Transformers in Aircraft Systems

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Questions and Answers

What causes eddy current loss in a transformer?

Excessive load on the transformer

High voltage supply

Induced emf in locally linked conducting parts (correct)

Dissipation of heat from the secondary winding

Which method is commonly employed to reduce eddy current loss?

Increased cooling systems

Applying a higher voltage

Using thicker metal sheets

Using laminated sheets of steel (correct)

What is the primary function of current transformers (CTs) in aircraft?

To measure the temperature of electrical components

To convert DC power to AC power

To generate electrical energy from hydraulic power

For power system wiring protection and power supply control (correct)

What is the formula for the efficiency of a transformer?

Efficiency = output / input Signup and view all the answers

What does a Transformer Rectifier Unit (TRU) convert?

115V AC to 28V DC Signup and view all the answers

What condition must be met for maximum efficiency in a transformer?

Cu loss equals iron loss Signup and view all the answers

What is a primary characteristic of autotransformers compared to two-winding transformers?

Only one winding common to both primary and secondary Signup and view all the answers

Which statement is true regarding the operation of a transformer?

It relies on mutual induction between two coils. Signup and view all the answers

Which of the following statements is true regarding transformer efficiency?

Transformers operate with full load efficiency of 95% to 98.5%. Signup and view all the answers

What principle underlies the operation of a transformer?

Mutual induction Signup and view all the answers

What happens to the current when a transformer raises the voltage in a circuit?

Current decreases Signup and view all the answers

What is the impact of high silicon content in the steel used for transformer cores?

Reduces hysteresis loss and increases permeability Signup and view all the answers

In an autotransformer, how is the power transformed inductively described?

Power transformed inductively is input multiplied by (1 - K). Signup and view all the answers

Which component primarily feeds a DC bus in an aircraft?

Transformer Rectifier Units (TRUs) Signup and view all the answers

What characteristic of a transformer allows it to function?

Presence of varying magnetic fields Signup and view all the answers

What type of electrical input can a transformer utilize?

Varying AC voltage Signup and view all the answers

What does the coefficient of coupling measure in a transformer?

The efficiency and effectiveness of magnetic flux transfer from primary to secondary Signup and view all the answers

If a transformer has 10,000 lines of magnetic force developed in its primary and only 8,500 cut across its secondary coil, what is the coefficient of coupling?

0.85 Signup and view all the answers

Which type of transformer is primarily used in transmission networks?

Power transformer Signup and view all the answers

In which situation would you typically use current transformers?

For measuring large alternating currents Signup and view all the answers

What is a common limitation of transformers regarding efficiency?

No transformer can achieve 100 percent efficiency due to leakage flux Signup and view all the answers

What type of transformer is used to measure high alternating voltages?

Potential transformer Signup and view all the answers

How is the induced e.m.f. in the primary winding of a transformer calculated?

By multiplying the induced e.m.f. per turn by the number of primary turns Signup and view all the answers

What role do instrument transformers play in measuring alternating currents and voltages?

They convert high values to measurable low ranges for standard instruments Signup and view all the answers

Study Notes

Transformers

Transformers are static electrical devices that convert electric power in one circuit into electric power of the same frequency in another circuit.
They can change voltage in a circuit, but this change is accompanied by a corresponding decrease or increase in current.
The basis of transformers is mutual induction between two circuits connected by a common magnetic flux.
Transformers transfer electric power from one circuit to another without changing the frequency.
Transformers usually consist of two coils that are not electrically connected but arranged so that the magnetic field circling one coil goes through the other.

Transformer Uses in Aircraft

Current transformers (CTs), also known as instrument transformers, are used for power system wiring protection and power supply control in commercial aircraft.
A Transformer Rectifier Unit (TRU) combines transformer and rectifier functions in one unit.
In aircraft, TRUs convert 115V AC power from the engine or an auxiliary power unit (APU) or a ground power unit (GPU) to 28V DC power for use by various electrical components.

