DC Generator Winding MCQ Quiz - Objective Question with Answer for DC Generator Winding - Download Free PDF

Explanation:

Four Pole Lap Wound Machine with Equal Air Gap Under Each Pole

Definition: A lap wound machine refers to the type of winding configuration in electrical machines where each coil's end is connected to the adjacent commutator segment, resulting in multiple parallel paths. In a four-pole machine, there are four magnetic poles, and the air gap under each pole is typically kept uniform to ensure consistent magnetic flux distribution.

Working Principle: In a lap wound machine, the armature winding is divided into as many parallel paths as the number of poles in the machine. For a four-pole machine, there will be four parallel paths. When the air gap under each pole is uniform, the magnetic flux density across the armature is more evenly distributed. This affects the current and voltage in the winding paths.

Analysis of the Correct Option:

Correct Option: Option 3: Current in each path will not be the same

In a lap wound machine with a uniform air gap under each pole, the magnetic flux distribution should ideally be symmetrical. However, in practical scenarios, even with an equal air gap, other factors such as slight manufacturing imperfections, uneven saturation of the magnetic core, or minor misalignments can cause variations in the flux linkage across different parts of the armature. Since the current in each path of a lap wound machine depends on the induced electromotive force (EMF) in the respective coils, these variations in flux linkage lead to unequal induced EMFs in the parallel paths. Consequently, the currents in the parallel paths will also differ slightly.

For example, if one path experiences a slightly higher flux density due to unavoidable non-uniformities, the induced EMF in that path will be higher, leading to a higher current in that path compared to the others. This imbalance in current distribution can cause uneven heating and may affect the machine's overall efficiency and performance. Therefore, even with an equal air gap under each pole, the current in each path of a four-pole lap wound machine will not be the same.

Important Information:

To further understand the implications of this condition, let us evaluate the other options:

Option 1: There will be reduced eddy currents

This option is incorrect. Eddy currents are induced circulating currents in the core material of the machine caused by changing magnetic fields. The uniformity of the air gap under each pole does not directly influence the eddy currents. Instead, eddy currents depend on factors such as the rate of change of magnetic flux, core material properties, and lamination thickness. Therefore, maintaining an equal air gap under each pole will not necessarily reduce eddy currents.

Option 2: There will be reduced hysteresis loss

This option is also incorrect. Hysteresis loss occurs in the magnetic core material due to the repeated magnetization and demagnetization cycles as the machine operates. Hysteresis loss depends on the properties of the core material, such as its hysteresis loop area, and the frequency of operation. While a uniform air gap can help ensure consistent magnetic flux distribution, it does not directly reduce hysteresis loss, which is determined by the material and operating conditions rather than the air gap uniformity.

Option 4: It will result in higher terminal voltage

This option is incorrect. The terminal voltage of a machine depends on the total induced EMF in the armature windings and the voltage drop across the internal resistances and brushes. While a uniform air gap can contribute to a more consistent flux distribution, it does not inherently result in a higher terminal voltage. The terminal voltage is primarily influenced by the machine design, operating speed, and load conditions.

Option 5: Not provided in the question

Since there is no information or specific context provided for Option 5, it is not considered relevant to the analysis. The correct answer remains Option 3.

Conclusion:

In a four-pole lap wound machine, even if the air gap under each pole is uniform, the practical imperfections and variations in flux linkage can lead to unequal induced EMFs in the parallel paths. This results in unequal currents in the paths, making Option 3 the correct answer. Understanding this phenomenon is crucial for the design and analysis of electrical machines to ensure their efficient and reliable operation. The other options are either unrelated to the air gap uniformity or are not directly impacted by it, as explained above.

Explanation:

Armature Winding Resistance in DC Machines

Definition: The armature winding resistance in a DC machine refers to the resistance offered by the winding of the armature, which is the rotating part of the machine. This resistance is a crucial parameter as it affects the performance and efficiency of the DC machine. For a typical 5 kW DC machine, this resistance is usually quite low.

Calculation and Approximate Value: The armature winding resistance can be estimated based on the power rating and the design of the machine. For a 5 kW DC machine, the armature winding resistance is typically in the range of a few ohms. Given the provided options, the correct approximate value is 0.15 ohms. This low resistance is essential to minimize power losses and ensure efficient operation of the machine.

