DC Generator Construction MCQ Quiz in मराठी - Objective Question with Answer for DC Generator Construction - मोफत PDF डाउनलोड करा

Construction of DC machines

1.) Yoke

• The outer frame of a dc machine is called a yoke. It is made up of cast iron or steel.
• It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.

2.) Poles and pole shoes

• Poles are joined to the yoke with the help of bolts or welding.
• Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in the air gap uniformly.

3.) Field winding

• They are usually made of copper. Field coils are former wounds placed on each pole and are connected in series.
• They are wound in such a way that, when energized, they form alternate North and South poles.

4.) Armature core

• The Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding.
• The armature is built up of thin laminated circular steel disks for reducing eddy current losses.
• It may be provided with air ducts for the axial airflow for cooling purposes. Armature is keyed (fixed) to the shaft.

5.) Armature winding

• It is usually a former wound copper coil that rests in armature slots. 
• Armature winding can be wound by one of two methods; lap winding or wave winding. Double-layer lap or wave windings are generally used.

6.) Commutator and brushes

• The function of a commutator, in a dc generator, is to collect the current generated in armature conductors.
• Brushes are usually made from carbon or graphite.
• The purpose of carbon brushes in DC motors is to reduce wear on the commutator as well as transfer electricity from outside the motor to the commutator.

DC Machine Construction:

The above figure shows constructional details of a simple 4-pole DC machine.

A DC machine consists of two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.

Yoke:

• The outer frame of a dc machine is called a yoke.
• It is made up of cast iron or steel.
• It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.

Poles and pole shoes:

• Poles are joined to the yoke with the help of bolts or welding.
• They carry field winding and pole shoes are fastened to them.
• Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in the air gap uniformly.

Field winding:

• They are usually made of copper.
• Field coils are former wounds placed on each pole and are connected in series.
• They are wound in such a way that, when energized, they form alternate North and South poles.

Armature core:

• Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding.
• The armature is built up of thin laminated circular steel disks for reducing eddy current losses.
• Armature core is made of silicon steel laminations which are insulated from each other by insulating varnish coating. These laminations are used to reduce eddy current losses.
• It may be provided with air ducts for the axial airflow for cooling purposes.
• The armature is keyed to the shaft.

Armature winding:

• It is usually a former wound copper coil that rests in armature slots.
• The armature conductors are insulated from each other and also from the armature core.
• Armature winding can be wound by one of the two methods; lap winding or wave winding.
• Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.

Commutator and brushes:

• In the case of the DC generator, the commutator is used to convert generated AC in armature into DC
• In the case of DC motor, the commutator is used to convert DC to A.C
• Due to the limitation of the commutator dc generators are not usually designed beyond 650 V

• The physical connection to the armature winding is made through a commutator-brush arrangement.
• The function of a commutator in a dc generator is to collect the current generated in armature conductors.
• While in the case of a dc motor, the commutator helps in providing current to the armature conductors.
• A commutator consists of a set of copper segments that are insulated from each other.
• The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft.
• Brushes are usually made from carbon or graphite.
• They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

DC Machine Construction:

The above figure shows constructional details of a simple 4-pole DC machine.

A DC machine consists of two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.

Yoke:

• The outer frame of a dc machine is called as yoke.
• It is made up of cast iron or steel.
• It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.

Poles and pole shoes:

• Poles are joined to the yoke with the help of bolts or welding.
• They carry field winding and pole shoes are fastened to them.
• Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in air gap uniformly.

Field winding:

• They are usually made of copper.
• Field coils are former wound and placed on each pole and are connected in series.
• They are wound in such a way that, when energized, they form alternate North and South poles.

Armature core:

• Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding.
• The armature is built up of thin laminated circular steel disks for reducing eddy current losses.
• Armature core is made of silicon steel laminations which are insulated from each other by insulating varnish coating. These laminations are used to reduce eddy current losses.
• It may be provided with air ducts for the axial air flow for cooling purposes.
• Armature is keyed to the shaft.

