Faculty of Agriculture , Engineering and Natural Sciences

Department of Electrical and Computer Engineering

TECP 3622: Electrical Machines Practical

THE SINGLE PHASE TRANSFORMER

STUDENT FULL NAME: Kornelius Martha T

SUDENT NO: 220013640

LAB TECHNICIAN: Mr Michael Nahole DATE: OCTOBER 2023

Other members:

KALOLA FERDINAND 219377370

SHIKONGO AUGUSTINUS S 221253521

Objectives:

The aim of this experiment is:

✔ To investigate the physical construction of the transformer.

✔ To determine the values of the components of the transformer equivalent model

experimentally.

Apparatus Used

• Variable AC Power Supply

• Transformer Trainer
• Digital Multimeter (DMM)
• Connecting Cables
• Notebook

Theory

Dc Resistance Test

What is DC resistance test of transformer?

The DC resistance of the transformer refers to the DC resistance value of each phase winding.

The purpose of measuring the DC resistance is to check whether there is an inter-turn short

circuit fault inside the transformer windings.

Why winding resistance?

● Calculation of the 𝐼 2 𝑅 component of the winding losses

● Calculation of winding temperature at the end of the temperature test cycle

● As a diagnostics tool for assessing possible winding damage or insulation breakdown.

When is winding resistance done?

● At the time of installation and commissioning to assess whether there is any damage that

occurred during transportation.

● For routine maintenance work to assess the tightness of tap windings at all tap positions.

Testing Procedure for winding resistance

DC winding resistance testing is performed during transformer commissioning, after the occurrence of
internal faults, and during periodic maintenance as recommended by the manufacturer. Winding
resistance tests are performed to assess the integrity of a transformer’s windings, tap changer, and
internal connections.

There are three primary methods for conducting winding resistance tests:

i. The voltmeter-ammeter method

ii. The bridge method
iii. And the micro-ohmmeter method.

The voltmeter-ammeter method is the most common, and it typically consists of connecting a dc source
across each separate phase for wye connected windings, or between each pair of phases for delta
connected windings, with all other terminals open. The measured current and voltage are used to
calculate the winding resistance, which is then compared amongst the three windings and with the
original data measured at the factory. Agreement within 5 percent is considered satisfactory.
 Figure 2;

Ohm’s law is the applied to get DC resistance. For this practical a DMM will be used.

Open and Short circuit tests

Open and short circuit tests are performed on a transformer to determine the:

1. Equivalent circuit of transformer

2. Voltage regulation of transformer

3. Efficiency of transformer

The power required for open circuit tests and short circuit tests on a transformer is equal to the power
loss occurring in the transformer.

Open circuit tests

The connection diagram for open circuit test on transformer is shown in the figure 3. A voltmeter,
wattmeter, and an ammeter are connected in LV side of the transformer as shown. The voltage at rated
frequency is applied to that LV side with the help of a variac of variable ratio auto transformer.

The HV side of the transformer is kept open. Now with the help of variac, applied voltage gets slowly
increased until the voltmeter gives reading equal to the rated voltage of the LV side. After reaching rated
LV side voltage, we record all the three instruments reading (Voltmeter, Ammeter and Wattmeter
readings).
 Figure 3: Open circuit test on a transformer

The ammeter reading gives the no load current 𝐼𝑒 . As no load current 𝐼𝑒 is quite small compared to rated
current of the transformer, the voltage drops due to this current that can be taken as negligible.

Since voltmeter reading V1 can be considered equal to the secondary induced voltage of the
transformer, wattmeter reading indicates the input power during the test. As the transformer is open
circuited, there is no output, hence the input power here consists of core losses in transformer and
copper loss in transformer during no load condition. But as said earlier, the no-load current in the
transformer is quite small compared to the full load current so, we can neglect the copper loss due to
the no-load current. Hence, can take the wattmeter reading as equal to the core losses in the
transformer.

Therefore it is seen that the open circuit test on transformer is used to determine core losses in
transformer and parameters of the shunt branch of the equivalent circuit of the transformer

Short Circuit Test on Transformer

The connection diagram for the short circuit test on the transformer is shown in the figure 4 below. A
voltmeter, wattmeter, and an ammeter are connected in HV side of the transformer as shown. A low
voltage of around 5-10% is applied to that HV side with the help of a variac (i.e. a variable ratio auto
transformer). We short-circuit the LV side of the transformer. Now with the help of variac applied voltage
is slowly increased until the wattmeter, and an ammeter gives reading equal to the rated current of the
HV side.