Importance of Low Armature Winding Resistance: A low armature winding resistance is critical for the following reasons:

• Minimizes I²R losses: Lower resistance reduces the power losses due to the current flowing through the armature windings.
• Improves efficiency: Lower resistance contributes to higher overall efficiency of the DC machine.
• Reduces voltage drop: Lower resistance helps in reducing the voltage drop across the armature, ensuring better performance.

Correct Option Analysis:

The correct option is:

Option 1: 0.15 ohms

This option correctly represents the approximate value of the armature winding resistance for a 5 kW DC machine. The value of 0.15 ohms aligns with the typical resistance expected for such machines, ensuring efficient operation with minimal losses.

Important Information

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

Option 2: 15 ohms

This value is significantly higher than the typical resistance for a 5 kW DC machine. Such a high resistance would result in substantial power losses and inefficiency. It is not practical for the operation of a 5 kW DC machine.

Option 3: 100 ohms

A resistance of 100 ohms is far too high for the armature winding of a 5 kW DC machine. This would lead to excessive I²R losses and a considerable voltage drop, severely impacting the machine’s performance and efficiency.

Option 4: 1 kilo ohms

This option is even more unrealistic. A resistance of 1 kilo ohms in the armature winding of a 5 kW DC machine would make it virtually inoperative due to the enormous power losses and voltage drop. Such a high resistance is not feasible for practical DC machine designs.

Conclusion:

Understanding the significance of the armature winding resistance in DC machines is crucial for their design and operation. For a 5 kW DC machine, the approximate value of the armature winding resistance is around 0.15 ohms, ensuring efficient performance with minimal power losses. Higher resistance values, as given in the other options, would lead to impractical and inefficient operation, highlighting the importance of maintaining low resistance in the armature winding for optimal machine performance.

The  correct answer is option "3".
Explanation:-

Lap winding:

• In lap winding, the number of parallel paths (A) is the same as the number of brushes and poles
• This winding is mainly used for low-voltage and high-current applications.
   

Wave winding:

• In wave winding, the number of parallel paths is two
• This winding is mainly used for high voltage and low current applications
   

Additional Information 

Concept:

Lap winding:

• In lap winding, the number of parallel paths (A) is the same as the number of brushes and poles
• This winding is mainly used for low voltage and high current applications

Wave winding:

• In wave winding, the number of parallel paths is two
• This winding is mainly used for high voltage and low current applications

Additional Information

Concept:

Windings in DC machine

1.) Lap winding

In this type of winding, the number of parallel paths is always the number of poles and the number of brushes.

A = P 

where, A = No. of parallel paths

P = No. of poles

In a lap winding for a DC machine, the number of poles is equal to the number of brushes. This is a fundamental characteristic of lap winding.

So, \(\rm \frac{(number\ of\ poles)}{(number\ of\ brushes)}= 1\)

This winding is used for high-current, low-voltage DC machines.

2.) Wave winding

In this type of winding, the number of parallel paths is always equal to 2

A = 2

This winding is used for high-voltage, low-current DC machines.

Given that:

DC generator wit Pole (P) = 6

Conductor = 480 (Z)

For lap winding armature resistance Ra = 0.06 Ω

When the generator is connected as lap winding then

\({R_a} = {R_{Lap}} = \frac{{Z/P}}{{m.p}} \cdot \frac{{\rho \ell }}{A} = \frac{Z}{{{P^2}}}\frac{{\rho \ell }}{A}\)        (Parallel path = m.p and m = 1)

Now, when the generator is connected as wave winding.

\({R_{wave}} = \frac{Z}{{\left( {{A_p}} \right)}} \cdot \frac{{\rho \ell }}{A}\)

In wave winding, the no. of parallel paths = A = 2

No. of conductors in each parallel path (AP) = \(\frac{Z}{2}\)

\({\left( {{R_a}} \right)_{wave}} = \frac{Z}{4}\frac{{\rho \ell }}{A}\)

\(\frac{{{{\left( {{R_a}} \right)}_{lap}}}}{{{{\left( {{R_a}} \right)}_{wave}}}} = \frac{{\left( {\frac{Z}{{{P^2}}}\frac{{\rho \ell }}{A}} \right)}}{{\left( {\frac{Z}{4} \cdot \frac{{\rho \ell }}{A}} \right)}} = \frac{4}{{{P^2}}} = \frac{4}{{{6^2}}} = \frac{1}{9}\)

⇒ (Ra)wave = 9 × (Ra)lap

= 9 × 0.06 = 0.54 Ω 

Alternate Method

 A²_lap × R_lap = A²_wave × R_wave

In lap winding A = P = 6

R_lap = 0.06 Ω

In wave winding A = 2

6² × 0.06 = 2² × Rwave

Rwave = 9 × 0.06 = 0.54 Ω 

Lap and wave winding:

Hence, statement 1 is false and statement 2 is true.