Armature winding:

• It is usually a former wound copper coil which rests in armature slots.
• The armature conductors are insulated from each other and also from the armature core.
• Armature winding can be wound by one of the two methods; lap winding or wave winding.
• Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.

Commutator and brushes:

• In the case of the DC generator, the commutator is used to convert generated AC in armature into DC
• In the case of DC motor, the commutator is used to convert DC to A.C
• Due to the limitation of commutator dc generators are not usually designed beyond 650 V

• Physical connection to the armature winding is made through a commutator-brush arrangement.
• The function of a commutator in a dc generator is to collect the current generated in armature conductors.
• While in case of a dc motor, commutator helps in providing current to the armature conductors.
• A commutator consists of a set of copper segments which are insulated from each other.
• The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft.
• Brushes are usually made from carbon or graphite.
• They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

Conclusion:

From the above concept of construction of DC Machine, it is clear that the damper winding is not used in it.

Damper winding is used in Synchronous machines for reducing hunting (both motor & generator) and proving induction torque at starting (motor).

The correct answer is (option 1) i.e. More than 1 hp.

Explanation:

Interpoles:

• Interpoles are similar to the main field poles and are located on the yoke between the main field poles.
• They have windings in series with the armature winding.
• The inter poles are tapering in shape i.e. having a broad base and less pole shoe area to reduce the extra air gap flux under the trailing pole tip
• The air gap under inter poles is more than the air gap under the main field poles to avoid the saturation.
• Generally, interpoles are used in more than 1 hp motor.

Connection diagram of inter poles for d.c machines is shown below,

 
Interpoles in DC machine has basically two functions:

• Automatic neutralization of cross magnetization due to armature reaction
• To reduce sparking at the commutator.
• To counter and cancel reactance voltage in the coil undergoing commutation

Important Points

In a DC machine, two kinds of magnetic fluxes are present (armature flux and main field flux). The effect of armature flux on the main field flux is called as armature reaction.

The effect of armature reaction can be reduced by following methods

a) By using compensating winding

b) By using commutation poles or inter-poles

c) By reducing the cross-section of pole pieces

The correct answer is option 4):(4)

Concept:

In Lap winding

The number of parallel path = Number of poles in Lap winding machine

The number of parallel path = No of brushes.

Calculation:

No of poles = 4

No of parallel path =  4

 No of brushes = 4

\(No \: of \: poles \over No\:of\:brushes \) × no. of parallel paths =  \(4 \over 4\)× 4

= 4

The above figure shows constructional details of a simple 4-pole DC machine.

A DC machine consists of two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.

Yoke:

• The outer frame of a dc machine is called as yoke.
• It is made up of cast iron or steel.
• It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.

Poles and pole shoes:

• Poles are joined to the yoke with the help of bolts or welding.
• They carry field winding and pole shoes are fastened to them.
• Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in air gap uniformly.

Field winding:

• They are usually made of copper.
• Field coils are former wound and placed on each pole and are connected in series.
• They are wound in such a way that, when energized, they form alternate North and South poles.

Armature core:

• Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding.
• The armature is built up of thin laminated circular steel disks for reducing eddy current losses.
• Armature core is made of silicon steel laminations which are insulated from each other by insulating varnish coating. These laminations are used to reduce eddy current losses.
• It may be provided with air ducts for the axial air flow for cooling purposes.
• Armature is keyed to the shaft.

Armature winding:

• It is usually a former wound copper coil which rests in armature slots.
• The armature conductors are insulated from each other and also from the armature core.
• Armature winding can be wound by one of the two methods; lap winding or wave winding.
• Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.

