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

Answer: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.

Answer:3. Increases to 600 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.

R_c ∝ N_c

R_c2 = R_c1 × Nc2Nc1

Given: R_c1 = 400Ω, N_c2 = 1800 rpm, N_c1 = 1200 rpm

R_c2 = 400 × 1800/1200

R_c2 = 600 Ω

Answer: 1. Increase

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 $= \dfrac{{{E_0} – {V_{fl}}}}{{{V_{fl}}}}$

E₀ = No-load voltage

V_fl = 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.

Answer:2. Very low 2 to 3 V

Explanation: 

The EMF induced (E_g) 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,

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

Answer: 3. 384 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

Answer:3. Increases to 250 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.

R_C ∝ N_C

R_C2 ∝ R_C1 × N_C2/N_C1

R_C2 = 200 × 1000/800 = 250Ω

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 

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.

Answer:2. Pole flux

Explanation: 

In a dc generator, generated emf is given as

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

E = V + I_a R_a

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

I_a is armature current

R_a is 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.

Answer: 3. Increasing 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.

Answer: 2. Residual flux

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.

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.
• Under load conditions, the load resistance must be more than the critical load resistance.