Nr. Questions Page Motor Theory 1. What are the standard AC supply voltages is South Africa? 2 2. What is phase voltage? 2 3.

What is the relationship between phase voltage and the line voltage

of a three-phase four-wire system? 2

4. What is meant by torque of a motor? 2 5. How is the torque of a motor produced? 2 6. What torques must be considered when driving a machine? 2 7. How can the initial starting torque needed for a driven machine be

found? 2

8. How is the accelerating torque determined? 2 9. What is meant by power of a motor? 3 10. How is the torque of a motor related to its power? 3 11. What is meant by the time rating of a motor? 3 12. What is the maximum continuous rating? 3 13. What is short time rating? 3 14. For what type of application does a short time rating apply? 3 15. What is meant by temperature rating of a motor? 3 16. How is the power required to drive an individual machine decided? 3 17.

What calculation is commonly used to determine the minimum power

required when the loads to be driven vary over a particular work cycle?

3

18. How is power decided for driving a lineshaft belted to a group of machines?

3

19. What is the power required for driving cranes, hoists, and winches? 3 20. How is the current required to supply a 3-phase motor at full-load

calculated? 3

21. How is the full-load current of a single-phase motor arrived at? 3 22. How is the full-load current of a DC motor calculated? 4 23. What is power factor? 4 24. If an induction motor is said to have a power factor of 0.8, what does

that mean? 4

25. How does lagging wattless current arise? 4 26. What is meant by power factor correction? 4 27. How can power factor be improved? 4 28. Why can a capacitor be used for improving the power factor? 4 29. Where are capacitors placed in an installation? 4 30. How is the insulation resistance of a motor winding checked? 5 31. If insulation resistance is low, what is the most likely cause? 5 32. How can motor winding be dried out? 5 33. What is the purpose of the motor enclosure? 5 34. What is the effect of dust on the winding? 5 Three-Phase Induction Motors 35. What is the simplest form of motor? 6

36. How does the 3-phase induction motor operate? 6 37. How are the stator and rotor cores constructed? 6 38. What is the basis of operation of all poly-phase motors? 6 39. What are the properties of this rotating magnetic field? 6 40. On what does the direction of rotation of an induction motor depend? 6 41. What is the phase sequence of the supply? 7 42. How are the phases connected round the stator? 7 43. How is the direction of rotation reversed? 7 44. What determines the speed of the rotating field? 7 45. Does induction motor speed correspond to the synchronous speed? 7 46.

Why does the rotor of an induction motor rotate at a lower speed

than the speed of the rotating field? 7

47. How is slip speed usually described? 7 48. What is the size of the air gap between the stator and the rotor? 7 49. What are the two principal kinds of induction motor? 7 50. How is the squirrel-cage rotor constructed? 7 51. What type of rotor winding is used in the slip ring motor? 7 52. On what does the starting torque of a squirrel-cage motor depend? 7 53. Why is the squirrel-cage motor the most widely used? 7 54. What is the principal drawback of the squirrel-cage motor? 7 55. What is meant by the pull-out torque of an induction motor? 7 56. What sort of squirrel-cage rotor gives both high starting and

running efficiencies? 8

57. How does the double-cage rotor work? 8 58. What other squirrel-cage constructions give the current-

displacement effect? 8

59.

What effect has the current-displacement rotor on pull-out torque and running power factor?

9

60. What is a dual-voltage motor? 9 61. What is a high-slip motor? 9 62. What are the applications of a high-slip motor? 9 63.

What are the effects of inserting resistance in the rotor circuit of an

induction motor? 9

64. Why does speed reduce when resistance is inserted in the rotor circuit?

9

65. Why is the starting torque improved when resistance is inserted in the rotor circuit?

9

66. What are the disadvantages of using a slip-ring motor for speed control?

9

67. How are slip-rings arranged on the motor shaft? 10 68. How is brush gear for slip ring arranged? 10 69. How are windings assembled in the slots? 10 70. What insulation is required in a motor? 10 71. How are the stator windings arranged in the slots? 11 72. How are the end-windings arranged? 11

73. What types of wire insulation are commonly used? 11 74.

If a motor is required to operate on a voltage different from that for

which it was designed, what would it be necessary to do? 11

75. Is it possible to change the power of an induction motor? 11 76.

Is it practical to rewind an induction motor for the same power on a

different frequency? 11

Induction Motor Control Gear 77. What are the main functions of control gear? 12 78. What devices are required to give adequate protection to the motor? 12 79. What provision must be made for short-circuit conditions in motor

circuits? 12

80. What are the usual forms of overload relay in motor control gear? 12 81. How do thermal relays work? 12 82. How does the magnetic overload relay operate? 12 83. How many overload relays are required in the control gear? 12 84.

