# DC Generator Principle MCQ || DC Generator Principle Questions and Answers

1. The nature of emf induced in the armature of a dc generator is _____ and it is converted into _____ by means of split rings.

1. DC : AC
2. AC : AC
3. DC : DC
4. AC : DC

Explanation:

• In DC generator, the commutator also known as split ring facilitates the collection of current from the armature conductors and converts the alternating emf induced (AC) in the armature conductors to unidirectional emf (DC) in the external load circuit.
• In a DC generator, the current is induced from the rotation of the armature within the field magnetic flux.
• In a DC motor, the alternating current is produced by the rotation of the commutator.

2. A dc shunt generator has a critical field resistance of 400 Ω at 1200 rpm. Now if the speed of the generator is increased to 1800 rpm what is the change in the critical resistance?

1. Increases to 900 Ω
2. Remains the same
3. Increases to 600 Ω
4. Decreases to 267 Ω

Explanation:

The speed for which the given field resistance acts as critical resistance is called the critical speed. The critical speed is directly proportional to critical resistance.

Rc ∝ Nc

Rc2 = Rc1 × Nc2Nc1

Given: Rc1 = 400Ω, Nc2 = 1800 rpm, Nc1 = 1200 rpm

Rc2 = 400 × 1800/1200

Rc2 = 600 Ω

3. A dc generator running at 1200 rpm has voltage regulation of 6%. When it runs at 1400 rpm, what will happen to its voltage regulation?

1. Increase
2. Decrease
3. Remains at 6%
4. Become zero

Explanation:

Voltage regulation: It is given by the percent change in the armature terminal voltage from no load to full load condition, with respect to the rated armature voltage.

%voltage regulation $= \dfrac{{{E_0} – {V_{fl}}}}{{{V_{fl}}}}$

Vfl = Full load voltage or rated voltage.

No-load voltage is the emf generated by the generator given as

$E = \frac{{\phi ZNP}}{{60A}}$ ——- (2)

The no-load voltage from equation(2) is directly proportional to the speed.

When the speed increases no-load voltage increases thereby voltage regulation also increases.

4. A 300 V of separately excited dc generator is run at rated speed without excitation, the 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:2. Very low 2 to 3 V

Explanation:

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

${E_g} = \frac{{P\phi ZN}}{{60A}}$

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

Eg ∝ 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

5. An eight-pole D.C. generator has a simple wave wound armature containing 32 coils of 6 turns each. Its flux per pole is 0.06 wb. The machine is running at 250 rpm. The induced armature voltage is

1. 96 V
2. 192 V
3. 384 V
4. 768 V

Explanation:

Total No, of conductor = coil × turn × 2 = 32 × 6 × 2 = 384

ϕ = Flux, N = speed, P = Number of poles, A = Number of parallel paths, Z = Number of conductors

Induced armature voltage $= \left( {\frac{{\phi ZN}}{{60}}} \right)\left( {\frac{P}{A}} \right)$

$= \frac{{0.06 \times 384 \times 250}}{{60}} \times \frac{8}{2}$

(For wave wound A = 2) = 384

6. A shunt generator has a critical field resistance of 200 ohm at a speed of 800 rpm. If the speed of the generator is increased to 1000 rpm, what is the change in critical field resistance of the generator?

1. Decrease to 160 ohm
2. Remains the same at 200 ohm
3. Increases to 250 ohm
4. Increases to 312.5 ohm

Explanation:

Critical resistance is the resistance of the field circuit at a given speed at which the generator just excites and starts voltage building while beyond this value generator fails to excite.

RC ∝ NC

RC2 ∝ RC1 × NC2/NC1

RC2 = 200 × 1000/800 = 250Ω

7. Identify the increasing order of voltage regulation of the following dc generator as same load

1. Differential compound, shunt and under compound
2. Shunt, differential compound and under compound
3. Under compound, shunt and differential compound
4. Shunt, under compound and differential compound

Answer:3. Under compound, shunt and differential compound

Explanation:

The voltage variation is higher in case of differential compound and lower for under compound generator

Under compound  < Shunt generator < Differential compound

8. The residual magnetism of a D.C. shunt generator can be regained by

1. Connecting the shunt field to the battery
2. Running the generator on no load for some time
3. Grounding the shunt field
4. Reversing the direction of rotation of the generator

Answer: 1. Connecting the shunt field to the battery

Explanation:

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 (Battery) for a short time to magnetize the poles.

9. In a DC generator, generated emf is directly proportional to _____

1. Armature current
2. Pole flux
3. Number of armature parallel paths
4. All of the above

Explanation:

In a dc generator, generated emf is given as

${E_g} = \frac{{P\phi ZN}}{{60A}}$

E = V + IRa

Where,

Z is conductor per pole

P is the number of poles

Φ is the flux per pole

N is the speed in rpm

A is the number of parallel paths

Iis armature current

Ris armature resistance

V is the terminal voltage

Thus, from the dc generator induced emf expressions, it is clear that the induced emf is directly proportional to the conductor per pole, speed, number of poles and flux per pole.

10. In a D. C. generator the critical resistance can be increased by ________

1. Increasing its field resistance
2. Decreasing its field resistance
3. Increasing its speed
4. Decreasing its speed

Explanation:

• The maximum field circuit resistance (for a given constant speed) at which the generator would just excite is known as its critical field resistance.

Critical Speed (NC):

• The critical speed of the DC generator is the minimum speed below which it fails to excite.
• It is the speed at which the given shunt field resistance represents the critical speed.
• In a D. C. generator, the critical resistance can be increased by Increasing its speed.

11. The slight curvature at the lower end of the OCC of a self-excited dc generator is due to

1. Magnetic inertia
2. Residual flux
3. High speed
4. High field resistance

Explanation:

• Self-excited dc generators whose field winding is energized by the current supplied by their own armature.
• Due to some residual magnetism, some of the flux is always present in the poles.
• When the armature is rotated by the means of the prime mover an e.m.f and current are produced.
• The slight curvature at the lower end of the OCC of a self-excited dc generator is due to Residual flux.

12. A shunt generator develops stable output voltage if

1. Magnetization characteristic is linear
2. Magnetization characteristic is non-linear
3. The field winding resistance is less than the critical resistance
4. The residual magnetism is equal to zero

Answer:3. The field winding resistance is less than the critical resistance

Explanation:

• There must be some residual magnetism in generator poles.
• Some amount of emf will be generated due to residual magnetism in the main poles. This small emf in turn produces additional flux to reinforce the original residual flux. This process is continued till the generator builds up the rated voltage.
• The connections of the field winding should be such that the field current strengthens the residual magnetism.
• The resistance of the filed circuit should be less than the critical resistance.
• Critical resistance is the total field resistance above which the generator fails to build up its voltage.
• In other words, the speed of the generator should be higher than the critical speed.
• Critical speed is the speed of the generator below which fails to build up its voltage.