Chapter 9 Transformers

航空发表于 2011-9-20 08:22:40

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航空发表于 2011-9-20 08:23:26

1 Chapter 9 Transformers 2 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. 3 9.1 Transformers • 9.1.1 Construction and Operation Figure 9-1 Simple Transformer 4 • 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. 5 • 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. 6 Where: VP = Primary voltage VS = Secondary voltage NP = Primary turns NS = Secondary turns S P S P V V N Turns Ratio  N  7 Categorise of Transformer Figure 9-2 Set up and Step down Transformers 8 • 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   9 • 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    10 • 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. 11 9.1.2 Types of Transformers • Three-phase transformers (isolation transformers). Figure 9-3 Primary Windings of Three-phase Transformers 12 9.1.2 Types of Transformers • Three-phase transformers (isolation transformers). Figure 9-4 Secondary Windings of Three-phase Transformers 13 Auto transformers 14 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. 15 Auto transformers (continue) • These transformers can be used to either step-up or step-down the applied voltage, depending on the winding configuration. 16 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. 17 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. 18 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. 19 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. 20 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. 21 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. 22 • 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. 23 • In AC power generation systems, this type of transformer can also be used as a sensor in a differential protection circuit, as shown below. 24 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. 25 • 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. 26 Figure 9-10 Typical TRUs on Aircraft 27 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. 28 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. 29 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. 30 END OF CHAPTER 9

linairm 发表于 2011-10-22 20:28:14

这个东东真好

liujunwei3980 发表于 2014-1-27 17:00:52

thanks a lot

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Chapter 9 Transformers