A variety of types of electrical transformer
are made for different purposes. Despite their
design differences, the various types employ
the same basic principle as discovered in
1831 by Michael Faraday, and share several
key functional parts.
Power transformers
Laminated core
This is the most common type of transformer,
widely used in electric power transmission
and appliances to convert mains voltage to
low voltage to power electronic devices. They
are available in power ratings ranging from
mW to MW. The insulated laminations minimizes
eddy current losses in the iron core.
Small appliance and electronic transformers
may use a split bobbin, giving a high level
of insulation between the windings. The rectangular
cores are made up of stampings, often in E-I
shape pairs, but other shapes are sometimes
used. Shields between primary and secondary
may be fitted to reduce EMI, or a screen winding
is occasionally used.
Small appliance and electronics transformers
may have a thermal cut out built into the
winding.
Toroidal
Doughnut shaped toroidal transformers are
used to save space compared to EI cores, and
sometimes to reduce external magnetic field.
These use a ring shaped core, copper windings
wrapped round this ring, and tape for insulation.
Toroidal transformers have a lower external
magnetic field compared to rectangular transformers,
and can be smaller for a given power rating.
However, they cost more to make, as winding
requires more complex and slower equipment.
They can be mounted by a bolt through the
center, using washers and rubber pads or by
potting in resin.
Autotransformer
An autotransformer has one winding which is
tapped at some point along the winding. Voltage
is applied across a portion of the winding,
and a higher voltage is produced across another
portion of the same winding. The equivalent
power rating of the autotransfomer is lower
than the actual load power rating. It is calculated
by:/Vin x load VA. For example, an auto transformer
used to adapt a 1000 VA load rated at 120
volts to a 240 volt supply has an equivalent
rating of at least:/240V x 1,000VA = 500VA.
However, the actual rating would have to be
at least 1000 VA.
For voltage ratios not exceeding about 3:1,an
autotransformer is cheaper, lighter, smaller
and more efficient than an isolating transformer
of the same rating. Large three-phase autotransformers
are used in electric power distribution systems,
for example, to interconnect 33 kV and 66
kV sub-transmission networks.
Variable autotransformer
By exposing part of the winding coils of an
autotransformer, and making the secondary
connection through a sliding carbon brush,
an autotransformer with a near-continuously
variable turns ratio can be obtained, allowing
for wide voltage adjustment in very small
increments.
Induction regulator
The induction regulator is similar in design
to a wound-rotor induction motor but it is
essentially a transformer whose output voltage
is varied by rotating its secondary relative
to the primary i.e. rotating the angular position
of the rotor. It can be seen as a power transformer
exploiting rotating magnetic fields. The major
advantage of the induction regulator is that
unlike variacs, they are practical for transformers
over 5 kVA. Hence, such regulators find windspread
use in high-voltage laboratories.
Polyphase transformer
For polyphase systems, multiple single-phase
transformers can be used, or all phases can
be connected to a single polyphase transformer.
For a three phase transformer, the three primary
windings are connected together and the three
secondary windings are connected together.
Examples of connections are wye-delta, delta-wye,
delta-delta and wye-wye. A vector group indicates
the configuration of the windings and the
phase angle difference between them. If a
winding is connected to earth, the earth connection
point is usually the center point of a wye
winding. If the secondary is a delta winding,
the ground may be connected to a center tap
on one winding or one phase may be grounded.
A special purpose polyphase transformer is
the zigzag transformer. There are many possible
configurations that may involve more or fewer
than six windings and various tap connections.
Grounding transformer
Grounding transformers are used to allow three
wire polyphase system supplies to accommodate
phase to neutral loads by providing a return
path for current to a neutral. Grounding transformers
most commonly incorporate a single winding
transformer with a zigzag winding configuration
but may also be created with a wye-delta isolated
winding transformer connection.
