GENERATORS and ALTERNATORS Flashcards
(30 cards)
What is Electromagnetic Induction ?
- Magnetism can be used to generate electricity by converting mechanical energy to electrical energy by Electromagnetic Induction.
- If a conductor is moved in a magnetic field the conductor will ‘cut through’ the invisible lines of flux. - When this happens an Electromotive Force EMF (voltage) is induced into the conductor as long as the conductor keeps moving. If the conductor stops the induced EMF ceases. It does not matter if the conductor or the magnetic field is moved as long as there is relative movement between the two.
- If the conductor is connected to a complete circuit then a current will flow in the circuit in
proportion to the induced EMF.
FLEMING’S RIGHT HAND RULE
The direction of the current can be determined by Fleming’s Right Hand Rule . To do so, align the first finger with the field from the North Pole to the South Pole. Point the thumb in the direction of rotation and the second finger will show the current direction.
The magnitude of the induced voltage can be affected in three ways:
The magnitude of the induced voltage can be affected in three ways:
– The rate of cutting of lines of force. (Speed)
– The strength of the magnetic field. (Flux density)
– The number of turns of wire. (Larger coil)
FARADAY’S LAW
LENZ’S LAW
Faraday’s law states:
When the magnetic flux through a coil is made to vary, a voltage is set up. The magnitude of this induced voltage is proportional to the rate of change of flux.
Lenz’s Law states:
A change of flux through a closed circuit induces a voltage and sets up a current. The direction of this current is such that its magnetic field tends to oppose the change in flux.This action produces a back EMF
SIMPLE GENERATOR
The simplest form of a generator is a single loop of wire turning in a fixed magnetic field produced
by a permanent magnet . The closed circuit is made by attaching rotating slip rings
to both ends of the loop which are in contact with stationery carbon brushes. Continuous
contact between the slip rings and the brushes is maintained by spring pressure. The brushes
are attached to cables which form a closed circuit.
– The rotating loop is known as the armature.
– The magnetic field is termed the field.
– In a simple generator the armature rotates in the field.
– An EMF is induced in the armature by electromagnetic induction.
To produce a DC output from the simple generator it is required to change the ……….. induced
into the armature to a DC output at the generator terminals. This is done by replacing the slip
rings with a …………………..
To produce a DC output from the simple generator it is required to change the AC EMF induced
into the armature to a DC output at the generator terminals. This is done by replacing the slip
rings with a Split Ring Commutator.
In an aircraft system the generator, load and battery are all in …………. with each other. The bus
bar is a distribution point. The generator output voltage is maintained ………………. than
battery voltage to maintain …………………..
In an aircraft system the generator, load and battery are all in parallel with each other. The bus
bar is a distribution point. The generator output voltage is maintained slightly higher than
battery voltage to maintain the battery charged.
Differentiate between a DC GENERATOR and an ALTERNATOR
DC GENERATOR ALTERNATOR
-Rotating Armature
-Stationary Field
-Converts AC to DC by means of a commutator
-Suffers from arcing and sparking at the commutator as the high load current has to flow through the commutator and brushes
ALTERNATOR
- Stationary Armature
- Rotating Field
- High load current taken from stationary armature eliminates arcing and sparking. Small
field current only flows through slip rings.
How does a voltage regulator of a generator work ?
–The only practical method of controlling the output voltage of a generator is to control the strength of the magnetic field by controlling the current flow in a coil wound around the magnetic pole pieces (field coil or field winding) Control of the current flow is achieved by a voltage regulator.
–A voltage regulator consists of:
» A variable resistance in series with the field coil. In older voltage regulators the variable resistance was achieved using a Carbon Pile. In modern voltage regulators it is achieved by employing an electronic solid state system of transistors, diodes and resistors. The net result is the same which ever is used.
» A control coil in parallel with the field coil and the armature. This is used to sense the generator output voltage and vary the resistance to control the current through the field coil, therefore controlling the voltage.
»The voltage regulator senses the output voltage of the generator or alternator and adjusts
the field current to maintain the correct output voltage irrespective of generator speed or
electrical load
An EMF is induced in a conductor rotating in a magnetic field by:
a. capacitive reaction.
b. the reverse current relay.
c. electro transmission.
d. electro magnetic induction.
D
Magnetic field strength is controlled by:
a. battery bus bar current.
b. current in the field coil.
c. current in the armature.
d. current flow to the battery.
B
If a conductor is placed in a magnetic field:
a. an EMF is induced in the conductor.
b. an EMF is induced in the conductor only when the conductor rotates.
c. the applied resistance assists the back EMF.
d. an EMF is induced in the conductor only when the conductor is stationary.
B
The output of a basic generator before commutation is:
a. AC
b. DC and after commutation is AC.
c. DC
d. synchronised AC and DC.
A
An internally excited generator is one where:
a. the field is produced within the distribution.
b. the field is initiated by a HT and LT coil.
c. the field is initiated by the battery.
d. the field is initiated within the generator.
D
A DC generator has a commutator whose purpose is to:
a. change AC to give a generator output of DC.
b. change DC to AC.
c. transmit the generator output to the electrical circuit and to cool the generator.
d. maintain a constant resistance.
A
Another name for a number of conductors rotating in a magnetic field is:
a. a capacitor.
b. an armature.
c. a condenser.
d. a commutator.
B
A generator is governed so that:
a. the EMF is constant and the rate of flow varies.
b. the rate of flow is constant and the EMF varies.
c. the generator voltage reduces generator temperature.
d. back EMF is equal and opposite to the applied EMF.
A
The voltage regulator:
a. senses cut out pressure and adjusts field current.
b. senses generator output pressure and adjusts field current.
c. senses generator output current and adjusts the field voltage.
d. senses back EMF.
B
The generator master switch is normally:
a. fitted with a mechanical safety catch.
b. in the field circuit which is connected in parallel with the generator output.
c. in the field circuit which is in parallel with the voltage regulator.
d. fitted in series with the commutator.
B
The voltage regulator:
a. provides a constant current flow from the generator with changes of generator speed.
b. senses current output.
c. maintains a steady generator voltage with changes of generator speed.
d. regulates the amount of current supplied by the battery to operate the generator.
C
Voltage is controlled in a generator by:
a. a reverse current relay.
b. moving the brushes.
c. a voltage regulator.
d. it is uncontrollable.
C
On aircraft, generator voltage is regulated by:
a. varying the generator field strength.
b. increasing and decreasing the load.
c. changing the generator speed.
d. changing generator load.
A
In an aircraft having a battery with a nominal voltage of 24v, generator output would be:
a. 24 volts.
b. 28amps.
c. 28 volts.
d. 24 amps.
C
