DC Generator Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. Which among the following conditions is not desirable while connecting two DC generators in parallel?

1. Voltage regulation of the same percentage
2. Equal voltage rating
3. Same current rating
4. Prime movers with the same percentage of speed regulation

Ans. The correct answer is c. Same current rating

Explanation: Aside from having the same current rating, the other three requirements are desirable for connecting two generators in parallel.

Ques. With respect to variation in separately excited DC generators, the variation in terminal voltage of the DC shunt generator will be

1. Very slow
2. Much rapid
3. Will remain constant
4. None of the above

Ans. The correct answer is b. Much rapid

Explanation: When a shunt generator is loaded, the terminal voltage drops significantly faster than in a separately-excited generator. The cause is a decrease in field current with increasing terminal voltage.

Ques. What will be the shaft power at the DC Generator?

1. The sum of rotational losses and mechanical power
2. No mechanical and load losses
3. Mechanical losses only
4. None of the above

Ans. The correct answer is a. The sum of rotational losses and mechanical power

Explanation: The shaft power is the sum of rotational losses and mechanical power.

Ques. By keeping _____, the characteristics of a separately excited DC generator are drawn.

1. Both Speed and field current variable
2. Both speed and field current constant
3. Speed variable but field current constant
4. None of the above

Ans. The correct answer is a. Both Speed and field current variable

Explanation: The characteristics of separately excited DC generators are drawn by keeping both the speed and field current variables.

Ques. At brush-commutator contact, the voltage drop is variable, and it is dependent on the armature current.

1. True
2. False

Ans. The correct answer is b. False

Explanation: The statement above is false because the voltage drop at the brush-commutator contact is variable and independent of the armature current.

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Short Answers Questions [2 Marks Questions]

Ques. What is a DC Generator?

Ans. A DC generator is an electrical machine whose main function is to convert mechanical energy into electricity. When a conductor generates a magnetic flux, emf is produced according to Faraday's law's electromagnetic induction principle.

Ques. Why is the armature of the DC generator laminated?

Ans. The armature of a DC motor is laminated to reduce eddy current loss. The armature is made of laminations of cylindrical or drum-shaped high-grade silicon steel. The circular path of Eddy Current is therefore interrupted when lamination is used. As a result, lamination reduces heating and armature damage.

Ques. What are the losses in the DC generator?

Ans. There are energy losses while converting mechanical energy to electrical energy, which means that the entire input is not turned into output. These losses are broadly categorized into three types:

• Copper Loss: These losses occur as current flows through windings and are classified into three types: armature copper loss, field copper winding loss, and brush resistance losses.
• Iron Losses: Eddy current losses and hysteresis loss occur as a result of current induction in the armature. These are also known as Core losses or Magnetic losses.
• Mechanical Losses: Mechanical losses are those that arise as a result of friction between the generator's components.

Ques. State the Faraday’s Law of Electromagnetic Induction.

Ans. According to Faraday's Law of Electromagnetic Induction, whenever a conductor is placed in a changing magnetic field or moved in a magnetic field, an emf is induced in the conductor.

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Long Answers Questions [3 Marks Questions]

Ques. What are the applications of DC generators?

Ans. The following are the applications of DC generators

• Using field regulators, the separately excited type DC generator is utilized for power and lighting purposes.
• Arc lamps employ series DC generators for reliable current generators, lighting, and boosters.
• Level compound DC generators are used to power hostels, workplaces, and resorts.
• Power is supplied to DC welding machines by compound DC generators.
• To compensate for the voltage loss in the feeders, a DC generator is required.

Ques. What are the advantages of DC generators?

Ans. The following are the advantages of DC generators

• A DC generator's design and construction are simple.
• They are used to power heavy machinery.
• These generators are useful for providing electricity to huge motors.
• DC generators supply electricity to heavy electric equipment.
• A direct current generator minimizes electric variations in the armature and provides a constant supply.

Ques. What are the disadvantages of DC generators?

