ELECTRICAL SYSTEM (SKAA 2032)
TUTORIAL 2
Transformer
1. What is principle of operation of transformer?
2. Based on construction how can you classify transformers?
3. If a 240V DC voltage is supplied to primary of a 10:1 transformer, what would be the voltage induced
in secondary of transformer? What else do you expect to happen?
4. Why cooling systems are needed for transformers? Give examples of the types of cooling systems
designed for transformer.
5. An ideal transformer has a turns ratio of 8:1 and the primary current is 3A when it is supplied at 240V.
Calculate the secondary voltage and current
6. A transformer primary winding is connected across a 415V supply, and has 750 turns. Determine how
many turns must be wound on the secondary side if an output of 1.66kV is required
7. An ideal transformer, with the primer connected to 240V main, supplies a 12V, 150W lamp. Calculate
the transformer turns ratio and the current taken from the supply.
8. An ideal transformer has a primary to secondary turns ratio of 1:15. (a) Calculate the primary voltage
necessary to supply a 240V load. (b) If the load current is 3A determine the primary current.
9. A single phase, 50Hz transformer has 25 primary turns and 300 secondary turns. When the primary
winding is connected to a 250V supply, determine (a) the maximum value of the flux in the core, and (b)
the voltage induced in the secondary winding.
10. A 12Ω resistor is connected across the secondary winding of an ideal transformer whose secondary
voltage is 120V. Determine the primary voltage if the supply (primary) current is 4A.
11. A single-phase 500V/100V 50Hz transformer has a maximum core flux of 75X10-4 Wb. Determine the
number of primary and secondary turns.
12. A single-phase transformer has a turns ratio of 10:1 and is fed from 2.5kV supply. The transformer
rating is 10kVA. By neglecting the losses, determine (a) the full-load secondary current, (b) the minimum
load resistance which can be connected across the secondary winding to give full-load kVA, (c) the
primary current at full-load kVA. (Hints: The full-load current is greatest current that a circuit is designed
to carry under specified conditions. Transformers are designed and cataloged by KVA ratings. A
transformer’s kVA rating indicates its maximum power output (or input) capacity)
13. A 10 kVA, single-phase transformer has a turns ratio of 12:1 and is supplied from a 2.4 kV supply.
Neglecting losses, determine (a) the full-load secondary current, (b) the minimum load resistance which
ELECTRICAL SYSTEM (SKAA 2032)
TUTORIAL 2
can be connected across the secondary winding without the kVA rating being exceeded, (c) the primary
current
14. A 20Ω resistance is connected across the secondary winding of a single-phase power transformer
whose secondary voltage is 150V. Calculate the primary voltage and the turns ratio is the supply current
is 5A, neglecting losses.
15. A 100kVA, 4000V/200V, 50Hz single-phase transformer has 100 secondary turns. Determine (a) the
primary and secondary current, (b) the number of primary turns, (c) the maximum value of the flux.
Electrical Machine
1. State the advantages of squirrel-cage motors compared with the wound type.
2. State the advantages of wound rotor motor compared with the cage type.
3. Name the two major parts in an induction motor and describe their functions.
4. What is the meaning of slip in AC motor?
5. Name any three industrial applications of three-phase induction motors.
6. The stator of a 3-phase, 4-pole induction motor is connected to a 50Hz supply. The motor runs at
1455 rpm at full load. Determine (a) the synchronous speed, and (b) the slip at full load.
7. A 3-phase, 60 Hz induction motor has 2 poles. If the slip is 2 percent at a certain load, determine (a)
the synchronous speed, (b) the speed of the rotor, and (c) the frequency of the induced emf in the rotor.
8. A three-phase induction motor is supplied from a 50 Hz supply and runs at 1200 rpm when the slip is
4 percent. Determine the synchronous speed.
ELECTRICAL SYSTEM (SKAA 2032)
TUTORIAL 2
Electrical Wiring
1. State 3 factors that determine the selection of the size of conductor.
2. What is the function of earthing/grounding and how it will provide safety to user?
3. What is the purpose of double insulation?
4. Explain how a wire fuse protects an appliance if it develops a fault.
5. Explain the working principle of magnetic-type circuit breaker in protecting appliances from damage.
6. State the advantages of a circuit breaker compared with a wire fuse.
7. State the safety measures to prevent electrical shock when working at a construction site that is
unknown for you.
8. What are the functions of distribution board?