Practical - 18 :- Identification of Step-Up and Step-Down Transformers

 Identification of Step-Up and Step-Down Transformers

1. Aim

To determine whether a given single-phase transformer is a step-up or a step-down transformer by measuring its input and output voltages.

2. Apparatus Required

• The transformer under test (with unidentified windings)

• An AC power supply (a variable AC supply, like a Variac, is preferred)

• Two AC voltmeters (or two multimeters set to measure AC voltage)

• Connecting wires

3. Theory

A transformer is a static device that transfers electrical energy from one circuit to another through electromagnetic induction, without a change in frequency. It consists of two or more coils, typically a primary winding and a secondary winding, wound on a magnetic core.

The operation is based on the transformer equation:

By applying a known AC voltage to one winding and measuring the resulting voltage on the other, we can compare Vp and Vs to identify the transformer type.

4. Circuit Diagram

(Note: V1 measures the primary voltage Vp, and V2 measures the secondary voltage Vs.)

5. Procedure

1. Take the given transformer. Arbitrarily label the terminals of one winding as the primary (input) and the other as the secondary (output).

2. Connect the AC power supply (Variac) across the primary winding.

3. Connect the first AC voltmeter (V1) in parallel with the primary winding to measure the input voltage ($V_p$).

4. Connect the second AC voltmeter (V2) in parallel with the secondary winding to measure the output voltage ($V_s$). Ensure the secondary side is an open circuit (no load is connected).

5. Important: Make sure the Variac is set to its minimum (zero volts) position before turning on the power supply.

6. Switch on the main AC power supply.

7. Slowly increase the voltage from the Variac to apply a safe, measurable voltage to the primary winding (e.g., 50V, 100V, or as specified by the lab instructor). Do not exceed the transformer's rating.

8. Record the reading on the primary voltmeter (V1) as $V_p$.

9. Record the reading on the secondary voltmeter (V2) as $V_s$.

10. Turn the Variac back to zero and switch off the power supply.

6. Observation Table

S. No.	Input Voltage (Vp​) (Volts)	Output Voltage (Vs​) (Volts)	Result (Vs​ vs Vp​)
1.			Vs [ > / < ] Vp

7. Calculations and Result

Compare the observed values of $V_p$ and $V_s$ from the observation table.

• Case 1: If Vs > Vp (Output voltage is greater than input voltage), the transformer is a Step-Up Transformer.

• Case 2: If Vs < Vp (Output voltage is less than input voltage), the transformer is a Step-Down Transformer.

• Case 3 (Unlikely for this test): If Vs \approx Vp, it is an Isolation Transformer (turns ratio 1:1).

8. Precautions

1. All connections must be neat and tight.

2. Always use an AC supply and set the multimeters to measure AC voltage ($\tilde{V}$). Using DC will not work and may damage the equipment.

3. Always start the experiment with the Variac at its minimum voltage position.

4. Do not apply a voltage higher than the rated voltage of the transformer windings.

5. Handle the live circuit with care to avoid electric shock.

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