Pole Pitch MCQ Quiz - Objective Question with Answer for Pole Pitch - Download Free PDF

Explanation:

Short Pitch Angle in Alternator

Definition: The short pitch angle, also known as the short pitch or coil pitch angle, is the angle by which the coil span is less than the pole pitch in electrical degrees in an alternator. The use of a short pitch coil helps to reduce harmonics in the generated voltage and improves the waveform quality.

Given Data:

• Number of poles (P) = 6
• Number of slots = 120
• Coil span = 18 slots

Step-by-Step Solution:

To calculate the short pitch angle, we must follow these steps:

Pole pitch is the number of slots per pole. It represents the slot distance for one complete pole.

Pole pitch = Total number of slots ÷ Number of poles
Pole pitch = 120 ÷ 6 = 20 slots

The short pitch is the difference between the pole pitch and the coil span.

Short pitch = Pole pitch - Coil span
Short pitch = 20 - 18 = 2 slots

One pole corresponds to 180° electrical. Hence, 1 slot in terms of electrical degrees can be calculated as:

Electrical degrees per slot = 180° ÷ Pole pitch
Electrical degrees per slot = 180° ÷ 20 = 9° per slot

Now, multiply the short pitch (in slots) by the electrical degrees per slot to find the short pitch angle:

Short pitch angle = Short pitch × Electrical degrees per slot
Short pitch angle = 2 × 9° = 18°

1. Calculate the Pole Pitch:
2. Determine the Short Pitch:
3. Convert the Short Pitch to Electrical Degrees:

Correct Option:

The correct option is:

Option 2: 18°

This result indicates that the short pitch angle of the given alternator is 18°, which aligns with the given data and calculations.

Additional Information

To further understand the analysis, let’s evaluate the other options:

This option is incorrect because it assumes a short pitch angle that does not match the given coil span and pole pitch data. Based on the calculations, the short pitch angle is 18°, not 15°.

This option is incorrect as it suggests a short pitch angle greater than the calculated value. A short pitch angle of 20° would imply a short pitch of more than 2 slots, which is inconsistent with the given data.

This option is incorrect because a short pitch angle of 0° would imply a full-pitch coil where the coil span equals the pole pitch. In the given data, the coil span is less than the pole pitch, resulting in a non-zero short pitch angle.

• Option 1: 15°
• Option 3: 20°
• Option 4: 0°

Conclusion:

Understanding the concept of short pitch angle is essential for analyzing alternator designs. The short pitch angle of 18° (Option 2) is calculated based on the given data of pole pitch and coil span. This angle is critical for reducing harmonics and improving the quality of the alternator's output voltage. Analyzing other options reveals that they do not align with the given data, further confirming the correctness of Option 2.

The correct answer is option 3): pole shoe

Concept:

• The pole core is an arrangement to provide lux and to support the field windings
• The pole faces shaped to fit the curvature of the armature are known as the pole shoe
• The pole shoe is stretched so as to provide a uniform air gap along the armature core and also to provide uniform flux distribution in the air gap.
• The Pole core basically carries a field winding which is necessary to produce the flux. It directs the flux produced through the air gap to the armature core, and to the next pole.
• Pole shoe enlarges the area of the armature core to come across the flux, which is necessary to produce larger induced emf. To achieve this, pole shoe has been given a particular shape.
• The functions of the pole shoe are to support the field coils to Spread out flux for better uniformity  and to reduce the reluctance of the magnetic path 

Additional Information

• A field winding is the insulated current-carrying coils on a field magnet that produce the magnetic field needed to excite a generator or motor. 
• Armature windings are the winding where a voltage is induced and power transfer between electrical and mechanical systems occur

Fractional pitch winding in DC machine

When the angular distance between the sides of a coil is less than the angular distance between the centers of adjacent field poles, the coil is termed to be a fractional pitch coil.

An armature winding made up of fractional pitch coils is termed a Fractional Pitch winding.

Advantages of fractional pitch winding

• It reduces the amount of copper needed for end connection when compared with the full-pitched coil.
• Due to poor performance of the brush, and poor undercutting of the commutator, incorrect spring pressure sparking at brush faces happen. To overcome this sparking fractional pitch winding is used.
• It improves the waveform of generated EMF i.e. generated EMF can be made to approximate to a sine wave more easily and the distorting harmonics can be reduced or eliminated. 

Pole Pitch: It is the angular distance between the two adjacent poles.

• It is always 180° electrical.

Coil Span: It is the angular distance between the two sides of the coil.

• It may be the full pitched or short pitch.
• If the coil span is equal to the Pole pitch then it is called a full pitch coil.

Here, e = E1 = E2 = E

∴ e = E1 + E2 = 2E

• If the coil span is less then one pole pitch than it is called short pitch coil.
• Here, one side of the coil is chorded by angle α.
   

Since one side of the coil is chorded by an angle α, therefore, EMF induced in the coil will be less than 2E and given by,

Pitch Factor(Kp): It is the ratio of EMF induced in the short pitch coil to the full pitch coil.

It is given by,

Where nth is the order of harmonics.

Note: In order to eliminate the nth order of harmonics, we have to equate the pitch factor with zero.

Pole pitch:

• It 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 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)

Important Points

Front pitch: The distance between the last side of the first coil and the 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:

• It 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 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)

Important Points

Front pitch: The distance between the last side of the first coil and the 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.

Concept:

• The pole pitch is defined as the peripheral distance between centers of two adjacent poles in dc machine.
• This distance is measured in terms of armature slots or armature conductors 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)

Calculation:

Given - 

Pole pitch = 2, 

Number of poles = 10

∴ Number of armature slots = 2 x 10

Number of armature slots = 20

Fractional pitch winding in DC machine

When the angular distance between the sides of a coil is less than the angular distance between the centers of adjacent field poles, the coil is termed to be a fractional pitch coil.

An armature winding made up of fractional pitch coils is termed a Fractional Pitch winding.

Advantages of fractional pitch winding

• It reduces the amount of copper needed for end connection when compared with the full-pitched coil.
• Due to poor performance of the brush, and poor undercutting of the commutator, incorrect spring pressure sparking at brush faces happen. To overcome this sparking fractional pitch winding is used.
• It improves the waveform of generated EMF i.e. generated EMF can be made to approximate to a sine wave more easily and the distorting harmonics can be reduced or eliminated. 

Pole Pitch: It is the angular distance between the two adjacent poles.

• It is always 180° electrical.

Coil Span: It is the angular distance between the two sides of the coil.

• It may be the full pitched or short pitch.
• If the coil span is equal to the Pole pitch then it is called a full pitch coil.

Here, e = E1 = E2 = E

∴ e = E1 + E2 = 2E

• If the coil span is less then one pole pitch than it is called short pitch coil.
• Here, one side of the coil is chorded by angle α.
   

Since one side of the coil is chorded by an angle α, therefore, EMF induced in the coil will be less than 2E and given by,

Pitch Factor(Kp): It is the ratio of EMF induced in the short pitch coil to the full pitch coil.

It is given by,

Where nth is the order of harmonics.

Note: In order to eliminate the nth order of harmonics, we have to equate the pitch factor with zero.

The correct answer is option 3): pole shoe

Concept:

• The pole core is an arrangement to provide lux and to support the field windings
• The pole faces shaped to fit the curvature of the armature are known as the pole shoe
• The pole shoe is stretched so as to provide a uniform air gap along the armature core and also to provide uniform flux distribution in the air gap.
• The Pole core basically carries a field winding which is necessary to produce the flux. It directs the flux produced through the air gap to the armature core, and to the next pole.
• Pole shoe enlarges the area of the armature core to come across the flux, which is necessary to produce larger induced emf. To achieve this, pole shoe has been given a particular shape.
• The functions of the pole shoe are to support the field coils to Spread out flux for better uniformity  and to reduce the reluctance of the magnetic path 

Additional Information

• A field winding is the insulated current-carrying coils on a field magnet that produce the magnetic field needed to excite a generator or motor. 
• Armature windings are the winding where a voltage is induced and power transfer between electrical and mechanical systems occur

Explanation:

Short Pitch Angle in Alternator

Definition: The short pitch angle, also known as the short pitch or coil pitch angle, is the angle by which the coil span is less than the pole pitch in electrical degrees in an alternator. The use of a short pitch coil helps to reduce harmonics in the generated voltage and improves the waveform quality.

Given Data:

• Number of poles (P) = 6
• Number of slots = 120
• Coil span = 18 slots

Step-by-Step Solution:

To calculate the short pitch angle, we must follow these steps:

Pole pitch is the number of slots per pole. It represents the slot distance for one complete pole.

Pole pitch = Total number of slots ÷ Number of poles
Pole pitch = 120 ÷ 6 = 20 slots

The short pitch is the difference between the pole pitch and the coil span.

Short pitch = Pole pitch - Coil span
Short pitch = 20 - 18 = 2 slots

One pole corresponds to 180° electrical. Hence, 1 slot in terms of electrical degrees can be calculated as:

Electrical degrees per slot = 180° ÷ Pole pitch
Electrical degrees per slot = 180° ÷ 20 = 9° per slot

Now, multiply the short pitch (in slots) by the electrical degrees per slot to find the short pitch angle:

Short pitch angle = Short pitch × Electrical degrees per slot
Short pitch angle = 2 × 9° = 18°

1. Calculate the Pole Pitch:
2. Determine the Short Pitch:
3. Convert the Short Pitch to Electrical Degrees:

Correct Option:

The correct option is:

Option 2: 18°

This result indicates that the short pitch angle of the given alternator is 18°, which aligns with the given data and calculations.

Additional Information

To further understand the analysis, let’s evaluate the other options:

This option is incorrect because it assumes a short pitch angle that does not match the given coil span and pole pitch data. Based on the calculations, the short pitch angle is 18°, not 15°.

This option is incorrect as it suggests a short pitch angle greater than the calculated value. A short pitch angle of 20° would imply a short pitch of more than 2 slots, which is inconsistent with the given data.

This option is incorrect because a short pitch angle of 0° would imply a full-pitch coil where the coil span equals the pole pitch. In the given data, the coil span is less than the pole pitch, resulting in a non-zero short pitch angle.

• Option 1: 15°
• Option 3: 20°
• Option 4: 0°

Conclusion:

Understanding the concept of short pitch angle is essential for analyzing alternator designs. The short pitch angle of 18° (Option 2) is calculated based on the given data of pole pitch and coil span. This angle is critical for reducing harmonics and improving the quality of the alternator's output voltage. Analyzing other options reveals that they do not align with the given data, further confirming the correctness of Option 2.

Pole pitch:

• It 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 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)

Important Points

Front pitch: The distance between the last side of the first coil and the 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.

Concept:

• The pole pitch is defined as the peripheral distance between centers of two adjacent poles in dc machine.
• This distance is measured in terms of armature slots or armature conductors 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)

Calculation:

Given - 

Pole pitch = 2, 

Number of poles = 10

∴ Number of armature slots = 2 x 10

Number of armature slots = 20

Fractional pitch winding in DC machine

When the angular distance between the sides of a coil is less than the angular distance between the centers of adjacent field poles, the coil is termed to be a fractional pitch coil.

An armature winding made up of fractional pitch coils is termed a Fractional Pitch winding.

Advantages of fractional pitch winding

• It reduces the amount of copper needed for end connection when compared with the full-pitched coil.
• Due to poor performance of the brush, and poor undercutting of the commutator, incorrect spring pressure sparking at brush faces happen. To overcome this sparking fractional pitch winding is used.
• It improves the waveform of generated EMF i.e. generated EMF can be made to approximate to a sine wave more easily and the distorting harmonics can be reduced or eliminated. 

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