What happens when one of the three lines supplying a three-phase

induction motor becomes open-circuited? 12

85. What currents flow in a single phasing delta-connected motor? 12 86. What currents flow in a single phasing star-connected motor? 12 87. Will normal overload relays trip on single phasing? 12 88. What special protection can be provided against single phasing? 12 89. What are alternatives to the use of overload releases? 13 90. How are built-in thermostatic overload protectors arranged? 13 91. How do built-in thermistors work? 13 92. When is direct-on-line starting used for three-phase squirrel-cage

motors? 13

93. What are the connections for direct-on-line starters? 13 94.

What methods are employed to reduce the starting current of

squirrel-cage motors? 13

95. When is primary-resistance starting employed? 14 96. When is the primary reactor method of starting employed? 14 97. When is star-delta starting used? 14 98. What are the connections for a star-delta starter? 14 99. When is auto-transformer starter used? 15 100. What does the simple auto-transformer starter consist of? 15 101. What are the connections for the Korndorfer system? 16

102.

What precautions should be observed when applying reduced voltage starting to a load with rising characteristics such as a fan?

16

103. What are the initial starting line current and motor torque when star-delta starting?

16

104.

What are the initial starting line current and motor torque when starting with primary resistance or primary reactance?

16

105.

What are the initial starting line current and motor torque when starting by auto-transformer?

16

106. What are the above values of initial starting current and torque 16

approximate? 107.

How do the various methods of starting on reduced voltage compare

as regards torque per ampere? 16

108. What mechanical methods of reducing starting current can be adopted?

16

109. What is sequence starting? 16 110. How are slip-ring motors started? 16 111.

What is the usual arrangement of connections for a hand-operated

slip-ring starter? 16

112. What are the essentials of a full automatic stator-rotor starter? 17 113. How is speed control of a slip-ring motor affected? 18 114. What is liquid resistance? 18 115. What is the advantage of a liquid resistance for starting purposes? 18 116. What is slip resistance? 18 117. What is meant by motors in synchronous tie? 18 Braking of AC Motors 118. When is braking of an electric motor required? 19 119. What are the principal ways of braking electric motors? 19 120. How is plugging applied to AC motors? 19 121. How is DC injection braking applied to induction motors? 19 122. Can DC injection braking be applied to other types of AC motors? 19 123. How is capacitor braking applied to induction motors? 19 Speed Control of AC Motors 124. What three phase motors are available for speed control? 20 125. How can induction motor be wound to give two or more speeds? 20 126. What is pole amplitude modulation? 20 127. How are p.a.m windings connected to give two motor speeds? 20 128. What are p.a.m motors used for? 20 129. What other form of speed control is used with induction motors? 20 130. What is a thyristor? 21 131. What are the disadvantages of thyristor controllers? 21 132. What are the three main types of 3-phase AC commutator motor? 21 133.

What is the effect of supplying the rotor winding with a voltage

from an external source? 21

134.

How does this method of speed variation compare in efficiency with rotor-resistance control of speed?

21

135. How does a variable speed stator fed commutator motor operate? 21 136. How does the variable speed rotor fed AC commutator motor

work? 21

137. How does the variable speed series AC commutator motor work? 22 138. What arrangements of control gear are required for squirrel cage

change-pole motors? 22

139. How is the stator-fed commutator motor controlled? 23 Synchronous Motors 140. What are three-phase synchronous motors? 24

141. How does the rotor of a salient pole synchronous motor operate? 24 142. How is the rotor of a synchronous induction motor constructed? 24 143. What are the applications of synchronous motors? 24 144. What control gear is required for synchronous motors? 24 145. What is a reluctance motor? 24 146. What are the applications of reluctance motors? 24 Single-Phase Motors 147. When are single-phase motors employed? 25 148. What type of single phase motors are in use? 25 149. What is the principle of the single phase induction motor? 25 150. How does the single-phase split phase induction motor work? 25 151. What are the applications of the split-phase induction motor? 25 152. What is a capacitor motor? 25 153. When is the capacitor-start motor used? 26 154. What type of capacitor is employed for capacitor-start motors? 26 155. What are the applications of the permanent split-capacitor motor? 26 156. What is the object of using two capacitors? 26 157. How is the single-phase induction motor switched from start to run? 26 158. How is the split-phase or capacitor induction motor reversed? 26 159. What is a shaded-pole motor? 26 160. How does the shaded-pole motor operate? 26 161. What are the applications of the shaded-pole motor? 26 162. Can the shaded-pole motor be reversed? 27 163. What is a universal motor? 27 164. How are universal motors started? 27 165. What are the applications of the universal motors? 27 166. What are repulsion motors? 27 167. How are repulsion motors reversed? 27 168. What are the applications of repulsion motors? 27 DC Motors 169. What are the two basic types of DC motors? 28 170. Why is a commutator essential in a DC motor? 28 171. On what does the speed of a DC motor depend? 28 172. What are the basic components in DC motor construction? 28 173. Why are interpoles used? 29 174. How are interpole windings connected? 29