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6 Answers

c172 alternator

Asked by: 14854 views Aircraft Systems

Hi

I browsed the whole forum but i couldn t find what I was looking for: a clear yet detailed explanation of the alternator found on se trainers such as the c172 or the pa28, with special reference to its components - rotating armature or rotating electromagnet? where is the electromagnet getting its trickle charge from? is part of the alternator output used to feed the trickle charged and if so, is it controlled by the VR?, how is AC output converted in DC? how should i interpret the ammeter?

 

Many Thanks & Happy Contrails

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6 Answers



  1. Terry Scott on Sep 06, 2012

    The field coil get juice from the battery when you turn on the master, its the one you turn off when it fails to reset the alternator. The current in the field coil makes the magnetic field that drives the electrons out and down the wires to the voltage regulator through a rectifyer into the buss to the battery and beyond. A rectifyer is one or more diodes made up of a silicon junction that only lets electrons pass in one direction. (Magic, they still are not sure if its electrons flowing one way or the holes where electrons need to be in the other that gives us the magic of electricity but that is a topic for a quantum physics blog).
    If the meter reads in voltage it should go up after starting to show charging and maybe drop a bit when the battery is full. If it reads in current it should behave the same way. They seem to be build that way. If it drops or pegs to the top do what you POH says to do! The soone go get it to read normal the less it will cost.

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  2. Terry Scott on Sep 06, 2012

    I just left you a long explanation and the internet ate it! Do what the POH says if it does not read right. Surf a car site for alternator basics. Same thinf pre computer chips.

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  3. diego on Sep 07, 2012

    Hi Terry Thanks for the concise explanation: to dissipate any doubt:

    is it a true statement that the stationary field coil surrounding the rotating armature behaves like an electromagnet as it gets its trickle charge from the battery through the close switch controlled by the battery master?

    that as the rotating armature driven by the engine spins within the electromagnetic field produced by the stationary field coil, cutting through the lines of flux, an induced alternating electrical current is generated?

    that the magnitude of the induced electrical current depends upon the strength of the electromagnetic field controlled by modulating the trickle charge to the stationary field coil, and upon the rotational velocity of the armature and thus on the engine RPM?

    that the main purpose of the battery is to store the energy required to jump start the stationary field coil and to supply the electrical system when the emf from the alternator drops below the charge of the battery, as it is the case during an alternator failure or when the alternator is not on line?

    Many Thanks for your willingness

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  4. diego on Sep 07, 2012

    Hi
    just to make sure that i have got that right, the path of the electricity when the engine is running is:

    – the negative charged electrons will travel from the negative post of the battery to the airframe
    – along the airframe to the earthing post on the device
    – through the device making it work
    – along the wire to the switch
    – through the switch, breakers, etc
    – back to the positive terminal of the battery.

    When the switch is off, no electrons will flow (even though the switch is on the “wrong side” of the device) as the circuit is not complete.

    The alternator will supply electrical power to the battery keeping it charged: this design, having the alternator supplying the battery which in turn supplies the electrical system, allows to compensate for the ripple effect as the armature revolves progressing through a zero output, max positive peak, zero output, max negative peak, back to zero.

    the alternator portion of the swicth controls the supply of power from the alternator to the battery, while the battery side controls the power from the battery to the distribution system – negative post –> airframe –> grounding lead –> bus.

    thanks

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  5. John D. Collins on Sep 07, 2012

    The typical alternator has a rotor and a stator. The rotor has a coil wound around it called the field and is attached to a set of slip rings that connect thru brushes to the field posts on the alternator. The stator is usually a fixed set of three coils that surround the rotating rotor coil. These coils don’t rotate and are oriented in 120 degree segments. When a current flows in the field coil on the rotor, it generates a magnetic field whose strength is a function of the amount of current flowing. Since the rotor is turning inside the alternator, the rotating magnetic field induces an AC current and voltage in the fixed stator coils. The three coils are out of phase with each other by 120 degrees, so the voltage output of the three coils are also out of phase with each other to the same degree. Each coil has a set of diodes that are used to rectify (converts AC to DC) the voltage. There are various configurations that the diodes are connected to one another, but the simplest is to tie the outputs together into a single waveform. The resulting voltage is the combination of the three out of phase voltages and has a significant amount of ripple. The combined alternator stator outputs are connected to the battery which acts as a large capacitor to smooth the voltage. In some installations, a capacitor is added to smooth the ripples even further.

    So where does the field get its current from. It gets its current from the voltage output of the regulator. The voltage regulator measures the buss voltage and if it is below the set point of the regulator (for a 14 volt system usually 14.25 volts +/- .25 volts or for a 28 volt system 28.5 volts +/- .5 volt), it will increase the voltage to the field of the alternator, thus increasing the current flow in the field and causing the alternator to generate a higher output voltage. When the voltage is above the regulator set voltage, the regulator reduces the voltage on the field, thereby reducing the current flow in the field and the resulting alternator output. Before you turn on the alternator, there is no current flowing in the alternator field and the battery is providing all the voltage and system current demand. The diodes in the alternator prevent it from having current flow backwards from the battery thru the alternator stator fields. When you turn on the alternator, current is allowed to flow from the battery to the voltage regulator, which in turn starts to provide a high voltage to the field. This starts the process of the alternator generating a voltage output and the regulator continues until the buss voltage reaches the set point value. If the RPM is too low and the demand for current is too high, the voltage regulator will reach its maximum output voltage (just below battery voltage) and the alternator will not be able to increase the voltage further until the load is reduced or the RPM increases. After startup, the battery is somewhat depleted and is in need of heavy charging. This will place an extra demand on the alternator to recharge the battery. As the battery gets recharged, it will demand less and less current. You can normally tell the health of the alternator if the voltage is near its setpoint value within a few minutes after starting and you don’t see a discharge on the battery. The setpoint is normally several volts higher than the battery voltage (12.5 battery 14.25 alternator setpoint or 24.5 volts battery verses the alternator setpoint of 28.5 volts).

    If the battery fails in flight, the alternator will normally keep running as it provides a voltage that can be used by the regulator to keep the system running. But if you turn off the alternator, you depend on the battery for starting the alternator running again.

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  6. diego on Sep 08, 2012

    Dear All
    many thanks for your explanations!
    i guess i had it wrong since i was under the impresson that the trickle charge from the battery to the VR was fed to the stator and that the rotating armature was actually the component supplying the emf! while if i understand it correctly, the stator consisting of the 3 coils spaced 120° apart and built around the rotor driven by the engine is the one supplying the AC! emf which is induced as the trickle charge is supplied to the rotating armature aka field coil, through the slip rings/brush assembly, generating an electromagnetic field and in which the same armature is spun – Faraday’s principle stating that whenver a conductor is moved into an electromagnetic field a certain emf is induced in the conductor – in this case the source of the electromagnetic field is represented by the rotor while the conductor by the stator! – and the magnitude of the emf is proportional to the relative velocty of the conductor with respect to the electromagnetic field and to the strength of the same field!
    Is that a correct statement?
    also what is the function of the BATT switch, the ALT switch i guess controls the trickle charge to the VR and then to the rotor through an alternator field relay – in case of an overvoltage the VR will trip the relay, cutting off the trickle charge to the field coil, causing the electromagnetic field to collapse and thus the output of the alternator to drop to zero! but the BATT switch what does it exactly control?
    Also how is the emf picked from the stator?
    And finally the right sequence for the CB’s is SOURCE –> CB –> switch –> item or source –> switch –> CB –> item?
    many thanks!!

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