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1-Describe how the Vibrating Contact Type Voltage Regulator maintains

the output generator at constant value (28Vdc)?When the supply generator voltage is at required voltage (28V), the

current will flow through voltage coil and back to the source. Current will also flow through normally close contact, to the

shunt field and back to the source. No current flow through the shunt resistance. The contact points remain closed as the

magnetic strength from the voltage coil is not enough to overcome spring tension to open the points.More current will flow

through the contact points rather than resistor to the shunt field and cause more voltage to be further increase. When

output voltage increased up to (31V) more than required voltage (28V), voltages coil enough strength to pull the contactpoint open. Now current must flow through shunt resistance, shunt field and back to source. Therefore cause less current

flow to the shunt field. Less voltage will be developed at the armature and the output voltage is reduced.When generator

output drop example (25V),the effect is voltage coil becomes weak, contact point closed by the spring tension and shunt

resistance will be shorted .out this allow more current to flow to the shunt field to strengthen its field. Therefore the

generator output will slightly increase, and this cycle repeated the contact will keep vibrating till its maintain vo ltage output

(28V)

2. Describe the operation of a Reverse Current Cut-Out Relay(RCCR) in DC generator

circuit when: a) Generator voltage is higher than the battery voltage.

b) Battery voltage is higher than the generator voltage.Relay consist of two coil wound on a core and a spring

controlled armature and contact assembly. The first one is shunt winding that is known as voltage coil connected across the

generator output. The second one that is in series with main supply line via contact assembly is known as series

winding.When the generator voltage exceeds the battery voltage, the shunt winding energise thus contact close and act asan automatic switch to connect the generator to the busbar and the battery is on charge. While the series winding aids the

shunt winding field in keeping the contacts firmly closed.When generator voltage falls below the battery voltage, there is a

momentary discharge of current from the battery make a reverse current through the cut-out relay series winding. The

shunt winding will oppose the series winding which reduce the core magnetization until the spring pull the contact opens. If

the contact remain closed, the generator will be motoring by battery current.

3-Describe how does the Carbon Pile Voltage Regulator (CPVR)maintain the output generator at constant

value (28Vdc)?The current generated from the regulator arrangement produces a magnetic field. When rotating, the rotor

causes a magnetic flux to be produced, and this flux is cut by the stator windings. This results in the generation of a voltage

and current output in the stator windings.A small part of current produced in the stator windings is passed to the shunt field

so as to provide excitation for the DC exciter itself. The amount of current flow through the exciter shunt field is controlled

by a resistance, which is made up of carbon discs or a carbon pile packed into a ceramic tube.The resistance of the carbon

disc is varied by pressure change. This pressure is controlled by a magnetic field produced by an electromagnet coil. The

current for this electromagnetic coil is supplied through the transformer and rectifier circuit from alternator output to the

terminal board. This means that as the load changes the alternator voltage also varies. The strength of the electromagnet

also increases or decreases due to this load change, resulting in the change in the compression on springs and thus the

resistance of the carbon pile.The resistance of the carbon is least when the pressure on springs is least and on the armature

greatest. This occurs only when there is low output voltage which causes solenoid to be weak. Due to this effect, low

resistance and more current flows to the shunt and high excitation is produced. This high excitation when fed to rotor

produces more voltage.In the same manner, the pressure will be low when strong solenoid field is present in the

arrangement. Due to this a small amount of current is conveyed to the shunt, mainly because of high resistance, resulting in

less excitation and reduced output voltage.

4-Describe the operation of a DC Brushless AlternatorA brushless alternator is composed of two alternators built

end-to-end on one shaft. Smaller brushless alternators may look like one unit but the two parts are readily identifiable on

the large versions. The larger of the two sections is the main alternator and the smaller one is the exciter. The exciter has

stationary field coils and a rotating armature (power coils). The main alternator uses the opposite configuration with a

rotating field and stationary armature. A bridge rectifier, called the rotating rectifier assembly, is mounted on a plate

attached to the rotor.Neither brushes nor slip rings are used, which reduces the number of wearing parts.High-output DC

alternators may be of the brushless type. That is, there is actually two alternators in the same housing. Once alternators

produces DC for the load, and the other controlled by the voltage regulator to provide excitation voltage for the output. An

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automatic voltage control device controls the field current to keep output voltage constant. In figure below, we have a

schematic of a typical brushless alternator. The exciter is stationary and receives field DC from the voltage regulator. If

output is low, the regulator will supply a larger amount of current than it does when output is high.The magnetic field of the

exciter stator is cut by the exciter rotor and the resulting three-phase AC is rectified by the three diodes which act as a half-

wave rectifier. The rectifier output of the exciter flows into the output field windings, which are also a part of the rotor. This

field creates north and south poles on the motor frame,a nd as it rotates within the three-phase stator, generates it

becomes DC for the output circuit. A capacitor is placed across the output of the alternator to prevent voltage surges fromdestroying the rectifying diodes.Varying the amount of current through the stationary exciter field coils varies the phase

output from the exciter. This output is rectified by a rotating rectifier assembly, mounted on the rotor, and the resultant DC

supplies the rotating field of the main alternator and hence alternator output. The result of all this is that a small DC exciter

current indirectly controls the output of the main alternator

5. Describe the whole operation of the aircrafts external power circuit below.Power is bought from a battery

cart or rectifier through a standard three-terminal external power plug. Two of the pins in the aircraft receptacle are larger

than the third, and are also longer. When the cart is plugged in, a solid contact is made with the two larger plugs. The

external power relay in the aircraft remains open, and no current can flow from the external source until the plug is forced

all the way into the receptacle, and the smaller pin makes contact. This small pin then supplies power through a reverse-

polarity diode to the external power relay that closes, connecting the external power source to the aircraft bus.The reverse-

polarity diode is used in the circuit to prevent an external power source with incorrect polarity from being connected to theaircrafts bus. The diode simply blocks current from flowing to the external power relay, if the applied power is connected

backwards or is offering reverse polarity.Depending on the system design, some external power sources can be connected

to charge the aircraft battery, while others isolate the battery from the external power. For systems that provide for battery

charging, it is quite possible that the aircraft battery can be so completely discharged that the battery contactor cannot get

enough current to close. This would prevent the external power source from charging the battery.To allow for battery

charging, a circuit consisting of a diode, a current-limiting resistor and a fuse is connected between the positive terminal of

the external power plug and battery side of the battery contactor. With this arrangement, enough current can flow from the

external power source to energize the battery contactor coil, so that it can close and allow the battery to be charged. A

diode is in the circuit to prevent the positive pin in the external power receptacle from having power, or being hot, when no

external power plug is connected.Today, with many of smaller aircraft having 12-volt systems while others have 24-volt

systems, it is extremely important to connect the correct voltage when using an external power source. Diode D1 prevents

the external power relay closing if the power source has the wrong polarity, but there is normally no protection against

improper voltage. Connecting the aircraft to the wrong voltage can severely damage sensitive electronic equipment.