Experiment5 Open circuit and Short circuit test and Autotransformer

Objectives: - To study two tests of open and short circuit test and apply in LVVL program. - To study how can be find six parameters of transformer using two tests. - To study what is an Autotransformer and types of connection.

Theory: The equivalent circuit parameters of a transformer can be determined by performing two tests:  Open-circuit test.  Short-circuit test.

Firstly// Open-circuit test:

Open circuit test or no load test on a transformer is performed to determine 'no load loss (core loss)' and 'no load current Io'. The circuit diagram for open circuit test is shown in figure below.

Figure 1 Circuit diagram for open circuit test Usually high (HV) winding is kept open and the low voltage (LV) winding is connected to its normal supply. A wattmeter (W), ammeter (A) and voltmeter (V) are connected to the LV winding as shown in the figure.

The ammeter reading gives the no load current Io. As Io itself is very small, the voltage drops due to this current can be neglected.

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The input power is indicated by the wattmeter (W). And as the other side of transformer is open circuited, there is no output power. Hence, this input power only consists of core losses and copper losses. As described above, no-load current is so small that these copper losses can be neglected. Hence, now the input power is almost equal to the core losses. Thus, the wattmeter reading gives the core losses of the transformer.

Figure 2 Parameters for open circuit test

Soc = Voc Ioc ,,, RcL = ,,, Qoc = √ ,,, XmL =

Secondly//Short circuit test This test is designed to determine the winding resistances and leakage reactances. It does not really matter on which side this test is performed. However, the measurement of the rated current suggests that, for safety purposes, the test be performed on the high-voltage side. The test arrangement with all instruments inserted on the high-voltage side with a short circuit on the low voltage side as shown in figure below

Figure 3 Circuit diagram for short circuit test

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Since the applied voltage is a small fraction of the rated voltage, both the core loss and the magnetizing currents are so small that they can be neglected.

The approximate equivalent circuit of the transformer as viewed from the high-voltage side is given in Figure. In this case, the wattmeter records the at full load.

Figure 4 Parameters for short circuit test

퐏퐬퐜 ퟐ ReH = RH +퐚 RL ReH = 퐈퐬퐜ퟐ X = X +퐚ퟐ X 퐕퐬퐜 eH H L ZeH = 퐈퐬퐜

ퟐ ퟐ XeH = √퐙퐞퐇 퐑퐞퐇

Autotransformer: Autotransformer is kind of electrical transformer where primary and secondary shares same common single winding. So basically it’s a one winding transformer. Autotransformers may be used for almost all applications in which we use a two-winding transformer. The only disadvantage in doing so is the loss of electrical isolation between the high- and low-voltage sides of the autotransformer.

Some of the advantages of an autotransformer compared with a two-winding transformer:

1- It delivers more power than a two-winding transformer of similar physical dimensions.

2- For a similar power rating, an autotransformer is more efficient than a two winding transformer.

3- It is cheaper in first cost than a conventional two-winding transformer of a similar rating.

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Possible ways to connect a two-winding transformer as an autotransformer:

Figure 5 Different Connection for autotransformer

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Experimental Procedures: Part1 : Determine the parameters from Open circuit test:

1- Ensure that the is switched off. Then Connect the circuit by (LVVL) program as shown in figure 6. 2- Switch on the power then use the indicator to set input voltage at 110V(by put the voltage control Knob 50%).

Figure 6 Connections for open circuit test

3- Open the metering window to read the value of Voc, Ioc, Poc, Qoc and Soc.

Figure 7 Measurement readings for open test

From the metering window:

Voc = 115.5 V Poc = 1.318 W Ioc = 0.013 A Qoc = 0.614 VAR Poc = 1.318 W Soc = 1.453 VA

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4- Calculation to find RC and Xm.

RcL = = = 10.121kΩ

XmL = = = 21.726kΩ

Part2 : Determine the parameters from Short circuit test:

1- Ensure that the power supply is switched off. Then Connect the circuit by (LVVL) program as shown in figure 10. 2- Switch on the power then use the indicator to set input voltage at 110V(by put the voltage control Knob 50%).

Figure 8 Connections for short circuit test

3- Open the metering window to read the value of Vsc, ISc, Psc, Qsc and Ssc.

Figure 9 Measurement readings for short test

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From the metering window:

Vsc = 110.9 V Psc = 124 W ISc = 1.155 A Qsc = 32.07 VAR PSc = 124 W Ssc = 128 VA

4- Calculation to find RC and Xm.

ReH = = = 92.95Ω

ZeH = = = 96.0173Ω

XeH = √ = √ = 24.075 Ω

Part3 : Autotransformer:  Step down autotransformer: 1- Ensure that the power supply is switched off. Then Connect the circuit by (LVVL) program as shown in figure 10. 2- Switch on the switch in the resistive load to get the resistor value 628Ω. 3- Switch on the power then use the indicator to set input voltage at 110V(by put the voltage control Knob 50%).

Figure 10 Step down autotransformer

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4- Open the metering window to read the value of Es, ESEC ,IPRI, ISEC and A.

Figure 11 Measurement readings for autotransformer

From the metering window:

ESEC=56.62 V IPRI =0.055 A Es = 115.4 V ISEC=0.091 A

A= Es/ESEC= 2

 Step up autotransformer: 1- Ensure that the power supply is switched off. Then Connect the circuit by (LVVL) program as shown in figure 12. 2- Switch on the switch in the resistive load to get the resistor value 628Ω. 3- Switch on the power then use the indicator to set input voltage at 110V(by put the voltage control Knob 50%).

Figure 12 Step up autotransformer

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1- Open the metering window to read the value of Es, ESEC ,IPRI, ISEC and A.

Figure 13 Measurement readings for autotransformer

From the metering window:

ESEC=208.2 V IPRI =0.694A Es = 112.7 V ISEC=0.335 A

A= Es/ESEC= 0.54

Exercises:

1- By using open and short test on transformer(7-8),(5-6) …Voltage control=50% ,find parameter:

cosϴ

2- Make autotransformer with turns ration =1.63 ,, and draw your connection.

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