(a) zero.
(b) very small, say about 2 or 3 V.
(c) about 100 V.
(d) 250 V.
Answer: (b) very small, say about 2 or 3 V.
Explanation:
CONCEPT: Residual Magnetism in DC Generators
In a DC generator, even when no field current is supplied, a
small amount of magnetism remains in the field poles.
This magnetism is called:
👉 Residual
Magnetism (Residual Flux)
It is present due to previous operation of the machine.
Why Residual Magnetism Is Important
Residual magnetism is necessary for:
✔ Initial voltage generation
✔ Self-excitation process
✔ Building up of terminal voltage
Without residual magnetism, the generator cannot generate
voltage.
GIVEN CONDITION
A DC generator:
- Rated
voltage = 250 V
- Running
at rated speed
- No
external field excitation (If = 0)
EXPLANATION: Initial Voltage Generation
When the armature rotates:
- Due
to residual flux (Φr), conductors cut magnetic field.
- According
to Faraday’s Law, EMF is induced.
- This
induced voltage is very small.
Typical value:
👉 About 2 to 3 V
This is called:
✔ Residual voltage
STEPWISE PROCESS: Voltage Build-Up (Self-Excitation)
Step 1: Residual Voltage Generation
- Armature
rotates
- Cuts
residual flux
- Produces
small voltage (2–3 V)
Step 2: Field Current Starts Flowing
- This
small voltage is applied to field winding
- Field
current flows:
Step 3: Increase in Field Flux
- Field
current produces more magnetic flux
- New
flux adds to residual flux
Step 4: Increase in Induced EMF
- Increased
flux → More EMF induced
- Voltage
rises
Step 5: Cumulative Process
This process repeats:
✔ More voltage → More field
current
✔ More field current →
More flux
✔ More flux → More
voltage
This is called:
👉 Cumulative
Build-Up
Step 6: Rated Voltage Reached
Finally, voltage reaches rated value:
👉 250 V
When:
- Field
circuit resistance balances
- Magnetic
saturation occurs
Then voltage becomes stable.
CONDITIONS FOR VOLTAGE BUILD-UP
For successful self-excitation, three conditions must be
satisfied:
1. Presence of Residual Magnetism
✔ Some magnetism must exist in
poles
2. Correct Field Connection
✔ Field winding must aid
residual flux
✔ (Not oppose it)
3. Field Resistance Must Be Below Critical Value
✔ Field resistance < Critical
resistance
Otherwise voltage will not build up.
If Residual Magnetism Is Lost
If residual magnetism is lost:
❌ No initial voltage
❌
No field current
❌
No voltage build-up
❌
Generator fails
Solution:
👉 Field flashing
(external DC supply)
IMPORTANT FORMULA
Generated EMF:
So:
EMF ∝ Flux ×
Speed
At constant speed:
So increasing flux increases voltage.
APPLICATION IN EXAMS
One-Line Answer
When a DC generator runs without excitation, a small voltage
(2–3 V) is generated due to residual magnetism, which initiates the
self-excitation process and builds up the terminal voltage.
FINAL CONCLUSION ⭐
When a 250 V DC generator is run at rated speed without
external excitation, the residual magnetism in the field poles produces a small
induced voltage of about 2 to 3 V. This voltage causes a small field current to
flow, increasing the magnetic flux and induced EMF. The process repeats
cumulatively until the rated terminal voltage is reached. This phenomenon is
called self-excitation of a DC generator.