c172 alternator
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Joined: Jul 2010
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From: france
c172 alternator
Dear All
i am not sure i am posting in the right section of the forum, anyway i am about to do my BFR and probably i will do it in a C172. As i was reviewing the systems, i came across the electrical one, and one doubt crept into my mind: how does the alternator work? does it have a stationary field coil supplied with a trickle charge from the battery and a rotating armature driven by the engine via a drive belt and coupled to a commutator turning AC into Dc or is the armature stationary and the coil rotating? In case of an overvoltage in excess of 31.5Vdc, a breaker will trip, discontinuing the trickle charge from the battery to the field coil, causing the electromagnetic field to collapse and then voltage of the alternature to drop, taking the alternator offline as reflected by a decrease in Volt ( to 0?) and by a discharge on the ammeter as the load placed on the battery is increased?
Many thanks for your replies.
Baobab72
i am not sure i am posting in the right section of the forum, anyway i am about to do my BFR and probably i will do it in a C172. As i was reviewing the systems, i came across the electrical one, and one doubt crept into my mind: how does the alternator work? does it have a stationary field coil supplied with a trickle charge from the battery and a rotating armature driven by the engine via a drive belt and coupled to a commutator turning AC into Dc or is the armature stationary and the coil rotating? In case of an overvoltage in excess of 31.5Vdc, a breaker will trip, discontinuing the trickle charge from the battery to the field coil, causing the electromagnetic field to collapse and then voltage of the alternature to drop, taking the alternator offline as reflected by a decrease in Volt ( to 0?) and by a discharge on the ammeter as the load placed on the battery is increased?
Many thanks for your replies.
Baobab72
Thread Starter
Joined: Jul 2010
Posts: 255
Likes: 0
From: france
C172 alternator
First of all, many thanks for your reply
Second, probably it will be a new C172 not the one with Garmin 1000 but a new model.
Third, i have no clue if my instructor knows about how a generator works, and i am almost positive that he will not go that far into details, however i am fascinated about the systems of an aircraft, whatever it is, and i do like to understand them in details.
So to sum it up, the armature is stationary and it is connected to the commutator converting AC into DC, while the field coil is driven by the engine through a belt and it is excited by the trickle charge from the battery?
Baobab72
Second, probably it will be a new C172 not the one with Garmin 1000 but a new model.
Third, i have no clue if my instructor knows about how a generator works, and i am almost positive that he will not go that far into details, however i am fascinated about the systems of an aircraft, whatever it is, and i do like to understand them in details.
So to sum it up, the armature is stationary and it is connected to the commutator converting AC into DC, while the field coil is driven by the engine through a belt and it is excited by the trickle charge from the battery?
Baobab72
Joined: Feb 2006
Aviation Qualifications: LAME
Posts: 36,145
Likes: 5,739
From: Falling off the end of the thread
CESSNA AIRCRAFT COMPANY
MODEL 172
MAINTENANCE MANUAL
ELECTRICAL POWER - GENERAL
1. Scope
A. This chapter gives the electrical units and components which control and supply electrical power for
the airplane systems. This includes the alternator, batteries, and relays.
B. Electrical energy for the airplanes is supplied by a 28-volt, direct current, single primary bus, negative
ground electrical system. A single 24-voltmain battery supplies power to the starting system and gives
a reserve source of power if an alternator failure were to occur. Airplanes that have the Garmin G1000
system have a second battery known as the Standby Battery. The Standby Battery is controlled and
monitored by the Standby Battery Controller and supplies power to the G1000 Essential Bus if there
is a failure of the main battery and alternator. A power junction box, also referred to as a Master
Control Unit (MCU), is attached to the forward left side of the firewall and includes electrical relays, an
alternator control unit (ACU), an ammeter sensor, an external power receptacle, fuses and/or circuit
breakers in a single box. An engine-driven alternator is the normal source of power during flight and
maintains a battery charge controlled by the ACU. The external power receptacle is used for ground
operation of the electrical equipment and helps the main battery during ground starts.
C. Electrical power is supplied to the two primary electrical busses through two 30A fuses, two 30A circuit
breakers, or two 40A circuit breakers in the junction box. These electrical busses supply power to two
avionics busses through 15A circuit breakers. The two avionics busses are controlled by an avionics
master switch.
