generator alternator in trainers

[baobab72]

Dear All

i have a couple of final questions about the generator/alternator used on trainers which might help to dissipate my doubts once for all, since i guess i had it wrong cos 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!!
Baobab72

[Whopity]

I think you should be aware that generators (dynamos) are quite different to alternators. In a DC generator the field winding is the Stator which is normally fed via a switch. The Rotor contains a number of coils all connected to a commutator (mechanical rectifier) which provides a pulsed DC output. As only a part of the rotor is ever in use the device is relatively inefficient.

In the case of an alternator, which is normally 3 phase, it is mechanically difficult to pick off the 3 phases from the rotor so the design is normally reversed. The Rotor is the field winding fed via slip rings from the Battery using the Alternator switch. The 3 windings of the Stator provide 3 phase ac which is fed to the semiconductor rectifier pack providing the DC output.

The Bat switch connects the Battery to the Busbar. The Alternator switch connects the field of the generator/alternator, with no field, no magnetism, no output.

Trickle charge is usually the term used to describe the charging of a battery not a field coil. By interrupting the field coil current, the output voltage of the generator can be controlled.

A switch is designed to switch a current, the points in it open and close quickly and are wide to prevent the arc from causing excessive pitting. A CB is designed to isolate a circuit not to switch it, if it is used as a switch it will have limited life.

The magnitude and frequency of the emf is proportional to the relative velocity. Alternators will be frequency wild unless operated at a constant speed, that is why the output is rectified to DC where it is more usable. AC power systems in aircraft did not really appear until the constant speed drive was developed.

[baobab72]

Many thanks for your prompt reply!

it was very detailed and it cornfirmed what i had in mind: the rotor is the one being supplied by the battery via the slip rings, generating the electromagnetic field, as it it spins, it induces the emf in the stator, consisting of 3 loops, three phases, from where the emf is fed to a bridge of diodes which convert the AC into DC. Therefore the ALT switch controls the supply of the field current from the battery to the field coil - the rotor - through the VR, while the BATT switch controls the supply of battery power to a dedicated bus.

IS THE ALTERNATOR POWERED FROM THE DC BATT BUS? Cos by looking at the C172 electrical system schematic, it looks like power from the battery is fed to a battery contactor - it does not show any relay/switch between the contactor and the battery: so i guess that from the contactor is made available to the MASTER switch consisting of a BATT half and of an ALT half. I believe that the BATT half controls the supply to the distribution system and also to the ALT half even though it is not shown on the schematic, so that when you flip the BATT switch to ON, it will connect it to the bus network and it will allow the juice to be provived through the ALT switch when set to ON to the VR.
Is this a correct statement?
Also how is powered taken from the stator?
And finally how is the battery kept charged cos the schematic is not really clear about that, I guess the battery must be constantly recharged since it supplies the field coil current apart from powering hot wired items.

Many Thanks

Baobab

[Whopity]

It is probably better to think of the BAT switch as a Battery Isolation switch which controls a contactor of some sort. Your statement appears correct.

The output from the Stator goes to the rectifier pack which is internal, so a single DC output terminal is normally provided with the frame as ground. This would normally go directly to the main Busbar. The battery would normally be connected to the DC bus, via the zero reading ammeter in Cessna aircraft. The manual may well show only major components.