Transformer, Rectifiers & Inverters
A transformer changes electrical energy of a given voltage into electrical energy at a different voltage level. It consists of two coils that are not electrically connected, but are arranged so that the magnetic field surrounding one coil cuts through the other coil. When an alternating voltage is applied to (across) one coil, the varying magnetic field set up around that coil creates alternating voltage in the other coil by mutual induction.
NOTE : A transformer can also be used with pulsating DC, but a pure DC voltage cannot be used, since only a varying voltage creates the varying magnetic field that is the basis of the mutual induction process.
A transformer consists of three basic parts :
An Iron Core which provides a circuit of low reluctance for magnetic lines of force,
A Primary Winding which receives the electrical energy from the source of applied voltage, and
A Secondary Winding which receives electrical energy by induction from the primary coil.
The primary and secondary of this closed core transformer are wound on a closed core to obtain maximum inductive effect between the two coils.
All the magnetic lines of force set up in the primary do not cut across the turns of the secondary coil. A certain amount of the magnetic flux, called leakage flux, leaks out of the magnetic circuit.
The measure of how well the flux of the primary is coupled into the secondary is called the “coefficient of coupling.”
There are two classes of transformers:
Voltage Transformers used for stepping up or stepping down voltages
Current Transformers used in instrument circuits.
In voltage transformers, the primary coils are connected in parallel across the supply voltage.
The primary windings of current transformers are connected in series in the primary circuit
There are many types of voltage transformers. Most of these are either
The factor that determines whether a transformer is a step-up or step-down type is the “turns” ratio.
The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding.
Where E is the voltage of the primary, E2 is the output voltage of the secondary, and N1 and N2 are the number of turns of the primary and secondary, respectively.
Step Up Transformer
The Step up transformer has more number of turns in the secondary winding than the primary winding, hence it increase or step's up the voltage. So the voltage in the secondary winding is more than the primary winding.
If the transformer losses are considered to be zero the power across the transformer is said to be constant. Hence the input power is equal to the output power. Hence as we have seen the voltage increase in the secondary winding so the current decreases in the secondary winding with respect to the primary winding keeping the power constant. The amount of voltage increase in the secondary winding depends on the Turns ratio.
Step Down Transformer
The Step down transformer has more number of turns in the primary winding than the secondary winding, hence it increase or step's down the voltage. So the voltage in the secondary winding is less than the primary winding.
If the transformer losses are considered to be zero the power across the transformer is said to be constant. Hence the input power is equal to the output power. Hence as we have seen the voltage decrease in the secondary winding so the current increases in the secondary winding with respect to the primary winding keeping the power constant. The amount of voltage decrease in the secondary winding depends on the Turns ratio.
NOTE : The frequency of the of the input and the output remains constant in both Step up or Step Down Transformer.
STEP UP & STEP DOWN TRANSFORMER
Transformers are devices that convert (or transfer) electrical energy from one circuit to another through inductively coupled electrical conductors. The transformer used as a power supply source can be considered as having an input (the primary conductors, or windings) and output (the secondary conductors, or windings).
A changing current in the primary windings creates a changing magnetic field; this magnetic field induces a changing voltage in the secondary windings. By connecting a load in series with the secondary windings, current flows in the transformer. The output voltage of the transformer (secondary windings) is determined by the input voltage on the primary and ratio of turns on the primary and secondary windings.
When an AC voltage is connected across the primary terminals of a transformer, an alternating current will flow and self induce a voltage in the primary coil that is opposite and nearly equal to the applied voltage. The difference between these two voltages allows just enough current in the primary to magnetize its core. This is called the exciting, or magnetizing, current. The magnetic field caused by this exciting current cuts across the secondary coil and induces a voltage by mutual induction.
If a load is connected across the secondary coil, the load current flowing through the secondary coil will produce a magnetic field which will tend to neutralize the magnetic field produced by the primary current.
