Introduction
A transformer is a device that transfers electrical energy from one circuit to another by electromagnetic induction (transformer action).
In its most basic form a transformer consists of:
- A primary coil or winding.
- A secondary coil or winding.
- A core that supports the coils or windings
The expanding and contracting magnetic field around the primary winding cuts the secondary winding and induces an alternating voltage into the winding.
This voltage causes alternating current to flow through the load. The voltage may be stepped up or down depending on the design of the primary and secondary windings.
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| Bagan 1 Basic transformer action. |
The Components of a Transformer
- The core, which provides a path for the magnetic lines of flux.
- The primary winding, which receives energy from the ac source.
- The secondary winding, which receives energy from the primary winding and delivers it to the load.
- The enclosure, which protects the above components from dirt, moisture, and mechanical damage.
Core Characteristics
air-core transformers are used when the voltage source has a high frequency (above 20 kHz).
Iron-core transformers are usually used when the source frequency is low (below 20 kHz).
A transformer whose core is constructed of laminated sheets of steel dissipates heat readily; thus it provides for the efficient transfer of power. The purpose of the laminations is to reduce certain losses.
The majority of transformers you will encounter in aircraft equipment contain laminated-steel cores. These steel laminations (see Figure below) are insulated with a non-conducting material, such as varnish, and then formed into a core.
There are two main shapes of cores used in laminated-steel-core transformers
- Hollow-Core Transformers
shaped with a hollow square through the center
- Shell-Core Transformers
each layer of the core consists of E and I-shaped sections of metal.
Transformer Windings
the transformer consists of two coils called windings which are wrapped around a core. The winding that is connected to the source is called the primary winding. The winding that is connected to the load is called the secondary winding.
The wire is coated with varnish so that each turn of the winding is insulated from every other turn.
In a transformer designed for high-voltage applications, sheets of insulating material, such as paper, are placed between the layers of windings to provide additional insulation.
Schematic Symbols for Transformers
The symbol of an air-core transformer is shown in Figure (A). Parts (B) and (C) show iron-core transformers. The bars between the coils are used to indicate an iron core.
How a Transformer Works
No-Load Condition
A no-load condition is said to exist when a voltage is applied to the primary, but no load is connected to the secondary.
however, a very small amount of current called exciting current flowing in the primary.
The amount of exciting current is determined by three factors :
- the amount of voltage applied (Ea),
- the resistance (R) of the primary coil's wire and core losses, and
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the XL which is dependent on the frequency of the exciting current.
This very small amount of exciting current serves two functions:
- Most of the exciting energy is used to maintain the magnetic field of the primary.
-
A small amount of energy is used to overcome the resistance of the wire and core losses which are dissipated in the form of heat (power loss).
Producing a Back-EMF
Flux leaves the primary at the north pole and enters the primary at the south pole. The Back-EMF induced in the primary has a polarity that opposes the applied voltage, thus opposing the flow of current in the primary. It is the Back-EMF that limits exciting current to a very low value.
Inducing a Voltage in the Secondary
During the time current is increasing in the primary, magnetic lines of force expand outward from the primary and cut the secondary., a voltage is induced into a coil when magnetic lines cut across it. Therefore, the voltage across the primary causes a voltage to be induced across the secondary.
Primary and Secondary Phase Relationship
Transformers in which the secondary voltage is in phase with the primary is referred to as like-wound transformers, while those in which the voltages are 180 degrees out of phase are called unlike-wound transformers
Coefficient of Coupling
The coefficient of coupling of a transformer is dependent on the portion of the total flux lines that cut both primary and secondary windings.
Lines of flux generated by one winding which do not link with the other winding are called leakage flux
leakage inductance is assumed to drop part of the applied voltage, leaving less voltage across the primary.
Turns and Voltage Ratios
EMF induced into the secondary will be the same as the EMF induced into each turn in the primary.
NP = number of turns in the primary
EP = voltage applied to the primary
ES = voltage induced in the secondary
NS=number of turns in the secondary
Turns Ratio Conventions
The American version of Turns Ratio convention is
The British version of turns ratio convention is
Turn ratio: N Secondary/N Primary
A transformer in which the voltage across the secondary is less than the voltage across the primary is called a step-down transformer.
A transformer that has fewer turns in the primary than in the secondary will produce a greater voltage across the secondary than the voltage applied to the primary. A transformer in which the voltage across the secondary is greater than the voltage applied to the primary is called a step-up transformer.