Principle Operation of a Transformer

A transformer changes electrical energy at a specific voltage to electrical energy at a different voltage level.
A transformer has two coils that aren't electrically connected but are arranged so that the magnetic field surrounding one coil passes through the other coil.
An alternating voltage applied to one coil creates a fluctuating magnetic field, producing a fluctuating voltage in the other coil via mutual induction.

Transformer Construction

Essentially, transformers involve two inductive windings wrapped around a laminated steel core.
The coils are insulated from each other, as well as from the steel core.
A transformer housing, called a tank, may encompass a winding and core assembly and contain bushings connecting to the terminals. It will have an oil conservator for coolant oil.

Transformer Types

(A) Based on construction*
Core type transformers: The windings surround a considerable portion of the core. Windings are cylindrical and often layered, each insulated from each other; materials such as paper, cloth, or mica can be insulating layers.
Shell type transformers: The core surrounds much of the windings; coils are wound and layered with insulator between them. These may have a simple rectangular or a distributed configuration.
(B) Based on purpose*
Step-up transformers: The primary coil has fewer turns than the secondary coil, increasing the voltage.
Step-down transformers: The primary coil has more turns than the secondary coil, decreasing the voltage.

Transformer Ratio

The turns ratio (N1/N2) determines whether a transformer is step-up or step-down.
The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding.

Voltage Transformation Ratio (K)

A constant K where E2/E1 = N2/N1 = K.
K signifies the voltage transformation ratio.
If N2 > N1, K > 1, and the transformer is a step-up transformer.
If N2 < N1, K < 1, and the transformer is a step-down transformer.

EMF Equation of a Transformer

E₁ = 4.44fΦ𝑚N₁
Φm is the maximum flux in the core in webers.
f is the frequency of the AC input in Hz.
N₁ is the number of turns in the primary winding.

Losses in a Transformer

(i) Core (or Iron) Loss*
This loss results from hysteresis and eddy currents within the transformer's core.
Core loss remains practically constant across all loads.
Core loss is minimized by using high-silicon steel, often laminated, for the core.
(ii) Copper Loss*
Copper loss arises from the ohmic resistance of the transformer windings.
The total copper loss (I²R loss) is directly proportional to the square of the current (I²) and the resistance (R).
This loss varies with the load on the transformer.

Methods to Overcome Core Losses

High silicon content in the steel, sometimes including heat treatment, reduces hysteresis loss.
Laminated steel sheets minimize eddy currents.

Transformer Efficiency

Transformer efficiency = (Output)/(Input).
Most transformers have high efficiencies, often between 95% and 98% at full load.
Efficiency is calculated as (Output)/(Output + Losses).

Condition for Maximum Efficiency

Maximum efficiency occurs when the copper loss equals the core loss.

Auto Transformers

Autotransformers have only one winding, a portion of which serves as both the primary and secondary.
They are simpler and cheaper than two-winding transformers.
Autotransformers are practical for situations where the voltage transformation ratio is close to unity.

Isolation Transformers

Isolation transformers provide a galvanic isolation between the source and load; there's no conductive path between them.
They isolate the load from the power source, reducing the risk of electrical shock and noise interference.
Primarily used for safety and signal isolation.

Transformer Tests

Open-circuit (no-load) test: Used to determine no-load loss (core loss).
Short-circuit test: Used to determine equivalent impedance, leakage reactance, and copper loss.

Transformer Rating in kVA

Transformer ratings are in kVA (kilovolt-amperes) and not kW due to the voltage and current characteristics of the loss components (iron and copper losses). Efficiency varies with load, but losses generally do not.

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Description

This quiz covers the fundamentals of transformers, including their function, operation, and essential uses in aircraft systems. You'll learn about mutual induction, current transformers, and the role of Transformer Rectifier Units in aviation. Test your understanding of how transformers help manage electric power in commercial aircraft.