This will reduce the self-induced (opposition) voltage in the primary coil and allow more primary current to flow. The primary current increases as the secondary load current increases, and decreases as the secondary load current decreases. When the secondary load is removed, the primary current is again reduced to the small exciting current sufficient only to magnetize the iron core of the transformer.
Types of Transformer
The transformer are also classified according to the type of core construction
Both these types of transformers can be either step up or step they only differ in the construction of the core of the transformer.
The magnetic core of the transformer is made up of laminations to form a rectangular frame. The laminations are cut in the form of L-shape strips. For avoiding the high reluctance at the joints where laminations are butted against each other, the alternate layer is stacked differently to eliminate continues joints.
The primary and secondary windings are interleaved to reduce the leakage flux. Half of each winding is placed side by side or concentrically on the leg of the core. For simplicity, the primary and secondary winding is located on the separate limbs of the core.
The insulation layer is provided between the core and lower winding and between the primary and the secondary winding. For reducing the insulation, the low winding is always placed near to the core. The winding is cylindrical, and the lamination is inserted later on it.
The laminations are cut in the form of a long strip of E’s, and I’s. To reduce the high reluctance at the joints where the lamination are butted against each other, the alternate layers are stacked differently to eliminate continuous joint.
The shell type transformer has three limbs or legs. The central limb carries the whole of the flux, and the side limb carries the half of the flux. Hence the width of the central limb is about to double to that of the outer limbs.
The primary and secondary both the windings are placed on the central limbs. The low voltage winding is placed near the core, and the high voltage winding is placed outside the low voltage winding to reducing the cost of insulation placed between the core and the low voltage winding. The windings are cylindrical, and the core laminations are inserted on it.
In core type transformer the core surrounds the windings whereas in shell type transformer the winding surrounds the core of the transformer. The core type transformer has two magnetic circuits whereas the shell type transformer has one magnetic circuit. Hence the losses in a core type transformer are more as compared to shell type transformer because the core type transformer consists two magnetic circuits, and output of core type is less then the shell type.
NOTE : In both Core & Shell type Transformer the low voltage winding is placed inside the high voltage one as the low voltage winding is easy to insulate as compared to the high voltage one.
CORE TYPE & SHELL TYPE CORE CONSTRUCTION
Current transformers are used in AC power supply systems to sense generator line current and to provide a current, proportional to the line current, for circuit protection and control devices. The current transformer is a ring-type transformer using a current carrying power lead as a primary (either the power lead or the ground lead of the AC generator). The current in the primary induces a current in the secondary by magnetic induction.
The sides of all current transformers are marked “H1” and “H2” on the unit base. The transformers must be installed with the “H1” side toward the generator in the circuit in order to have proper polarity.
The Current transformers are step up transformers as they should be able to detect large current in the primary, since they are step up transformer they reduce the current in the secondary winding. So we can measure high amount of current , however the secondary of the transformer should never be left open while the system is being operated; to do so could cause dangerously high voltages,since they are step up transformers, and could overheat the transformer. Therefore, the transformer output connections should always be connected with a jumper when the transformer is not being used but is left in the system.
In circuit applications normally requiring only a small step up or step-down of voltage, a special variant of transformer design is employed and this is known as an auto-transformer. It consists of a single winding tapped to form primary and secondary parts.
When a voltage is applied to the primary terminals current flow through the portion of the winding spanned by the terminals. The magnetic flux due to this current will flow through the core and will therefore, link with the whole of the winding. Those turns between the primary terminals act in the same way as the primary winding of a conventional transformer, and so they produce a self-induction voltage in opposition to the applied voltage.
The voltage induced in the remaining turns of the winding will be additive, thereby giving a secondary output voltage greater than the applied voltage. When a load circuit is connected to the secondary terminals, a current due to the induced voltage will flow through the whole winding and will be in opposition to the primary current from the input terminals .
Since the turns between the primary terminals are common to input and output circuits alike they carry the difference between the induced current and primary current and they may therefore be wound with smaller gauge wire than the remainder of the winding
Auto-transformers may also be designed for use in consumer circuits requiring three phase voltage at varying levels.
