11. Pole shoe of a D.C. machine is laminated for the purpose of
Decreasing hysteresis loss
Decreasing eddy current loss
Decreasing both hysteresis and eddy current loss
Manufacturing ease
Answer:2. Decreasing eddy current loss
Explanation:
The pole shoes are generally made of thin strips called laminations to reduce eddy current loss.
The thickness of the transformer core laminations is usually in the order of 0.25 mm to 0.5 mm.
The thickness of silicon steel lamination is defined by varnish + silicon steel + varnish.
Insulating material between laminations is required for electrical isolation, so as to reduce eddy current loss.
The process of bunching all laminations is called core staggering.
12. The insulating material used between the commutator segments is normally ___________
Graphite
Paper
Mica
Insulating varnish
Answer: 3. Mica
Explanation:
The insulating material used between the commutator segments is normally mica as it has a very high dielectric stress of around 1180 kV/cm.
13. Which of the following parts is NOT connected to a DC machine?
Armature
Damper windings
Commutator
Carbon brushes
Answer:2. Damper windings
Explanation:
Damper winding is used in Synchronous machines for reducing hunting (both motor & generator) and proving induction torque at starting (motor). It is not used in DC machines.
14. Which of the following is not a function of the magnetic frame in a DC generator?
It provides low reluctance path for flux.
It carries the magnetic flux produced by the poles in the DC generator.
It acts as a protective cover for the whole machine
It provides mechanical support for the poles in the DC generator.
Answer:1. It provides low reluctance path for flux.
Explanation:
The outer frame or yoke serves a double purpose in DC machine:
(i) It provides mechanical support for the poles and acts as a protecting cover for the whole machine and
(ii) It carries the magnetic flux produced by the poles.
In small generators where cheapness rather than weight is the main consideration, yokes are made of cast iron
For large machines usually cast steel or rolled steel is employed.
The modern process of forming the yoke consists of rolling a steel slab around a cylindrical mandrel and then welding it at the bottom.
The feet and the terminal box etc. are welded to the frame afterward. Such yokes possess sufficient mechanical strength and have high permeability
15. Which of the following relation is TRUE for progressive winding of a DC machine?
YF = YB – 2
YF = YB + 1
YF = YB – 1
YF = YB + 2
Answer: 1. YF = YB – 2
Explanation:
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,
YB = YF ± 2
YB =YF + 2 for progressive winding
YB = YF – 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}$
16. In a four-pole D. C. machine
All the four poles are North poles
Alternate poles are North and South
All the four poles are South poles
Two North poles follow two South poles
Answer:2. Alternate poles are North and South
Explanation:
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.
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.
17. Copper brushes in D. C. machine are used
Where low voltage and high currents are involved
Where high voltage and small currents are involved
In both of the above cases
In none of the above cases
Answer: 1. Where low voltage and high currents are involved
Explanation:
The brush in a DC machine is a component that collects the current from the moving parts and feeds it to the stationary circuit
Brushes are usually made from carbon or graphite, but copper brushes are preferred in low voltage high current requirements.
Brush contact loss attributes to the resistance between the surface of the brush and the commutator
This loss is mostly enclosed in armature copper loss
As copper losses depend on load current, brush contact losses also depend on load current; These are directly proportional to the current
18. The main Function of the Bearing in the DC generator is
Reduce armature reaction
Reduce hysteresis loss
Reduce Friction loss
Reduce Airflow loss
Answer: 3. Reduce Friction loss
Explanation:
The bearings in a DC generator may be ball or roller bearings these are fitted in the end housings. Their function is to reduce friction between the rotating and stationary parts of the machine. Mostly high carbon steel is used for the construction of bearings as it is a very hard material.
19. The shaft in a DC Generator is made up of ______
Mild Steel
Hard Steel
Copper
Mica
Answer:1. Mild Steel
Explanation:
The shaft in DC Generator is made of mild steel with a maximum breaking strength. The shaft is used to transfer mechanical power from or to the machine. The rotating parts like the armature core, commutator, cooling fan, etc. are keyed to the shaft.
20. The function of _______ is to collect the current from the commutator of the machine.
Pole shoe
Yoke
Brush
Armature winding
Answer:3. Brush
Explanation:
The function of the brush is to collect the current from the commutator of ‘ the machine. They are made of carbon and are placed in brush holders. The brushes rest on the commutator segments against the spring tension. A lever is used to adjust the spring tension.
The commutator and brush arrangement is the connecting link between the armature winding and the external circuit. The purpose of bearings is to provide free and smooth rotation of the armature. Ball bearings are frequently.
