McqMate
Chapters
| 1. |
An air gap is usually inserted in magnetic circuits to |
| A. | increase m.m.f. |
| B. | increase the flux |
| C. | prevent saturation |
| D. | none of the above |
| Answer» C. prevent saturation | |
| 2. |
The relative permeability of a ferromagnetic material is |
| A. | less than one |
| B. | more than one |
| C. | more than 10 |
| D. | more than 100 or 1000 |
| Answer» D. more than 100 or 1000 | |
| 3. |
The unit of magnetic flux is |
| A. | henry |
| B. | weber |
| C. | ampereturn/weber |
| D. | ampere/metre |
| Answer» B. weber | |
| 4. |
Permeability in a magnetic circuit corresponds to______ in an electric circuit. |
| A. | resistance |
| B. | resistivity |
| C. | conductivity |
| D. | conductance |
| Answer» C. conductivity | |
| 5. |
Point out the wrong statement. Magnetic leakage is undesirable in electric machines because it |
| A. | lowers their power efficiency |
| B. | increases their cost of manufacture |
| C. | leads to their increased weight |
| D. | produces fringing |
| Answer» A. lowers their power efficiency | |
| 6. |
Relative permeability of vacuum is |
| A. | 1 |
| B. | 1 H/m |
| C. | 1/4JI |
| D. | 4n x 10-' H/m |
| Answer» A. 1 | |
| 7. |
Permanent magnets are normally made of |
| A. | alnico alloys |
| B. | aluminium |
| C. | cast iron |
| D. | wrought iron |
| Answer» A. alnico alloys | |
| 8. |
Energy stored by a coil is doubled when its current is increased by percent. |
| A. | 25 |
| B. | 50 |
| C. | 41.4 |
| D. | 100 |
| Answer» C. 41.4 | |
| 9. |
Those magnetic materials are best suited for making armature and transformer cores which have____permeability and_______hystersis loss. |
| A. | high, high |
| B. | low, high |
| C. | high, low |
| D. | low, low |
| Answer» C. high, low | |
| 10. |
The rate of rise of current through an inductive coil is maximum |
| A. | at 63.2% of its maximum steady value |
| B. | at the start of the current flow |
| C. | after one time constant |
| D. | near the final maximum value of current |
| Answer» B. at the start of the current flow | |
| 11. |
When both the inductance and resistance of a coil are doubled the value of |
| A. | time constant remains unchanged |
| B. | initial rate of rise of current is doubled |
| C. | final steady current is doubled |
| D. | time constant is halved |
| Answer» A. time constant remains unchanged | |
| 12. |
The initial rate of rise of current through a coil of inductance 10 H when suddenly connected to a D.C. supply of 200 V is_______Vs |
| A. | 50 |
| B. | 20 |
| C. | 0.05 |
| D. | 500 |
| Answer» B. 20 | |
| 13. |
A material for good magnetic memory should have |
| A. | low hysteresis loss |
| B. | high permeability |
| C. | low retentivity |
| D. | high retentivity |
| Answer» D. high retentivity | |
| 14. |
Conductivity is analogous to |
| A. | retentivity |
| B. | resistivity |
| C. | permeability |
| D. | inductance |
| Answer» C. permeability | |
| 15. |
In a magnetic material hysteresis loss takes place primarily due to |
| A. | rapid reversals of its magnetisation |
| B. | flux density lagging behind magnetising force |
| C. | molecular friction |
| D. | it high retentivity |
| Answer» D. it high retentivity | |
| 16. |
Those materials are well suited for making permanent magnets which have______retentivity and _______coercivity. |
| A. | low, high |
| B. | high, high |
| C. | high, low |
| D. | low, low |
| Answer» B. high, high | |
| 17. |
If the area of hysteresis loop of a material is large, the hysteresis loss in this material will be |
| A. | zero |
| B. | small |
| C. | large |
| D. | none of the above |
| Answer» C. large | |
| 18. |
Hard steel is suitable for making permanent magnets because |
| A. | it has good residual magnetism |
| B. | its hysteresis loop has large area |
| C. | its mechanical strength is high |
| D. | its mechanical strength is low |
| Answer» A. it has good residual magnetism | |
| 19. |
Silicon steel is used in electrical machines because it has |
| A. | low co-ercivity |
| B. | low retentivity |
| C. | low hysteresis loss |
| D. | high co-ercivity |
| Answer» C. low hysteresis loss | |
| 20. |
Conductance is analogous to |
| A. | permeance |
| B. | reluctance |
| C. | flux |
| D. | inductance |
| Answer» A. permeance | |
| 21. |
The property of a material which opposes the creation of magnetic flux in it is known as |
| A. | reluctivity |
| B. | magnetomotive force |
| C. | permeance |
| D. | reluctance |
| Answer» D. reluctance | |
| 22. |
The unit of retentivity is |
| A. | weber |
| B. | weber/sq. m |
| C. | ampere turn/metre |
| D. | ampere turn |
| Answer» B. weber/sq. m | |
| 23. |
Reciprocal of reluctance is |
| A. | reluctivity |
| B. | permeance |
| C. | permeability |
| D. | susceptibility |
| Answer» B. permeance | |
| 24. |
While comparing magnetic and electric circuits, the flux of magnetic circuit is compared with which parameter of electrical circuit ? |
| A. | E.m.f. |
| B. | Current |
| C. | Current density |
| D. | Conductivity |
| Answer» B. Current | |
| 25. |
The unit of reluctance is |
| A. | metre/henry |
| B. | henry/metre |
| C. | henry |
| D. | 1/henry |
| Answer» D. 1/henry | |
| 26. |
A ferrite core has less eddy current loss than an iron core because |
| A. | ferrites have high resistance |
| B. | ferrites are magnetic |
| C. | ferrites have low permeability |
| D. | ferrites have high hysteresis |
| Answer» D. ferrites have high hysteresis | |
| 27. |
Hysteresis loss least depends on |
| A. | volume of material |
| B. | frequency |
| C. | steinmetz co-efficient of material |
| D. | ambient temperature |
| Answer» D. ambient temperature | |
| 28. |
Laminated cores, in electrical machines, are used to reduce |
| A. | copper loss |
| B. | eddy current loss |
| C. | hysteresis loss |
| D. | all of the above |
| Answer» B. eddy current loss | |
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