The working principle of DC generator is based on the Faraday’s law of electromagnetic induction. According to this law, when the magnetic flux liked to a conductor or coil changes an EMF is induced in the conductor or coil. The magnitude of this induced EMF is given by,

$$mathrm{mathit{e}:=:mathit{N}frac{mathit{dphi }}{mathit{dt}}:cdot cdot cdot (1)}$$

Where, $phi$ is the flux linkage of the coil and N is the number of turns in the coil.

In case of a DC generator, the magnetic flux ($phi$) remains stationary and the coil rotates. The EMF induced where the coil is rotating and flux is stationary, is known as dynamically induced EMF.

In order to understand the working principle of a DC generator, we consider a single loop DC generator (i.e. N = 1) as shown in above figure. Here, the coil is rotated by some prime mover (a source of mechanical energy), and there is a change in the magnetic flux linkage to the coil.

Let $phi$ be the average magnetic flux produced by each magnetic pole of the machine, then the average induced EMF in the generator is given by,

$$mathrm{mathit{E_{av}}:=:frac{mathit{dphi }}{mathit{dt}}:=:mathrm{Flux: cut: per:sec:by: the :coil}}$$

$$mathrm{Rightarrow mathit{E_{av}}:=:mathrm{Flux: cut: in :one :rotation:times :No.:of: rotations: per: sec}}$$

$$mathrm{Rightarrow mathit{E_{av}}:=:mathrm{left ( Flux:per:poletimes No.:of:poles right )}:times :mathrm{No.:of :rotations :per: sec}}$$

$$mathrm{therefore mathit{E_{av}}:=:mathit{phi :times P:times :n}:cdot cdot cdot (2)}$$

Where, P is the total number of poles in the generator and n is the speed of the coil in rotation per second. The expression in the Equation-(2) gives the average induced EMF in a single loop DC generator.

The following points explain the working principle of a DC generator −

• Position 1 − The induced EMF is zero because, the movement of coil sides is parallel to the magnetic flux.

• Position 2 − The coil sides are moving at an angle to the magnetic flux, and hence a small EMF is generated in the loop.

• Position 3 − The coil sides are moving at right angle to the magnetic flux, therefore the induced EMF is maximum.

• Position 4 − The coil sides are cutting the magnetic flux at an angle, thus a reduced EMF is induced in the coil sides.

• Position 5 − No flux linkage with the coil side and the coil sides are moving parallel to the magnetic flux. Therefore, no EMF is induced in the coil.

• Position 6 − The coil sides move under a pole of opposite polarity and hence the polarity of induced EMF is reversed. The maximum EMF will induce in this direction at position 7 and zero when it is at position 1. This cycle repeats with rotation of the coil.

In this way, EMF is induced in a DC generator. Though, this induced EMF is alternating in nature, which is then converted in the unidirectional EMF by using a device called commutator.

The direction of induced EMF in the armature conductor of the DC generator is determined by the Fleming’ right hand rule (FRHR) which we discussed in the module-1 (basic concepts) of this tutorial.