1. Which of the following machines do NOT use equalizer rings?
4-pole duplex wave-wound DC machine
6-pole simplex lap-wound DC machine
4-pole simplex wave-wound DC machine
4-pole simplex lap-wound DC machine
Answer:3. 4-pole simplex wave-wound DC machine
Explanation:
Equalizer rings are used in lap wound machines to avoid unequal distribution of current at the brushes, thus reducing sparking at the brushes.
As in lap winding number of the parallel paths is more, there would be severe sparking at brushes due to differences in currents in different parallel paths.
But in wave winding number of parallel paths is 2 and the sparking at commutation is less. So, equalizer rings are used in lap winding to avoid any unequal distribution.
The same thing is with duplex windings, with more parallel paths, so more sparking, so equalizer rings are required.
2. ________ is used for neutralizing armature reaction while inter pole winding is used for improving commutation.
Armature winding
Field Winding
Pole Shoe
Compensating Winding
Answer:4. Compensating Winding
Explanation:
Compensating winding is used in DC machines to neutralize the cross magnetizing effect of armature reaction.
The machines which are subjected to large changes in load will experience a sudden shift in the magnetic neutral axis.
This statically induced emf in armature may strike an arc between the commutator segments and result in a flash-over in the whole commutator segments.
Compensating winding is also known as pole face winding.
Compensating winding is embedded in the slots in pole faces. It is connected in series with the armature winding so that their mmfs are proportional to the same current.
Compensating winding is placed in the main poles to cancel the effect of the armature reaction and to maintain uniform flux distribution under the main poles.
Compensating winding is connected in series with the armature winding in such a way that the current flowing in them is directly opposite to the current flowing in the armature located just below the pole faces.
3. The material for commutator brushes in D.C generator is generally __________
Mica
Copper
Cast iron
Carbon
Answer:4. Carbon
Explanation:
Brushes are used to collect the current due to the induced emf from the armature coils in a DC generator.
Brushes are made up of carbon to reduce the wear and tear of the commutator.
When a brush is at any particular segment, it shorts out that particular coil and draws current from the rest of the coils, If emf induce in that coil, the huge current will flow.
To prevent this, brushes are positioned in such a way that the coil being short-circuited has no emf in it to prevent heat loss in the brushes and sparking at commutator segments.
Hence brushes are placed along the magnetic neutral axis.
4. For a 4-pole simplex wave-wound DC generator, number of parallel paths will be ____________.
8
2
4
1
Answer:2. 2
Explanation:
m is the multiplicity of the winding
m = 1 for simplex winding
m = 2 for duplex winding
m = 3 for triplex winding
Calculation:
Given that, simplex wave winding i.e. m = 1
Number of parallel paths = 2
5. The commutator segments of the DC Generator are connected to the armature conductors by means of
Copper lugs
Resistance wires
Insulation pads
Brazing
Answer: 1. Copper lugs
Explanation:
Mechanical lugs are available bolted with two screws or with four screws. They are typically ring-shaped and are usually made of high-grade electrolyte copper or aluminum, so these are also called copper lugs.
They are suitable for big wires and cables and are available in three varieties: plain type, sight hole type (which is similar to the plain types, but with a bigger hole), and connector type.
Designed to be easily installed and removed for repairs or maintenance, copper lugs are generally used when permanent, direct-fastening methods are not feasible or necessary.
Copper Lugs may be provided with a safety covering to avoid shocks or protective covering to avoid damage to the copper lugs, cables, connectors, and to prevent short-circuiting.
6. In a DC generator the commutator converts ________.
AC to DC current
variable DC to constant DC
constant DC to variable DC
DC to AC current
Answer: 1. AC to DC current
Explanation:
In the case of the DC generator, the commutator is used to convert generated AC in armature into DC.
In the case of DC motor, the commutator is used to convert DC to A.C
Due to the limitation of the commutator dc generators are not usually designed beyond 650 V
7. For both lap and wave windings, there are as many commutator bars as the number of
Slots
Armature conductors
Windings elements
Poles
Answer:2. Armature conductors
Explanation:
A commutator consists of a set of copper segments that are insulated from each other.
The number of segments is equal to the number of armature conductors. Each segment is connected to an armature coil and the commutator is keyed to the shaft.
8. The field coils of the D.C generator are usually made of –
Mica
Copper
Cast Iron
Carbon
Answer:2. Copper
Explanation:
The field winding of the DC generator is usually made of copper. Enamelled copper wire is used for the construction of field or exciting coils.
Field coils are former wound and placed on each pole and are connected in series.
They are wound in such a way that, when energized, they form alternate North and South poles.
9. Armature core of DC Generator is made up of
Mica
Copper
Cast Iron
Silicon Steel
Answer:4. Silicon Steel
Explanation:
The armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding.
The armature is built up of thin laminated circular steel disks for reducing eddy current losses.
The armature core is made of silicon steel laminations which are insulated from each other by an insulating varnish coating. These laminations are used to reduce eddy current losses.
It may be provided with air ducts for the axial airflow for cooling purposes.
The armature is keyed to the shaft.
10. Why are armature windings made on the rotor of a DC machine?
For production of voltage
To make rotation easy
For electromechanical conversion
For commutation
Answer:4. For commutation
Explanation:
The rotating part of the DC machine is called the armature
The armature winding of a dc machine is placed on the rotor to facilitate commutation
Commutation is a process of converting the alternating voltage produced in the winding into a direct voltage at the brushes or direct voltage into alternating voltage.
