N2 Electrical Trade Theory|The Easy Way! 119 8.3 Transformer tappings


During peak periods, such as normal factory working hours, early in the morning, late in the aſternoon and during cold spells in winter, the extra load causes a voltage drop at the consumer’s point.


In order to compensate for this volt drop, tappings are made on the secondary side of supply transformers.


Tap changing switches are fitted to increase the voltage during peak periods and to reduce it when the load reduces. Tap changing switches can be of the manual or automatic types.


Exercise 8.2 1. Write down the equation of a transformer.


2. What is the most import factor to bear in mind when doing calculations involving three-phase transformers?


3. Write down the relationship that exists between the line and phase voltage and the line and phase current in a delta-connected winding.


4. Write down the relationship that exists between the line and phase voltage and the line and phase current in a star-connected winding.


5. A three-phase delta-star transformer is connected to a 2,2 kV supply. Te secondary phase voltage is measured and found to be 220 V. Determine; 5.1 the primary phase voltage, 5.2 the secondary line voltage,


5.3 the primary phase current when it draws its full load current of 120 A from the supply, and


5.4 the full load rating of the transformer in kVA.


(2 200V) (381 V)


(69,28 A) (457 kVA


6. A three-phase transformer has a delta-connected primary and a star- connected secondary. Te transformer supplies a line current of 300 amperes to a certain load. If the primary line voltage is 6 600 V and the secondary line voltage 380 V, determine; 6.1 the primary and secondary phase voltages, 6.2 the transformer ratio, and 6.3 the primary line current.


(6 660 V) (30:1)


(17,3 A)


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