D. The operation of the main battery and alternator system is controlled by the MASTER ALT BAT switch.
The switch is an interlocking split rocker and is found on the left side of the switch panel. The right
half of the rocker controls the main battery and the left half controls the alternator. It is possible
in this configuration for the main battery to be online without the alternator. However, operation of
the alternator without the main battery is not possible. The BAT MASTER switch, when operated,
connects the main battery contactor coil to ground so that the contacts close and supply power to the
system from the main battery only. The ALT MASTER switch, when ON, applies positive voltage to
the ACU and to the alternator contactor coil at the same time, which then applies field voltage to the
alternator field and supplies power to the electrical system from the alternator.
E. The operation of the Standby Battery, if installed, is controlled by a three-position STDBY BATT switch.
Normal flight operation is with the switch in the ARM position that lets the standby battery charge from
the G1000 Essential Bus. If there is an alternator failure, the standby battery controller will not let the
standby battery discharge to the G1000 Essential Bus until the depletion or failure of the main battery.
It is necessary during preflight to do an "energy level" acceptance test. Refer to the Pilot's Operating
Handbook, Chapter 4, Starting Engine, for details of the "energy level" acceptance test.
F. The main battery ammeter is controlled by a sensor found in the power junction box. In flight, without
the use of external power, the meter shows the quantity of current that flows to or from the battery.
With a low battery and the engine at cruise speed, the ammeter will show a large positive output and a
charge of the main battery. When the main battery is fully charged, the ammeter will show a minimum
charge rate.
G. The main battery is a 24-volt, 12.75 Amp-hour (5-hour rate), flooded lead-acid type. The battery is
installed in the front-left side of the firewall.
2. Tools, Equipment and Materials
NOTE: Equivalent substitutes can be used for the following items:
24-00-00 Page 1
© Cessna Aircraft Company Jul 3/2006
CESSNA AIRCRAFT COMPANY
MODEL 172
MAINTENANCE MANUAL
ALTERNATOR CONTROL UNIT - MAINTENANCE PRACTICES
1. General
A. The Alternator Control Unit (ACU) is found inside the power junction box, also referred to as a Master
Control Unit (MCU) or J-Box. The alternator system includes the ACU, Alternator Contactor, and
alternator field circuit. The ACU functions are as follows:
(1) Alternator Voltage Regulation - The ACU controls the alternator field circuit to supply a main bus
voltage of approximately 28.5 volts.
(2) Low Voltage Annunciation - The ACU monitors the main bus voltage in the power junction box
and supplies an output for low voltage (less than 24.5 +0.35 or -0.35 volts) for the annunciation.
(3) Over-voltage Protection - The ACU monitors the main bus voltage in the power junction box and
disengages the aircraft ALT FIELD circuit breaker. This removes the power from the alternator
system if there is an over-voltage condition greater than 31.75 +0.5 or -0.5 volts.
(4) Reverse Alternator Current Protection - The ACU monitors the alternator output current and
disengages the aircraft ALT FIELD circuit breaker. This removes the power from the alternator
system if there is a reverse alternator current.
(5) Excess Field Current Protection - The ACU monitors the alternator field current and disengages
the aircraft ALT FIELD circuit breaker. This removes the power from the alternator system if
there is an excessive field current.
2. Alternator Control Unit Removal/Installation
A. Remove the Alternator Control Unit. Refer to Power Junction Box - Maintenance Practices,
Component Removal/Installation.
B. Install the Alternator Control Unit. Refer to Power Junction Box - Maintenance Practices, Component
Removal/Installation.
3. Over-voltage Protection Circuit Test
A. General.
(1) The ACU Over-voltage Protection Circuit must be tested in accordance with the time limits in
Chapter 5, Inspection Time Limits. Use one of the two procedures that follow to do the test of
the Over-voltage Protection Circuit. The recommended procedure uses the Lamar TE04 MCU
Test Set. The external battery procedure can be used if a TE04 test set is not available.
B. Over-voltage Protection Circuit Test with the Lamar TE04 MCU Test Set
(1) Use a Lamar TE04 MCU Test Set and do steps 4.2, 4.3.A, 4.3.B, and 4.3.I in the Lamar’s TE04
MCU Test Set instructions LI-0021(refer to Electrical Power - General, Tools, Equipment, and
Materials).