Leakage transformers
A leakage transformer, also called a stray-field
transformer, has a significantly higher leakage
inductance than other transformers, sometimes
increased by a magnetic bypass or shunt in
its core between primary and secondary, which
is sometimes adjustable with a set screw.
This provides a transformer with an inherent
current limitation due to the loose coupling
between its primary and the secondary windings.
The output and input currents are low enough
to prevent thermal overload under all load
conditions—even if the secondary is shorted.
Uses
Leakage transformers are used for arc welding
and high voltage discharge lamps. It acts
then both as a voltage transformer and as
a magnetic ballast.
Other applications are short-circuit-proof
extra-low voltage transformers for toys or
doorbell installations.
Resonant transformer
A resonant transformer is a transformer in
which one or both windings has a capacitor
across it and functions as a tuned circuit.
Used at radio frequencies, resonant transformers
can function as high Q_factor bandpass filters.
The transformer windings have either air or
ferrite cores and the bandwidth can be adjusted
by varying the coupling. One common form is
the IF transformer, used in superheterodyne
radio receivers. They are also used in radio
transmitters.
Resonant transformers are also used in electronic
ballasts for fluorescent lamps, and high voltage
power supplies. They are also used in some
types of switching power supplies. Here often
only one winding has a capacitor and acts
as a tank circuit. The transformer is driven
by a pulse or square wave for efficiency,
generated by an electronic oscillator circuit.
Each pulse serves to drive resonant sinusoidal
oscillations in the tuned winding, and due
to resonance a high voltage can be developed
across the secondary.
Applications:
Intermediate frequency transformer in superheterodyne
radio receiver
Tank transformers in radio transmitters
Tesla coil
Oudin coil
D'Arsonval apparatus
Ignition coil or induction coil used in the
ignition system of a petrol engine
Electrical breakdown and insulation testing
of high voltage equipment and cables. In the
latter case, the transformer's secondary is
resonated with the cable's capacitance.
Constant voltage transformer
By arranging particular magnetic properties
of a transformer core, and installing a ferro-resonant
tank circuit, a transformer can be arranged
to automatically keep the secondary winding
voltage relatively constant for varying primary
supply without additional circuitry or manual
adjustment. Ferro-resonant transformers run
hotter than standard power transformers, because
regulating action depends on core saturation,
which reduces efficiency. The output waveform
is heavily distorted unless careful measures
are taken to prevent this. Saturating transformers
provide a simple rugged method to stabilize
an AC power supply.
Ferrite core
Ferrite core power transformers are widely
used in switched-mode power supplies. The
powder core enables high-frequency operation,
and hence much smaller size-to-power ratio
than laminated-iron transformers.
Ferrite transformers are not used as power
transformers at mains frequency since laminated
iron cores cost less than an equivalent ferrite
core.
Planar transformer
Manufacturers etch spiral patterns on a printed
circuit board to form the "windings" of a
planar transformer, replacing the turns of
wire used to make other types. Some planar
transformers are commercially sold as discrete
components. Other planar transformers are
one of many components on a printed circuit
board. A planar transformer can be thinner
than other transformers, which is useful for
low-profile applications or when several printed
circuit boards are stacked. Almost all planar
transformers use a ferrite planar core.
Oil cooled transformer
For large transformers used in power distribution
or electrical substations, the core and coils
of the transformer are immersed in oil which
cools and insulates. Oil circulates through
ducts in the coil and around the coil and
core assembly, moved by convection. The oil
is cooled by the outside of the tank in small
ratings, and in larger ratings an air-cooled
radiator is used. Where a higher rating is
required, or where the transformer is used
in a building or underground, oil pumps are
used to circulate the oil and an oil-to-water
heat exchanger may also be used. Some transformers
may contain PCBs where or when its use was
permitted. For example, until 1979 in South
Africa. substitute fire-resistant liquids
such as silicone oils are now used instead.