In addition to the power loss caused by imperfect coupling, transformers are subject to “copper” and “iron” losses.
The resistance of the conductor comprising the turns of the coil causes copper loss.
The iron losses are of two types called hysteresis loss and eddy current loss.
Hysteresis loss is the electrical energy required to magnetize the transformer core, first in one direction and then in the other, in step with the applied alternating voltage.
Eddy current loss is caused by electric currents (eddy currents) induced in the transformer core by the varying magnetic fields. To reduce eddy current losses, cores are made of laminations coated with an insulation, which reduces the circulation of induced currents
LOSSES IN A TRANSFORMER
Transformer Rectifier Unit (TRU)
Transformer Rectifier Units (T.RU.'s) are combinations of static transformers and rectifiers , and are utilized in some a.c. systems as secondary supply units, and also as the main conversion units in aircraft having rectified a.c. power systems.
Aircraft T.R.U. are designed to operate on a regulated three phase input of 200 volts at a frequency of 400 Hz and to provide a continuous d.c. output of 110 A at approximately 26 volts.
The unit consists of a transformer and two three-phase bridge rectifier assemblies mounted in separate sections of the casing. The transformer has a conventional star wound primary winding and secondary windings wound in star and delta. Each secondary winding is connected to individual bridge rectifier assemblies made up of six silicon diodes, and connected in parallel.
These terminals, together with all others associated with input and output circuits, are grouped on a panel at one end of the unit. Cooling of the unit is by natural convection through gauze-covered ventilation panels and in order to give warning of overheating conditions, thermal switches are provided at the transformer and rectifier assemblies, and are connected to independent warning lights.
The switches are supplied with d.c. from an external source (normally one of the busbars) and their contacts close when temperature conditions at their respective locations rise to approximately 150°C and 200°C.
TRANSFORMER RECTIFER UNIT (TRU)
Many devices in an aircraft require high amperage, low voltage DC for operation. This power may be furnished by DC engine driven generators, motor generator sets, vacuum tube rectifiers, or dry disk or solid-state rectifiers. However, in aircraft with AC systems, a special DC generator is not desirable since it would be necessary for the engine accessory section to drive an additional piece of equipment. Hence rectifiers are used.
A rectifier is a device that transforms alternating current into direct current by limiting or regulating the direction of current flow.
The principal types of rectifiers are dry disk and solid state. Dry disk and solid-state rectifiers, are an excellent source of high amperage at low voltage.
Solid-state, or semiconductor, rectifiers have replaced virtually all other types; and, since dry disk and motor generators are largely limited to older model aircraft.
Half Wave Rectifier
The basic concept of a half wave rectifier. When an AC signal is on a positive swing as shown in illustration A of the input signal, the polarities across the diode and the load resistor will also be positive. In this case, the diode is forward biased and can be replaced with a short circuit as shown in the illustration. The positive portion of the input signal will then appear across the load resistor with no loss in potential across the series diode.
Illustration B now shows the input signal being reversed. Note that the polarities across the diode and the load resistor are also reversed. In this case, the diode is now reverse biased and can be replaced with an equivalent open circuit. The current in the circuit is now 0 amperes and the voltage drop over the load resistor is 0 volts. The resulting waveform for a complete sinusoidal input can be seen at the far right of Figure. The output waveform is a reproduction of the input waveform minus the negative voltage swing of the wave. For this reason, this type of rectifier is called a half-wave rectifier.
HALF WAVE RECTIFIER
Full Wave Rectifier
More common use of the diode as a rectifier. This type of a rectifier is called a full-wave bridge rectifier. The term “full-wave” indicates that the output is a continuous sequence of pulses rather than having gaps that appear in the half wave rectifier.
Illustration C shows the initial condition, during which, a positive portion of the input signal is applied to the network. Note the polarities across the diodes. Diodes D2 and D4 are reverse biased and can be replaced with an open circuit. Diodes D1 and D3 are forward biased and act as an open circuit. The current path through the diodes is clear to see, and the resulting waveform is developed across the load resistor.
