question electric

Its common to equate electrics with the flow of water in tubes, at least it works up to a point. Voltage can be compared to water pressure, current compared to the flow rate.

An electrical resistance is like a water wheel, it restricts the flow of the electricity but it does work, usually producing heat. In a resistive circuit current and voltage are in phase, that is to say more water pressure leads to an increased flow rate leads to the water wheel going round faster and more work is done. Of course, in an AC circuit the water rushes backwards and forwards and the wheel changes direction with the water flow but the analogy is still sound.

A capacitive circuit is comparable to a bath, with bathwater and you in it. As you slosh backwards and forwards the water goes with you. At one point, where the movement of the water has stopped for a second, it's all hanging over the tap end of the bath (electrical equivalent, high voltage or potential, no current) then it whooshes back past your thighs (electrical equivalent, low voltage and high current) then it hanges over the head end of the bath (high potential, no current) then it whooshes back etc. What we have here is voltage and current out of phase, when one is big the other is small.

If we put the two circuits together we have two opposing effects. In the resistive circuit more volts means more current but, if we put a capacitor in the circuit it creates a current that can oppose and reduce the original flow leading to a less efficient water wheel. This opposing force is called capacitive reactance.

The effect, then, of putting a capacitor in a circuit is to make the resistive devices less efficient. That's why capacitive reactance is seen as 'bad' and that's why we try to cancel out the effect with the opposite effect, inductive reactance.

Source: (http://www.atpforum.eu/showthread.php?t=321)

Assume a constant speed DC generator providing a constant output voltage. If the electrical load increases, the voltage regulator will increase the intensity of the excitation current.

The load current is the current that flows through the wires connecting the electrical components (loads) of the aircraft. Remember these components are connected in parallel. Switch on the electrics ! then switch on the anti collision light ! being wired in parallel means the total resistance in the circuit will go down. current flow increases. Switch on the electric fuel pump, same thing will happen. You are increasing load current.

The value of induced voltage in the armature of the generator is not the same as the output voltage . The armature has resistance and voltage is required to make the electicity flow through it. There will be a voltage drop across the armature. Example induced voltage 16 v, armature resistance 2 ohms current flow 1 amp , therefore voltage drop across armature I x R = 1x 2 = 2 volts. output voltage 16-2 =14 v.

The armature of the of the generator is connected in series with the aircraft loads. All current flowing through the aircraft circuits flow through the armature. If you increase load current, this will increase the current flow through the armature. it resitance stays the same . Example resitance 2 ohms current flow 2 amps. This will increases the voltage drop across the armature which now = 2 x 2 = 4 volts. Induced voltage still 16 volts - 4volts voltage drop means output voltage = 12 v.

Aircraft generators are required to deliver a constant voltage, example 14v. to maintain the output voltage as the load current increases the induced voltage must be increased. This is achieved by increasing current flow throught the field coils via variable resistors connected in series with the field coil.

Increasesd current flow through the field increases its magnetic field and the induced voltage is increased. example increased to 18v - 4v voltage drop across the armature = 14v constant generator output.

On reducing load current (switch off electrical equiptment) the reverse happens.

The objectives for the ATPL exams require you to know that output voltage is controlled by changing current flow through

the field coils. This current flow is also called the exitation or energising current.


When two DC generators are operating in parallel, control of load sharing is achieved by an equalising circuit which, in conjunction with the voltage regulators, varies the field excitation current of the generators.

In order that DC generators will achieve equal load sharing when operating in parallel, it is necessary to ensure that: their voltages are almost equal.

The voltage regulator of a DC generator is connected in series with the shunt field coil

The type of windings commonly used in DC starter motors are series wound.

The primary purpose of bonding the metallic parts of an aeroplane is to:

a) Provide a single earth for electrical devices b) Provide safe distribution of electrical charges and currents <-- Marked Correct

If one of the 12 cells of a lead-acid battery is dead, the battery is unserviceable.

A test to assess the state of charge of a lead acid battery would involve comparing the on-load and off-load battery voltages.

When carrying out battery condition check using the aeroplane's voltmeter, a load should be applied to the battery in order to give a better indication of condition.

R1 and R2 resistances are connected in parallel. The value of the equivalent resistance (Req) so obtained is given by the following formula:

1/Req = 1/R1 + 1/R2

An aircraft electrical circuit which uses the aircraft structure as a return path to earth, may be defined as a single pole.

The true statement among the following in relation to the application of Ohm's law is current in a circuit is directly proportional to the applied electromotive force.

Fuse is not resettable and a circuit breaker is resettable.