Cast resin transformer
Cast-resin power transformers encase the windings
in epoxy resin. These transformers simplify
installation since they are dry, without cooling
oil, and so require no fire-proof vault for
indoor installations. The epoxy protects the
windings from dust and corrosive atmospheres.
However, because the molds for casting the
coils are only available in fixed sizes, the
design of the transformers is less flexible,
which may make them more costly if customized
features are required.
Isolating transformer
An isolation transformer links two circuits
magnetically, but provides no metallic conductive
path between the circuits. An example application
would be in the power supply for medical equipment,
when it is necessary to prevent any leakage
from the AC power system into devices connected
to a patient. Special purpose isolation transformers
may include shielding to prevent coupling
of electromagnetic noise between circuits,
or may have reinforced insulation to withstand
thousands of volts of potential difference
between primary and secondary circuits.
Instrument transformer
Instrument transformers are typically used
to operate instruments from high voltage lines
or high current circuits, safely isolating
measurement and control circuitry from the
high voltages or currents. The primary winding
of the transformer is connected to the high
voltage or high current circuit, and the meter
or relay is connected to the secondary circuit.
Instrument transformers may also be used as
an isolation transformer so that secondary
quantities may be used without affecting the
primary circuitry.
Terminal identifications indicate one end
of each winding, indicating the same instantaneous
polarity and phase between windings. This
applies to both types of instrument transformers.
Correct identification of terminals and wiring
is essential for proper operation of metering
and protective relay instrumentation.
Current transformer
A current transformer is a series connected
measurement device designed to provide a current
in its secondary coil proportional to the
current flowing in its primary. Current transformers
are commonly used in metering and protective
relays in the electrical power industry.
Current transformers are often constructed
by passing a single primary turn through a
well-insulated toroidal core wrapped with
many turns of wire. The CT is typically described
by its current ratio from primary to secondary.
For example, a 1000:1 CT would provide an
output current of 1 amperes when 1000 amperes
were passing through the primary winding.
Standard secondary current ratings are 5 amperes
or 1 ampere, compatible with standard measuring
instruments. The secondary winding can be
single ratio or have several tap points to
provide a range of ratios. Care must be taken
that the secondary winding is not disconnected
from its low-impedance load while current
flows in the primary, as this may produce
a dangerously high voltage across the open
secondary and may permanently affect the accuracy
of the transformer.
Specially constructed wideband CTs are also
used, usually with an oscilloscope, to measure
high frequency waveforms or pulsed currents
within pulsed power systems. One type provides
a voltage output that is proportional to the
measured current. Another, called a Rogowski
coil, requires an external integrator in order
to provide a proportional output.
A current clamp uses a current transformer
with a split core that can be easily wrapped
around a conductor in a circuit. This is a
common method used in portable current measuring
instruments but permanent installations use
more economical types of current transformer.
Potential transformer
Voltage transformers) are a parallel connected
type of instrument transformer, used for metering
and protection in high-voltage circuits or
phasor phase shift isolation. They are designed
to present negligible load to the supply being
measured and to have an accurate voltage ratio
to enable accurate metering. A potential transformer
may have several secondary windings on the
same core as a primary winding, for use in
different metering or protection circuits.
The primary may be connected phase to ground
or phase to phase. The secondary is usually
grounded on one terminal.
There are three primary types of voltage transformers(VT):
electromagnetic, capacitor, and optical. The
electromagnetic voltage transformer is a wire-wound
transformer. The capacitor voltage transformer
uses a capacitance potential divider and is
used at higher voltages due to a lower cost
than an electromagnetic VT. An optical voltage
transformer exploits the electrical properties
of optical materials. measurement of high
voltages is possible by the potential transformers.
〈〉
Combined instrument transformer
A combined instrument transformer encloses
a current transformer and a voltage transformer
in the same transformer. There are two main
combined current and voltage transformer designs:
oil-paper insulated and SF6 insulated. One
advantage of applying this solution is reduced
substation footprint, due to reduced number
of transformers in a bay, supporting structures
and connections as well as lower costs for
civil works, transportation and installation.
