1. What should be the range for the width of the commutator in a good design?
a) < 4 mm
b) > 4 mm
c) 3-4 mm
d) < 3 mm
Explanation: For a good design of the commutator, the width of the commutator should be greater than 4 mm. If the value goes below 4 mm, we should not choose that value
2. The design of commutator and the brushes of DC machines are interconnected.
a) true
b) false
Explanation: The design of brushes and commutators are interconnected, because in the calculation of the length of the commutator, the width of the brush, number of brushes are used. In the same way, brush values are also used in calculation of commutator loss.
3. What is the formula for calculation of brush current of DC machine for wave winding?
a) brush Current = 2*(Armature Current) / P
b) brush Current = (Armature Current) / P
c) brush Current = Armature Current
d) brush Current = P/2*(Armature Current)
Explanation: Brush Current = 2*(Armature Current) / P is the formula for the calculation of brush current of DC machine for lap winding.
4. What is the formula for brush contact loss of DC machines?
a) brush contact loss = armature current + brush contact voltage
b) brush contact loss = armature current – brush contact voltage
c) brush contact loss = armature current * brush contact voltage
d) brush contact loss = (armature current + brush contact voltage) / 2
Explanation: For calculation of the brush contact loss, first the armature current is obtained. Then the voltage through the brush contacts is calculated and the product gives the brush contact loss.
5. What is the formula for total commutator loss for DC machine?
a) brush contact loss + brush friction loss
b) brush contact loss – brush friction loss
c) brush contact loss * brush friction loss
d) brush contact loss / brush friction loss
Explanation: Brush contact loss = brush contact loss = armature current * brush contact voltage
Brush friction loss = (Coefficient f friction) * Brush Pressure * Area of the brush*Commutator voltage.
6. How is the selection of number of poles made in AC and DC machines?
a) any number of poles can be used for both AC and DC machines
b) fixed number of poles in both AC and DC machines
c) fixed number of poles in DC machines and any number of poles in AC machines
d) fixed number of poles in AC machines and any number of poles in DC machines
Explanation: When it comes to DC machines, any number of poles can be made use of, but it should be within a range. Whereas in AC machines the number of poles is fixed by supply frequency and speed.
7. How many considerations are present in the selection of number of poles?
a) 4
b) 5
c) 6
d) 7
Explanation: There are 7 consideration in selection of number of poles. They are frequency, weight of iron parts, weight of copper, length of commutator, labor charges, flash over, distortion of field form.
8. What is the formula for frequency of flux reversals?
a) f = p*n
b) f = p/n
c) f = n/p
d) f = (p*n)/2
Explanation: Frequency is directly proportional to the number of poles (p). It is also proportional to the speed of the machine as well.
9. What is the range of frequency during the selection of number of poles?
a) 20-50 Hz
b) 25-40 Hz
c) 25-50 Hz
d) >50 Hz
Explanation: While selecting the number of poles, the lowest value of frequency should be minimum 25 Hz. The highest value of frequency should be limited to 50 Hz
10. What is the relation of hysteresis loss and weight of iron parts with respect to increase of number of poles?
a) decrease in hysteresis loss, increase in weight
b) decrease in hysteresis loss, decrease in weight
c) increase in hysteresis loss, increase in weight
d) increase in hysteresis loss, decrease in weight
Explanation: With larger number of poles, the area of cross section can be reduced, henceforth decreasing the hysteresis loss. Also by increasing pole number, weight of iron parts is reduced.