Commutator and brushes:

• In the case of the DC generator, the commutator is used to convert generated AC in armature into DC
• In the case of DC motor, the commutator is used to convert DC to A.C
• Due to the limitation of commutator dc generators are not usually designed beyond 650 V

• Physical connection to the armature winding is made through a commutator-brush arrangement.
• The function of a commutator in a dc generator is to collect the current generated in armature conductors.
• While in case of a dc motor, commutator helps in providing current to the armature conductors.
• A commutator consists of a set of copper segments which are insulated from each other.
• The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft.
• Brushes are usually made from carbon or graphite.
• They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

Terminologies of Armature Winding :

Pole pitch: It is defined as the number of armature slots per pole. For example, if there are 36 conductors and 4 poles, then the pole pitch is 36/4=9.

Coil span or coil pitch (Y_s): It is the distance between the two sides of a coil measured in terms of armature slots.

Front pitch (Y_F): It is the distance, in terms of armature conductors, between the second conductor of one coil and the first conductor of the next coil. OR it is the distance between two coil sides that are connected to the same commutator segment.

Back pitch (Y_B): The distance by which a coil advances on the back of the armature is called the back pitch of the coil. It is measured in terms of armature conductors.

Resultant pitch (Y_r): The distance, in terms of armature conductor, between the beginning of one coil and the beginning of the next coil is called the resultant pitch of the coil.

The armature winding of a dc machine is wound by one of the two methods:

• Lap winding
• Wave winding

/
The difference between these two is merely due to the end connections and commutator connections of the conductor. 

Lap Winding:

• In lap winding, the successive coils overlap each other. In a simplex lap winding, the two ends of a coil are connected to adjacent commutator segments.
• The winding may be progressive or retrogressive. A progressive winding progresses in the direction in which the coil is wound. In progressive winding Y_B > Y_F  and Y_C = +1.The following image shows progressive simplex lap winding.

• The opposite way is retrogressive. A retrogressive winding progresses in the opposite direction in which the coil is wound. In retrogressive winding YB < YF  and YC = -1.The following image shows progressive simplex lap winding. 
• The following image shows retrogressive simplex lap winding.

Wave Winding:

• In wave winding, a conductor under one pole is connected at the back to a conductor which occupies an almost corresponding position under the next pole which is of opposite polarity.
• In other words, all the coils which carry emf in the same direction are connected in series.
• The following diagram shows a part of simplex wave winding.

Relations between Pitches for Lap Winding:

• In a simplex lap winding, the various pitches should have the following relation:
• The back and front pitches are odd and are of opposite signs. They differ numerically by 2,
• Y_B = Y_F ± 2
• Y_B =Y_F + 2 for progressive winding
• Y_B = Y_F - 2 for retrogressive winding
• Both YB and YF  should be nearly equal to pole pitch.
• Average pitch = \(\frac{(Y_B +Y_F)}{2}\). It equals pole pitch\(= \frac{Z}{P}\) 

The correct answer is (0ption 2) i.e. Brushes.

Explanation:

DC Machine Construction:

The above figure shows the construction details of a simple 4-pole DC machine.

A DC machine consists of two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.

Yoke:

• The outer frame of a dc machine is called as yoke.
• It is a stationary part
• It is made up of cast iron or steel.
• It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.

Poles and pole shoes:

• Poles are joined to the yoke with the help of bolts or welding.
• They carry field winding and pole shoes are fastened to them.
• Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in air gap uniformly.

Field winding:

• They are usually made of copper.
• Field coils are former wound and placed on each pole and are connected in series.
• They are wound in such a way that, when energized, they form alternate North and South poles.

Armature core:

• Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding.
• The armature is built up of thin laminated circular steel disks for reducing eddy current losses.
• Armature core is made of silicon steel laminations which are insulated from each other by insulating varnish coating. These laminations are used to reduce eddy current losses.
• It may be provided with air ducts for the axial air flow for cooling purposes.
• Armature is keyed to the shaft.

Armature winding:

• It is usually a former wound copper coil which rests in armature slots.
• The armature conductors are insulated from each other and also from the armature core.
• Armature winding can be wound by one of the two methods; lap winding or wave winding.
• Double-layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.

