1. In lap winding, the number of brushes is always
Explanation:
A. 2
B. 4
C. 2P
D. P
Answer: D. P
Explanation:
- The brushes in Dc machines are used for collecting the current from the commutator segment.
- The brushes are made from carbon.
- In Dc machines Number of brushes are equal to the number of parallel paths or the number of poles.
- For wave winding, a number of parallel paths are equal to 2 and the number of brushes is required also 2.
2. In the case of lap winding resultant pitch is
A. Multiplication of Front and back pitch
B. Division of the front pitch by back pitch
C. Sum of back pitch and front pitch
D. Difference between front and back pitches
Answer: D. Difference between front and back pitches
Explanation:
- For simplex lap winding range of the winding pitch of (2,-2). For progressive winding pitch = Forward pitch - Backward pitch.
- For simplex lap winding, commutator pitch is equal to 1 and -1.
- The resultant pitch in lap winding is equal to the difference between the back and front pitches.
3. A dummy coil may required in a DC machine, if the machine is
A. lap wound
B. wave wound
C. either lap or wave wound
D. none of the above
Answer: B. wave wound
Explanation:
- In wave winding, there are dummy coils are used.
- The dummy coil provides the mechanical balance to the rotor only and dummy coils are physically connected but electrically not connected.
4. The purpose of providing dummy coils in dc machine armature is to
A. Reduce eddy currents
B. Increase voltage induced
C. Decrease armature resistance
D. Provide mechanical balance for the motor
Answer: D. Provide mechanical balance for the motor
Explanation:
- The equalizer rings are used in lap winding to reduce the unequal current distribution at brushes and reduce the sparking between the carbon brushes and commutator.
- In wave winding, there are dummy coils are used.
- The dummy coil provides the mechanical balance to the rotor only and dummy coils are physically connected but electrically not connected.
5. Dummy coil in a d.c. generator is provided to reduce
A. Mechanical unbalance
B. Hysteresis losses
C. Armature reaction
D. Tendency of reversal
Answer: A. Mechanical unbalance
Explanation:
- In wave winding, there are dummy coils are used.
- The dummy coil provides the mechanical balance to the rotor only and dummy coils are physically connected but electrically not connected. Dummy coils are used in DC machines for reducing mechanical unbalance.
6. The function of dummy coils in DC machines is to
A. improve commutation
B. reduce machine cost
C. mechanically balance the armature
D. increase efficiency
Answer: C. mechanically balance the armature
Explanation:
- For lap winding, the number of parallel paths(A) is equal to the number of poles(P) and a number of brushes required in lap winding are equal to the number of poles.
- In lap winding, there are equalizer rings are required for good commutation. The equalizer rings are used in lap winding because there are more parallel paths in lap winding than in wave winding.
- The equalizer rings are used in lap winding to reduce the unequal current distribution at brushes and reduce the sparking between the carbon brushes and commutator.
- In wave winding, there are dummy coils are used.
- The dummy coil provides the mechanical balance to the rotor only and dummy coils are physically connected but electrically not connected. Dummy coils are used in DC machines for reducing mechanical unbalance.
7. The number of parallel path in a 8-pole duplex lap winding is equal to
A. 4
B. 8
C. 16
D. 32
Answer: C. 16
Explanation:
- A winding which is having a number of parallel paths equal to the number of poles is called simplex lap winding.
- A winding which is having a number of parallel paths is equal to twice the number of parallel paths.
- A. winding which is having the number of parallel paths is equal to thrice the number of poles.
- For the above Question, the number of poles is 8 and the winding is duplex so the number of parallel paths is equal to the 2 x 8 =16.
- For lap winding, the number of parallel paths(A) is equal to the number of poles(P) and a number of brushes required in lap winding are equal to the number of poles.
- In lap winding, there are equalizer rings are required for good commutation. The equalizer rings are used in lap winding because there are more parallel paths in lap winding than in wave winding.
8. In the DC machine, the fractional pitch winding is used
A. To reduce the Harmonic in generated EMF
B. Improve Cooling
C. Increase EMF
D. To reduce the copper losses
Answer: A. To reduce the Harmonic in generated EMF
Explanation:
Use of fractional pitch winding
- Make the winding stiffer.
- It reduces the distorting harmonics and produces sinusoidal harmonics.
- Save Cu wire & fewer copper losses and reduce cost.
- Easier to manage, reduces end turns to build up and shorten the overall length of armature core which reduces the flux leakage
- Fractional Pitch winding is not used to reduce size of the machine.
Additional Information
Advantages of Short Pitch Coil:
- Reduces the amount of copper needed for End Connection when compared with Full Pitched Coil.
- 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.
- Eddy current and hysteresis losses are reduced.
Coil pitch or coil span:
It is defined as the angular distance between the two coil sides of one coil. It is expressed in terms of electrical space degree.
9. The number of parallel paths in an 8 pole, duplex wave wound armature in DC generator is
A. 2
B. 4
C. 8
D. 16
Answer: C. 8
Important Points about Lap and Wave Windings
Wave winding:
For simplex wave winding, the number of parallel paths between the brushes is always 2.
For multiplex wave, the winding number of the parallel path is ‘2m’
Where m is the multiplicity of the winding
m = 1 for simplex winding
m = 2 for duplex winding
m = 3 for triplex winding
For duplex wave winding number of parallel paths are
A = 2 x 2
A = 4
Lap winding:
For Simplex Lap Windings
Number of parallel paths (A) = P
For Duplex Lap Windings
Number of parallel paths (A) = 2P
For Triplex lap windings
Number of parallel paths (A) = 3P
10. The number of parallel path in a simplex wave windings are equal to
A. Two
B. Number of poles
C. Number of poles pairs
D. No of coils
Answer: A. Two
Explanation:
Wave winding:
For simplex wave winding, the number of parallel paths between the brushes is always 2.
For multiplex wave, the winding number of the parallel path is ‘2m’
Where m is the multiplicity of the winding
m = 1 for simplex winding
m = 2 for duplex winding
m = 3 for triplex winding