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

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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
  volt-amperes (VA). The following relationship
  exists between the turns ratio, voltage, and
  current.
• where IS Secondary Current
• IP Primary Current

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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 12, and inputs of 240 V and 5
  amps, the outputs will be, respectively

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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 three-phase
  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