Part C: Calculations/Drawings (complete questions first)
Complete each question on a separate sheet. Show all working. The marks allocated for each question are shown alongside each question.
1 A single-phase transformer is wound with 80 turns on the secondary winding and the primary supply voltage is 240 volts, 50 hertz. The minimum core flux is 0.002 Weber. Determine:
(a) the secondary voltage (b) the primary turns (c) the secondary load current for a primary load current of 10 amperes.
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2 A single-phase transformer is used to reduce a 240 volt supply to 12 volts to operate a doorbell. If the primary winding has 480 turns, how many turns must be added to the secondary winding so the transformer can be used for a 30 volt doorbell?
3 A 240/32 volt swimming pool light transformer has a core loss current of 0.5 ampere and a magnetising current of 5 amperes. Use a scale of 20 millimetres per ampere to determine the value and power factor of the transformer's no-load current.
4 A 240/40 volt transformer supplies two different secondary loads. One load (a) draws a current of 12 amperes at a lagging power factor of 0.94. The other load (b) draws a current of 12 amperes at a leading power factor of 0.94. With both loads disconnected, the primary current is found to be 2 amperes at a power factor of 0.173.
Marks
7 An autotransformer has a step-down ratio of 2:1 and the primary is connected to a 415 volt supply. A motor has a normal rated voltage of 415 volts and a starting current of 100 amperes.
Determine, with the autotransformer in circuit:
(a) the motor terminal voltage
(b) the starting current drawn from the transformer secondary (assume that the starting current is proportional to voltage)
(c) the current drawn from the 415 volt supply
(d) the current in the common portion of the transformer winding. 4
8 The design requirements of a 6600 V/400 V 50 Hz single-phase core-type transformer are: Approximate emf/tum: 15 V Maximum flux density: 1.5 T
Find a suitable number of primary and secondary turns, and the nett cross-sectional area of the core. 3
9 Two single-phase 33 kV/11 kV transformers rated at 400 kVA and 500 kVA are connected in parallel to supply 800 kVA at a pf of 0.85 lagging. The transformers have percent impedances of 5.38% and 6.1% respectively.
Calculate the load on each transformer. 5
10 The input voltage, current and power to a single-phase load are measured by means of a voltmeter, ammeter and wattmeter respectively, each of these instruments being single-range type. They are connected to the lines by means of a current transformer of ratio 100:5 and a voltage transformer of ratio 50:1.
Draw a circuit diagram to show how the CT, VT and meters are connected into the load circuit. Neglecting instrument and transformer errors, calculate the load kVA and power factor if the voltmeter, ammeter and wattmeter indicate values of 200 V, 2.73 A, and 436 W.
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9080V Electrical Machines: Assessment guide OTEN 2010
Marks
3
5
Detelinine the primary current when both the loads are connected. 6
5 A 10 kVA transformer has an iron loss of 250 watts and a half-load copper loss of 80 watts. Determine the full-load efficiency at 0.8 power factor. 6 The open circuit voltage of a transformer is 240 volts. Determine the full-load voltage if the percentage regulation is 8%.
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9080V Electrical Machines: Assessment guide 0 OTEN 2010
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