Question

If residual magnetism is NOT present in a d.c. generator the induced emf at zero speed
1. 10% of rated voltage
2. 50% of rated voltage
3. Zero
4. Same as the rated voltage

Answer 1

Correct Answer - Option 3 : Zero

Concept:

• Residual magnetism is the small magnetic field left in the iron cores of the shunt fields when the generator is at rest
• Without it, the fields would have to be flashed with a DC current in order to start the generator generating; Hence to start the generator, it is necessary for the DC shunt generator
• Once it is destroyed, it can be restored by connecting its field finding to a DC source for a short time to magnetize the poles
• So, we need to connect the field winding to a battery (DC source)

 

Explanation:

In dc generators, induced emf is given by

\({E_b} = \frac{{\phi ZNP}}{{60A}}\)

Induced emf is directly proportional to speed, it will be zero at zero speed.