Question

A 300 V of separately excited dc generator is run at rated speed without excitation, terminal voltage on no load condition will be:
1. Zero
2. Very low 2 to 3 V
3. 300 V
4. Half voltage 150 V

Answer 1

Correct Answer - Option 2 : Very low 2 to 3 V

Separately excited DC Generator without Excitation:

If the generator is run at any particular speed without field excitation, some EMF will be generated due to residual magnetism in the main poles.

This power of magnetism after the applied magnetism is removed is called residual magnetism of a substance.

The EMF induced (E_g) in DC generator is given by,

\(E_g=\frac{Pϕ ZN}{60A}\)

If the number of poles (P), conductor (Z), Parallel Path (A) is constant,

E_g ∝ ϕN

Since the residual magnetism flux is 2% to 3% of main field flux.

In that case, the EMF induced will also be 2% to 3% of actual induced EMF.

As we have an EMF rating of 300 V,

Hence, EMF at no load due to residual magnetism will be (2% to 3%) of 300 V = 2 to 3 V