Ans. The main disadvantages of DC generators are

• A significant drawback of a DC generator is that it cannot be utilized together with a transformer.
• The efficiency of a DC generator is quite poor.
• Eddy current losses, copper losses, mechanical losses, and other losses occur when the current flows through the generator.
• DC generators placed at a long distance may face significant voltage drops.

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Very Long Answers Questions [5 Marks Questions]

Ques. What is the Working principle of a DC generator? How Does it Work?

Ans. The concept of electromagnetic induction is the main principle for the working of a DC generator. The technique of producing an electromotive force as a result of a changing magnetic field is known as electromagnetic induction. The following describes how a DC generator works:

• The magnetic field changes as the machine turns. The conductor placed in the field cut the changing magnetic field lines. 
• As a result, the magnetic flux linkage of the conductor changes, causing an EMF in the coil. 
• A direct current is generated by the induced EMF flowing in the closed circuit.
• The direction of the current may be calculated using Fleming's right-hand rule. When the magnetic field changes, the current produced in the conductor varies.
• Consider an armature revolving clockwise and a conductor traveling at a left angle upwards. The conductor's movement will reverse downwards after a half swing of the armature.
• Once the armature has completed a half rotation, the conductor's motion will be reversed downward. As a result, the current direction in each armature will alternate.
• However, during a current reversal, the armature conductor connections are reversed. As a result, the terminals will have a unidirectional current.

Ques. What are the different parts of a DC generator?

Ans. The different parts of a DC generator are

• Stator: Stators, which are made up of two magnets with opposing polarities, are used to produce magnetic fields surrounding coils. These magnets are placed in the rotor region.
• Rotor: The cylindrical armatures of DC machines are built of slotted iron laminations. The laminate protects the armature core and reduces eddy current losses.
• Armature Core: The outside surface of a cylindrical armature core features slots for winding the armature. These grooves go along the outside of the body.
• Armature Winding: The current in an armature is amplified by joining series and parallel windings to form a closed circuit. The actual power conversion takes place here.
• Field Coils: Field coils are arranged over the pole core, each in series with the coils on the opposing pole. As current travels through the field coils, neighboring poles become polarised in the opposite direction.
• Yoke: The yoke is the exterior hollow cylindrical frame of a DC generator. It is in charge of moving magnetic flux from the poles to the electrical transformer. It also supports main poles and inter-poles while offering a low reluctance path.
• Poles: A pole's purpose is to hold field windings. Field windings are wrapped around poles and connected in series or parallel to armature windings.
• Pole Shoe: Pole shoes are generally used to keep the field coil from dropping and uniformly distribute the magnetic flux.
• Commutator: When placed in an armature winding, commutators convert alternating current to direct current. It is made of copper segments that are separated from one another by mica sheets. It is mounted on a machine's shaft. The primary purpose of a commutator is to link stationary external circuits to armature conductors through brush connections.
• Brush: A brush ensures that electrical contacts are formed between the commutator and the external load circuit.

Ques. What are the different types of DC generators?

Ans. The different types of DC generators are

• Permanent magnet-type DC generator: There are no field windings near the poles in this type of DC generator. Here, the poles generate constant fields. This is very small and is used in dynamos, cycles, and so forth. The magnets generate flux, which decreases with time and affects the features of the machine.
• Separately excited DC generator: The field windings of this generator are energized by a separate DC source. The flux produced at the poles is determined by the field current, and the saturated region remains constant.
• Self-excited DC generator: The generator supplies power to the field winding. When the electric generator is turned off, a small voltage is generated in the rotor, causing an electromotive force in the armature and current to flow through the field windings.

The self-excited DC generator is further classified as

• Shunt-wound generators: The field winding of this generator is connected across the armature windings in parallel, commonly known as a shunt circuit, that helps in applying a full terminal voltage across it.
• Series wound generators: The field coils are connected in series with the armature windings in this generator. The armature current is carried by the field windings in series.
• Compound wound generators: On each pole of a Compound Wound Generator, there are two sets of field winding. One is connected in series with a few turns of thick wire, while the other is connected in parallel with several turns of thin wire and armature windings.

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