Pulse transformer
A pulse transformer is a transformer that
is optimised for transmitting rectangular
electrical pulses. Small versions called signal
types are used in digital logic and telecommunications
circuits, often for matching logic drivers
to transmission lines. Medium-sized power
versions are used in power-control circuits
such as camera flash controllers. Larger power
versions are used in the electrical power
distribution industry to interface low-voltage
control circuitry to the high-voltage gates
of power semiconductors. Special high voltage
pulse transformers are also used to generate
high power pulses for radar, particle accelerators,
or other high energy pulsed power applications.
To minimize distortion of the pulse shape,
a pulse transformer needs to have low values
of leakage inductance and distributed capacitance,
and a high open-circuit inductance. In power-type
pulse transformers, a low coupling capacitance
is important to protect the circuitry on the
primary side from high-powered transients
created by the load. For the same reason,
high insulation resistance and high breakdown
voltage are required. A good transient response
is necessary to maintain the rectangular pulse
shape at the secondary, because a pulse with
slow edges would create switching losses in
the power semiconductors.
The product of the peak pulse voltage and
the duration of the pulse is often used to
characterise pulse transformers. Generally
speaking, the larger this product, the larger
and more expensive the transformer.
Pulse transformers by definition have a duty
cycle of less than 0.5; whatever energy stored
in the coil during the pulse must be "dumped"
out before the pulse is fired again.
RF transformer
There are several types of transformer used
in radio frequency work. Steel laminations
are not suitable for RF.
Air-core transformer
These are used for high frequency work. The
lack of a core means very low inductance.
Such transformers may be nothing more than
a few turns of wire soldered onto a printed
circuit board.
Ferrite-core transformer
Ferrite-core transformers are widely used
in stages in superheterodyne radio receivers.
They are mostly tuned transformers, containing
a threaded ferrite slug that is screwed in
or out to adjust IF tuning. The transformers
are usually canned for stability and to reduce
interference.
Transmission-line transformer
For radio frequency use, transformers are
sometimes made from configurations of transmission
line, sometimes bifilar or coaxial cable,
wound around ferrite or other types of core.
This style of transformer gives an extremely
wide bandwidth but only a limited number of
ratios can be achieved with this technique.
The core material increases the inductance
dramatically, thereby raising its Q factor.
The cores of such transformers help improve
performance at the lower frequency end of
the band. RF transformers sometimes used a
third coil to inject feedback into an earlier
stage in antique regenerative radio receivers.
In RF and microwave systems, a quarter-wave
impedance transformer provides a way of matching
impedances between circuits over a limited
range of frequencies, using only a length
of transmission line. The line may be coaxial
cable, waveguide, stripline or microstripline.
Balun
Baluns are transformers designed specifically
to connect between balanced and unbalanced
circuits. These are sometimes made from configurations
of transmission line and sometimes bifilar
or coaxial cable and are similar to transmission
line transformers in construction and operation.
Audio transformer
Audio transformers are those specifically
designed for use in audio circuits to carry
audio signal. They can be used to block radio
frequency interference or the DC component
of an audio signal, to split or combine audio
signals, or to provide impedance matching
between high and low impedance circuits, such
as between a high impedance tube amplifier
output and a low impedance loudspeaker, or
between a high impedance instrument output
and the low impedance input of a mixing console.
Audio transformers that operate with loudspeaker
voltages and current are larger than those
which operate at microphone or line level,
carrying much less power.
Being magnetic devices, audio transformers
are susceptible to external magnetic fields
such as those generated by AC current-carrying
conductors. "Hum" is a term commonly used
to describe unwanted signals originating from
the "mains" power supply. Audio transformers
used for low-level signals, such as those
from microphones, often include magnetic shielding
to protect against extraneous magnetically
coupled signals.
