C172R electrical system

Hi Folks 

i wrote a dissertation on the electrical system of the C172r and i would like to know if what i wrote is correct!

The electrical system of the C172 is a 28 volt direct current, single primary bus, negative ground electrical system powered by a 28vdc 60amp engine driven alternator and by a 24vdc lead acid battery and using the fuselage for grounding.
The main power source during normal operation is represented by the alternator comprised of an axial rotor and of a stator; the rotor driven by the engine through the alternator belt basically consists of an electromagnet self excited by a trickle charge from a Voltage Regulator or VR while the stator built around the rotor basically consists of several coils of copper wire wound in slots in a soft iron frame: as the rotor spins inside the stator as a result of an eletromagnetic induction phenomenon it induces in the stator coils a certain induced alternating current due to the reversing of the polarity of the rotating electromagnet and the magnitude of which depends upon the rotational velocity of the rotor and thus upon the engine RPM's and upon the strenght of the electromagnetic field and thus upon the trickle charge - about 2amps- supplied through a set of slip rings mounted on the rotor and modulated by the VR in such a manner to maintain a constant bus voltage of 28.5vdc, increasing the trickle charge and thus the strength of the electromagnetic field and the magnitude of the induced current as the electrical load increases and/or the engine slows down, or decreasing the trickle charge as the load decreases and/or the engine spools up.
The induced alternating current from the stator coils is fed through a rectifier consisting of a diode pack blocking the flow in one direction while allowing it in the opposite direction, converting the alternating current into direct current supplied to the primary bus through a 60 amp ALT cb.
In addition to modulating the trickle charge alternator the VR provides as well:
- Over-voltage Protection - monitoring the main bus voltage and eventually interrupting the trickle charge to the rotor should the bus voltage exceed 31.5vdc as detected by an over-voltage sensor, causing the electromagnetic field to collapse and thus the output of the generator to drop to zero, resulting in the battery supplying the electrical system as reflected by a discharge indication on the ammeter and triggering a HIGH VOLTAGE warning light. The overvoltage sensor can be reset by cycling the MASTER switch off and then back on provided that there is no indication of electrical fire and/or smoke.
- Excess Field Current Protection - monitoring the alternator field current and tripping
the ALT FIELD circuit breaker in case of an excessive field current in excess of 5 amps.
- Alternator Output Over-voltage Protection - monitoring the output of the alternator and eventually tripping the ALT cb isolating the alternator from the bus network should the power output exceed 60amps probably due to an issue wiith the grounding of the alternator itself.
Alternator operation is controlled by the ALT side of the MASTER switch that when flipped to the ON position powers through the over-voltage relay the VR which in turn supplies the trickle charge to the axial rotor flashing the electromagnetic field bringing the alternator online.
The battery located in the nose section of the aircraft is designed to chemically store the electrical energy required to start the engine and as a back up to the alternator in case of an alternator failure.
Power from the main battery is made available to the starter contactor solenoid and to the bus nework via the battery contactor basically consisting of a plunger type solenoid: as the BATT side of the split rocker MASTER switch is flipped to the ON position, the coil of the contactor is grounded allowing battery power to be applied to the solenoid driving the plunger down, completing a circuit and allowing battery power to be supplied to the starter contactor and through the ammeter to the PRIMARY BUS.
The battery contactor has three poles: one connected to the battery allowing battery power to be supplied to the clock and whenever the oil pressure sensor detects a rise in oil pressure to the hobbsmeter at all times whenever the battery is connected; one connected to the BATT side of the MASTER switch controlling the grounding of the contactor coil and thus the operation of the same solenoid and one supplying battery power whenever the solenoid is energized to the starter contactor solenoid powering the electrical starter and controlled by the IGNITION switch and through the ammeter to the PRIMARY BUS. An additional terminal on the battery contactor solenoid allows to supply external power from an external power unit plugged through a receptacle to the starter, paralleling the battery in cranking the engine up. Besides a so called battery contactor closing circuit allows to shunt a small charge in the contactor area, allowing to close the solenoid even if the battery charge is low.
Distribution-wise the electrical system is comprised of a primary bus and of an avionics bus powered by the primary bus via a toggle AVIONICS PWR switch/cb controlling both the flow of electrical current from the primary bus to the avionics bus - when flipped up the avionics bus is powered, when down it is unpowered - and serving as a cb as well isolating the avionics bus and automatically resetting the switch to the down position in case of a short regardless of the MASTER switch position.
To monitor the electrical system besides the LOW VOLTAGE light, the crew is provided with an ammeter providing an indication of the rate of charge/discharge of the battery.
Prior to starting the engine and bringing the engine driven alternator online, as the battery is supplying the whole electrical system including the starter, the ammeter shows a minus (-) left needle deflection, indicating that the battery is not being recharged. As the engine is started and the alternator is brought online, as the voltage on the PRIMARY BUS increases above 24 volts, the current flows from the bus to the battery through the starter contactor and the battery contactor due to the stronger electromotive force on the PRIMARY BUS with respect to the battery output - 24 vs 28 volts - , recharging the battery as reflected by a plus (+) needle deflection on the ammeter. The deflection at first more prominent as the alternator is recharging the battery after the start up, should decrease gradually as the battery is becoming recharged, stabilizing itself after approximately 30' of flight within two needle widths, regardless of the load placed on the alternator.
Should the alternator fail as the voltage on the PRIMARY BUS drops below the output of the battery, the load is placed once again on the battery as reflected by a discharging indication on the ammeter.

kind regards

diego

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