When a conductor cuts the flux of a magnetic field an electromotive force (EMF) is induced in the conductor.

A component consuming 80 watts at 8 amps would require a supply of 0.01 kv

POWER (WATTS) = VOLTS x AMPERES

80 = V x 8

V = 10

10/1000 = 0.01 kv

In aeronautics, the most commonly used batteries are NiCd because?

a) Their output voltage is more constant than lead-acid batteries b) They weigh less than lead-acid batteries <-- Marked Correct c) Their electrolyte is neither corrosive nor dangerous d) They are cheaper than lead-acid batteries

The advantages of grounding the negative pole of the aircraft structure are:

1) Weight saving

2) Easy fault detection

4) Reduction of short-circuit risk

On board present aircraft, the batteries used are mainly Cadmium-Nickel. Their advantages are:

3) Good charging and discharging capability at high rating.

4) Wider permissible temperature range.

5) Good storage capability.

6) Sturdiness owing to its metal casing.

The advantages of Nickel-Cadmium compared with Lead-Acid batteries are:

3) Reduced charging time

4) Constant output voltage

What are the advantages of a NICd battery?

1) More compact

2) Longer shelf life

3) An even voltage over total range before rapid discharge

The voltage of a fully charged lead-acid battery cell is 2.2 V

A current limiter fuse in a DC generation system is used to allow a short term overload before rupturing.

The purpose of a battery protection unit is generally to isolate the battery:

1) From the bus when the battery charge is deemed satisfactory

2) When there is a battery overheat condition

3) In case of an internal short circuit

Advantages of NICAD batteries: Even voltage before rapid discharge.

A magnetic circuit breaker is a protection system that has a quick tripping response.

Ohm's Law V = I x R

To reverse the direction of rotation of a shunt type (parallel field) DC electric motor, it is necessary to reverse the polarity of either the stator or the rotor.

In a simple electrical circuit, if the power consuming devices are connected in parallel, the total current consumed is equal to the sum of the currents taken by the devices.

The total resistance of a number of power consuming devices connected in series is equal to the sum of the individual resistances.

Electromotive force is measured in Volts.

An aircraft has an accumulator with a capacity of 40 Ah. Assuming that it will provide its nominal capacity and is discharged at the 10 hour rate: It will pass 4A for 10 hours.

A significant increase in battery temperature is an indication of "Thermal Runaway".

Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. In other words, the term "thermal runaway" is used whenever a process is accelerated by increased temperature, in turn releasing energy that further increases temperature.

Batteries have a negative temperature co-efficient, that is, as temperature rises, resistance decreases, therefore current flow increases with the possibility of thermal runaway if temperature cannot be dissipated.

The electrolyte in a nicked cadmium battery is "Potassium Hydroxide".

You can use circuit breakers in AC or DC circuits.

The essential difference between aircraft AC alternators and DC generators (dynamos) is that induced (output) windings of the alternators are fixed (stator), and the dynamos have a fixed inductor (field) coil.

If the frequency of the supply in a series capacitive circuit is increased, the current flowing in the circuit will "Increase".

Capacitive reactance reduces current flow. Capacitive reactance is inversely proportional to frequency. As frequency rises capacitive reactance reduces and thus the current flow will increase.

A typical transformer has two windings, the primary and the secondary which are not electrically connected. In an auto transformer, there is only one winding that acts as the primary and a portion of it as the secondary winding. The primary and secondary windings are electrically connected.

An autotransformer is an electrical transformer with only one winding. The auto prefix refers to the single coil acting on itself rather than any automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where electrical connections are made. An autotransformer can be smaller, lighter and cheaper than a standard dual-winding transformer however the autotransformer does not provide electrical isolation.

An autotransformer has a single winding with two end terminals, and one or more terminals at intermediate tap points. The primary voltage is applied across two of the terminals, and the secondary voltage taken from two terminals, almost always having one terminal in common with the primary voltage. The primary and secondary circuits therefore have a number of windings turns in common. Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. In an autotransformer part of the current flows directly from the input to the output, and only part is transferred inductively, allowing a smaller, lighter, cheaper core to be used as well as requiring only a single winding.

One end of the winding is usually connected in common to both the voltage source and the electrical load. The other end of the source and load are connected to taps along the winding. Different taps on the winding correspond to different voltages, measured from the common end. In a step-down transformer the source is usually connected across the entire winding while the load is connected by a tap across only a portion of the winding. In a step-up transformer, conversely, the load is attached across the full winding while the source is connected to a tap across a portion of the winding.

