2A672 Vol 1 Self Test

What is CEMF?

A voltage induced into a coil or conductor in the opposite direction of the applied voltage. It opposes the original current and prevents the current from building up to an unlimited value.

What PC cards contain the sensing circuits for the AC and DC power supply?

AC card senses AC output and the DC card senses DC output.

What protective circuits are designed to stop generator output?

All protective circuits stop generator output since K16 contacts connect CR6 and K16 to ground.

What's the purpose of S55?

Allows an 8- to 10-second delay in order for oil pressure to build up during engine start. If oil pressure switch (S46) doesn't close within 8 to 10 seconds, S55 closes and L6 shuts down the unit.

What is the difference between a stable and unstable atom?

An atom having eight valence electrons is considered stable because it has the maximum electrons its outer shell can hold. However, an atom with less than eight valence electrons is considered unstable because it is missing electrons and loses its electrons to another atom much easier.

How many PN junctions are in the unijunction transistor?

One PN junction with two cathodes

What must you accomplish after repairing a unit?

Operations check.

What is placed in series with a zener diode to protect it?

A resistor.

How is the output pulse sent to the exciter field from the regulator card powered?

By a rectified 110 VDC signal, that is derived from the output of the AC generator through a transformer

How is P-type material made?

By adding an impurity having less than four electrons in its valence band like aluminum. Aluminum has less than four valence electrons; therefore, it has an absence of electrons (holes) and will try to fill its holes from other atoms.

How is a junction diode forward biased?

By connecting a positive potential to the anode and the negative potential to the cathode. Once the barrier voltage is reached, electrons flow from the cathode to the anode.

Explain the formula for voltage, current, and resistance in a series circuit.

RT = R1 + R2 + R3, and so forth. IT = I1 = I2 = I3, and so forth. ET = E1 + E2 + E3, and so forth.

Briefly describe how a basic transformer operates.

The AC power source is connected to the primary winding of a transformer and creates an expanding and collapsing magnetic field. This field induces an opposite voltage into the secondary winding of the transformer and applies that voltage to the load. This principle is called mutual induction.

Which part of the transistor is the control circuit?

The E-B junction or the control circuit.

Which portion of the transistor handles 95 percent of the current flow?

The E-C circuit, also known as the load circuit.

What happens when the threshold voltage is applied to the emitter of the UJT?

The UJT turns on and a current starts to flow from the emitter to B1 to B2, as well as B1 to the emitter.

The unit is running at governed speed and the automatic voltage control circuit is energized; what voltage reading do you get at test receptacle J8-P?

115 VAC.

What happens when S13 is placed in the GENERATE position?

12 VDC are supplied to the electronic governor control unit, which sends a signal to the governor actuator and engine RPM increases to governed speed.

At what frequency does the overfrequency relay trip?

415 to 425 Hz.

What is the turns ratio of a transformer if the primary winding has 600 turns and the secondary winding has 150 turns?

4:1.

If a transformer has a turns ratio of 4(P):1(S) and the voltage in the primary is 20 volts, what is the voltage in the secondary winding?

5 volts.

What happens when an N-type material is doped with a P-type material?

A PN junction is formed. As soon as the two materials are joined, the holes and electrons start pairing up. This continues until both sides are neutralized and there is an equal number of electrons in the N material as there are holes in the P material or vice versa.

What controls the firing time of the UJT?

A capacitor and resistor located in series with the UJT.

Explain the difference between conductors, insulators, and semiconductors.

A conductor is a material having less than four valence electrons; an insulator has more than four valence electrons, but less than eight; and a semiconductor has four valence electrons.

What component senses engine speed for the engine speed governor?

A magnetic pick-up.

What effect does a shorted resistor have on a series circuit?

A shorted series resistor has a higher than normal current flow and no voltage development across it. If there are additional resistors in the circuit, they will have higher than normal voltage development because they are the only resistance left in the circuit, and the sum of their voltage developments must equal the applied voltage.

What are the differences between a mixture, element, and compound?

An element is the simplest form of matter and cannot be broken down any further. A mixture is a combination of two or more elements, but there is no chemical interaction. A compound is two or more elements chemically combined to form another substance.

What happens if a silicon crystal is doped with an impurity like phosphorus?

An extra electron is added to the silicon crystal, making a new N-type material.

How does resistance in the E-C circuit affect operation?

An increase in resistance causes more voltage development across the E-C and less voltage is available to the load. A decrease in resistance has the opposite effect.

What is the difference between an open and a short?

An open is a break in the path for current flow. A short is a loss of resistance and results in maximum current flow.

What effect does an open resistor have on a parallel circuit?

An open resistor in a parallel branch stops current flow in that branch and causes total current flow to decrease and total resistance to increase. Total voltage in the circuit remains the same.

