Question

When speed of DC generator decreases, the armature current
A) Increases
B) Decreases
C) Do not change
D) Increases and decreases continuously

Answer 1

Hint: The speed of DC motor can be changed by changing the current in the field or current in armature. When the current in the field is decreased then the flux through armature also decreases. This leads to the decrease in the back emf which leads to flow of the armature current through it. More the flux passing more the back emf will generate and less the current will flow in the armature.

Complete step by step solution:
When the speed of the DC generator decreases, the armature current increases. This is due to the generation of back emf in armature. Back emf is the potential which is generated due to self induction of the coils. More the magnetic flux passing through the coil, more will be the back emf generated. This back emf in turns generates self induced current in the opposite direction of the armature current.

This is also stated in faraday’s law which states that total emf generated is equal to rate of change in flux in opposite direction i.e. $$\xi = - {{d\phi } \over {dt}}$$ where $$\phi $$=total flux passing through it.

In a generator the mechanical energy is converted into electrical energy. The rotation performed creates the flux around the armature. As the speed decreases, the flux also decreases and in case back emf also decreases. This leads to an increase in armature current. But to gain more electrical energy the speed of the DC generator should be low.

Note: The coil in the armature of the DC generator generates the current which is indirectly related to the speed of the armature. Either it is a generator or motor, the basic principle behind them remains the same : that change in magnetic flux generates induced current in it. Also the induced current produces another back emf which opposes the main current.