AC generators Flashcards
What are AC waveforms also known as?
Alternating current waveforms vary with time, so they can be called “time-dependent signals” or “periodic signals”
What is the amplitude also known as?
Voltage
Which axis is for the amplitude?
The Y
Which axis is for time?
X
What types of AC generator is there?
The permanent magnet generator
The rotating armature generator
The rotating field generator
How does a permanent magnet generator work?
The engine driven rotor of the machine is a permanent magnet and as the magnet is rotated its magnetic field cuts the stationary output windings producing an alternating voltage (and current) output.
How does a rotating armature work?
In this arrangement, the armature is wound on the rotor where the winding moves through a stationary magnetic field. It is very similar to the DC generator except the commutator is replaced with Slip Rings. Like the DC generator, the stator windings are fed with a DC current to produce a stationary magnetic field and the loop of wire or rather loops of wires mounted on a former, induce an EMF which is then fed to the load via the slip rings and brushes.
What is a weakness of the rotating armature generator?
The weakness of this type of generator is the relatively low limit output because of arc over and shorts from high voltages due to the difficulty of insulating the slip rings and brushes. Also, a combination of current load (demand) and friction on the brushes generate heat which limits the output, so is seldom used on aircraft.
How does a rotating field generator work?
The rotating field type generator switches the output and field around. Instead of magnetising the stator, the rotor is magnetised through the slip rings and brushes.
Does the rotating field generator solve the problem of heat production in rotating armature?
Since the voltage applied to the rotating field is low voltage DC, the problem of high voltage arc-over at the slip rings does not exist.
In a rotating field generator, what do the slip rings do?
The slip rings only carry the field supply which is the smaller DC voltage and current.
What does the stator of a rotating field generator do?
The stator of this type of generator, holds the windings that are cut by the rotating magnetic field. The voltage generated in the armature as a result of this cutting action is the AC power that will be applied to the load.
Summerise the two main types of AC generation.
There are two main types of generators called rotating field and rotating armature.
Rotating field has a field winding on the rotor and armature in the stator.
Rotating armature has an armature in the rotor and field winding in the stator.
Rotating armature is only used for DC generators, as it is impossible to get DC output on stationary armatures.
Rotating field is advantageous for AC generators, especially at voltages of 11 kV or more.
What is an issue with two phase AC generators?
There are vibrational and uneven heating problem associated with such machines as proper two-phase balance may not be achieved.
Moreover, system becomes complex and the cost of energy compared to a three-phase generation system will increase.
In an AC system, a star connection has how many wires?
Four wires