(2) If the ACU TRIP indicator on the TE04 MCU Test Set does not illuminate in step 4.3.I, the Overvoltage
Protection Circuit is not operational.
(a) Replace the ACU.
(b) Do this test again.
(3) If the ACU TRIP indicator does illuminate in step 4.3.I, the Over-voltage Protection Circuit is
operational.
(a) Complete the Lamar procedure 4.3.I.
(b) Remove the TE04 MCU Test Set.
(c) Continue with step D in this section.
24-60-01 Page 201
© Cessna Aircraft Company Jan 1/2008
MODEL 172
MAINTENANCE MANUAL
ELECTRICAL POWER - GENERAL
1. Scope
A. This chapter gives the electrical units and components which control and supply electrical power for
the airplane systems. This includes the alternator, batteries, and relays.
B. Electrical energy for the airplanes is supplied by a 28-volt, direct current, single primary bus, negative
ground electrical system. A single 24-voltmain battery supplies power to the starting system and gives
a reserve source of power if an alternator failure were to occur. Airplanes that have the Garmin G1000
system have a second battery known as the Standby Battery. The Standby Battery is controlled and
monitored by the Standby Battery Controller and supplies power to the G1000 Essential Bus if there
is a failure of the main battery and alternator. A power junction box, also referred to as a Master
Control Unit (MCU), is attached to the forward left side of the firewall and includes electrical relays, an
alternator control unit (ACU), an ammeter sensor, an external power receptacle, fuses and/or circuit
breakers in a single box. An engine-driven alternator is the normal source of power during flight and
maintains a battery charge controlled by the ACU. The external power receptacle is used for ground
operation of the electrical equipment and helps the main battery during ground starts.
C. Electrical power is supplied to the two primary electrical busses through two 30A fuses, two 30A circuit
breakers, or two 40A circuit breakers in the junction box. These electrical busses supply power to two
avionics busses through 15A circuit breakers. The two avionics busses are controlled by an avionics
master switch.
D. The operation of the main battery and alternator system is controlled by the MASTER ALT BAT switch.
The switch is an interlocking split rocker and is found on the left side of the switch panel. The right
half of the rocker controls the main battery and the left half controls the alternator. It is possible
in this configuration for the main battery to be online without the alternator. However, operation of
the alternator without the main battery is not possible. The BAT MASTER switch, when operated,
connects the main battery contactor coil to ground so that the contacts close and supply power to the
system from the main battery only. The ALT MASTER switch, when ON, applies positive voltage to
the ACU and to the alternator contactor coil at the same time, which then applies field voltage to the
alternator field and supplies power to the electrical system from the alternator.
E. The operation of the Standby Battery, if installed, is controlled by a three-position STDBY BATT switch.
Normal flight operation is with the switch in the ARM position that lets the standby battery charge from
the G1000 Essential Bus. If there is an alternator failure, the standby battery controller will not let the
standby battery discharge to the G1000 Essential Bus until the depletion or failure of the main battery.
It is necessary during preflight to do an "energy level" acceptance test. Refer to the Pilot's Operating
Handbook, Chapter 4, Starting Engine, for details of the "energy level" acceptance test.
F. The main battery ammeter is controlled by a sensor found in the power junction box. In flight, without
the use of external power, the meter shows the quantity of current that flows to or from the battery.
With a low battery and the engine at cruise speed, the ammeter will show a large positive output and a
charge of the main battery. When the main battery is fully charged, the ammeter will show a minimum
charge rate.
G. The main battery is a 24-volt, 12.75 Amp-hour (5-hour rate), flooded lead-acid type. The battery is
installed in the front-left side of the firewall.
2. Tools, Equipment and Materials
NOTE: Equivalent substitutes can be used for the following items:
24-00-00 Page 1
© Cessna Aircraft Company Jul 3/2006
CESSNA AIRCRAFT COMPANY
MODEL 172
MAINTENANCE MANUAL
ALTERNATOR CONTROL UNIT - MAINTENANCE PRACTICES
1. General
A. The Alternator Control Unit (ACU) is found inside the power junction box, also referred to as a Master
Control Unit (MCU) or J-Box. The alternator system includes the ACU, Alternator Contactor, and
alternator field circuit. The ACU functions are as follows:
(1) Alternator Voltage Regulation - The ACU controls the alternator field circuit to supply a main bus
voltage of approximately 28.5 volts.
(2) Low Voltage Annunciation - The ACU monitors the main bus voltage in the power junction box
and supplies an output for low voltage (less than 24.5 +0.35 or -0.35 volts) for the annunciation.
(3) Over-voltage Protection - The ACU monitors the main bus voltage in the power junction box and
disengages the aircraft ALT FIELD circuit breaker. This removes the power from the alternator
system if there is an over-voltage condition greater than 31.75 +0.5 or -0.5 volts.
(4) Reverse Alternator Current Protection - The ACU monitors the alternator output current and
disengages the aircraft ALT FIELD circuit breaker. This removes the power from the alternator
system if there is a reverse alternator current.
(5) Excess Field Current Protection - The ACU monitors the alternator field current and disengages
the aircraft ALT FIELD circuit breaker. This removes the power from the alternator system if
there is an excessive field current.
2. Alternator Control Unit Removal/Installation
A. Remove the Alternator Control Unit. Refer to Power Junction Box - Maintenance Practices,
Component Removal/Installation.
B. Install the Alternator Control Unit. Refer to Power Junction Box - Maintenance Practices, Component
Removal/Installation.
3. Over-voltage Protection Circuit Test
A. General.
(1) The ACU Over-voltage Protection Circuit must be tested in accordance with the time limits in
Chapter 5, Inspection Time Limits. Use one of the two procedures that follow to do the test of
the Over-voltage Protection Circuit. The recommended procedure uses the Lamar TE04 MCU
Test Set. The external battery procedure can be used if a TE04 test set is not available.
B. Over-voltage Protection Circuit Test with the Lamar TE04 MCU Test Set
(1) Use a Lamar TE04 MCU Test Set and do steps 4.2, 4.3.A, 4.3.B, and 4.3.I in the Lamar’s TE04
MCU Test Set instructions LI-0021(refer to Electrical Power - General, Tools, Equipment, and
Materials).
(2) If the ACU TRIP indicator on the TE04 MCU Test Set does not illuminate in step 4.3.I, the Overvoltage
Protection Circuit is not operational.
(a) Replace the ACU.
(b) Do this test again.
(3) If the ACU TRIP indicator does illuminate in step 4.3.I, the Over-voltage Protection Circuit is
operational.
(a) Complete the Lamar procedure 4.3.I.
(b) Remove the TE04 MCU Test Set.
(c) Continue with step D in this section.
24-60-01 Page 201
© Cessna Aircraft Company Jan 1/2008
Last edited by NutLoose; 6th May 2011 at 12:26.
Joined: Jan 2008
Posts: 1,191
Likes: 0
From: lancs.UK
Strictly speaking, there is no armature in an alternator. Instead, there is a rotor with laminated-iron poles and a continuous coil terminated in slip-rings. these can be cylindrical or end-face (a disc surrounded by a concentric ring)
the rotor is surrounded by a stator,-again, laminated-iron, with multiple poles upon which the output-coils are wound..
By energising the rotor ,it will induce a current in the stator windings. the energising input is regulated to give the desired output-voltage at normal operating speeds and to disconnect the machine when output-voltage is less than the connected battery....with a blocking-diode fitted, the main output is safely left connected directly to the battery,as no back-flow can take place.
I assume that in aircraft applications it is disconnected via the master switch and thus fully isolated.
Normally, the battery is used to provide initial excitation current and in solid-state circuits it can be arranged to be the "earthy" side of the warning-lamp. as the machine starts generating, the output opposes the battery-voltage and the light goes outdue to the battery and generator-voltages reducing the current-flow.
in summary, the generating-coils are stationary, the magnet (electromagnet) revolves. output is AC , normally rectified by a diode-pack....regulation of field-current can be solid-state or electro-mechanical.
IMHO, the former is more reliable and i think that early 20th century equipment belongs in the museum.
(ducks incoming)
the rotor is surrounded by a stator,-again, laminated-iron, with multiple poles upon which the output-coils are wound..
By energising the rotor ,it will induce a current in the stator windings. the energising input is regulated to give the desired output-voltage at normal operating speeds and to disconnect the machine when output-voltage is less than the connected battery....with a blocking-diode fitted, the main output is safely left connected directly to the battery,as no back-flow can take place.
I assume that in aircraft applications it is disconnected via the master switch and thus fully isolated.
Normally, the battery is used to provide initial excitation current and in solid-state circuits it can be arranged to be the "earthy" side of the warning-lamp. as the machine starts generating, the output opposes the battery-voltage and the light goes outdue to the battery and generator-voltages reducing the current-flow.
in summary, the generating-coils are stationary, the magnet (electromagnet) revolves. output is AC , normally rectified by a diode-pack....regulation of field-current can be solid-state or electro-mechanical.
IMHO, the former is more reliable and i think that early 20th century equipment belongs in the museum.
(ducks incoming)
Thread Starter
Joined: Jul 2010
Posts: 255
Likes: 0
From: france
alternator c172
many thanks for your replies.
Hopefully this time i get it right.
The alternator represents the primary source of DC power when the engine is running and it basically consists of a stationary armature and of a rotating field coil driven by the engine through a drive belt and trickle charged from the battery through the ACU and an alternator contactor field, generating an electromagnetic field: as the coil revolves around the stationary armature, it induces as a result of an electromagnetic induction phenomenon a certain amount of induced current into the armature then converted into a direct current flow by rectifier, and the magnitude of which depends upon the strength of the electromagnetic field and thus of the trickle charge and upon the rotational velocity of the field coil with respect to the armature.
The alternator is used in conjunction with an ACU or Alternator Control Unit housed in a power junction box also referred to as the Master Control Unit or MCU located on the forward left side of the firewall and designed to provide a constant 28.5VDC supply to the Main Bus system, by increasing the trickle charge and thus the strength of the electromagnetic field as the load on the electrical system is increased or as the engine spools down or by decreasing it as the load on the electrical system is decreased or as the engine spools up.
In addition the ACU will provide a low voltage indication should the main bus voltage supply drop below 24.5VDC; over-voltage protection should the voltage supply to the main bus exceed 31.5VDC forcing an ALT FIELD CB open, cutting off the trickle charge to the field coil, causing the electromagnetic field to collapse and thus the output of the alternator to drop as reflected by a drop on the voltmeter and by a discharge indication on the ammeter as the battery copes with the extra load and reverse current protection.
Alternator operation is controlled by the ALT half of the split rocker switch that when turned ON it will energize the ACU and the alternator contactor field, restoring the trickle charge to the field coil and thus flashing the electromagnetic field back.
anything to review, or add to it?
Thanks
Baobab72
Hopefully this time i get it right.
The alternator represents the primary source of DC power when the engine is running and it basically consists of a stationary armature and of a rotating field coil driven by the engine through a drive belt and trickle charged from the battery through the ACU and an alternator contactor field, generating an electromagnetic field: as the coil revolves around the stationary armature, it induces as a result of an electromagnetic induction phenomenon a certain amount of induced current into the armature then converted into a direct current flow by rectifier, and the magnitude of which depends upon the strength of the electromagnetic field and thus of the trickle charge and upon the rotational velocity of the field coil with respect to the armature.
The alternator is used in conjunction with an ACU or Alternator Control Unit housed in a power junction box also referred to as the Master Control Unit or MCU located on the forward left side of the firewall and designed to provide a constant 28.5VDC supply to the Main Bus system, by increasing the trickle charge and thus the strength of the electromagnetic field as the load on the electrical system is increased or as the engine spools down or by decreasing it as the load on the electrical system is decreased or as the engine spools up.
In addition the ACU will provide a low voltage indication should the main bus voltage supply drop below 24.5VDC; over-voltage protection should the voltage supply to the main bus exceed 31.5VDC forcing an ALT FIELD CB open, cutting off the trickle charge to the field coil, causing the electromagnetic field to collapse and thus the output of the alternator to drop as reflected by a drop on the voltmeter and by a discharge indication on the ammeter as the battery copes with the extra load and reverse current protection.