Commutator and brushes:

• Brushes are a stationary part.
• In the case of the DC generator, the commutator is used to convert generated AC in armature into DC
• In the case of DC motor, the commutator is used to convert DC to A.C
• Due to the limitation of commutator dc generators are not usually designed beyond 650 V
• Physical connection to the armature winding is made through a commutator-brush arrangement.
• The function of a commutator in a dc generator is to collect the current generated in armature conductors.
• While in case of a dc motor, commutator helps in providing current to the armature conductors.
• A commutator consists of a set of copper segments which are insulated from each other.
• The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft.
• Brushes are usually made from carbon or graphite.
• They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

Conclusion:

From the above concept of construction of DC Machine, it is clear that brushes, poles and pole shoes and yoke are stationary parts.

DC Machine:

The above figure shows constructional details of a simple 4-pole DC machine.

A DC machine consists of two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.

Yoke:

• The outer frame of a DC machine is called a yoke.
• It is made up of cast iron or steel.
• It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.

Poles and pole shoes:

• Poles are joined to the yoke with the help of bolts or welding.
• They carry field winding and pole shoes are fastened to them.
• Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in the air gap uniformly.
• DC machine consists of alternate poles such as North and South poles known as Heteropolar structure.

Field winding:

• They are usually made of copper.
• Field coils are former wounds and placed on each pole and are connected in series.
• They are wound in such a way that, when energized, they form alternate North and South poles.

Heteropolar structure: 

• The induced emf in a conductor goes through a cyclic change in voltage as it passes under north and south pole polarity alternately. Thus, induced emf in the conductor alternates in magnitude.
• For a constant velocity of sweep (rotation of conductor) the induced emf is directly proportional to the flux density under which it is moving. If the flux density variation is sinusoidal in space, then a sine wave voltage is generated.

In DC machines, The armature is mounted on bearings and is free to rotate. It is mounted in the magnetic field produced by permanent magnets or current passing through coils of wire, which are called field coils. When a current passes through the armature coil, forces act on the coil and result in rotation. The north and south poles are shown on the stationary part of the machine.

Armature core:

• Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding.
• The armature is built up of thin laminated circular steel disks for reducing eddy current losses.
• Armature core is made of silicon steel laminations which are insulated from each other by insulating varnish coating. These laminations are used to reduce eddy current losses.
• It may be provided with air ducts for the axial air flow for cooling purposes.
• Armature is keyed to the shaft.

Armature winding:

• It is usually a former wound copper coil which rests in armature slots.
• The armature conductors are insulated from each other and also from the armature core.
• Armature winding can be wound by one of the two methods; lap winding or wave winding.
• Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.

Commutator and brushes:

• In the case of the DC generator, the commutator is used to convert generated AC in armature into DC
• In the case of DC motor, the commutator is used to convert DC to A.C
• Due to the limitation of commutator dc generators are not usually designed beyond 650 V

• Physical connection to the armature winding is made through a commutator-brush arrangement.
• The function of a commutator in a dc generator is to collect the current generated in armature conductors.
• While in the case of a dc motor, the commutator helps in providing current to the armature conductors.
• A commutator consists of a set of copper segments which are insulated from each other.
• The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft.
• Brushes are usually made from carbon or graphite.
• They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

Additional Information

Homopolar structure:

• In a homopolar machine, the conductors are arranged in such a manner that they always move under one polarity (either N or S). Since the conductor cuts the magnetic flux of the same polarity, the induced emf in the conductors is not alternating in nature.
• Synchronous machines and Induction machines construction comes under this category.

• There are two windings in a DC machine; Armature winding is rotating, and field winding is stationary
• Armature winding is responsible for torque and field winding is responsible for flux in the machine
• So, armature current and field current should be independent of each other
• This can be achieved only when the two windings namely armature and field winding are perpendicular to each other
• Thus, armature winding and field winding is drawn at an angle of 90 degrees