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

1. The dummy coils in DC machines are used for

1. Increase the efficiency
2. Improving commutation
3. Reducing cost of machine
4. Maintaining mechanical balance

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.

2. ______ is used as the power source in electric arc welding.

1. DC Series generator
2. Differentially wound compound generator
3. Separately excited DC generator
4. Cumulative compound DC generator

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

3. The pole pitch of a 10 pole motor is 2. Find the number of slots.

1. 10
2. 50
3. 5
4. None of these

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

4. A DC shunt generator delivers 480 A at 200 V. Its armature and field winding resistance are 0.02 Ω and 40 Ω respectively. What will be the value of generated emf?

1. 200.7V
2. 209.7V
3. 220.7V
4. 215.7V

Explanation:

Given –

V = 200 volt

Load current (I) = 480 A

Rsh = 40 Ω

Ra= 0.02 Ω

∴ Field current If = 200/40 = 5A

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

Ia = 485 A

Now generated emf is calculated as

Eg = 200 + 485 × 0.02 = 209.7 V

5. A 36-slot, 4-pole, dc machine has a simplex lap winding with two conductors per slot. The back pitch and front pitch adopted could be respectively

1. 15, 13
2. 19, 17
3. 21, 19
4. 23, 21

Answer: 2. 19, 17

Explanation:

No. of slots S = 36

P = 4

Back Pitch

Yb = (2S / P) +1

Yb = ((2 x 36) / 4) + 1

Yb = 19

Front Pitch

Yf = (2S / P) -1

Yf = ((2 x 36) / 4) – 1

Yf = 17

6. For a DC welding Motor-Generator set, which type of DC generator is used?

1. Differential compound DC generator
2. DC Series generator
3. DC Shunt generator
4. Cumulative compound DC generator

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

7. In DC machines, the distance between two adjacent poles can be defined as:

1. Pole size
2. Armature length
3. Pole-pitch
4. Coil length

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)

8. The terminal voltage of a long shunt dc generator could be determined as:

1. V = Eg + Ia Ra
2. V = Eg – IaRa
3. V = Eg + Ia(Ra + Rs)
4. V = Eg – Ia(Ra + Rs)

Answer: 4. V = Eg – Ia(Ra + Rs)

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– Ia(Ra + Rs)

Where,

Eg = Generated EMF

Ia = Armature current

R= Armature resistance

Rs = Series field resistance

9. What is the relation between coil-pitch and pole-pitch in pitch winding?

1. Coil pitch > Pole pitch
2. Coil pitch = Back pitch
3. Coil pitch = Pole pitch
4. Coil pitch < Pole pitch

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

10. Flat compound characteristic of a DC compound generator is obtained by connecting a:

1. Variable resistance in parallel with shunt load
2. Variable resistance across series field
3. Fixed resistance in parallel with shunt load
4. Fixed resistance across series field

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

11. For a 4 – pole, 45 slot dc armature, 180 electrical degree coil span will be obtained for the coils occupying which of the following slots.

1. 1 and 12
2. 1 and 11
3. 1 and 10
4. 1 and 9

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.

12. Shunt generators are generally used for

1. Arc welding
2. Charging batteries
3. Regenerative braking of dc locomotives
4. Elevators

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)

13. Compound generators are used to supply power to:

1. Incandescent lamps
2. Railway circuits
3. Elevator motors
4. Railway circuits, incandescent lamps and elevator motors

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

14. The application of D.C. shunt generator is _____.

1. Welding generator set
2. Booster
3. Electroplating
4. Electric traction

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)

15. If a DC compound machine connected as a generator is run as a motor, the series field connections must be in _______, as the armature current ______.

1. Reverse direction; remains same
2. Reverse direction; reverses
3. Forward direction; reverses
4. Forward direction; remains same

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.

16. For a d.c. shunt generator to self excite, the conditions to be satisfied are that there must be some residual magnetism in the field magnet, it must be in the proper direction and the shunt field resistance must be

1. Above the critical field resistance
2. Equal to the critical field resistance
3. Less than the armature resistance
4. Less than the critical field resistance

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.

17. The armature current in a D.C. shunt generator is given as

1. Eg + V/Ra
2. Ea/Ra
3. Eg – V/Ra
4. IL – Ish

Answer:3. Eg – V/Ra

Explanation:

In DC Shunt Motor

Eg is generated EMF

V is terminal voltage.

IL is load current

Ia is armature current

Ish is shunt field current

Ra is armature resistance

Rsh is shunt field resistance.

Apply KVL to above circuit,

Eg = V + IaRa = IshRsh + IaRa

Now, we have to find the relation between Eg, V & IaRa

Eg = V + IaRa

Eg – V = IaRa

Ia = Eg – V/Ra

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