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Electrics > DC Alternators/Generators > Flashcards

DC Alternators/Generators Flashcards

1
Q

DC Alternators Components

A

Diodes
Rotary electromagnet
Stator is the field coils
Rectifier unit

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2
Q

How do we increase power generation capability in alternator

A

3 pairs of coils for 3-phase AC

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3
Q

Benefits of DC Alternator

A

No commutator and brush system so less weight less maintenance

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4
Q

Reverse current protection in DC alternator

A

Provided by the diodes in rectifier unit

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5
Q

Voltage Control in DC Alternator

A

Depends on speed of rotor
Number of turns of wire
Strength of magnetic field

RPM increases root increase - voltage regulator adjusts voltage supply to electromagnet to reduce/increase strength as required

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6
Q

Field Current/Excitation Current

A

Refers to the current passed through the field to control the output via the voltage regulator

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7
Q

Difference Between DC Generator and Alternator

A

DC Gen has -
Rotating Armature
Stationery field
Split Ring commutator
Low Power/weight ratio
High Maintenance
Can self excite

DC Alternator -
Stationery Armature
Rotating field
AC/DC bridge rectifier unit
High power to weight ratio
Low Maintenance
Cannot self excite

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8
Q

Rectifier Bridges

A

Made of diodes
Pass in one direction
Block half the since curve of AC
AC to DC
Sensitive to Static E limit current flow

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9
Q

Disadvantage of both DC Alternator and generator

A

Low engine RPM low current to exciter winding = weak magnetic field reducing V output

Issues with loads/consumers consistent power

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10
Q

Advantages of DC Alternator over DC generator

A

Save weight
Smaller Parts
Less brush wear and arcing due to lower current
Higher power/weight ratio

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11
Q

Output voltage on DC generator and alternator vary with…

A

RPM of engine driving generator
Loads attached
Field strength
Number of windings on rotor and stator

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12
Q

Starter Generators Function?

A

Unit starts as motor supplied with current via field changeover relay

Engine running self sustaining speed motor circuit isolated from power

Functions as conventional DC generator and connected to bus bar

Typically 300a AT 28V

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13
Q

Voltage Control in DC Systems

A

Varying current through exciter field to vary strength of magnetic field

  • Increase exciter current if output too low
  • Decrease exciter current if output is too high
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14
Q

Reverse Current Cut Out Relay

A

If generator V falls below bus bar voltage reverse current could occur

Disconnects generator from busbar or battery when reverse current flows

Redundancy usually back up reverse current circuit breaker

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15
Q

DC Gen Series Wound

A

Has poor voltage regulation not used for fluctuating loads

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16
Q

DC Generators Shunt Wound

A

Field Windings parallel with armature/loads

Output V decrease with increasing load

Used in Light DC generators that need lots of initial power

17
Q

DC Generator Compound Wound

A

Connected in series and parallel with armature

Expensive to make

Constant output with changes in load

18
Q

What is a Bus Bar?

A

Low resistance conductor service as a distribution point for loads

Consumers connected in parallel to busbar

19
Q

Essential Bus Bar Controls?

A

Compass/Radio/Flight controls/Flight instruments

20
Q

Non Essential Bus Bar

A

Galley Services

21
Q

Hot Bus Bar

A

Connected directly to the battery - hard wired cannot be switched off

Emergency lights etc

22
Q

Battery Bus Bar

A

Connected to battery
Checks/Start up
Can be disconnected

23
Q

What is a contractor

A

Mechanically latched solenoid
High voltage/high load
Connects generators to bus bars
Used as bus tie breakers

24
Q

Armature runaway

A

Rotor runaway current and torque decrease to try and slow
Everything decreases

25
Q

Rotor seizure

A

Everything increases - current/torque

26
Q

DC Generator summary

A

Has a rotating armature
Stationary field
Split ring commutator for AC to DC
Generated current has to pass through brushes and split ring commutator (arching and sparking)
Low power to weight ratio
High maintenance

27
Q

DC Alternator Summary

A

Rotating field magnetic
Stationary armature
AC to DC via a bridge rectifier unit
Only field current has to pass through brushes and slip rings
High power to weight ration

28
Q

Voltage Regulator

A

RPM load changes so voltage fluctuation therefore voltage sensitive variable resistor varies to provide constant field coil current to maintain voltage

Resistors in series with shunt wound field coil

29
Q

Starter Generator

A

Operate as starter motor to start engine using battery then operates as DC generator after start

+ Supplies 300A at 28V
+ Saves weight

Electrics (15 decks)

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