Chapter 9 Transformers

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Chapter 9 Transformers
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Chapter 9 Transformers
• Function: Transformers are extremely
versatile devices that can be used to either
step up and step down AC voltages or to
step up and step down AC current. They can
also allow AC to pass and block DC.
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9.1 Transformers
• 9.1.1 Construction and Operation
Figure 9-1 Simple Transformer
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• The most common type of transformer is
the voltage transformer, which consists of
two windings, the primary winding and the
secondary winding. The windings are not
electrically connected together, which is a
safety feature in AC electrical circuits, but
are wound on the same laminated soft iron
core.
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• If an AC voltage is applied to the primary
winding, the resultant changing flux links
with the secondary winding. The changing
flux is concentrated by the iron core and
causes an EMF to be induced in the
secondary winding. The magnitude of the
EMF is proportional to the ratio of the
number of turns between the primary and
secondary windings.
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Where:
VP = Primary voltage
VS = Secondary voltage
NP = Primary turns
NS = Secondary turns
S
P
S
P
V
V
N
Turns Ratio  N 
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Categorise of Transformer
Figure 9-2 Set up and Step down Transformers
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• Transformers are also extremely efficient
(i.e. the amount of power in is
approximately equal to the amount of
power out), and they are rated in voltamperes
(VA). The following relationship
exists between the turns ratio, voltage, and
current.
• where IS = Secondary Current
• IP = Primary Current
P
S
S
P
S
P
I
I
N
N
V
V  
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• If the voltage is stepped up, the current is
stepped down. For example, if a transformer
has a turns ratio of 1:2, and inputs of 240 V
and 5 amps, the outputs will be, respectively:
P
S
P
S
N
N
V
V 
240 480 volts
1
V 2 S   
S
P
P
S
N
N
I
I 
5 2.5 amps
2
I 1 S   
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• Transformers also consist of inductive
components, so it is important that they are
operated at their correct frequency and
voltage. Any under-frequency condition
results in the primary current increasing and
the transformer overheating.
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9.1.2 Types of Transformers
• Three-phase transformers (isolation
transformers).
Figure 9-3 Primary Windings of Three-phase Transformers
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9.1.2 Types of Transformers
• Three-phase transformers (isolation
transformers).
Figure 9-4 Secondary Windings of Three-phase Transformers
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Auto transformers
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Auto transformers (continue)
• Auto transformers are a special type,
since they have no electrical isolation
between the primary and secondary
windings. A single continuous winding is
wound on a laminated iron core, where
part of the winding is used as the primary,
whilst the other part is used as the
secondary, as shown below.
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Auto transformers (continue)
• These transformers can be used to
either step-up or step-down the applied
voltage, depending on the winding
configuration.
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Auto transformers (continue)
• In a step-down device, the whole of the winding
serves as the primary winding, whilst the lower half
of the winding serves as the secondary winding. In
this case, there are fewer turns in the secondary than
in the primary: so the voltage is stepped-down, but
the current is stepped-up. This configuration is
typically used to power aircraft instruments where
the voltage is stepped down from 115 V 400 Hz to 26
VAC.
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Auto transformers (continue)
• The disadvantage of this format is that the
full voltage is placed across the load if the
coil goes open circuit, since there is no
voltage isolation between the two
windings.
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Auto transformers (continue)
• Conversely, in a step-up auto transformer, the
lower half of the coil is used as the primary, and the
entire coil is used as the secondary. In this case, the
secondary has more turns than the primary, so the
transformer steps-up the voltage and steps-down the
current. On aircraft, this arrangement is typically
used in windshield anti-icing systems.
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Auto transformers (continue)
• Conversely, in a step-up auto transformer, the lower
half of the coil is used as the primary, and the entire
coil is used as the secondary. In this case, the
secondary has more turns than the primary, so the
transformer steps-up the voltage and steps-down the
current. On aircraft, this arrangement is typically
used in windshield anti-icing systems.
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Auto transformers (continue)
• If the output from the auto transformer can be
varied via a moveable tapping, as shown
below, it is also known as a variac and is
typically used on the flight deck to control the
intensity of ultra-violet lighting.
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Current transformers
• Current transformers differ from the voltage
transformer, because the primary circuit consists
of a supply feeder cable rather than a winding
connected across a supply, as shown below.
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• In this arrangement, the alternating magnetic field associated
with the load current is linked to the current transformer
secondary winding via a laminated soft iron core, through which
the feeder (primary) passes. The secondary current is used to
feed a meter and typically registers the current flowing from an
AC generator to the busbar or load. The secondary current can
additionally be used to supply power meters and to monitor the
load-sharing in an electrical circuit.
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• In AC power generation systems, this type of
transformer can also be used as a sensor in a
differential protection circuit, as shown below.
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9.2 Transformer Rectifier Units
• A transformer rectifier unit (TRU) is used to
convert AC into relatively smooth DC. An
example of a simple TRU circuit is that which
is used in a car battery charger, as shown
below.
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• This device takes the mains 240 VAC and converts it
to approximately 14 VDC to charge the battery. This is
achieved by a transformer, which first steps down the
AC voltage to a reasonable level and then converts it
via a bridge rectifier assembly into DC.
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Figure 9-10 Typical TRUs on Aircraft
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Typical TRUs on Aircraft
• The TRU that is fitted to an aircraft is
typically supplied with 200V 400Hz threephase
AC, Which is stepped-down
through a three-phase star-star wound
transformer and changed to 28 V DC by a
six-rectifier bridge assembly. The output
from the TRU is then fed to the aircraft's
DC busbars.
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Overheat protection
• When operating, most TRUs are cooled
by air from a thermostatically controlled
cooling fan. If the TRU overheats (150°-
200°) due to fan or other failure, a
warning light illuminates on the flight deck.
The TRU should then be switched off,
either manually or automatically.
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Reverse Current protection
• When the TRUs are operating in parallel with
some other power source, the failure of a
rectifier in a TRU can cause a reverse current
to flow into it and may even cause a fire.
Reverse current protection in the failed TRU
is designed to sense the fault current when it
reaches approximately 1 amp, and
disconnect the TRU automatically from the
DC bus bars.
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END OF CHAPTER 9

linairm

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liujunwei3980

thanks a lot