In basic electrical engineering, a transformer is a device primarily used to convert which of the following types of electrical supply?

Introduction / Context:
Transformers are fundamental components in power systems and electrical engineering. They are seen at power stations, on distribution poles, and inside many electronic devices. A transformer operates on the principle of electromagnetic induction and works only with alternating current. The main role of a transformer is to change voltage levels efficiently, either stepping voltage up for transmission or stepping it down for safe use in homes and industries. This question asks you to identify which type of electrical conversion a transformer is primarily designed to perform.

Given Data / Assumptions:
• The device in question is a transformer with primary and secondary coils wound on a magnetic core. • It is used in conventional AC power systems. • The options include conversions between direct current and alternating current, voltage changes, and energy conversion from mechanical to electrical. • Ideal transformer theory and standard textbook usage are assumed.

Concept / Approach:
A transformer operates when an alternating current flows through the primary coil, creating a changing magnetic flux in the core. This changing flux induces an alternating voltage in the secondary coil according to Faradays law of electromagnetic induction. The ratio of the number of turns in the coils determines whether the secondary voltage is higher or lower than the primary voltage. Importantly, transformers do not convert DC to AC or AC to DC; they require AC to function at all. They also do not directly convert mechanical energy into electrical energy, which is the job of generators. Therefore, the correct description of a transformer is a device that converts low voltage AC to high voltage AC or high voltage AC to low voltage AC, depending on the winding ratio.

Step-by-Step Solution:
Step 1: Recall that transformers are used in power transmission to step up voltages for long distance lines and step down voltages near consumers. Step 2: Recognise that these processes involve changing the magnitude of an alternating voltage, not changing AC to DC or DC to AC. Step 3: Understand that a transformer requires a time varying magnetic flux, which is produced by alternating current, to induce voltage in the secondary coil. Step 4: Note that DC, which does not vary with time, would not create the changing magnetic field needed for transformer action. Step 5: Compare the options and identify the one that explicitly states conversion between low voltage AC and high voltage AC, either stepping up or stepping down. Step 6: Select that option as it matches the standard function of a transformer.

Verification / Alternative check:
If you look at typical ratings on a distribution transformer, you might see values like 11 kV slash 415 V, indicating that the transformer takes 11 kilovolt AC on the primary side and provides 415 volt AC on the secondary side. No change from AC to DC occurs inside the transformer core itself. Devices such as rectifiers or power electronic converters are required to convert AC to DC or DC to AC. Generators, not transformers, are responsible for converting mechanical shaft power into electrical energy. These real world labels and roles confirm that transformers are AC voltage changing devices rather than current type changing or mechanical energy converting devices.

Why Other Options Are Wrong:
Option A, converting DC to AC, describes the function of an inverter, not a transformer. Option B, low voltage DC to high voltage DC, may be achieved using DC DC converters and switching circuits, but not directly by a simple transformer alone. Option D, converting mechanical energy into electrical energy, is the function of electrical generators or dynamos, not transformers. Because transformers require AC and mainly change its voltage level, these options do not accurately describe their primary purpose.

Common Pitfalls:
Students sometimes loosely say that a transformer changes electricity from high to low or vice versa without specifying that the current remains alternating. This can lead to confusion between transformers and rectifiers or inverters. Another pitfall is assuming that the large size of power transformers means they generate electricity themselves, when in reality they only transform voltages. Remembering the phrase transformer transforms AC voltage levels but does not create electricity helps keep these roles clear in your mind.

Final Answer:
The correct choice is Low voltage alternating current into high voltage alternating current (or vice versa), because a transformer is specifically designed to step up or step down AC voltages using electromagnetic induction, without changing AC into DC or converting mechanical energy into electrical energy.