Transformers in Aircraft Systems

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.