O.C AND S.C TEST OF A SINGLE PHASE TRANSFORMER
OBJECT: To determine % efficiency and % voltage regulation of a single phase transformer at a different p.f. and different load without actually loading transformer i.e. by (analitically) finding iron and copper losses from open circuit and short circuit tests.
APPARATUS:
Sr No. Item No. reqd. Range Remarks
1. Single phase transformer
2. Ammeter
3. Voltmeter
4. Wattmeter
5. D.P. Switch
6. Variac
THEORY: Mainly two types of losses occur in transformer.
1. Copper losses which occur in the transformer winding. Copper loss varies according to the square of the current.
2. Iron loss which consists of hysteresis & eddy current losses. Iron loss occur in iron core & hence it is called as iron loss. Iron loss remain constant, from no load to full load. Hence it is called constant loss.
There are two tests:
1. Short circuit test.
2. Open circuit test.
These two tests enable to determine % efficiency & % voltage regulation to be calculated without actually loading the transformer. The results are far more accurate than are possible with direct measurement of input & output power and voltages. Also the required to carry out these tests is very small, compare to the full load output of the transformer.
The open circuit tests is carried out with no load on the transformer. One winding, usually the low voltage side. To get better reading is supplied at it�s rated voltage and the other side (H.V. side) is kept open circuited. The secondary side may be kept open circuited connecting a voltmeter because of 8 times high resistance. The input the transformer on open circuit gives the iron losses because Cu. losses due to small no load current is neglected. In the short circuit test the secondary winding, usually L.V. side is short circuited. Low voltage (about 5% normal) is applied to the primary (H.V.side). The input power to the transformer gives Cu. losses. The core losses are very small because of low voltage resistance (Re) and equivalent reactance (Xe) referred to the primary side can also be determined, from which the voltage regulation & % efficiency of the transformer can be calculated for any condition of leading.
Input during O.C. test = Iron loss = W0
Wsc = Input power of S.C. test = Cu. loss
Wpp = Primary voltage at S.C. test.
Ip =Primary current at S.C. test.
Re = Equivalent reffered to H.V. side.
Ze = Equivalent impedance reffered to H.V. side.
Xe = Equivalent reactance reffered to H.V. side.
V2 = High voltage side.
I2 = High voltage side (rated current)
Then,
Ze = Vp = ohms
Ip
Re = Wsc = ohms
I2p
X2e = Z2e � R2e = ohms
% efficiency = out put x 100
input
= V2 I 2 cos? x 100
????????V2 I2 cos? + W0 +Wcu
&, % reg. = I2 Re2 cos? + I2 Xe2 cos? x100
V2
�+ve� sign for lagging power factor.
� �ve� sign for leading power factor.
PROCEDURE:
1. Set up the connections as per circuit diagram.
2. For O.C. test the L.V. winding was connected to supply and H.V. was kept open.
3. Then gradually the supply voltage was increased with the help of " variac " and adjusted to rated voltage (L.V. side). Then the reading of " Ammeter, voltmeter & wattmeter were noted."
4. For S.C. test L.V. side is short circuited &H.V. side was connected to supply through " variac."
5. Then very reduce voltage (5 to 10% of rated voltage) was applied with the help of variac and value of voltage was adjusted so that there is full load current in the primary. Then reading of Ammeter, Voltmeter & wattmeter were noted.
Then, Iron losses, copper losses etc. were calculated and then effieciency and " regulation were calculated for different loads and at different power factor conditions.
6. Plot the graph of
a. % load v/s % efficiency
b. % load v/s % regulation.
OBSERVATION TABLE :
1. O.C. test:
Sr. No Primary voltage in volts. (Vp) Primary current in amp. (Ip) Wattmeter reading (Iron loss) in watts Secondary voltage in volts (Vs)
1.
2. S.C. test :
Sr.No Vp in volts Ip in amps Wcu in watts Is in amp Re=Wcu
I2p
In ohms Ze=Vp
Ip
In ohms X2e =Z2e-R2e in ohms
1.
3. Obs. Table
Sr. No V2 in
volt I2in amp % Load I22 Re in watts
%reg. At unity p.f. %reg. At .8 p.f. lg. %reg. At .8 p.f. lead %effi.
.8 le-ad .8 lg. 1
1.
2.
3.
4.
CALCULATIONS:
CONCLUSION: