 education

# PAST QUESTION ON EEC 123(MACHINE 1)

``EEC 123``

## With the aid of an appropriate diagram explain the operation of a basic transformer

Do not make the mistake of just drawing diagrams alone… Endeavour that you explain,
Just like something as this;
The basic things that should be on your transformer are ferromagnetic core, flux, current flowing, voltage, number of turns (i.e winding) and load

## (2)Explain the characteristics of transformer when carrying load.

When an applied voltage is applied to the primary side of a transformer, current I1 flows generating a flux that produce current I2 that opposes the current I1, so as yo maintain the flux value needed for the transformer. Noting that with the aid of flux the transformer is able to transfer current from the primary side to the secondary side which hereby induces an E. M. F in the both side E1 and E2.The number of turns on the primary side and secondary side can be increase or reduce to determine how the current will be i.e increased in the primary side will give more current flowing than an increase in the secondary side. The voltage produce at the secondary side may be connected to a load.

1c) A 440/110V single phase transformer draw 5 amps on no load at 0.9 power factor. Calculate the values of(I) magnetizing current (iii) and core component of current.
440=V1 P.F=0.9
110=V2
I0=5A
Let’s solve

2a) Explain how the short circuit test is carried out on a power transformer.

2b) sketch the connection of a transformer in delta start.

2c) An autotransformer is supplied at 240V. If the secondary is tapped to give 60V with a load current of 12A, calculate the other two circuit current and show the direction of current and voltage.

3a) With the aid of a diagram, explain the construction of a D.C machine.

3b) Explain the function(s) of pole assembly and commutator in a D.C machine.

3c) List three types of winding used in transformer.
Primary winding : This is the winding that is found at the primary side of the transformer.
Secondary winding :This is the winding that is found at the secondary side of the transformer.
tertiary winding :This is the winding that is found between the primary and secondary side of the transformer

4a) Deduce the equation for armature Torque of a D.C machine.

4b) Explain commutation in D.C machine.

4c) list two causes of sparking at the brushes of the D.C machine apart from ARMATURE.