9.O.C. & S.C TEST ON 1 - Ø TRANSFORMER

CIRCUIT DIAGRAM FOR O.C. & S.C TEST ON 1 - Ø TRANSFORMER

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O.C. & S.C TEST ON 1 - Ø TRANSFORMER

AIM :- To conduct O.C & S.C Test on a given single phase transformer and to predetermine the characteristics and to determine the equivalent circuit parameters , maximum efficiency, Power factor, regulation at constant loads.

Rating of the transformer:-

Apparatus Required:-

For Open Circuit Test

THEORY:

Since efficiency is the ratio of the output power to input power, it can be evaluated, if output and input power are measured with watt meters incorporated in the primary and secondary.

O.C & S.C TEST ON 1-Ф TRANSFORMER

Since efficiency is the ratio of the output power, it can be evaluated, if output and input power, are measured with Wattmeter’s incorporated in the primary and secondary circuits. During a load test. But as experience shows, such a direct load test fails to give accurate results, since the efficiency is usually very high, and even a smaller error in one of the instruments gives wrong results, hence the usual practice is to measure the internal loss and copper loss by indirect methods and based upon this data, the efficiency at any desired load and power factor is determined. The methods adopted are

1. Open circuit test and2. Short circuit test

Voltage regulation can be determined from the data obtained from a short circuit test. These are discussed briefly.

O .C TEST :

The purpose of this is to determine No-load loss (or) Core loss and no-load I0 which is helpful in finding X0 and R0 one winding of the transformer usually high voltage winding is left open and the other is connected to the supply with normal voltage and frequency.

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2 KVA 1 -Phase Step down Transformer 230V / 115V

2 KVA 1- Phase Step down Transformer 230V / 115V

S.No. Name of the apparatus

Range Type Qty.

1 Voltmeter 0-150V MI 1

2 Ammeter 0-2A MI 1

3 Wattmeter 150V/2.5A LPF 1

4 Transformer 2KVA -- 1

5 Dimmerstat 0-270/8A W.W 1

A Wattmeter, Voltmeter and Ammeter are connected to primary winding as the no—load current is very small i.e 2 to 10% of full load current, negligible cu losses occurs. Rated voltage at normal frequency is applied across the primary and the input current and input power is measured.

Let, V1 – Primary applied voltage. I0 – Input current. W0 – Input powerAs discussed earlier, with the secondary open circuited and rated voltage applied (secondary copper loss is zero, and primary copper loss is negligible small)

Hence , W0 = Iron loss

Power Input W0 =V1I0 Cos Ø0

Cos Ø0=W0/V1I0

Also,

IC = I0 Cos Ø and IM =I0 SinØ

V1,I0 and W0 are measured during the open circuit test, hence IC and IM Can be evaluated.

Hence we have R0=V2 /IC and X0=V1/IM

Thus, the parameters R0 and X0 also can be determined.

S . C TEST :

In this test we can find out equivalent impedances, leakage reactance and transformer resistance.Cu losses at full load which is used to find out efficiency.

This test is generally performed on high voltage side (primary) and low voltage side (secondary) short circuited with a thick conductor .Voltage, current and wattmeter are connected in order to measure voltage and power. A reduced voltage at normal frequency which can be obtained from a regulating transformer is applied across the primary .The voltage is adjusted such that the rated current flows through each winding, the applied voltage, input current and input power are measured.

Let,

Vsc - primary applied voltage

Isc – short circuited current through primary

Ino – Input power

As discussed earlier, The Iron loss which occurs, during the sc test, is quite small .Since the applied voltage is only a very small friction of the rated voltage (Iron loss is a friction of voltage) and it is usually ignored. The primary and windings carry rated currents, and hence full load copper loss occurs We have,

Input power = Total full load copper loss

Therefore, the waltmeter reading

Wsc = Full load copper loss.

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PROCEDURE:-

1. Connect the circuit as shown in circuit diagram.

2. Close the DPST switch.

3. Apply 115V, 50Hz supply to the L.V. side of transformer with the help of auto

transformer.

4. Note down the readings of ammeter, Voltmeter, Wattmeter.

5. Do the calculations and find the shunt parameters.

Tabulation for O.C. Test:-

V0 I0 W0 Cos Ø0 = WO/VOIO R0 =VO/IOcos Ø0 X0 =VO/IOsin Ø0 Z0

For Short Circuit Test

Apparatus:-

S.No.

Name of the apparatus Range Type Qty.

1 Voltmeter 0-30V MI 1

2 Ammeter 0-10A MI 1

3 Wattmeter 75V/10A UPF 1

4 Transformer 2KVA -- 1

5 Dimmerstat 0-270/8A W.W 1

PROCEDURE:-

1. Connect the circuit as shown in circuit diagram.

2. Close the DPST switch.

3. Vary the dimmerstat until full rated current flows through the transformer.

4. Note down the readings of ammeter, voltmeter and wattmeter.

5. Make the necessary calculations and draw the graph.

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Tabulation for S.C. Test:-

Vsc Isc

Wsc Cos Øsc = Wsc/ Vsc Isc Rsc = W/ I2SC Zsc = V sc/ISC Xsc

P.F Voltage drop due to I2R2CosØ2

Voltage drop dueI2X2 SinØ2

Regulation

Lead Lag

0.1

0.2

0.4

0.6

0.8

1.0

P.F CosØ Load Iron losses Copper losses Out put Input Efficiency

CosØ=0.8

¼

½

¾

Full load

CosØ=0.6

¼

½

¾

Full load

CosØ=0.2

¼

½

¾

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Full load

Model Graphs:-

MODEL CALCULATION FOR FULL LOAD EFFICIENCY:

Iron loss (W0)

Copper loss (WSC) = (ISC*Full load)2*R01

= Out put =KVA*Full load*CosƟ

=

Input = Out put+ iron loss + copper loss

=

Efficiency =Output/input *100

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=

MODEL CALCULATION FOR REGULATION:

Regulation : I2R2CosØ2 + I2X2SinØ2/V0 * 100

I2R02Cos Ø2 =

I2X02SinØ2 =

LagRegulation : I2R2Cos Ø2+I2X2SinØ2/ V0

=

Lead Regulation ;I2R2CoØs2-I2X2SinØ2/V0

=

Precautions:-

1. Dimmetstat should be at minimum position at the time of starting.

2. All the connections should be tight.

RESULT:

Hence the O.C &S.C test is obtained and the regulation and efficiency of two similar transformer at different power factor is obtained successfully.

VIVA VOCE:

1.OC test is conducted on which side of transformer and why?

2.SC test is conducted on which side of transformer and why?

3.Which losses will be given by OC test?

4. Which losses will be given by SC test?

5.What is the formula for regulation of a transformer?

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