1.While applying Fleming's right-hand rule to find the direction of induced e.m.f., the thumb points towards
A. direction of induced e.m.f.
B. direction of flux
C. direction of motion of the conductor if forefinger points in the direction of generated e.m.f.
D. direction of motion of conductor, if forefinger points along the lines of flux
Answer: D. direction of motion of conductor, if forefinger points along the lines of flux
Explanation:
- Fleming's Right-hand Rule shows the direction of induced current when a conductor attached to a circuit moves in a magnetic field.
- It can be used to determine the direction of current in a generator's windings.
- If the thumb, the forefinger, and the middle finger of the right hand are bent at right angles to one another with the thumb pointed in the direction of motion of a conductor relative to a magnetic field and the forefinger in the direction of the field, then the middle finger will point in the direction of the induced electromotive force.
2. Which of the following could be lamina-proximately the thickness of laminations of a D.C. machine ?
A. 0.005 mm
B. 0.05 mm
C. 0.5 mm
D. 5 mm
Answer: C. 0.5 mm
Explanation:
- Eddy current loss is basically I2 R loss present in the core due to the production of eddy currents in the core, because of its conductivity.
- Eddy current losses are directly proportional to the conductivity of the core.
- Eddy current losses can be reduced by either by adding silica content (4% - 5 %) to steel or by using a laminated core instead of a solid core.
- In that case, eddy current losses are directly proportional to the square of the frequency.
- In order to reduce the eddy current losses, we use laminations
- In a DC machine, laminations are used to reduce eddy current losses and for insulation purposes. The approximate thickness of laminations is 0.5 mm.
- The stator frame consists of laminations of silicon steel, usually with a thickness of about 0.5 millimetre.
3. The armature of D.C. generator is laminated to
A. reduce the bulk
B. provide the bulk
C. insulate the core
D. reduce eddy current loss
Answer: D. reduce eddy current loss
Explanation:
- When an alternating magnetic field is applied to a magnetic material, an emf is induced in the material itself according to Faraday’s law of Electromagnetic induction.
- Since the magnetic material is a conducting material, these EMF’s circulates current within the body of the material. These circulating currents are called Eddy currents. They are produced when the conductor experiences a changing magnetic field.
- The process of lamination involves dividing the core into thin layers held together by insulating materials.
- Due to lamination effective cross-section area of each layer reduces and hence the effective resistance increases.
- As effective resistance increases, the eddy current losses will get decrease.
- the Eddy current loss is proportional to the square of the frequency.
- To minimize the eddy current loss we increases the resistance in the path of eddy current by laminating it.
- Eddy current losses are directly proportional to area of armature or more precisely the path of motion.
- In laminated armature eddy current losses are reduced to very less or '0' quantity. That is why armature of DC machines (either motor or generator) is laminated.
4. The resistance of armature winding depends on
A. length of conductor
B. cross-sectional area of the conductor
C. number of conductors
D. all of the above
Answer: D. all of the above
Explanation:
- Resistance of each conductor is depending on resistivity of the material, length and area of cross section of the conductor.
- The total armature resistance depends on the number of conductors.
5. The field coils of D.C. generator are usually made of
A. mica
B. copper
C. cast iron
D. carbon
Answer: B. copper
Explanation:
Field coil of D.C generator is usually made up of copper because it has good electrical conductivity, good thermal conductivity and corrosion resistance.
Important Points:
A DC machine consists of two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.
Yoke:
- The outer frame or body of a dc machine is called as yoke.
- It is made up of cast iron or cast steel.
- It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.
Poles and pole shoes:
- 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 air gap uniformly.
Field winding:
- They are usually made of copper.
- 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.
Armature core:
- 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.
- Armature core is made of silicon steel laminations which are insulated from each other by insulating varnish coating. These laminations are used to reduce eddy current losses.
- It may be provided with air ducts for the axial air flow for cooling purposes.
- Armature is keyed to the shaft.
Armature winding:
- It is usually a former wound copper coil which rests in armature slots.
- The armature conductors are insulated from each other and also from the armature core.
- Armature winding can be wound by one of the two methods; lap winding or wave winding.
- Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.
Commutator and brushes:
- Physical connection to the armature winding is made through a commutator-brush arrangement.
- The function of a commutator in a dc generator is to collect the current generated in armature conductors.
- While in case of a dc motor, the commutator helps in providing current to the armature conductors.
- A commutator consists of a set of copper segments which are insulated from each other.
- The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft.
- Brushes are usually made from carbon or graphite.
