70 Chapter 4 • Alternating current motors


6. What is a star-delta starter?


7. How is the voltage across the windings of a three-phase motor reduced when making use of a star-delta starter?


8. Explain briefly how a star-delta motor starter works. 9. What is an auto-transformer starter?


10. How is connection between a wound rotor and the external rotor resistors made?


11. What does SANS 10142 stipulate with regard to accessibility of a manually operated motor starter?


12. What does SANS 10142 stipulate with regard to a manually operated disconnecting device supplying a motor?


13. What does SANS 10142 stipulate with regard to undervoltage release incorporated in motor starters?


4.10 Comparison of single-phase and three-phase induction motors


Motor Advantages


Single- phase


Relative advantages and disadvantages Disadvantages


• Single-phase supplies are readily available, e.g. houses and offices.


• The electrical installation is simple. • The electrical control is simple.


• Available in small sizes only. • They are less efficient. • They are not self starting. • For the same output they are more expensive.


• For the same output the physical size is bigger.


• For the same output they have a lower starting torque.


Tree- phase


• They are self starting. • For the same output they have a higher starting torque.


• They are more efficient. • They are available from small to very large.


• For the same output they are cheaper. • For the same output the physical size is smaller.


• Three-phase supplies are usually only found in large installations.


• The electrical installation is more difficult. • The electrical control is more complicated.


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