UNIT 1 ELECTRICAL POWER SOURCES

AIMS OF THE UNIT:-to identify the function of an electrical power sources

-to understand the grammar point ‘ Present Simple Tense’

TASKS

1 Do your best to answer the brainstorming questions. 2 Read the text for general understanding.

3 Make up questions to the text. 4 Find the sentences with the new words in the text. Give the Kazakh or Russian

equivalents of the words.

5 Write sentences with the new vocabulary. 6 Do the given exercisesfor better remembering the tеxt.7 Study grammar point ‘Present simple tense`,do the tasks.

8 Speak on the topic. Given schemes will help you to remember and understand the

topic. 9Find more information about the topic. Do some research, create slideshow or a

project work and present them.

Brainstorming questions

 

1 What is APU?

2 What is GTCP-95?

 

Aircraft electrical power sources are divided into two categories: primary power sources and secondary power sources. Some aircraft have auxiliary power units. When engine-driven generators don`t operate, external power is not available, or the engine-driven generator fails they furnish electrical power. APUs are used for ground maintenance as well as emergency in-flight power loss. Using the pneumatic starting system, the gas-turbine APU provides compressed air to start engines and for air conditioning.

There are many types and configurations of gas-turbine units. One of them is GTCP-95.

The GTCP-95 is a gas-turbine power plant unit (referred to as an APU). It furnishes electricity, starting air, and air conditioning. The gas-turbine engine of the APU requires only the aircraft battery and fuel for starting. The engine consists of two main sections and four main systems. The two main sections include an accessory assembly and a compressor and turbine assembly. The four main systems consist of an electrical system, a fuel control system, a bleed-air system, and a lubrication system. The engine develops power by compressing ambient air with a two-stage centrifugal compressor. Compressed air, mixed with fuel and ignited, drives a radial, inward-flow turbine wheel. The gas-turbine electrical system provides automatic actuation of the various circuits.They control fuel, ignition, engine starting, acceleration, and monitoring.

The electrical system consists of the following components: holding relays, oil pressure switch, centrifugal switch assembly, hour meter, and harness assembly. The ignition portion consists of an exciter and ignition plug. It is controlled by the multiple centrifugal switch. Ignition is only required during starting and automatically cuts out at 95% engine rpm.

Centrifugal speed sequence switch controls the sequence of operation of various electrical components. The input drive shaft turns a knife-edged fulcrum (fly weight support) and a pair of flyweights pivots on the knife edges. Each flyweight has a toe.Itlies under the outer race of a ball bearing on theactuating shaft. As the centrifugal switch turns,centrifugal force causes the flyweights to pivot.It movesthe actuating shaft to the right against the lever arm.The three electrical switches actuate at 35%, 95%, and106% of turbine speed. The functions of the switches are as follows:

Þ; 35%—turns off starter motor

Þ; 95%—arms load control circuits, starts hourmeter, turns off ignition

Þ; 106%—stops unit (overspeed protection)

As the drive shaft rotates, the flyweights moveoutward. This action forces the actuating shaft to movethe lever arm, actuating the switches. The lowestpercent speed adjustment spring acts on the lever armduring the actuating of all switches. Changing itssetting affects the setting of the 95% and 106%switches. A drift in setting of the 106% switch affectsonly the 106% switch.

The starter provides initial power for rotating the components of the gasturbine to self-sustaining speeds. It rotates the compressorto a speed high enough for correct airflow forcombustion. Also, the starter assists acceleration afterlight-off. The starter motor rating is 1.5 horsepower at 14volts at 5,000 rpm. The starter has a duty cycle of 1minute on and 4 minutes off. The starter motorarmature shaft is splined and pinned to the clutchassembly.

The starter clutch assembly performs twofunctions:

Þ As a friction clutch, it prevents excessive torquebetween the starter and accessory drive gears to protectboth.

Þ; As an overrunning clutch, it provides the meansof automatically engaging the starter with the gear trainfor starting. The clutch automatically releases it whenthe unit has reached a condition allowing it toaccelerate and run without assistance.The friction clutch section provides overtorqueprotection. The assembly will slip at 135 to 145inch-pounds of torque. The starter normally is de-energized by the centrifugal switch at 35%. If the switch does not cut outat this speed, the starter may fail from overheating or itmay fail mechanically from overspeed. If theoverrunning clutch does not release properly,mechanical overspeed failure of the starter will result.