What PC card controls fault indicator lights DS3 thru DS6 and DS9?

Annunciator card.

What turns the SCR on?

Applying a small positive potential to the gate terminal.

Briefly explain how a capacitor charges in an AC circuit.

As AC voltage starts to increase, the capacitor starts to charge. When the applied voltage is at its peak, the charge voltage across the capacitor is at its maximum.

The generator set is operating under a 100-amp load and voltage is normal on all three phases. You turn S3 to B then to C phase and there's no indication of a load on C; where do you start your troubleshooting?

At phase selector switch (S3) or current transformer (CT3).

What purpose does the failsafe feature of the electric speed control governor serve?

Causes the output to go to zero any time the signal from the magnetic pick-up is less than 3 VAC.

What are the three types or configurations of transistor amplifiers?

Common base, common emitter, and common collector.

How does the engine cranking motor receive potential to engage the engine ring gear?

Depressing PB4 will energize the engine crank relay closing contacts ECR-1 energizing the engine crank solenoid. The engine crank solenoid energizes the cranking motor and drives the cranking motor pinion into the engine ring gear.

If one diode is connected into an AC circuit, what happens to the AC output?

During the positive half cycle cycle of AC sine wave, the diode is forward biased and allows current to flow. During the opposite cycle, current is blocked. The result is half wave DC output.

What's the purpose of the K14 memory time delay relay in the undervoltage relay circuit?

During voltage buildup the transistor provides a 4- to 12-second time delay to prevent tripping until generator voltage stabilizes. It also prevents the undervoltage circuit from activating above 102 VAC.

What happens if the E-B current is decreased in a transistor amplifier?

E-C current also decreases.

What are the formulas for voltage and current in a parallel circuit?

ET = E1 = E2 = E3, and so forth. IT = I1 + I2 + I3, and so forth.

What type of cold weather starting system is incorporated in the B809A generator set?

Electric grid, air intake heater.

How many system PC cards are contained in the system output fault monitor module (SOFM)?

Five.

What two potentiometers send a DC voltage signal to the engine speed governor (ESG) to determine the engine operating speed set point?

Frequency adjust trim potentiometer and speed control potentiometer

Where is the largest amount of current flow in the UJT?

From B1 to the emitter.

What relay is de-energized during high voltage operation to lockout the DC contactors?

High sense relay (HSR).

How does the low coolant level circuit operate?

If the coolant level in the radiator drops below the sensor, LLS sends a signal to the coolant module (A2). The low coolant light comes on and L6 shuts the unit down.

What is the main difference between induced voltage in a DC circuit and an AC circuit?

In a DC circuit, the opposition to current flow only occurs when the circuit is turned on or off. However, in an AC circuit, the current is continually changing and generating a CEMF to oppose the change in current.

What is meant by the term hole charge?

In a P-type material, the missing electron creates a hole. The hole takes on a positive charge in comparison to the electron.

How is the bipolar transistor turned off?

Interrupting or removing the biasing voltage.

What is the purpose of the left hand rule?

It is a simple means to identify the direction of the lines of force around a conductor.

What is the principle of operation of a zener diode?

It is a zener diode that operates in the reverse-bias mode. It blocks voltage until the avalanche breakdown point is reached and then allows current to flow. Once the breakdown point is reached, the zener diode maintains a constant voltage drop.

How do you calculate the rate of charge of a capacitor?

It is based on time constants. One time constant is the product of resistance and capacitance. It takes five time constants to fully charge a capacitor so TC = 5 (R x C).

How is capacitance determined?

It is determined by the size of the plates, the type dielectric material used, and the thickness of the dielectric.

How does the voltage regulator sense generator output?

It is sensed by a line drop compensation system which automatically adjust the generator output by sensing current flow through current. It receives voltage input at terminals L and K of the regulator. This voltage is in proportion to generator output and causes the amplifier to turn on the appropriate transistors to adjust generator output.

If you increase the number of turns in a coil, what effect will this have on the magnetic field?

It will increase the strength of the magnetic field.

What happens when S69 is flipped up to the GENERATOR position?

K2 is energized when the 24-volt power supply switch (S69) is placed in the GENERATOR position and twenty-four VDC are available from the 24-volt power supply and battery charger through E pin to the aircraft

What is the first component you should check if the load contactor K1 failed to close during operation?

Load contactor circuit breaker CB5.

What is a coulomb?

One coulomb is equal to the electric charge of 6.24 x 10-18 electrons or protons. If an object has a positive charge of one coulomb, it has 6.24 x 10-18 protons in it.

What fault conditions are sensed by the engine card?

Overspeed, low oil pressure, high coolant temperature, and low fuel.

What component is malfunctioning if you have the unit running with the load contactor closed and output is 134 VAC?

Overvoltage/undervoltage relay (K26).

