City of Westminster College

Scenario: City of Westminster College:

Electrical circuits consist mainly of series/parallel combinations using common components, irrespective of complexity these networks can be reduced to aid in solving current flow and/or voltages developed.

The majority of this assignment looks at different techniques and how they are used in solving networks.

Task 1

a) Below is a series/parallel combinational circuit that is not uncommon in a building services engineering.

● Use Ohms’ Law to determine the voltage across and the current through the load RL.

                                         4 ohms.                                     A                               




        95 volts.

                                12 ohms.                            8 ohms                                RL 
                                                                                                                        5 ohms.






                                                                                            B

Task 2

a) Describe the construction of a single phase transformer. Explain the relevance of Lenz’s Law and Faraday’s Laws of electromagnetic induction to the operation of the transformer. Give two examples where they would be used in the Building Services Industry.

b) Detail the equations relating current, voltage, power, and turns ratio in a simple single phase transformer.

c) A 23O volt to 24 volt bell transformer is constructed with 6OO turns on the primary winding. Calculate the number of secondary turns and the primary and secondary currents when the transformer supplies a 24 volt 12 watt bell. (Include diagram).

d) A 1MVA transformer has an impedance of 4.5% what is the maximum fault level on the secondary side of the transformer?

Task 3
A steel ring of mean diameter 120mm is uniformly wound with 1500 turns of wire. When a current of 0.30 A is passed through the coil a flux density of 1.5 T is set up in the steel. Find the relative permeability of the steel under these conditions.

Task 4
Describe how an electrical installation is designed to ensure safety of users and maintainers. The safety of staff working during the construction phase also needs to be considered.

Task 5

A small AC motor in the Hospital displays the following values. Series connected resistance of 28W and an inductance of 0.45H. It is connected to a 230V 50 Hz supply. Calculate the following circuit parameters
a) The circuit impedance
b) The current and the circuit phase angle
c) The voltage drop across the resistor and
d) The voltage drop across the inductor.
e) The circuit power factor.

Task 6

Considering the circuit in Task 4 evaluate how the power factor could be improved illustrating your answer with a phasor diagram and detailing how the kVA consumption would be affected.

Sample Solution