Effect of a Load
When a load device is connected across the secondary winding of a transformer, current flows through the secondary and the load.
flux links both windings, it is called mutual flux.
The inductance which produces this flux is also common to both windings and is called mutual inductance.
When a load resistance is connected to the secondary winding, the voltage induced into the secondary winding causes current to flow in the secondary winding.
The number of flux lines developed in a core is proportional to the magnetizing force (in ampere-turns) of the primary and secondary windings
Therefore:
IPNP = ISNS
Step-Up and Step-Down Transformers
A transformer that increases the voltage from primary to secondary (more secondary winding turns than primary winding turns) is called a step-up transformer. Conversely, a transformer designed to do just the opposite is called a step-down transformer.
Power Relationship between Primary and Secondary Windings
If the voltage is doubled in the secondary, the current is halved in the secondary. Conversely, if the voltage is halved in the secondary, current is doubled in the secondary.
with the exception of the power consumed within the transformer, all power delivered to the primary by the source will be delivered to the load.
Transformer Losses
Copper Loss
Eddy-Current Loss
Hysteresis Loss
Heat and Noise
One loss is due to the dc resistance in the primary and secondary windings. This loss is called copper loss or 12R loss.
The two other losses are due to eddy currents and to hysteresis in the core of the transformer. Copper loss, eddy-current loss, and hysteresis loss result in the undesirable conversion of electrical energy into heat energy.
Whenever current flows in a conductor, power is dissipated in the resistance of the conductor in the form of heat.
Copper loss = 12R
The effect of this is that if the current (I) is doubled, for example, the amount of copper loss will quadruple.
The core of a transformer is usually constructed of some type of ferromagnetic material because it is a good conductor of magnetic lines of flux.
To minimize the loss resulting from eddy currents, transformer cores are laminated.
When a magnetic field is passed through a core, the core material becomes magnetized.
The energy used to turn each domain is dissipated as heat within the iron core. This loss, called hysteresis loss, can be thought of as resulting from molecular friction.
Eddy current loss and hysteresis loss are both losses from the magnetic core of the transformer.
Noise is primarily a nuisance effect, but heat is a potentially serious problem because winding insulation will be damaged if allowed to overheat.
Heat-dissipating "radiator" tubes on the outside of the transformer case provide a convective oil flow path to transfer heat from the transformer's core to ambient air:
Class A: No more than 55° Celsius winding temperature rise, at 400 Celsius (maximum) ambient air temperature.
Class B: No more than 80° Celsius winding temperature rise, at 40° Celsius (maximum) ambient air temperature.
Class F: No more than 115° Celsius winding temperature rise, at 40° Celsius (maximum) ambient air temperature.
Class H: No more than 150° Celsius winding temperature rise, at 40° Celsius (maximum) ambient air temperature
Transformer Efficiency
The input power is equal to the product of the voltage applied to the primary and the current in the primary. The output power is equal to the product of the voltage across the secondary and the current in the secondary.
Transformer Ratings
When a transformer is to be used in a circuit, more than just the turns ratio must be considered. The voltage, current, and power-handling capabilities of the primary and secondary windings must also be considered.
Power Transformers
Power transformers are used to supply voltages to the various circuits in electrical equipment.
Audio-Frequency Transformers
Audio-frequency (AF) transformers are used in AF circuits as coupling devices.
Radio-Frequency Transformers
Radio-frequency (RF) transformers are used to couple circuits to which frequencies above 20,000 Hz are applied.
Isolation Transformers
transformers also provide an extremely useful feature called isolation. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass.
Current Transformers
The Current Transformer (or CT) is a step-up device (with respect to voltage), which is what is needed to step down the power line current.
Because CTs are designed to be powering ammeters, which are low-impedance loads, and they are wound as voltage step-up transformers, they should never, ever, be operated with an open-circuited secondary winding.
Linear Variable Differential Transformer
A linear variable differential transformer (LVDT) has an AC driven primary wound between two secondaries on a cylindrical air core form.
The LVDT is a displacement or distance measuring transducer.
A ferrite core is suitable at these frequencies.
A three-phase transformer is made of three sets of primary and secondary windings, each set wound around one leg of an iron core assembly. Essentially it looks like three single-phase transformers sharing a joined core as in Figure
Phase shift
the winding configurations are not of the same type. In other words, a transformer connected either Y- Δ or Δ -Y will exhibit this 30O phase shift, while a transformer connected Y - Y or Δ - Δ will not.