Magnetic Circuits - Short Question and Answer
Points : magnetic circuits short question and answer
Q. 1. Does a magnetic circuit consume energy?
Ans. Though energy is required in order to set up a magnetic flux, no energy is necessary to continue it. This supplied energy is store in magnetic field. While magnetic field collapses (by opening circuit switch), the stored magnetic energy is return to circuit Though, energy is consumed in an electric circuit.
Q. 2. Why does leakage occur in a magnetic circuit?
Ans. In a practical magnetic circuit, large amount of flux is confine to intended path (i.e. iron path and air gap path in magnetic circuit) but a small amount of flux inevitably Leaks through surrounding air. This stray flux is termed as leakage flux. Leakage flux is useless or even harmful in an electrical machine.
Q.3. There is no magnetic insulator but there are a number of electric insulators. clarify. Ans. There is no magnetic insulator. as, flux can be set up still in air (the best identified magnetic insulator) by reasonable m.m.f. There are a large number of electric insulators. as, air is a extremely good electric insulator and current cannot pass through it. Q. 4. Why is a needed to keep air gaps in magnetic circuits as small as feasible? Ans. Magnitude of AT necessary for air gap is much greater than that necessitate for iron part of magnetic circuit. It is as reluctance of air is very large as compare to that accessible through iron. In nature, every effort should be made to keep air-gaps in magnetic circuits as short as feasible in order to ease leakage, fringing and total reluctance of circuit. In rotate electrical machines, we try to remain the air gap among the rotor and stator as short as feasible to remain the m.m.f. of poles to a minimum. By reducing the m.m.f., we can ease size of pole winding which enables us to decrease the dimensions and cost of machine. Q. 5. Which material would you select for making electromagnets? Ans. Energy necessary to demagnetise 1 m3. of a material is different for different materials. For example:
(i) For soft iron, energy required is 10 J/m3.
(ii) For Alnico, energy necessary is 50,000 J/m3.
Electromagnets are made of that materials which necessitate small energy for demagnetisation.
For this reason. soft iron is use to create electromagnets. Q. 6. Why is air gap flux termed as useful flux? Ans. The majority practical magnetic circuits have one or more air gaps. as, an air-gap is necessary to permit the rotation of rotor in case of an electrical machine or to permit movement in case of moving-coil instruments. Air-gap flux is link among stationary and moving parts and is thus termed as useful flux. Q.7. Why is air-gap essential in practical magnetic circuits? Ans. In mainly of magnetic circuits, bulk of space is taken through magnetic material (e.g. iron etc.) excepting for small air gaps which are present also through intention or by essential.
(i) In electromechanical energy change devices, magnetic flux must infuse a stationary as well as rotary mass of magnetic material, therefore making an air gap necessary.
(ii) In some devices an air gap might be insert intentionally to mask non- linear relationship among B and H.
Q. 8. What is permeability of a material?
Ans. Permeability of a material can be thought of as a measure of how well magnetic field lines concentrate inside its boundaries, under influence of magnetising force. Ferromagnetic material has a large permeability and flux density in such a material is much greater than in a nonmagnetic material while equally experience similar magnetising force.
Fig shows flux in iron bar, which has a high permeability, is densely concentrated (large value of B) as the lines are spares in Low-permeability air surrounding bar.
(i) No energy is expended in a magnetic circuit. in other words, energy is required to create flux but not to preserve it. When current flows through an electric circuit, energy is consumed so long as current flows. The expended energy is dissolute in form of heat.
(ii) Reluctance of a magnetic circuit is not steady rather is depends upon flux density (B) in magnetic circuit. Though, resistance of an electric circuit is almost constant. it is because value of resistivity vanes very slightly by temperature. Q. 10. Hysteresis refers to the lagging of B behind H in a magnetic circuit. Is it a time lag? Ans. Hysteresis way lagging of B behind H in a magnetic material. But this is not a time lag. The remaining flux density does not turn into zero after the passage of time. The residual magnetism becomes zero simply upon the application of a proper coercive force. Q. 11. What is cause of hysteresis? Ans. Hysteresis in an electrical machine results in following harmful effects:
(i) It results in a power loss, therefore raising temperature of machine.
(ii) It reduces the competence of machine. Q. 12. Permanent magnets are prepared of materials having large hysrteresis loop. Why? Ans. Large hysteresis loop of a material means that it will contain large residual flux density and coercive force. This means that energy necessary to demagnetize such a material would be extremely large. Q.13. What do you mean by curie temperature? Ans. While temperature of a magnetic material is raised beyond a certain value, called curie temperature, thermal agitation destroys position of domains. Material loses its magnetism over this temperature and becomes in essence paramagnetic again until the temperature is lowered through the curie point. The curie temperature for iron is on 700°C. Q. 14. How will you decrease hysteresis loss in an electrical machine? Ans. HysteresIs loss in an electrical machine can be reduced in following ways:
(i) As hysteresis loss depends upon flux density, flux density in iron part should be limited to a value that will not cause extreme loss.
(ii) Frequency of reversal of magnetism should be low. Though, this factor is not in control of designer because in mainly instances frequency is fixed by cause of supply.
(iii) Through use special steels containing silicon e.g silicon steel. These alloys contain extremely low hysteresis loss. Q. 15. What is the meaning of the term hysteresis? Ans. Hysteresis is the term use to describe behaviour of a uneven whose value changes in one method when it is increasing and in a different way while it is decreasing. Hysteresis is associated by many physical phenomena and we see that magnetic flux density in a ferromagnetic material is an example. If we are given a exacting- value of H, we cannot determine the corresponding value of B unless we know whether B (and H) are increasing or decreasing. Q. 16. What is importance of magnetic circuits? Ans. Magnetic circuits constructed of ferromagnetic materials constitute essential parts of many pieces of electrical apparatus.