After reaching the rated current of the HV side, we record all the three instrument readings (Voltmeter,
Ammeter and Watt-meter readings). The ammeter reading gives the primary equivalent of full load
current IL. As the voltage applied for full load current in a short circuit test on the transformer is quite
small compared to the rated primary voltage of the transformer, the core losses in the transformer can
be taken as negligible here.
 FIGURE 4: short circuit test on transformer

Let’s say, voltmeter reading is 𝑉𝑠𝑐 . The watt-meter reading indicates the input power during the test. As
we have short-circuited the transformer, there is no output; hence the input power here consists of
copper losses in the transformer. Since the applied voltage 𝑉𝑠𝑐 is short circuit voltage in the transformer
and hence it is quite small compared to the rated voltage, so, we can neglect the core loss due to the
small applied voltage. Hence the wattmeter reading can be taken as equal to copper losses in the
transformer. Let us consider wattmeter reading is 𝑃𝑠𝑐 .

𝑃𝑠𝑐 =𝑅𝑒 𝐼𝐿2

Where, Re is equivalent resistance of transformer.

If, Ze is equivalent impedance of transformer.

𝑉𝑆𝐶
Then, 𝑍𝑒 =
𝐼𝐿

Therefore, if equivalent reactance of transformer is Xe.

Then, 𝑋𝑒2 =𝑍𝑒2 - 𝑅𝑒2

These values are referred to the HV side of the transformer as the test is conducted on the HV side of
the transformer. These values could easily be converted to the LV side by dividing these values with the
square of transformation ratio.

Hence the short-circuit test of a transformer is used to determine copper losses in the transformer at
full load. It is also used to obtain the parameters to approximate the equivalent circuit of a
transformer.

DC resistance testing

Procedures

1. The construction of the transformer is examined; the windings, the core ad laminations
2. The following parameters are then recorded as they appear on the transformer that is being
tested

𝑉𝑝 =230 V
𝑉𝑠 =15.6 V
𝑆=100 VA
3. The rated current is calculated as follow
4. By using the DMM, DC resistance of the transformer on the secondary and primary side is
measured and recorded.
Transformer DC
resistance(R)
Primary side Secondary side
Transformer 1 16.6 0.7
Transformer 2 16.6 0.5
Transformer 3 17.9 0.7

5. The AC supply is connected to the single phase transformer as shown in the figure below

6. The variable AC supply is turned slowly until rated voltage which is 230 V
7. V1 and V2 is recorded
V1=230.3 V
 V2= 16.5 V

8. The variable AC supply is then switched off .

9. Transformer ration is calculated
𝑉1 230.3
a= 𝑉2 = 16.5
= 13.95757576
= 14

DISCUSSION

Is this transformer step up or step down?

The transformer is a step down

Open circuit test

procedures

1. The low side of the three phase transformer is connected to the AC supply
2. The AC supply is slowly adjusted to rated voltage of the transformer low side
3. The transformer are measured and recorded, as indicated in the table below

𝑉𝑂𝐶 𝐼𝑂𝐶 𝑃1
15.75 V 0.86 A 12 W

4. The AC power supply is then switched off

5. The following values are then calculated
i. 𝑃𝑜𝑐 = 12W

ii. 𝑃𝐹= COS∅ = 0.885

𝑉2
iii. 𝑅𝑐 = 𝑃𝑜𝑐 = 20.67 Ω
𝑜𝑐

𝑉𝑜𝑐
iv. 𝑋𝑚 = 𝐼 ; sin ∅ = sin 27.75 , therefore 𝑋 15.75
𝑜𝑐 sin ∅ 𝑚= =39.33Ω
0.86 sin 27.75

short circuit test

Procedures

1. The low side of a single transformer is short circuited through the ammeter
2. The high side of the single phase transformer is then connected to the variable AC supply
3. The AC supply is slowly adjusted until the current measured on the short circuited side is equal
to the rated value
4. The parameters 𝑉𝑠𝑐 , 𝐼𝑠𝑐 and 𝑃1 are measured and recorded as shown in the table below

𝑉𝑠𝑐 𝐼𝑠𝑐 𝑃1
64.3 V 0.442 A 5W

5. The following values are calculated

i. 𝑃𝑠𝑐 = 5W

𝑉 64.3
ii. 𝑍𝑒𝑞 = 𝐼𝑠𝑐 = 0.442 =145 Ω
𝑠𝑐

𝑃 5𝑊
iii. 𝑅𝑒𝑞 = 𝐼2𝑠𝑐 = 0.4422 𝐴 = 25.59 Ω
𝑠𝑐

Below is the per phase equivalent circuit of this transformer showing all parameters and
their per phase values: