Why is HV kept open in OC test of transformer? Also, why is LV shorted in SC test of the transformer? (Generally this is preferred - why?)
By definition, an OC test of a transformer is an Open Circuit test. Why would you connect the HV windings? The open circuit test primarily is used to determine the core (magnetic) losses of the transformer, as with no load, you don't have to worry about copper (winding) losses.
For an SC test, the low voltage side is shorted in order to measure the impedance of the transformer. The high voltage winding is connected to a variable voltage supply, and the supply is increased until the high voltage leg reads it's maximum current. You don't need full rated voltage to achieve full current this way, and it makes it a simple matter to measure the transformers impedance, as well as the actual copper (winding) loss at full load since the magnetic structure of the transformer isn't driven into saturation (or anywhere near it, the magnetic losses are extremely low when a transformer is excited in this manner).
- In short circuit test we have to pass rated current through the short circuited side.
- Therefore we select High Voltage (HV) side normally for short circuiting, because it's rated current is less and therefore it is easy to short.
- Since the winding are short circuited it takes very less voltage to let the rated current flow through the secondary side, i.e., short circuited winding
- When the voltage is less, the flux produced due to it, is less
- Therefore, flux linking with core, 'phi' is very less
- Hence, cos(phi), that is the power factor, is more. This is also the reason for "When we connect watt-meter in SC test it gives us approximately Copper loss only and Iron loss is negligible."
Since, flux linkage is negligible therefore, iron loss is negligible and hence whatever power is consumed, almost all of that is used to meet Copper loss.
- In open circuit test we have to apply rated secondary voltage
- Therefore, we generally select Low Voltage side for secondary
- In Transformer, Primary Current Ip, is the sum of no load current I0 and the current due to effect of ‘load on secondary' I2".
- I0 is primary no load current and is approximately 5% of rated primary current, which is very small value
- I2" is dependent on secondary current I2 and since, secondary is open circuited, therefore I2 = 0 and this implies I2" = 0
- Thus Ip, which is sum of I0 and I2", is very small
- Therefore, Copper loss is negligible.
- And since we have applied rated voltage, therefore normal flux linkage takes place and inductance of circuit increases.
- This gives low power factor and also Iron loss is shown on the watt-meter (approx.)