O.C and S.C Tests on Transformers

1. Open-circuit or No-load Test. This test is performed to determine core or iron loss, P_iand no-load current I₀. This test is helpful in determination of magnetizing component I_m’energy component I_e and so no-load resistance R₀ being given as V₁/I_e and no-load reactance given as V₁/I_m.

In this test secondary (usually high voltage) winding is left open, all metering instruments (ammeter, voltmeter and wattmeter) are connected on primary side and normal rated voltage is applied to the primary (low voltage) winding, as illustrated below   

(source:Google Images)

Iron loss P₁ = Input power on no-load – W₀ watts (wattmeter reading)

No-load current = ₀ amperes (ammeter reading)

Angle of lag, Ø₀ = cos^-1 W_o/V₁I_o

I­_e = I₀ cos Ø₀ and     I_m = √I²_o - I²_e

Caution: Since no load current I₀ is very small, therefore, pressure coils of watt meter and the volt meter should be connected such that the current taken by them should not flow through the current taken by them should not flow through the current coil of the watt meter.

2. Short-circuit or Impedance Test. This test is performed to determine the full-load copper loss and equivalent resistance and reactance referred to secondary side. 

In this test, the terminals of the secondary (usually the low voltage) winding are short – circuited, all meters (ammeter, voltmeter and wattmeter) are connected on primary side and a low voltage, usually 5 to 10 % of normal rated primary voltage at normal frequency is applied to the primary, as shown in fig below. The applied voltage to the primary, say V_s’ is gradually increased till the ammeter A indicates the full load current of the side in which it is connected. The reading W_s of the wattmeter gives total copper loss (iron losses being negligible due to very low applied voltage resulting in very small flux linking with the core) at full load. Le the ammeter reading be I_s.

(Source: Google Images)

          

Full load copper loss, P_c= I²_s R₁ = W_s

Equivalent resistance referred to primary, R'₁  = Ws/I²_s

Equivalent impedance referred to primary,  = Z'₁  = V_s/I_s                  

Equivalent impedence referred to primary, X'₁ = √(Z'₁)²  - (R'₁)²'

3. Sumpner’s Test or Back to Back Test. This test is mainly performed to determine temperature rise. In this test two transformers are loaded fully, in similar way as two dc machines in a regenerative test, and the power required from the supply is just to meet the iron and full load copper losses of the two transformers.