- They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.
6. The commutator segments are connected to the armature conductors by means of
A. copper lugs
B. resistance wires
C. insulation pads
D. brazing
Answer: A. 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 safety covering to avoid shocks or protective covering to avoid damage to the copper lugs, cables, connectors, and prevent short-circuit.
Application:
The commutator segments are connected to the armature conductors by means of copper lugs.
7. In a commutator
A. copper is harder than mica
B. mica and copper are equally hard
C. mica is harder than copper
D. none of the above
Answer: C. mica is harder than copper
Explanation:
Commutator:
- In the case of the DC generator, the commutator is used to convert generated AC in armature into DC.
- In the case of the DC motor, the commutator is used to convert DC to A.C.
- Due to the limitation of commutator dc generators are not usually designed beyond 650 V
- The physical connection to the armature winding is made through a commutator-brush arrangement.
- The function of a commutator in a dc generator is to collect the current generated in armature conductors.
- While in the case of a dc motor, the commutator helps in providing current to the armature conductors.
- A commutator consists of a set of copper segments which are insulated from each other.
- The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft.
- In a commutator mica is harder than copper
8. In D.C. generators the pole shoes are fastened to the pole core by
A. rivets
B. counter sunk screws
C. brazing
D. welding
Answer: B. counter sunk screws
Explanation:
In DC generators, the pole shoes are fastened to the pole by countersunk screws.
A countersunk screw is a type of fastening that sits flush with the surface of the material it occupies.
Yoke:
- It is the outer covering of the DC generator and is made of cast steel or cast iron. It serves two purposes:
- Provides a path for pole flux.
- Provides mechanical support to the whole machine.
Pole Shoe:
- It distributes the flux in the air gap uniformly.
- In DC generators, the pole shoes are fastened to the pole by countersunk screws.
Field Windings:
- It is made of copper and wound around each pole core in such a way that adjacent North and South poles develop when the field winding is excited.
Armature Windings:
- Armature winding is made of copper and placed inside the slots of the armature core.
- Each conductor in the winding is insulated from the other and also from the armature core.
Commutator:
- A commutator is also known as a mechanical rectifier.
- It collects the current generated in the armature winding.
9. According to Fleming's right-hand rule for finding the direction of induced e.m.f., when middle finger points in the direction of induced e.m.f., forefinger will point in the direction of
A. motion of conductor
B. lines of force
C. either of the above
D. none of the above
Answer: B. lines of force
Explanation:
Fleming's right-hand rule:
- According to Fleming's right-hand rule, the thumb, forefinger, and middle finger of the right hand are stretched to be perpendicular to each other.
- The thumb represents the direction of the movement of the conductor.
- The forefinger represents the direction of the magnetic field (lines of force), then the middle finger represents the direction of the induced current.
- It is used used to determine the direction of current in a generator's windings.
Important Information
Fleming's left-hand rule:
- According to Fleming's left-hand rule, if the thumb, forefinger, and middle finger of the left hand are stretched to be perpendicular to each other and if the forefinger represents the direction of the magnetic field, the middle finger represents the direction of the current, then the thumb represents the direction of the force.
- Fleming's left-hand rule is applicable for motors.
Maxwell's corkscrew rule:
- The Maxwell Screw Rules is also called Maxwell's Corkscrew Rule.
- Imagine a right-handed screw being turned so that it bores its way in the direction of the current in the wire.
- The direction of rotation gives the direction of the magnetic field and the direction of thumb give the direction of current
Ampere's swimming rule:
- Ampere's swimming rule states that "If a man swims along the wire carrying current such that his face is always towards the magnetic needle with current entering his feet and leaving his head, then the north pole of the magnetic needle is always deflected towards his left hand."
10. Fleming's right-hand rule regarding direction of induced e.m.f. correlates
A. magnetic flux, direction of current flow and resultant force
B. magnetic flux, direction of motion and the direction of e.m.f. induced
C. magnetic field strength, induced voltage and current
D. magnetic flux, direction of force and direction of motion of conductor
Answer: B. magnetic flux, direction of motion and the direction of e.m.f. induced
Explanation:
Fleming's right-hand rule:
- According to Fleming's right-hand rule, the thumb, forefinger, and middle finger of the right hand are stretched to be perpendicular to each other.
- The thumb represents the direction of the movement of the conductor.
- The forefinger represents the direction of the magnetic field (lines of force), then the middle finger represents the direction of the induced current.
- It is used used to determine the direction of current in a generator's windings.