Question 1

Why is AC voltage required for a transformer to operate?

Accepted Answer

A DC voltage across the primary winding is constant, so the resulting magnetic flux is also constant with zero rate of change. According to Faraday's Law, an induced voltage equals the flux's rate of change multiplied by the number of windings. Only AC voltage produces a magnetic flux with a non-zero rate of change, inducing voltage on the secondary winding.

Question 2

How does the turns ratio determine transformer voltage and current relationships?

Accepted Answer

The turns ratio equals the number of primary turns divided by secondary turns. The voltage ratio across primary to secondary equals the turns ratio, while the current ratio is the inverse. For example, a 0.1 turns ratio steps up 120 volts to 1200 volts but reduces 10 amps to 1 amp, demonstrating how turns ratio controls voltage and current transformation.

Question 3

What is mutual induction and how does it work in transformers?

Accepted Answer

Mutual induction occurs when voltage across the primary winding causes current to flow, creating a magnetic field. Magnetic flux couples through the core to the secondary winding, inducing voltage there. This principle enables transformers to transfer power between windings without direct electrical connection, providing the foundation for transformer operation.

Question 4

How do real transformers differ from ideal transformers?

Accepted Answer

Ideal transformers transfer energy without loss, but real transformers experience leakage inductance from magnetic flux that doesn't contribute to energy transfer between windings. Additionally, winding resistances cause power dissipation and heating, while core loss resistance generates heat from magnetic flux induced in the core, reducing overall efficiency.

Question 5

What does the DC winding resistance test measure and why is it important?

Accepted Answer

The DC winding resistance test measures the resistance of each primary and secondary winding by applying DC voltage and recording voltage and current readings. Dividing voltage by current yields winding resistance, which is essential for creating the equivalent circuit model of the transformer and understanding its real-world performance characteristics.

Question 6

What parameters does the open-circuit test determine?

Accepted Answer

The open-circuit test measures core loss resistance and mutual reactance by applying voltage to the primary while leaving the secondary open. Using open-circuit voltage, current, real power, and power factor measurements, engineers calculate core loss resistance from voltage and power, and mutual reactance similarly, characterizing the transformer's magnetic properties.

Question 7

How does electrical isolation in transformers enhance safety?

Accepted Answer

When input and output terminals don't share a common connection, transformers transfer power with complete physical separation between primary and secondary sides. This isolation prevents hazardous high-voltage electricity from reaching safe low-voltage circuits and people, making transformers critical for safe operation of laboratory instruments and medical devices.

Transformateurs monophasés - Quiz d'évaluation