Lab Report 4: Load Test on Single Phase Transformer

1. OBJECTIVES

• To understand the different tests on a single-phase transformer.
• To be able to plot the T vs S curve.
• To understand the relation between torque and slip on an induction machine.

2. INSTRUMENTS REQUIRED

• Wattmeter
• Ammeter
• Voltmeter
• Transformer
• Induction Machine

3. THEORY

Open Circuit Test on Transformer

The open circuit test on a transformer assesses core losses by applying a rated voltage to the primary winding while leaving the secondary open. This helps determine the transformer’s efficiency and core characteristics under no-load conditions, which is crucial for optimizing its design and performance.

[Image of Open Circuit Test on Transformer]

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

Short Circuit Test on Transformer

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.

After reaching the rated current of the HV side, we record all three instrument readings (Voltmeter, Ammeter, and Wattmeter readings). The ammeter reading gives the primary equivalent of the full load current \(I_L\). As the voltage applied for the 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.

Transformer

The transformer is a device that steps up or steps down voltage. In a step-up transformer, the output voltage is increased, and in a step-down transformer, the output voltage is decreased. The transformer works on the principle of Faraday’s law of electromagnetic induction and mutual induction.

Induction Machine

An induction machine is a versatile AC electric motor known for its simplicity and reliability. Its working principle relies on electromagnetic induction. A single-phase AC supply voltage is used to generate a rotating magnetic field in the stator. This rotating magnetic field induces currents in a conductor or squirrel cage rotor, creating torque and causing the rotor to turn.

Torque-Slip Characteristics of a Single-Phase Induction Motor

At a slip of unity, both forward and backward fields develop equal torque, but their directions are opposite to each other, so the net torque produced is zero; hence the motor fails to start. From this, we can say that these motors are not self-starting, unlike the three-phase induction motor. There must be some means to provide the starting torque. If by some means, we can increase the forward speed of the machine, the forward slip decreases, the forward torque will increase, and the reverse torque will decrease, as a result of which the motor will start.

4. CIRCUIT DIAGRAMS FOR EXPERIMENT

[Image of Circuit diagram for open circuit test]

5. OBSERVATION TABLE

i. For Short Circuit Test

• \(V_{sc}\) = 10 volt
• \(W_{sc}\) = 26 watt
• \(I_{sc}\) = 2.6 Amps

ii. For Open Circuit Test

• \(V_o\) = 220 volt
• \(I_o\) = 0.27 Amps
• \(W_o\) = 34 Watt

iii. For T-S Characteristics

S.N	Amps (I)	Watt (W)	Torque (N-m)	Speed (RPM)	P.F = W / (V x I)
1.	1.52	237	0.44	2856	0.708
2.	1.61	249	0.502	2838	0.702
3.	1.69	270	0.55	2820	0.72
4.	1.78	280	0.61	2796	0.71
5.	1.86	294	0.667	2772	0.71
6.	1.96	312	0.728	2745	0.72

6. CALCULATION

Supply voltage = 220V

\((P.F)_1 = \frac{237}{220 \times 1.52} = 0.708\)

\((P.F)_2 = \frac{249}{220 \times 1.61} = 0.702\)

\((P.F)_3 = \frac{270}{220 \times 1.69} = 0.72\)

\((P.F)_4 = \frac{280}{220 \times 1.78} = 0.71\)

\((P.F)_5 = \frac{294}{220 \times 1.86} = 0.71\)

\((P.F)_6 = \frac{312}{220 \times 1.96} = 0.72\)

7. GRAPH

T-S curve

A graph plotting Torque (Y-axis) vs. Speed (X-axis) based on the observation table data.

Scale:
X-axis: 10 small divisions = 50 rpm
Y-axis: 10 small divisions = 0.05 N-m

8. RESULT

The load test on a single-phase transformer was done with the help of an ammeter, voltmeter, and transformer. The T-S characteristics of the induction machine were studied with the help of an induction machine. While increasing the mechanical load resistance, there was a decrease in rpm speed.

9. DISCUSSION

Hence, the above experiment was performed with the help of a transformer and an induction machine. The load test on the transformer is done by a short-circuit test and an open-circuit test. The short circuit test was done by supplying voltage on the high voltage winding, while the open circuit test was done by supplying voltage on the low voltage winding. In the short circuit test, a supply voltage = 10V, power = 26W, and current = 2.6A was observed. In the open circuit test, a supply voltage = 220V, power = 34W, and current = 0.27A was observed. For the T-S characteristics of the induction machine, it was done by applying a mechanical load. While increasing the mechanical load resistance, there was a decrease in rpm speed, and power and current also changed. The power factor was calculated by using apparent power and active power.

At a current of 1.52 A, a power of 237W, a Torque of 0.44 N-m, a Speed of 2856 rpm, and a power factor of 0.708 was observed. For currents of 1.61, 1.69, 1.78, 1.86, and 1.96 Amps, the respective power, torque, speed, and power factor values were observed as:

• 249W, 0.502 N-m, 2838 rpm, 0.702
• 270W, 0.55 N-m, 2820 rpm, 0.72
• 280W, 0.61 N-m, 2796 rpm, 0.71
• 294W, 0.667 N-m, 2772 rpm, 0.71
• 312W, 0.728 N-m, 2745 rpm, 0.72

10. CONCLUSION

Hence, the load test on a single-phase transformer was done and the T-S characteristics of the induction machine were observed.

11. PRECAUTIONS

• Readings of the values should be noted carefully.
• In the short circuit test, the voltage should be supplied on the high voltage winding.
• The circuit should be checked carefully before powering it on.