Armature winding of DC machine:

Modern dc machines employ two general types winding

1. Lap winding
2. Wave winding 

In the wave winding, the end of one coil is connected to the starting of another coil of the same polarity as that of the first coil.

• Back pitch YB the distance between the top and bottom coil sides of a coil measured around the back of armature is called back pitch.
• Front pitch YF the distance between the two coil sides connected to the same commutator segment is called front pitch.
• Winding or resultant pitch YR the distance between the starts of the two consecutive coils measured in terms of coil sides is called resultant pitch.
• Commutator pitch YC the distance between the two commutator segments to which the two ends of a coil are connected is called commutator pitch.
• In wave winding back pitch and front pitch, both are odd and are of the same sign.
• Back pitch and front pitch are nearly equal to pole pitch and maybe equal or differ by ± 2, + for progressive winding, - for retrogressive winding.
• Resultant pitch YR = YB + YF.
• Commutator pitch = average pitch = (YB + YF) / 2.

The armature winding of dc machine:

Modern dc machines employ two general types winding

1. Lap winding
2. Wave winding 

The most commonly used windings are simplex lap and wave windings.

Simplex wave winding:

• In this winding, we connect the end of one coil to the starting of another coil of the same polarity as that of the first coil.
• In this type of winding the coil, side progresses forward around the armature to another coil side and goes on successively passing through N and S pole till it returns to a conductor lying under the starting pole.
• This winding forms a wave with its coil, that’s why we call it as wave winding.

​

Important points:

• Back pitch Y_B the distance between the top and bottom coil sides of a coil measured around the back of armature is called back pitch.
• Front pitch Y_F the distance between the two coil sides connected to the same commutator segment is called front pitch.
• Winding or resultant pitch Y_R the distance between the starts of the two consecutive coils measured in terms of coil sides is called resultant pitch.
• Commutator pitch Y_C the distance between the two commutator segments to which the two ends of a coil are connected is called commutator pitch.
• In simplex wave winding back pitch and front pitch, both are odd and are of the same sign.
• Back pitch and front pitch are nearly equal to pole pitch and may be equal or differ by ± 2, + for progressive winding, - for retrogressive winding.
• Resultant pitch Y_R = Y_B + Y_F.
• Commutator pitch = average pitch = (YB + Y_F) / 2.

Brushes:

The function of brushes in the DC machine is to collect current from commutator segments. Therefore, the number of brushes in the DC machine equal to the number of parallel paths.

Wave winding:

• In wave winding, the number of parallel paths is two. 
• Therefore the number of brushes is also two.

Lap winding:

• In lap winding, the number of parallel paths is equal to A
• Therefore the number of brushes is also equal to A = P

​

In lap winding the resultant pitch should be approximately equal to the difference between the back and front pitches.

YR = YB - YF

In wave winding the resultant pitch should be approximately equal to the sum of the back pitch and front pitch.

YR = YB + YF

Armature Winding of DC Machine:

Modern dc machines employ two general types winding

1. Lap winding
2. Wave winding 

Lap winding:

In lap winding, the conductors are joined in such a way that their parallel paths and poles are equal in number.

Back pitch (Yb): It is the span of the coil from the back end.

Front pitch (Yf): It is the span of the coil from the front end.

Resultant pitch (YR): Distance between the starts of two successive coils.

Formula:

YR = Yb - Yf = ± 2m

m = multiplicity factor

Yb = (2S / P) + 1

Yf = (2S / P) - 1

S = No. of slots

P = No. of poles

Yb ≠ Yf, Yf and Yb are odd in successful winding design.

Wave winding:

In the wave winding, the end of one coil is connected to the starting of another coil of the same polarity as that of the first coil.

Important Points

• Back pitch YB the distance between the top and bottom coil sides of a coil measured around the back of armature is called back pitch.
• Front pitch YF the distance between the two coil sides connected to the same commutator segment is called front pitch.
• Winding or resultant pitch YR the distance between the starts of the two consecutive coils measured in terms of coil sides is called resultant pitch.
• Commutator pitch YC the distance between the two commutator segments to which the two ends of a coil are connected is called commutator pitch.
• In wave winding back pitch and front pitch, both are odd and are of the same sign.
• Back pitch and front pitch are nearly equal to pole pitch and maybe equal or differ by ± 2, + for progressive winding, - for retrogressive winding.
• Resultant pitch YR = YB + YF.
• Commutator pitch = average pitch = (YB + YF) / 2.