(i) Each generator, every motor and every transformer (except those which operate at high frequency) make use of magnetic circuits.
(ii) Watt-hour meters, telephone receivers, loudspeakers, relays for power, telegraph circuit and certain type of circuit breakers every one depend upon magnetic flux restricted to paths of finite length and cross-section.
Q.3. There is no magnetic insulator but there are a number of electric insulators. clarify. Ans. There is no magnetic insulator. as, flux can be set up still in air (the best identified magnetic insulator) by reasonable m.m.f. There are a large number of electric insulators. as, air is a extremely good electric insulator and current cannot pass through it. Q. 4. Why is a needed to keep air gaps in magnetic circuits as small as feasible? Ans. Magnitude of AT necessary for air gap is much greater than that necessitate for iron part of magnetic circuit. It is as reluctance of air is very large as compare to that accessible through iron. In nature, every effort should be made to keep air-gaps in magnetic circuits as short as feasible in order to ease leakage, fringing and total reluctance of circuit. In rotate electrical machines, we try to remain the air gap among the rotor and stator as short as feasible to remain the m.m.f. of poles to a minimum. By reducing the m.m.f., we can ease size of pole winding which enables us to decrease the dimensions and cost of machine. Q. 5. Which material would you select for making electromagnets? Ans. Energy necessary to demagnetise 1 m3. of a material is different for different materials. For example:
(i) For soft iron, energy required is 10 J/m3.
(ii) For Alnico, energy necessary is 50,000 J/m3.
Electromagnets are made of that materials which necessitate small energy for demagnetisation.
For this reason. soft iron is use to create electromagnets. Q. 6. Why is air gap flux termed as useful flux? Ans. The majority practical magnetic circuits have one or more air gaps. as, an air-gap is necessary to permit the rotation of rotor in case of an electrical machine or to permit movement in case of moving-coil instruments. Air-gap flux is link among stationary and moving parts and is thus termed as useful flux. Q.7. Why is air-gap essential in practical magnetic circuits? Ans. In mainly of magnetic circuits, bulk of space is taken through magnetic material (e.g. iron etc.) excepting for small air gaps which are present also through intention or by essential.
(i) In electromechanical energy change devices, magnetic flux must infuse a stationary as well as rotary mass of magnetic material, therefore making an air gap necessary.
(ii) In some devices an air gap might be insert intentionally to mask non- linear relationship among B and H.
Fig shows flux in iron bar, which has a high permeability, is densely concentrated (large value of B) as the lines are spares in Low-permeability air surrounding bar.
(i) No energy is expended in a magnetic circuit. in other words, energy is required to create flux but not to preserve it. When current flows through an electric circuit, energy is consumed so long as current flows. The expended energy is dissolute in form of heat.
(ii) Reluctance of a magnetic circuit is not steady rather is depends upon flux density (B) in magnetic circuit. Though, resistance of an electric circuit is almost constant. it is because value of resistivity vanes very slightly by temperature. Q. 10. Hysteresis refers to the lagging of B behind H in a magnetic circuit. Is it a time lag? Ans. Hysteresis way lagging of B behind H in a magnetic material. But this is not a time lag. The remaining flux density does not turn into zero after the passage of time. The residual magnetism becomes zero simply upon the application of a proper coercive force. Q. 11. What is cause of hysteresis? Ans. Hysteresis in an electrical machine results in following harmful effects:
(i) It results in a power loss, therefore raising temperature of machine.
(ii) It reduces the competence of machine. Q. 12. Permanent magnets are prepared of materials having large hysrteresis loop. Why? Ans. Large hysteresis loop of a material means that it will contain large residual flux density and coercive force. This means that energy necessary to demagnetize such a material would be extremely large. Q.13. What do you mean by curie temperature? Ans. While temperature of a magnetic material is raised beyond a certain value, called curie temperature, thermal agitation destroys position of domains. Material loses its magnetism over this temperature and becomes in essence paramagnetic again until the temperature is lowered through the curie point. The curie temperature for iron is on 700°C. Q. 14. How will you decrease hysteresis loss in an electrical machine? Ans. HysteresIs loss in an electrical machine can be reduced in following ways:
(i) As hysteresis loss depends upon flux density, flux density in iron part should be limited to a value that will not cause extreme loss.
(ii) Frequency of reversal of magnetism should be low. Though, this factor is not in control of designer because in mainly instances frequency is fixed by cause of supply.
(iii) Through use special steels containing silicon e.g silicon steel. These alloys contain extremely low hysteresis loss. Q. 15. What is the meaning of the term hysteresis? Ans. Hysteresis is the term use to describe behaviour of a uneven whose value changes in one method when it is increasing and in a different way while it is decreasing. Hysteresis is associated by many physical phenomena and we see that magnetic flux density in a ferromagnetic material is an example. If we are given a exacting- value of H, we cannot determine the corresponding value of B unless we know whether B (and H) are increasing or decreasing. Q. 16. What is importance of magnetic circuits? Ans. Magnetic circuits constructed of ferromagnetic materials constitute essential parts of many pieces of electrical apparatus.
(i) Each generator, every motor and every transformer (except those which operate at high frequency) make use of magnetic circuits.
(ii) Watt-hour meters, telephone receivers, loudspeakers, relays for power, telegraph circuit and certain type of circuit breakers every one depend upon magnetic flux restricted to paths of finite length and cross-section.
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