Question 1

Transformer core choice by application: For audio-frequency coupling (AF transformers) and low-frequency power conversion (mains transformers), which core type is most commonly used due to high permeability and low magnetizing current?

Accepted Answer

Introduction / Context: Transformer core selection is driven by frequency. Audio transformers (roughly 20 Hz to 20 kHz) and grid-power transformers (50/60 Hz) require materials with high permeability to concentrate flux and reduce magnetizing current and size. The right choice improves efficiency, bandwidth, and reduces distortion and heating. Given Data / Assumptions: Application: audio coupling and low-frequency power. Goal: low magnetizing current, acceptable losses, economical construction. We compare air, iron/steel laminations, and ferrite cores. Concept / Approach: Laminated silicon-steel (commonly called iron-core) offers very high permeability and manageable losses at audio and mains frequencies. Air-cores lack permeability, making them impractically large. Ferrites excel at higher frequencies (tens of kHz to MHz) but have different loss characteristics at low frequency and are uncommon for bulk 50/60 Hz power unless specially designed and large. Step-by-Step Solution: Identify frequency range: AF and 50/60 Hz.Select material with high mu_r and low loss at these frequencies: laminated iron/steel.Reject air-core for poor coupling and size; ferrite primarily for HF switch-mode.Therefore, choose iron-core. Verification / Alternative check: Commercial mains transformers (EI, UI, toroidal) and classic audio output transformers all use laminated iron or grain-oriented steel, confirming the standard practice. Why Other Options Are Wrong: Air-core: Requires massive turns and current; inefficient at low frequency. Ferrite-core: Optimized for high-frequency SMPS; not typical for 50/60 Hz power transformers. All of the above: Not true; only iron-core fits the general case stated. Powdered-iron core: Used in RF inductors/filters, not typical for AF or mains transformers. Common Pitfalls: Assuming ferrite is always better; ignoring that core losses and permeability vary strongly with frequency. Final Answer: iron-core

Question 2

Variable transformer concept (Variac principle): Which transformer configuration is commonly constructed as a variable transformer for smoothly adjustable AC output voltage?

Accepted Answer

Introduction / Context: Bench power control and many laboratory/test setups require a continuously adjustable AC output. Commercial “Variac” devices provide this functionality. Knowing the internal configuration helps with proper selection, safety, and application limits. Given Data / Assumptions: Goal: variable, smooth AC output from a fixed AC input. Mechanical adjustment (slider/brush) is acceptable. Isolation from mains is not necessarily provided. Concept / Approach: A variable transformer is typically an autotransformer with a single continuous winding and a movable contact that varies the effective turns and hence the output voltage ratio. Because primary and output share a winding, these units are compact and efficient but do not provide isolation. Multiple taps give step changes only, not smooth variation, while center-tapped and multi-winding designs serve different purposes (e.g., dual polarities, multiple secondaries). Step-by-Step Solution: Relate “variable output” to “variable number of effective turns.”Recognize that a sliding contact on a single winding accomplishes this.Identify this as an autotransformer implementation (Variac).Therefore select “autotransformer.” Verification / Alternative check: Device datasheets and teardown photos of Variac units show a single toroidal/stacked core with one winding and a carbon brush wiper—the hallmark of an autotransformer. Why Other Options Are Wrong: Multiple-tap transformer: Only provides discrete steps, not continuous adjustment. Center-tapped transformer: Supplies split voltages but not variable output. Multiple-winding transformer: Offers independent secondaries, not continuous adjust. Current transformer: Used for measurement, not variable-voltage supply. Common Pitfalls: Assuming a Variac gives isolation; it does not. Users must follow isolation and grounding practices when powering DUTs. Final Answer: autotransformer

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