkages.
When coil sides AB and CD are moving parallel to the magnetic field, the coil sides do not cut the flux. Instead, they move parallel to the flux. Hence, no emf is induced in the coil.
As the coil rotates further, the rate of change of flux linkage increases steadily, till position 2 is reached where θ = 900. This can be seen in the below waveform.
At this angle, the coil sides AB and CD are perpendicular to the magnetic field. Now, the flux linked with the coil is minimum but the rate of change of flux is maximum. Hence, maximum emf is induced in the coil at position 2.
As the coil rotates further from θ = 900 to θ = 1800, the rate of change of flux linkage reduces steadily, til position 4 is reached. Here, again the coil sides become parallel to the magnetic flux lines. The rate of change of flux linkage will be minimum. So no emf is induced when it reaches position 4.
Thus during the first half rotation of the coil from θ = 00 to θ = 1800, no emf is induced at position 0, then increases and becomes maximum at position 2, then decreases and no emf is induced at position 4.
The direction of the emf induced can be found by applying Fleming’s Right-hand rule. By applying this law, the direction of the flow of current is found to be ABFGCD. The load current flows from F to G.
In the next half rotation of the coil, that is, from θ = 1800 to θ = 3600, the variation in the emf induced is similar to that of the first half rotation. No emf is induced at position 4, then increases and becomes maximum at position 6, then decreases, and no emf is induced at position 8.
But the direction of current induced gets reversed in the second half rotation. The path of current flow is from DCFGBA. It is just the reverse of the current flow during the first half rotation of the coil.
If this rotation of the coil is continued, the change in emf is repeated and becomes alternately positive and negative. Such an emf is called an alternating emf. It has both positive and negative half-cycles and is called a bidirectional output.
Working of DC Generator with split rings or commutator
Since, we are dealing with the working principle of a DC generator, where the flow of current is unidirectional, we need some changes in the circuit. So replace the slip rings with the split rings in dc generator.
Split rings are made up of conducting cylinder which is cut into two segments insulated from each other by a thin layer of mica or some insulating materials. The split rings are also called as a commutator. The coil ends are joined to these segments on which the carbon brushes are rested to tap the current to the outside load.
During the first half rotation of the coil, the current flows from ABFGCD. The brush 1 will get in contact with the commutator segment E and brush 2 will be in contact with the segment H. So the current flows from F to G in the load.
During the second half rotation of the coil, the current gets reversed and the path of flow is DCFGBA. The brush 1 will get in contact with the commutator segment H and brush 2 will be in contact with the segment E. So the current flows from F to G in the load.
For each half rotation of the coil, the commutator segments change their position. The current through the load flows in the same direction, from F to G. Thus the current thus produced is unidirectional but not continuous like a pure DC current.
An important thing to be noted here is that the current induced while using the split rings is also alternating in nature. But because of the commutation process or rectifying process, we get the output current as a unidirectional one. Thus Both in DC generator and AC generator the induced voltage is alternating in nature.
The emf induced in a dc generator is given by an equation