Answer MET Question 36

Question: With reference to transformer answer following:

(a) Explain the construction of a lighting transformer used onboard.

(b) What is all day efficiency of a transformer?

(c) How are iron losses in the transformer made very negligible?

(d) In a short-circuit test performed on a transformer iron losses are negligible, why?

Answer: a. Transformer is a static device which

i. transforms electric power from one circuit to another in a.c system only.
ii. does not alter frequency.
iii. Has two circuit electrically saparated from each other. But magnetically linked with a common magnetic core of low reluctance.
iv. work on the principle of mutual inductance.

The 440/220 V lighting transformers may be located inside the main switchboard or, more likely, will be separately mounted nearby.

A step-down transformer of 440/220 V  is used for general supplies, wound delta—star, would permit earthing of the neutral point for the low-voltage supply with the higher voltage system supplying essential machinery having an unearthed neutral. Star winding has the advantage of the neutral point which is available for an earth connection if required or for a fourth wire. The delta winding is useful for unbalanced loading but has no neutral point. Earthing of the low voltage neutral reduces over-voltages when a fault occurs and at the same time causes sufficient fault current to operate protective devices.

The material for the construction is good silicon steel, thin laminated sheets. good silicon steel reduces hysteresis loss and thin laminated sheetsreduces eddy currents.

b. All day efficiency:  All day efficiency of transformer is the ratio of output in KW-hr to input in KW-hr for 24hrs. Which is less than ordinary efficiency of transformer.

Commercial efficiency of a transformer is given by;

$\displaystyle \small \mathrm{\eta = \frac{Output\ in\ watts}{Input\ in\ watts}\times 100 }$

However this does not reflect the actual condition as the load varies throughout the day for a distribution transformer.

All day efficiency  is given by;

$\displaystyle \small \mathrm{\eta = \frac{Output\ in\ KWH}{Input\ in\ KWH}\times 100\ for\ 24\ hours}$

c. How are iron losses in the transformer made very negligible:

The iron losses are constant in a induction machine, they can only be taken care at the construction stage. To reduce the iron loses in a transformers, the core must be made of a material having high permeability and high resistivity because high permeability carry high magnetic flux, and high resistivity provides low conductivity which prevent eddy currents.

d. In a short-circuit test performed on a transformer iron losses are negligible: since the voltage applied is a small % of the normal voltage the mutual flux created will also be small percentage of normal value, which will lead to a very negligible core loss. Thus the watt meter reading will represent the entire copper loss for both primary and secondary at full load.