experiment5 open circuit and short circuit test and...

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Experiment5

Open circuit and Short circuit test and Autotransformer

Objectives:

- To study two tests of transformer 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 voltage (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 copper loss at full load.

Figure 4 Parameters for short circuit test

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.

ReH = 𝐏𝐬𝐜

𝐈𝐬𝐜𝟐

ZeH = 𝐕𝐬𝐜

𝐈𝐬𝐜

XeH = √𝐙𝐞𝐇𝟐 𝐑𝐞𝐇

𝟐

ReH = RH +𝐚𝟐 RL

XeH = XH +𝐚𝟐 XL

https://www.electrical4u.com/electrical-power-transformer-definition-and-types-of-transformer/

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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 power supply 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

Ioc = 0.013 A

Poc = 1.318 W

Poc = 1.318 W

Qoc = 0.614 VAR

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:



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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4- Calculation to find RC and Xm.

ReH =

=

= 92.95Ω

ZeH =

=

= 96.0173Ω

XeH = √

= √ = 24.075 Ω

Part3 : Autotransformer:

Step down autotransformer:



2- Switch on the switch in the resistive load to get the resistor value 628Ω.


control Knob 50%).

Figure 10 Step down autotransformer

Vsc = 110.9 V

ISc = 1.155 A

PSc = 124 W

Psc = 124 W

Qsc = 32.07 VAR

Ssc = 128 VA

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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


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.


control Knob 50%).

Figure 12 Step up autotransformer

ESEC=56.62 V

Es = 115.4 V

IPRI =0.055 A

ISEC=0.091 A

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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


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.

ESEC=208.2 V

Es = 112.7 V

IPRI =0.694A

ISEC=0.335 A

experiment5 open circuit and short circuit test and...

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