During the negative portion of the applied signal, the diodes will reverse their polarity and bias states. The result is a network shown in illustration D. Current now passes through diodes D4 and D2, which are forward biased, while diodes D1 and D3 are essentially open circuits due to being reverse biased. Note that during both alternations of the input waveform, the current will pass through the load resistor in the same direction. This results in the negative swing of the waveform being flipped up to the positive side of the time line.
FULL WAVE RECTIFIER
Silicon- Controlled Rectifier / Thyristors
Silicon-controlled rectifiers (or thyristors) are three terminal devices which can be used for switching and AC power control. Silicon-controlled rectifiers can switch very rapidly from a conducting to a non-conducting state. In the off state, the silicon-controlled rectifier exhibits negligible leakage current, while in the on state the device exhibits very low resistance. This results in very little power loss within the silicon controlled rectifier even when appreciable power levels are being controlled.
Silicon-controlled rectifiers have anode and cathode connections; control is applied by means of a gate terminal.
SILICON CONTROLLED RECTIFIER (SCR) OR THYRISTOR
The Silicon Control Rectifier can be considered as two transistor connected together at their common regions. i.e PNP and NPN connected together as shown in the figure. By joining we have three junction J1, J2 and J3. and two terminals.
When terminal 1 is supplied with positive voltage the J1 and J3 junction are in forwards bias and J2 in reverse bias, whereas when terminal 1 is supplied with negative terminal the junction J1 and J3 are in Reverse bias and the junction J2 is forwards bias.
Once switched into the conducting state, the silicon controlled rectifier will remain conducting (i.e. it is latched in the on state) until the forward current is removed from the device. In DC applications this necessitates the interruption (or disconnection) of the supply before the device can be reset into its non-conducting state.
Where the device is used with an alternating supply, the device will automatically become reset whenever the main supply reverses. The device can then be triggered on the next half-cycle having correct polarity to permit conduction.
In normal use, a silicon-controlled rectifier is triggered into the conducting (on) state by means of the application of a current pulse to the gate terminal. The effective triggering of a silicon controlled rectifier requires a gate trigger pulse having a fast rise time derived from a low-resistance source. Triggering can become erratic when insufficient gate current is available or when the gate current changes slowly.
SCR VS DIODE VS TRANSISTOR
Dry Disk Rectifiers
Dry disk rectifiers operate on the principle that electric current flows through a junction of two dissimilar conducting materials more readily in one direction than it does in the opposite direction. This is true because the resistance to current flow in one direction is low, while in the other direction it is high. Depending on the materials used, several amperes may flow in the direction of low resistance but only a few milliamperes in the direction of high resistance.
Three types of dry disk rectifiers may be found in aircraft:
the copper oxide rectifier,
the selenium rectifier,
magnesium copper-sulfide rectifier.
Copper Oxide Rectifiers
The copper oxide rectifier consists of a copper disk upon which a layer of copper oxide has been formed by heating. It may also consist of a chemical copper oxide preparation spread evenly over the copper surface. Metal plates, usually lead plates, are pressed against the two opposite faces of the disk to form a good contact. Current flow is from the copper to the copper oxide.
COPPER OXIDE DRY DISK RECTIFIER
The selenium rectifier is formed on an aluminium sheet which serves both as a base for the rectifying junction and as a surface for the dissipation of heat.
The rectifying junction covers one side of the base with the exception of a narrow strip at the edges and a small area around the fixing hole which is sprayed with a layer of insulating varnish. A thin layer of a low- melting point alloy, referred to as the counter electrode, is sprayed over the selenium coating and insulating varnish. Contact with the two elements of the rectifying junction, or barrier layer, is made through the base on one side and the counter electrode on the other.
Mechanical pressure on the rectifying junction lends to lower the resistance in the reverse direction and this is prevented in the region of the mounting studs by the layer of varnish.
