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Starter-Generators

A dual function starter-generator mounted on the top right hand side of the engine (when viewed from the front of the engine) supplies power to the it respective main electrical bus, i.e. left gen./left bus.

When the starter-generator operates in generator mode it can supply up to 200 amperes of direct current at a normal main bus voltage of 28.5 volts D.C.

Operation of each generator is controlled by separate Generator Control Units (GCU's) located under the cabin floor forward of the pilots seat.

Generator control units provide:

  • voltage regulation,
  • reverse current sensing,
  • over voltage protection,
  • generator load sharing, and
  • ground fault sensing.

If the GCUs take a generator offline or a fault occurs with the generator during flight, illumination of the left or right GEN OFF annunciator light will occur telling the pilot that the line contactor for that generator is open.

Right GEN OFF light illuminated. This would indicate the line contactor for the generator is open

During normal 2 engine operation both generators will be online and providing D.C. current to the main buses.

A paralleling circuit in each GCUs will sense the voltage from the opposite generator and adjust its respective generator to match the opposite generator to within 25 amperes for load sharing purposes.

If the GCUs are out of adjustment, the paralleling circuitry may be unable to function correctly causing one of the generators to be carrying the entire load.

This would be clearly identified by uneven generator amp. indications on the ammeters.

Uneven amp. indications on ammeter Indicating no load sharing between GCU’s

Another function of the GCU’s is Overvoltage protection.

Once again through the paralleling circuitry, if either GCU senses more than 32 volts from either generator it will automatically open the line contactor on the respective generator totally disabling it.

There are 3 ways the pilot can manually switch off the generators, however 2 of them require shutting down the respective engine.

The first method is by placing the generator switch to the centre OFF position.

The second method is by pressing the red ENGINE STOP button.

3 position generator switches (shown in the OFF position) And the ENGINE STOP buttons

The third method is by pressing the Firewall Shutoff button. This button serves 4 purposes:

  • It shuts off the fuel at the firewall,
  • It closes the bleed air valve,
  • It disables the generator, and
  • Arms the fire bottle located in the MLG wheel well.

It will shut the engine down too though.

Firewall shutoff button will manually open line contactor for Generator. It will also shutdown the engine too (discussed later!).

During start it is important that the generator switches are in the OFF position even though the generators are automatically disabled below 60% RPM.

If the switches are left on during start the rapidly accelerating engine at 60% RPM will cause a voltage surge will occur too rapidly for the GCUs to control causing a voltage surge at the Main bus and in the opposite generator possibly taking it offline.

The generator switches should be positioned to the on position only after the engine RPM has stabilised.

It is good practice to move the switch to the RESET position prior to switching the generator on this allows the GCU to build up the generator voltage to 28.5 volts prior to the generator be turned on.

3 position generator switches. GEN ON, OFF and RESET. For an EPU start always have the Battery switches ‘ON’ and the Generator switches ‘OFF’. Generator switches can be turned ‘ON’ after EPU disconnection (see text below).

During a normal shutdown the Engine stop buttons being pushed will disable the generators stopping any reverse current flow through the generators as the engines wind down.

For any other generator shutdowns however, the GCUs will sense this reverse current flow and opens the line contactor at approximately 150 amperes of reverse current.

During EPU starts a lockout circuit prevents the EPU and generators from providing power to the main electrical bus at the same time.

Therefore when an EPU is connected to the aircraft and is supplying power, the generators will not come online even with the generator switches ON.

It is good practice to leave the generator switches off during an EPU start however and should be standard operating procedure for all Conquests.

It is standard practice however, to have the Battery switches on during an EPU start to prevent the engine computers from entering Manual Mode (Manual Mode will be discussed in a later section).

 
Starter-Generators


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