Concept:

The armature winding of the DC machine is classified into two types:

1) Lap winding: 

• A = P
• (I_a)_L =  A(I_P)

2) Wave winding: 

• A = 2
• (Ia)w = 2(IP)

where A= No. of parallel paths

P= No. of poles

A= No. of parallel paths

(Ia)_L  = Armature current in Lap winding

(Ia)_w = Armature current in Wave winding

 I_P = Current in parallel path

 Calculation:

Given, For Lap winding : 

• A = P = 6 
• (Ia)L =  A(IP)

Given, For Wave winding : 

• A = 2
• (Ia)w = 2(IP)

By the power conservation theorem, total power remains constant irrespective of the connection type of the armature resistance.

 (P)_L = (P)_W

(Ia)2_L (Ra)_L = (Ia)2w  (Ra)_w

(6I_P)² (0.05) = (2IP)2 (Ra)w

(Ra)w = 9 × 0.05

(Ra)w = 0.45Ω

Pole pitch:

• The pole pitch is defined as the peripheral distance between centers of two adjacent poles in dc machine.
• This distance is measured in term of armature slots or armature conductor come between two adjacent pole centers.
• This is naturally equal to the total number of armature slots divided by the number of poles in the machine.
• A pole pitch in an electrical machine is equal to 180° electrical.

Calculation:

Given that,

Number of coils = 16

As the winding is two-layer winding

⇒ Total number of conductors = 2 × 16 = 32

Pole pitch = Number of conductors/number of poles = 32/4 = 8

Armature windings are mainly of two types: lap winding and wave winding

Lap winding:

• Lap winding is the winding in which successive coils overlap each other. It is named “Lap” winding because it doubles or laps back with its succeeding coils.
• In this winding, the finishing end of one coil is connected to one commutator segment and the starting end of the next coil situated under the same pole and connected with the same commutator segment.
• Generally in lap winding number of parallel paths are equals to number of poles.

Types of Lap winding:

Simplex lap winding: A winding in which the number of parallel paths between the brushes is equal to the number of poles is called simplex lap winding.

• Number of parallel paths (A) = P

Duplex lap winding: A winding in which the number of parallel paths between the brushes is twice the number of poles is called duplex lap winding.

• Number of parallel paths (A) = 2P

Triplex lap winding: A winding in which the number of parallel paths between the brushes is thrice the number of poles is called triplex lap winding.

• Number of parallel paths (A) = 3P

Wave winding:

• It  the armature winding in which two coils are connected in series and follow each other on the surface of the armature like waves such that there are only two paths for the current flow irrespective of the number of poles in the circuit.
• For simplex wave winding, number of parallel paths between the brushes is always 2.
• For multiplex wave winding number of parallel path are ‘2m’

Where, m is the multiplicity of the winding

m = 1 for simplex winding

m = 2 for duplex winding

m = 3 for triplex winding

• Coils are typically wound with enameled copper wire, sometimes termed magnet wire.
• The winding material must have a low resistance, to reduce the power consumed by the field coil, but more importantly to reduce the waste heat produced by ohmic heating.
• The enameled wire basically refers to an aluminium or copper wire which has been given a coating.
• This thin layer of insulation mates it useful for building transformers, motor, inductors, hard disk actuators, speakers, electromagnets, etc.
• The enameled copper wire is electrolytic – refined copper.
• Since these wires come with a coating they are also popularly called magnet wires.
• This type of copper wire is most extensively used in constructing transformers and motors.
• It finds use in the application which needs insulated wires that are tightly coiled.
• The main reason for using this coating or ‘enamel’ is to prevent the wire from getting caught in accidental short circuits.
• The enameled copper wire is also useful because it can be soldered easily.
• Magnet wires will produce electromagnetic fields if one winds them to create coils; these get reenergized in the process.
• The enameled copper wire is used to convert electrical energy to mechanical energy for making motors, office HVAC, etc.
• The main reason for using copper is because of its high electrical conductivity.
• So the use of this metal (Cu) as compared to other types of metals is more because it improves energy efficiency for motors.
• Thus enameled copper wire is used for generator, motor winding, and field windings.