Open Circuit and Short Circuit Test of Transformer

Open circuit and closed-circuit test on transformer are conducted to predict the performance analysis without actual loading it.

Open Circuit Test on Transformer

Open circuit test on transformer is conducted to determine the core losses of the transformer. As core losses is proportional to frequency thus these losses are treated as constant during actual operation, because frequency is constant during actual operation.

Core losses :-

Hysteresis Loss (P_h) α f.B_mⁿ (n = Steinment Exponent)

Eddy Current Loss (P_c) α f².B_m².t²

Why open circuit test is performed on low voltage side?

In open circuit test high voltage side is kept open circuited while at low voltage side rated voltage at rated frequency is given with the help of auto transformer to determine no load losses of transformer i.e. core losses. This test is carried out with the instrument (voltmeter, wattmeter and ammeter) placed on low voltage side because it is easier to supply low voltage (rated) then high voltage and also instrument used are economic in cost at low voltage level and it is safer to conduct the test at low voltage level.

Open circuit test of transformer procedure

In this test voltage is given to low voltage side with the help of auto-transformer. Now with the help of autotransformer applied voltage gets slowly increased up to rated voltage. Now record all the three instruments (voltmeter, ammeter and wattmeter) reading.

Since transformer secondary is open circuited means, it is on no load hence only no-load current flow in the transformer. So, the ammeter reading gives the no load current (I_o) in a transformer and also no-load current in transformer is limited to 5% of full load primary current and the value of primary copper losses is ignored at such small current.

Primary copper loss = I₁².R₁ (here I₁ is primary current)

No load current (I_o) = 5% I₁ = 0.05I₁

Primary copper loss at such current = (0.05I₁)².R₁ = 0.0025.I₁².R₁, which is very small quantity so it can be ignored.

Therefore, wattmeter reading is taken to represent only core loss.

P_oc = V_oc²/R_c

P_oc = V_oc. I_oc. Cosɸ_o

ɸ_o = Cos^-1 (P_oc/ V_oc.I_oc) i.e. the Power factor angle of transformer operating on no load.

X_m =R_c/tanɸ_o

Since, transformer is on no load means current in secondary winding is zero so equivalent circuit can be redrawn as:

Since, no load current in transformer is very small, the primary impedance drop at such low current is very small and can be neglected an therefore the voltmeter reading is taken to represent the voltage across the shunt exciting branch.

Short Circuit Test on Transformer

Short circuit test is to be performed on transformer at rated current to determine full load copper loss. Since full load copper losses is equal to I²R_eq and R_eq is independent of frequency, so we can say that short circuit test may not be carried out necessarily at rated frequency.

Why Short Circuit Test is performed on high voltage side?

In short circuit test supply is given to high voltage side with the help of auto-transformer while low voltage side get short circuited with a thick conductor. Short circuit test is carried out with the instruments placed on high voltage side because the rated current on high voltage side is lower than at the low voltage side and therefore low-cost instruments may use.

Short Circuit Test of Transformer Procedure

In the above discussion we see that supply is given to high voltage side around 5% to 10% of rated voltage is required to circulate full load current. So, a low voltage is applied to high voltage side with help of variac. Now applied voltage get slowly increased with the help of variac until ammeter show rated current. After reaching rated current record all three instruments (voltmeter, ammeter and wattmeter) reading. Ammeter reading gives the full load primary current. Since the applied voltage required to circulate the full load current is very small i.e.5% to 10% of rated voltage, the core loss at such a low voltage is very small and can be ignored and therefore wattmeter reading is taken to represent only the full load copper loss.

P_cu= I². R_eq.

Also, the exciting current at such low voltage is neglected and therefore the ammeter reading is taken to represent the current flowing through the secondary winding as well and therefore drops across the shunt branch can be neglected. So equivalent circuit of transformer on short circuit test can be redrawn as

V_sc = I_sc. Z_eq