Question

A 250 V DC generator is run at rated speed with no excitation. The open circuit voltage will be

• A. Zero
• B. Very small, about 2 or 3 V
• C. About 100 V
• D. 250 V

Hint:

A DC generator cannot "self-start" or build up voltage if it loses its residual magnetism. In such cases, you need "field flashing" (applying an external DC source briefly) to restore it.

Answer 1

The Correct Option is B

This question explores the concept of "Residual Magnetism" in self-excited DC generators. 1. The Role of Excitation: In a DC generator, the output voltage ($E_g$) is given by $E_g = \frac{\phi Z N}{60} \times \frac{P}{A}$. This shows that voltage is directly proportional to the magnetic flux ($\phi$) and speed ($N$).

2. No Excitation Scenario: "No excitation" means there is no current flowing through the field windings to create a magnetic field. Theoretically, if the flux ($\phi$) were zero, the voltage output would be zero.

3. Residual Magnetism: In practice, the iron core of the field poles retains a small amount of magnetism from previous operations, known as

residual magnetism. Even without field current, this tiny amount of flux exists.

4. Resulting Voltage: As the armature rotates at rated speed ($N$) through this residual flux, it cuts the weak magnetic lines of force, inducing a very small EMF. This voltage is typically just

2 to 3 Volts. This small voltage is crucial because it provides the initial current to the field winding to begin the "voltage build-up" process.