Source: (http://en.wikipedia.org/wiki/Autotransformer) (http://in.answers.yahoo.com/question/index?qid=20081130030226AA0AlWT)

In an aircraft electrical system where AC generators are not paralleled mounted, the changeover relay allows power supply to the faulty AC generators busbar.

Pulling the fire shutoff handle causes a number of devices to disconnect. In respect of the AC generator it can be said that the:

a) Exciter control relay and the generator breaker open <-- Correct

b) Exciter control relay, the generator breaker and the tie breaker open

When a persistent over-excitation fault is detected on only one AC generator, the protection device opens the:

a) exciter breaker and generator breaker <-- Correct

b) exciter breaker, generator breaker and tie breaker

When a persistent top excitation limit fault on an AC generator connected to the mains with another AC generator, the over-excitation protection device opens:

a) The exciter breaker and the generator breaker

b) The exciter breaker, the generator breaker and the tie breaker <-- Correct

If AC generators are connected in parallel, the reactive loads are balanced by regulating the energising current.

If generators are to be paralleled frequency is adjusted by adjusting CSD output torque to balance the real load.

The reactive loads are trimmed by adjusting the energising current from the voltage regulator.

Going back to DC generators, voltage regulation is achieved by changing the voltage in the voltage coil, which changes the carbon pile pressure, which changes the current in the shunt field. This then is the reactive load (wasted power) that adjusts the energising current. The same principle applies in AC generators (alternators) but they use solid state circuitry.


On starting, in a brushless AC generator with no commutator rings, the generator is activated by a set of permenant magnets.

Regarding three-phase AC generators, the following conditions must be met for paralleling AC generators:

Correct Statements:

1. Equal voltage

3. Equal frequencies

4. Same phase rotation

5. Voltages of same phase

Incorrect Statements:

2. Equal current

The frequency of an AC generator is dependent on the number of pairs of poles and the speed of the rotor.

The frequency of the induced AC current depends on the revolution of the rotor. Since rate of revolution is Revolutions Per Minute (RPM) and frequency is expressed as cycles per second, the frequency of the induced AC current will be the rate of revolution of the rotor divided by 60.

In addition to above, if two magnets are used (two pole pairs) the frequency is doubled (tripled for three). Thus putting all of this in a formula we get:

Frequency (Hz) = RPM x number of pole pairs / 60

In order to produce an alternating voltage of 400 Hz, the number of poles (should have been pole pairs) required in an AC generator running at 6000 rpm is 4.

Frequency (Hz) = RPM x number of pole pairs / 60

400 = 6000 x Pole pairs / 60

Pole pairs = 400/6000 x60 = 4

The speed of a synchronous four-pole motor fed at a frequency of 400 Hertz is "12000 revolutions per minute".

The speed of a synchronous motor depends upon the frequency of the supply and the number of pole pairs (pairs of rotors).

The formula is a rearrangement of the formula used for generators (as mentioned above):

Synchronous speed (RPM) = Frequency (Hz) x 60 / Number of pole pairs

= 400 x 60 / 2

= 12,000 RPM

The advantages of alternating current on board an aircraft are:

Correct Statements:

3) Flexibility in use

4) Lighter weight of equipment

5) Easy to convert into direct current

6) Easy maintenance of machines


1) Simple connection 2) High starting torque

The frequency of the current provided by an alternator depends on its rotation speed.

Frequency wild in relation to a AC generation system means the generator output frequency varies with engine speed.

When operating two AC generators unparalleled, the phase relationship of each generator is unimportant.

As regards the Generator Control Unit (GCU) of an AC generator, it can be said that:

Correct Statements:

2. Modern GCUs are provided with a permanent indication to record the failure.

3. All the commands originating from the control panel are applied via the GCU, except dog clutch release.


1. The GCU controls the AC generator voltage

4. The Auxiliary Powr Unit (APU) provides the excitation of the AC generator as soon as the APU starts up

To ensure correct load sharing between AC generators operating in parallel both real and reactive loads must be matched.

With AC electrics current flow is affected not just by resistance but also inductive and capacive reactance. The electrical 'loads' on the circuit are real loads ( resistive) and reactive loads (inductive & capacitive).

If the AC generators are paralleled ie two generators conected to the same bus bar the real loads and reactive loads must be balanced.

The real loads are balanced by making slight adjustment to the torque of CSDU and reactive loads are balanced by adjustment of the excitation current to the generator field coils.


Real load sharing in a parallel AC system is achieved by automatic adjustment of the torque on each generator rotor via the CSD unit.

When the AC generators are connected in parallel, the reactive loads are balanced by means of the energising current.

