1) Why does the short-circuit test essentially show only i2Rlosses and not excitation losses in a transformer?

2) Why does the open-circuit test essentially show only excitation losses and not i2Rlosses?

3)  What are transformer taps? Why are they used?

Answer:

Short-circuit test essentially show only i ² R losses and not excitation losses in a transformer because of the following reason that

In case of short circuit test, LV winding is short circuited, and voltage is applied to HV winding such that full load rated current flows in secondary, this voltage is very small compared to rated voltage. As the value of current is rated, it will give considerable amount of copper losses.

SC test is mainly performed to find out the copper loss at rated. It is performed on the HV side and the LV side is shorted.The reason for performing the SC test on the HV side is that the rated current is smaller in HV side when compared to the LV Very low voltage is required to produce that rated current,So core loss is neglected,And only copper loss is considered in SC test.

In SC test we supply voltage to HV side so that rated current flows in HV side .a primary voltage of approximately 5-12% is sufficient to flow rated current in both the windings . Hence rated current will also flow in LV side . As a result , the losses will include rated copper losses+Core losses (at a fraction of rated primary voltage) .Since the core loss occurring here is negligible as compared to rated voltage core losses , so the losses incurred here are approximated to rated copper losses.

 Open-circuit test essentially show only excitation losses and not i ² R losses because of the following reason that

when we perform open circuit test in transformers, the secondary is open circuited and rated voltage is applied to primary side. A very small amount of current flows through this primary side (known as no load current) which is required to keep the transformer magnetized and cope up with hysteresis and eddy current losses. so, these are known as core losses and as primary side is a low voltage winding, no. of turns are small. As value of no load current is about 5% of rated current and primary winding has very less resistance, copper losses can't be calculated. therefore it gives only core losses.

it is mainly performed to find out the core loss at rated voltage,so we perform OC test on the LV side and the HV side is opened. Because rated voltage in LV side is small when compared to the HV side.And also as these test are performed under no load conditions,no load current is much lesser than the normal current,only a small value of magnetizing current is we neglect the copper loss and consider only the core loss in OC test.

Consider a 220/3300 V , 33 KVA transformer. During OC test we will give rated supply(220 V here) to Low Voltage side keeping the High voltage side open. It is because here the current flowing will be no load current which is only a fraction of full load current. For this reason, we can neglect the variable copper losses and whatever power is injected that may be considered as core losses and the magnetic flux in the transformer is flowing on the iron core, and the iron core has magnetoresistance to the magnetic flux, which will generate heat so that this is why open-circuit test gives iron core losses only.

 Transformer taps? Why are they used?

A transformer tap is a connection point along a transformer winding that allows a certain number of turns to be selected. it means, a transformer with a variable turns ratio is produced, enabling voltage regulation of the output. The tap selection is made with a tap changer mechanism.

Tappings are provided on a transformer winding for selecting/cutting out a certain number of turns on the transformer winding thus obtaining a variable turns ratio.

This is done in order to maintain the output voltage within desirable limits because the equipments work satisfactorily at a somewhat fixed voltage and voltage variation is a common phenomenon in the power system.

Suppose we have a transformer as follows:

No. of primary turns = 1500

No. of secondary turns = 150

Turns ratio = 1500:150 = 10:1

If 150 V is applied to the primary, 10 V will be available at secondary.

Now we assume that the voltage has suddenly drops to 75 V. and we still want 10V to be available at the secondary. If cut out 750 turns from the primary, the new turns ratio will be

750:100 = 7.5 : 1

Now with 75 V applied to the primary, we still get 10 V at the secondary. The transformer tappings serve this purpose of cutting out the turns.