Audio transformers were originally designed
to connect different telephone systems to
one another while keeping their respective
power supplies isolated, and are still commonly
used to interconnect professional audio systems
or system components, to eliminate buzz and
hum. Such transformers typically have a 1:1
ratio between the primary and the secondary.
These can also be used for splitting signals,
balancing unbalanced signals, or feeding a
balanced signal to unbalanced equipment. Transformers
are also used in DI boxes to convert high-impedance
instrument signals to low impedance signals
to enable them to be connected to a microphone
input on the mixing console.
A particularly critical component is the output
transformer of a valve amplifier. Valve circuits
for quality reproduction have long been produced
with no other audio transformers, but an output
transformer is needed to couple the relatively
high impedance of the output valve(s) to the
low impedance of a loudspeaker. Most solid-state
power amplifiers need no output transformer
at all.
Audio transformers affect the sound quality
because they are non-linear. Harmonic distortion
is added to the original signal, especially
odd-order harmonics with an emphasis on third-order
harmonics. When the incoming signal amplitude
is very low there is not enough level to energize
the magnetic core. When the incoming signal
amplitude is very high the transformer saturates
and adds ringing harmonics. Another non-linearity
comes from limited frequency response. For
good low-frequency response a relatively large
magnetic core is required; high power handling
increases the required core size. Good high-frequency
response requires carefully designed and implemented
windings without excessive leakage inductance
or stray capacitance. All this makes for an
expensive component.
Early transistor audio power amplifiers often
had output transformers, but they were eliminated
as advances in semiconductors allowed the
design of amplifiers with sufficiently low
output impedance to drive a loudspeaker directly.
Loudspeaker transformer
In the same way that transformers are used
to create high voltage power transmission
circuits that minimize transmission losses,
loudspeaker transformers can be used to allow
many individual loudspeakers to be powered
from a single audio circuit operated at higher-than
normal loudspeaker voltages. This application
is common in public address applications.
Such circuits are commonly referred to as
constant voltage speaker systems. Such systems
are also known by the nominal voltage of the
loudspeaker line, such as 25-, 70- and 100-volt
speaker systems. A transformer steps up the
output of the system's amplifer to the distribution
voltage. At the distant loudspeaker locations,
a step-down transformer matches the speaker
to the rated voltage of the line, so the speaker
produces rated nominal output when the line
is at nominal voltage. The loudspeaker transformers
commonly have multiple primary taps, allowing
the volume at each speaker to be adjusted
in steps.
Output transformer
Valve amplifiers almost always use an output
transformer to match the high load impedance
requirement of the valves to a low impedance
speaker.
Small signal transformer
Moving coil phonograph cartridges produce
a very small voltage. In order for this to
be amplified with a reasonable signal-noise
ratio, a transformer is usually used to convert
the voltage to the range of the more common
moving-magnet cartridges.
Microphones may also be matched to their load
with a small transformer, which is mumetal
shielded to minimise noise pickup. These transformers
are less widely used today, as transistorized
buffers are now cheaper.
Interstage and coupling transformers
In a push-pull amplifier, an inverted signal
is required and is obtained from a transformer
with a center-tapped winding, used to drive
two active devices in opposite phase. These
phase splitting transformers are not much
used today.
Other types
Hedgehog
Hedgehog transformers are occasionally encountered
in homemade 1920s radios. They are homemade
audio interstage coupling transformers.
Enamelled copper wire is wound round the central
half of the length of a bundle of insulated
iron wire, to make the windings. The ends
of the iron wires are then bent around the
electrical winding to complete the magnetic
circuit, and the whole is wrapped with tape
or string to hold it together.
