Types of DC Generator MCQ || Types of DC generator Questions and Answers

Answer:4. Maintaining mechanical balance

Explanation: 

• The dummy coils are similar to other coils except their ends are cut, short and taped. They do not connect with the commutator bars.
• The dummy coils are simply to provide mechanical balance for the armature.
• As they do not connect with commutator bars, they do not affect the electrical characteristics of the winding.

Answer: 1. DC Series generator

Explanation: 

Applications of series generator:

• Used for supplying field excitation current in DC locomotives for regenerative breaking
• Used as boosters to compensate the voltage drop in the feeder in various types of distribution systems such as railway service
• Used in series arc lightening

Answer: 4. None of these

Explanation: 

Pole Pitch = (Number of armature slots) / (Number of poles)

Calculation:

Given –

Pole pitch = 2,

Number of poles = 10

∴ Number of armature slots = 2 x 10

Number of armature slots = 20

Answer: 2. 209.7V

Explanation: 

Given –

V = 200 volt

Load current (I) = 480 A

R_sh = 40 Ω

R_a= 0.02 Ω

∴ Field current If = 200/40 = 5A

∴ Armature current, Ia = I_f + I = 480 + 5

I_a = 485 A

Now generated emf is calculated as

Eg = 200 + 485 × 0.02 = 209.7 V

Answer: 2. 19, 17

Explanation: 

No. of slots S = 36

P = 4

Back Pitch

Y_b = (2S / P) +1

Y_b = ((2 x 36) / 4) + 1

Y_b = 19

Front Pitch

Y_f = (2S / P) -1

Y_f = ((2 x 36) / 4) – 1

Y_f = 17

Answer: 1. Differential compound DC generator

Explanation: 

• In welding, we use a differentially compound generator because of its drooping characteristics
• The generator used in welding purposes should have high current and low voltage which can be obtained by the differential compound generator
• We can’t get low voltage on high current for a dc shunt generator
• The differential compound generator provides a higher voltage at no load to initiate the arc and once the arc is formed i.e. low resistance and it provides a smaller voltage

Motor-Generator set:

• They consist of a (3-phase) motor, directly coupled to a DC generator
• As the motor speed depends mainly on the mains frequency, these units are relatively insensitive to variations in the supply voltage
• They can be remotely controlled by varying the excitation current
• The efficiency of a welding motor-generator is less and it is in the range of 50 to 60%
• The cost of a motor-generator set is high

Answer: 3. Pole-pitch

Explanation: 

• Pole Pitch is defined as the peripheral distance between centers of two adjacent poles in the DC machine.
• This distance is measured in terms of armature slots or armature conductors that come between two adjacent pole centers.
• A pole pitch in the electrical machine is equal to 180° electrical.
• This is naturally equal to the total number of armature slots divided by the number of poles in the machine.

∴ Pole Pitch = (Number of armature slots) / (Number of poles)

Answer: 4. V = E_g – I_a(R_a + R_s)

Explanation: 

If series winding is connected in series with the armature and then the shunt winding is put in parallel to the combination, then such a combination will form DC long shunt compound type of generator.

The terminal voltage of DC long shunt compound generator can be determined as

V = E_g – I_a(R_a + R_s)

Where,

E_g = Generated EMF

I_a = Armature current

R_a = Armature resistance

R_s = Series field resistance

Answer: 3. Coil pitch = Pole pitch

Explanation: 

Front pitch: The distance between the last side of the first coil and first side of the second coil is called the front pitch

Back pitch: The distance between the two sides of a coil in the back of the armature is called back pitch

Coil pitch: The distance measured in terms of armature slots or armature conductors between two sides of a coil

Pole pitch: The distance measured in terms of armature slots or armature conductors between two adjacent pole centers.

In pitch winding

Coil pitch = Pole pitch

Answer:2. Variable resistance across the series field

Explanation: 

External characteristic of DC compound wound generator is drawn between the terminal voltage and the load current. By adjusting the no. of amp-turns in the series field winding we can get the following external characteristics.

• If the series turns are so adjusted that with the increase in load current the terminal voltage also increases, then the generator is called over compounded.
• If the load is reduced, the terminal voltage will get decrease.
• If the series turns are so adjusted that with the increase in load current the terminal voltage remains constant, then the generator is called flat compounded.
• The flat compound characteristic of a DC compound generator is obtained by connecting a variable resistance across a series field.
• If the series field winding has a lesser no. of turns then the rated terminal voltage becomes less than the no-load voltage, then the generator is called under compounded

Answer: 1. 1 and 12

Explanation: 

Slots per pole =45/4=11.25

The value has to be a whole number

So the value 11.25 is rounded to the next greater integer, which is 12.

Now 12 × 4 = 48 slots will be there of which 3 slots will be having dummy coils.

∴ The Coil span should be 12. Hence the coils should be in slots 1 and 12.

Answer:2. Charging batteries

Explanation: 

Shunt generators are used where the main requirement is a constant voltage over a narrow load range.

Applications of shunt generator:

• Used for general lighting
• Used to charge the battery because they can be made to give constant output voltage
• Used for giving the excitation to the alternators
• Used for small power supply (such as a portable generator)

Answer: 4. Railway circuits, incandescent lamps and elevator motors

Explanation: 

Applications of compound generator:

• Cumulative compound generators (over compounded) are generally used for lighting, power supply purpose, and for heavy power services like railways because of their constant voltage property; these are also used for driving a motor like an elevator motor.
• The flat compounded generators are generally used for small distance operations, such as power supply for hotels, offices, homes, and lodges
• The differential compound wound generators are used for arc welding where huge voltage drop and constant current is required because of their large demagnetization armature reaction

Answer: 3. Electroplating

Explanation: 

Shunt generators are used where the main requirement is a constant voltage over a narrow load range.

Applications of shunt generator:

• Used for general lighting
• Used to charge the battery because they can be made to give constant output voltage
• Used for giving the excitation to the alternators
• In process of Electroplating
• Used for small power supply (such as a portable generator)

Answer:2. Reverse direction; reverses

Explanation: 

If the series winding is connected in series with the armature and then the shunt winding is put in parallel to the combination, then such a combination will form DC long shunt compound type of generator.

The terminal voltage of DC long shunt compound generator can be determined as

V = Eg – Ia(Ra + Rs)

Where,

Eg = Generated EMF

Ia = Armature current

Ra = Armature resistance

Rs = Series field resistance

For the generator to run as a motor, the series field connections must be in the reverse direction, as the armature current reverses.

Answer: 4. Less than the critical field resistance

Explanation: 

The following conditions must be satisfied for voltage build-up in a self-excited DC generator.

• There must be a sufficient residual flux in the field poles.
• The field terminals should be connected in such a way that the field current increases flux in the direction of residual flux.
• The field circuit resistance should be less than the critical field circuit resistance.

Note: If there is no residual flux in the field poles, disconnect the field from the armature circuit and apply a dc voltage to the field winding. this process is called Flashing the field. It will induce some residual flux in the field poles.

Answer:3. Eg – V/Ra

Explanation: 

In DC Shunt Motor

E_g is generated EMF

V is terminal voltage.

I_L is load current

I_a is armature current

Ish is shunt field current

R_a is armature resistance

R_sh is shunt field resistance.

Apply KVL to above circuit,

E_g = V + I_aR_a = I_shR_sh + I_aR_a

Now, we have to find the relation between E_g, V & I_aR_a

E_g = V + I_aR_a

E_g – V = I_aR_a

Ia = Eg – V/Ra