When does the hour meter receive potential and begin operating?

Placing master switch S1 in the closed position allows potential to reach the hour meter.

What's the probable cause when you place the load contactor switch (S5) in the CLOSED position and the contactor light comes on, but goes out when you release the switch?

Plug interlock relay (K2) or fuse interlock relay (K17) contacts are not closing.

What is forward breakover voltage?

The amount of current required to gate the SCR.

How does the schematic diagram of a bipolar transistor differentiate between an NPN and a PNP transistor?

The arrow on the emitter always points towards the N-type material.

What is meant by holding current?

The current required to turn an SCR on or off.

What is meant by electrostatic force or electrostatic field of a capacitor?

The electric field created and stored between the two plates of the capacitor as it charges.

Briefly describe electron organization within an atom.

The electrons revolve around the nucleus in orbiting shells. Each shell is assigned a letter and a maximum capacity. The shell closest to the nucleus is identified as K shell, and holds a maximum of two electrons, L is the next shell, and can hold a maximum of eight electrons, etc.

What are the three leads of a bipolar transistor called?

The emitter, collector, and base.

How are voltage, current, and resistance calculated in a series-parallel circuit?

The formulas for series and parallel circuits apply. Reduce the parallel circuit to an equivalent series circuit.

What happens if CR3 is defective?

The generator exciter field wouldn't build up, and there would be no output voltage when S1 is placed in the MANUAL position

What happens if S53 fails to close?

The hour meter and fuel gauge wouldn't operate. S13 wouldn't receive potential and the unit wouldn't accelerate to governed speed.

To what does the term "lines of force" refer?

The magnetic field that extends around a magnet.

What determines the position of the engine throttle actuator?

The magnitude of DC control voltage applied to the torques motor.

Explain what is meant by the characteristics of high permeability and low reluctance.

The material has little opposition to the lines of force; therefore, it easily passes the magnetic lines of force.

You press S30 and the overvoltage and underfrequency lights don't come on. You start swapping light bulbs, but it doesn't fix the problem. What do you check?

The memory time delay relay (K14).

What factors affect the inductance of a coil?

The number of turns in the coil, the diameter of the core, the length of the coil, and the type of material used.

What is capacitive reactance?

The opposition a capacitor applies to changes in voltage in a circuit.

What happens in a differential amplifier if one transistor turns on more?

The other transistor turns off more.

What components in the engine safety circuit must be manually reset after tripping?

The overspeed switch (S22) and emergency shutdown latch.

What is the intrinsic standoff ratio?

The percent of voltage required to fire or turn on the UJT.

What are the subatomic particles of an atom?

The proton, neutron, and electron.

What are the three methods used to calculate resistance in a parallel circuit?

The reciprocal, equal resistance, and product sum methods.

How does the regulator card keep generator output constant during varying loads?

The regulator card senses the selected output voltage of the unit and varies the output pulse to the exciter field.

What happens when a positive potential is applied to the base of Q1?

The transistor turns on.

What factors affect resistance?

The type of material, area, and the temperature of the material.

What is the difference between a short circuit and an unwanted ground?

The unwanted ground involves a portion of the circuit instead of a single component.

What effect will a zener diode have if it is connected in the circuit in the forward-bias position?

The zener diode acts like a regular diode.

What is the difference between the power rating of the primary winding and the power rating of the secondary winding?

There is no difference. The power rating of a transformer is always the same in both windings.

For what is a bridge rectifier used?

To change both half cycles of alternating current into DC.

What's the function of the fuel injector pump?

To distribute and deliver an accurate amount of fuel to each injector nozzle in properly timed relationship.

What's the purpose of CR17?

To prevent current flow to the hour meter (M4) and engine-generator control switch (S13) when the panel light switch (S4) is closed.

How can you best determine where you should start checking a circuit for a malfunction?

Trace the circuit in question back to the place where you know there was power or pressure.

In a circuit using a bridge rectifier to change AC to DC, how many diodes are forward biased during each cycle of AC?

Two diodes are forward biased during each opposite cycle of alternating current.

Briefly explain what happens to Q2, Q3, and Q4 once Q1 is turned on.

Voltage drop across R25 increases so Q2 turns off more. This in turn applies less voltage to the base of Q3. Voltage at the base of Q4 increases and causes it to turn on more. Current now flows through the emitter and collector of Q4 to the exciter field, and generator output increases.

What are some of the uses of a zener diode?

Voltage reference, voltage regulator, and spike protector.

What components control exciter field strength?

Voltage regulator (VR1) and manual voltage control rheostat (R61).

What switch in the output circuit determines the contactor that will energize?

Voltage selector switch (VSS).

When should a load test be performed on a B809 Generator?

When carbon deposits build up and performed as part of any operational check following major maintenance actions to the engine, generator, or DC power supplies.