In practice a number of rectifying elements may be connected in series or parallel to form what is generally referred to as a rectifier stack.
When connected in series the elements increase the voltage handling ability of a rectifier and when connected In parallel the ampere capacity is increased.
An inverter is used in some aircraft systems to convert a portion of the aircraft’s DC power to AC. This AC is used mainly for instruments, radio, radar, lighting, and other accessories. These inverters are usually built to supply current at a frequency of 400 cps, but some are designed to provide more than one voltage; for example, 26 volt AC in one winding and 115 volts in another. There are two basic types of inverters: the rotary and the static. Either type can be single phase or multiphase.
The multiphase inverter is lighter for the same power rating than the single phase, but there are complications in distributing multiphase power and in keeping the loads balanced
There are many sizes, types, and configurations of rotary inverters. Such inverters are essentially AC generators and DC motors in one housing. The generator field, or armature, and the motor field, or armature, are mounted on a common shaft that will rotate within the housing.
Permanent Magnet Rotary Inverter
A permanent magnet inverter is composed of a DC motor and a permanent magnet AC generator assembly. Each has a separate stator mounted within a common housing. The motor armature is mounted on a rotor and connected to the DC supply through a commutator and brush assembly. The motor field windings are mounted on the housing and connected directly to the DC supply. A permanent magnet rotor is mounted at the opposite end of the same shaft as the motor armature, and the stator windings are mounted on the housing, allowing AC to be taken from the inverter without the use of brushes.
The generator rotor has six poles, magnetized to provide alternate north and south poles about its circumference. When the motor field and armature are excited, the rotor will begin to turn. As the rotor turns, the permanent magnet will rotate within the AC stator coils, and the magnetic flux developed by the permanent magnets will be cut by the conductors in the AC stator coils. An AC voltage will be produced in the windings whose polarity will change as each pole passes the windings. This type inverter may be made multiphase by placing more AC stator coils in the housing in order to shift the phase the proper amount in each coil. As the name of the rotary inverter indicates, it has a revolving armature in the AC generator section.
Inductor Type Rotary Inverter
Inductor-type inverters use a rotor made of soft iron laminations with grooves cut laterally across the surface to provide poles that correspond to the number of stator poles. The field coils are wound on one set of stationary poles and the AC armature coils on the other set of stationary poles.
When DC is applied to the field coils, a magnetic field is produced. The rotor turns within the field coils and, the poles on the rotor align with the stationary poles, a low reluctance path for flux is established from the field pole through the rotor poles to the AC armature pole and through the housing back to the field pole. In this circumstance, there will be a large amount of magnetic flux linking the AC coils.
When the rotor poles are between the stationary poles, there is a high reluctance path for flux, consisting mainly of air; then, there will be a small amount of magnetic flux linking the AC coils. This increase and decrease in flux density in the stator induces an alternating current in the AC coils. The number of poles and the speed of the motor determine the frequency of this type of inverter. The DC stator field current controls the voltage.
In many applications where continuous DC voltage must be converted to alternating voltage, static inverters are used in place of rotary inverters or motor generator sets. The rapid progress made by the semiconductor industry is extending the range of applications of such equipment into voltage and power ranges that would have been impractical a few years ago. Some such applications are power supplies for frequency sensitive military and commercial AC equipment, aircraft emergency AC systems, and conversion of wide frequency range power to precise frequency power.
The use of static inverters in small aircraft also has increased rapidly in the last few years, and the technology has advanced to the point that static inverters are available for any requirement filled by rotary inverters.
For example, 250 VA emergency AC supplies operated from aircraft batteries are in production, as are 2,500 VA main AC supplies operated from a varying frequency generator supply. This type of equipment has certain advantages for aircraft applications, particularly the absence of moving parts and the adaptability to conduction cooling. AC output Field winding DC Flux lines.
Static inverters, referred to as solid-state inverters, are manufactured in a wide range of types and models, which can be classified by the shape of the AC output waveform and the power output capabilities. One of the most commonly used static inverters produces a regulated sine wave output.