Alternator operation is controlled by the ALT half of the split rocker switch that when turned ON it will energize the ACU and the alternator contactor field, restoring the trickle charge to the field coil and thus flashing the electromagnetic field back.
anything to review, or add to it?
Thanks
Baobab72
Joined: Dec 2004
Posts: 832
Likes: 0
From: East Anglia
One key thing is that an alternator generates AC voltage.
There is a separate box that rectifies this to convert it to DC. This is usually solid state and sometimes it is built into the alternator housing, sometimes it is a seperate box or integral with the regulator which controls the voltage.
I think you said this but wanted to make sure.
There is a separate box that rectifies this to convert it to DC. This is usually solid state and sometimes it is built into the alternator housing, sometimes it is a seperate box or integral with the regulator which controls the voltage.
I think you said this but wanted to make sure.
Joined: Jan 2008
Posts: 1,191
Likes: 0
From: lancs.UK
Dynamo = Fixed field-coils in the casing, rotating armature has multiple windings the terminations of which are diametrically opposite segments of a commutator.....each coil is positioned so that brushes contactthe segments just as the field -poles induce current on the armature......the output is DC.
the alternator is, in some senses an inside-out dynamo.-as previously stated, the armature's GENERATING FUNCTION is replaced by a stator (the casing or carcase of the machine) The rotor,as it's name suggests, rotates inside and concentric to, the stator.....often it contains residual magnetism which can "kick-off" the generation,when spun. Primarily, it uses a single winding and although it still needs slip-rings and brushes to energise the spinning rotor (field coil equivalent) the contact is continuous, therefore, unlike a dynamo, the ring/brush arrangement has a very long service life, whereas the dynamo's segments wear the brushes more rapidly. (old trick, blow conductive dust out with a blast from an airline and it sometimes revives a machine which had been "tracking-down" through excess deposits )
In all cases the ALTernator output is ALTernating current and is rectified either internally or externally (diode-pack) (a 3-phase bridge, iirc ) and a regulator which senses machine output or battery-voltage and regulates the rotor-field accordingly. (there is a slight difference between the machine and battery due to wiring/connector/battery internal resistance) Temperature can also be compensated for as it helps extend the service life of the battery.
Lucas, AC Delco, Paris-Rhone ,Ducellier and many other Alternator manufacturers use the nomenclature above (stator / rotor) I believe theirs are the correct names.
the alternator is, in some senses an inside-out dynamo.-as previously stated, the armature's GENERATING FUNCTION is replaced by a stator (the casing or carcase of the machine) The rotor,as it's name suggests, rotates inside and concentric to, the stator.....often it contains residual magnetism which can "kick-off" the generation,when spun. Primarily, it uses a single winding and although it still needs slip-rings and brushes to energise the spinning rotor (field coil equivalent) the contact is continuous, therefore, unlike a dynamo, the ring/brush arrangement has a very long service life, whereas the dynamo's segments wear the brushes more rapidly. (old trick, blow conductive dust out with a blast from an airline and it sometimes revives a machine which had been "tracking-down" through excess deposits )
In all cases the ALTernator output is ALTernating current and is rectified either internally or externally (diode-pack) (a 3-phase bridge, iirc ) and a regulator which senses machine output or battery-voltage and regulates the rotor-field accordingly. (there is a slight difference between the machine and battery due to wiring/connector/battery internal resistance) Temperature can also be compensated for as it helps extend the service life of the battery.
Lucas, AC Delco, Paris-Rhone ,Ducellier and many other Alternator manufacturers use the nomenclature above (stator / rotor) I believe theirs are the correct names.
Thread Starter
Joined: Jul 2010
Posts: 255
Likes: 0
From: france
C172 alternator
Thanks once again for the replies.
So, to sum it up, the armature is the stator and is outside, and the field coil is the rotor driven by the engine via the drive belt and it spins inside the stator-armature?
The resulting current is AC which is then converted by a solid state rectifier into DC.
Are those correct statements?
baobab72
So, to sum it up, the armature is the stator and is outside, and the field coil is the rotor driven by the engine via the drive belt and it spins inside the stator-armature?
The resulting current is AC which is then converted by a solid state rectifier into DC.
Are those correct statements?
baobab72