On-board electrical systems are protected against faults of the following type:

Correct Statements:

1) AC generator over-voltage

2) AC generator under-voltage

4) Over-speed

5) Under-frequency


3) Over-current

6) Undue vibration of AC generators

The purpose of a voltage regulator is to control the output voltage of the generator at varying loads and speeds.

An AC generator driven by a CSD unit requires a voltage controller to maintain constant voltage under load.

On the flight deck, an oil operated CSD unit is normally provided with means of monitoring the oil over-temperature and low oil pressure. High temp and low oil pressure are the main reasons for its disconnection.

The measured output power components of a constant frequency AC system are KVA and KVAR.

KVA = Total or apparent power.

KVAR = Idle, reactive or wattless power.

A 3 phase AC generator has 3 separate stator windings spaced at 120 degrees.

A relay is an electromagnetically/magnetically operated switch.

Electrical bonding of an aircraft is used to:

Correct Statements:

1) Protect the aircraft against lightning "EFFECTS" (not from lightning "STRIKE")

3) Reduce radio interference on radio communication systems

4) Set the aircraft to a single potential


2) Reset the electrostatic potential of the aircraft to a value approximating 0 volt

A 'trip free' type circuit breaker is a circuit protection device which will not allow the contacts to be held closed while a current fault exists in the circuit.

Static dischargers:

Correct Statements:

2) Are placed on wing and tail tips to facilitate electrical discharge

4) Are located on wing and tail tips to reduce interference with the on-board radio communication systems to a minimum

5) Limit the risks of transfer of electrical charges between the aircraft and the electrified clouds


1) Are used to set all the parts of the airframe to the same electrical potential

3) Are used to reset the electrostatic potential of the aircraft to a value approximately 0 volts

The services connected to a supply bus-bar are normally in parallel, so that isolating individual loads decreases the bus-bar current consumption.

In a two generator system, a differential relay will ensure that generator voltages are almost equal before the generators are paralleled.

In a generator, the Constant Speed Drive (CSD):

Correct Statements:

1) May be disconnected from the engine shaft

3) Is a hydro-mechanical system

6) May be disconnected in flight


2) May be disconnected from the generator

4) Is an electronic system

5) May not be disconnected in flight

On detection of a persistent over voltage/fault on an AC generator connected to the aircraft AC bus-bars, the on-board protection device opens the exciter breaker and the generator breaker.

When an underspeed fault is detected on an AC generator connected to the aircraft AC busbar, the protection device opens generator breaker.

On detection of a persistent phase imbalance between an AC generator connected to the main bus bars and other AC generators, the protection device that opens is/are the tie breaker(s).

The question describes a parallel system. The fault is to do with the phase relationship. To parallel AC generators the voltage frequency and phase rotation of each generator must be matched. In the question there is a phase problem, so you cannot parallel the generators. To isolate it the Bus Tie breaker opens.

This does not mean that the generator cannot be connected to a bus bar on its own, thus only the tie breaker opens.


In an alternator rotor coil you can find:

a) AC <-- Correct

b) Three phase Ac.

A capacitor in parallel with breaker points induces a very high voltage across the secondary windings.

The conditions to be met to activate a shunt generator are:

Correct Statements:

1) Presence of a permanent field

4) Minimum rotation speed


2) Closed electrical circuit

3) Generator terminals short-circuited

The shunt wound generator produces an output in which the voltage decreases slightly with increase in load.

The power required for field excitation of the main rotor in modern constant frequency alternators is directly controlled by the volatge regulator.

The main features of a unipole system are:

Correct:

1) Lighter

2) Easier fault finding

4) Less likely to short circuit

Incorrect:

3) More likely to short circuit

5) It is not a single wire system

In an electrical circuit the reverse current relay will open When battery voltage exceeds generator voltage.

GCB connects generator to its busbar.

Generator cut-out (reverse current relay) contacts are kept open by:

a) Magnetism

b) Spring Tension <-- Correct

A Series wound electrical motor is used as a starter motor.

If the load on a DC generator is reduced the voltage regulator will "Decrease" the current in the exciter field.

If an AC system is operating at a lower than designed frequency, the transformer could over heat due to current overload.

Transistor:

A diode allows current to flow in one direction only.

A ZENER diode is used for voltage stabilisation.

A squirrel cage tacho generator system uses a three phase AC whose frequency varies with the speed of the engine delivered to a three phase syschronous motor and drag cup.

What are the conditions necessary in order to parallel DC generators?

a) voltage must be the same b) voltage must be nearly the same <-- marked correct c) voltage and frequency must be the same

question electric

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