Variometer and variocoupler
A variometer is a type of continuously variable
air-core RF inductor with two windings. One
common form consisted of a coil wound on a
short hollow cylindrical form, with a second
smaller coil inside, mounted on a shaft so
its magnetic axis can be rotated with respect
to the outer coil. The two coils are connected
in series. When the two coils are collinear,
with their magnetic fields pointed in the
same direction, the two magnetic fields add,
and the inductance is maximum. If the inner
coil is rotated so its axis is at an angle
to the outer coil, the magnetic fields do
not add and the inductance is less. If the
inner coil is rotated so it is collinear with
the outer coil but their magnetic fields point
in opposite directions, the fields will cancel
each other out and the inductance will be
very small or zero. The advantage of the variometer
is that the inductance can be adjusted continuously,
over a wide range. Variometers were widely
used in 1920s radio receivers. One of their
main uses today is as antenna matching coils
to match longwave radio transmitters to their
antennas.
The vario-coupler was a device with similar
construction, but the two coils were not connected
but attached to separate circuits. So it functioned
as an air-core RF transformer with variable
coupling. The inner coil could be rotated
from 0° to 90° angle with the outer, reducing
the mutual inductance from maximum to near
zero.
The pancake coil variometer was another common
construction used in both 1920s receivers
and transmitters. It consists of two flat
spiral coils suspended vertically facing each
other, hinged at one side so one could swing
away from the other to an angle of 90° to
reduce the coupling. The flat spiral design
served to reduce parasitic capacitance and
losses at radio frequencies.
Pancake or "honeycomb" coil vario-couplers
were used in the 1920s in the common Armstrong
or "tickler" regenerative radio receivers.
One coil was connected to the detector tube's
grid circuit. The other coil, the "tickler"
coil was connected to the tube's plate circuit.
It fed back some of the signal from the plate
circuit into the input again, and this positive
feedback increased the tube's gain and selectivity.
Rotary transformer
A rotary transformer is a specialized transformer
used to couple electrical signals between
two parts that rotate in relation to each
other, as an alternative to slip rings which
are prone to contact noise.
Homemade transformers
Transformers may be wound at home using commercial
transformer kits, which contain laminations
& bobbin. Firm clamping of laminations and
varnish help to avoid buzz.
It is possible to make the transformer laminations
by hand too. Such transformers are encountered
at times in third-world countries, using laminations
cut from scrap sheet steel, paper slips between
the laminations, and string to tie the assembly
together. The result works, but is usually
noisy due to poor clamping of laminations.
See also
Three-phase
Three-phase electric power
Buck–boost transformer
Tap changer
Transformer
Motor-generator
References
^ "The basics of autotransformers". ECMwebs. 
^ Donald G. Fink, H. Wayne Beatty, Standard
Handbook for Electrical Engineers, 11th Edition,
McGraw Hill, 1978 ISBN 0-07-020974-X pp. 10-44
and 10-45
^ T. J. Gallagher and A. J. Pearmain: "High
Voltage - Measurement, Testing and Design",
ISBN 0-471-90096-6
^ Philips PM3311 Oscilloscope service manual
Section 6.2.16 [describing the switch mode
power supply],"The unregulated d.c. voltage
is applied in the form of pulses to a resonant
transformer T1602..."
^ "Planar Transformer on a PCB"
^ ANSI IEEE Standard C57.12.00 General Requirements
for Liquid-Immersed Distribution, Power and
Regulating Transformers, 2000
^ page 9
^ UNEP Chemicals. Guidelines for the Identification
of PCBs and Materials Containing PCBs. United
Nations Environment Programme. p. 2. Retrieved
2007-11-07. 
^ [1]
^ [2]
^ "Measurement Canada Standard Dwg. No.3400
D3 Delta Connected CTs". MEASUREMENT CANADA.
Retrieved 12 December 2012. 
^ Network Protection & Automation, AREVA 2002
^ "The combined transformer - an optimal solution
för HV substations, p. 1". Retrieved 9 July
2013. 
^ "Combined instrument transformer, PVA 123,
p. 2". Retrieved 9 July 2013. 
^ Robjohns, Hugh. "Analogue Warmth – The
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