Module 3.17 — AC Generators

What is an AC generator?

A machine that converts mechanical energy into alternating electrical energy.

What is another name for an AC generator?

An alternator.

What is the basic operating principle of an AC generator?

Electromagnetic induction.

What type of energy enters an AC generator?

Mechanical energy.

What type of energy leaves an AC generator?

Alternating electrical energy.

What is electromagnetic induction?

Generation of an EMF by changing magnetic flux linkage.

What is required for a conductor to generate an EMF?

Relative movement that causes it to cut magnetic flux.

What happens when a conducting loop rotates in a magnetic field?

An alternating EMF is induced.

What happens to the induced EMF as a loop continues rotating?

Its magnitude and polarity vary periodically.

What type of waveform is produced by a simple rotating loop?

A sinusoidal alternating waveform.

Why does a rotating loop produce alternating rather than constant EMF?

The direction and rate of flux cutting change throughout rotation.

What is a rotor?

The rotating assembly of an electrical machine.

What is a stator?

The stationary assembly of an electrical machine.

What are the two principal types of AC generator described by BA?

Rotating-armature and rotating-field alternators.

What is a rotating-armature alternator?

An alternator whose output windings rotate through a stationary magnetic field.

What is a rotating-field alternator?

An alternator whose magnetic field rotates past stationary output windings.

What is the main difference between the two alternator types?

Whether the output winding or magnetic field rotates.

Which alternator type is commonly used on aircraft according to BA?

Rotating-field.

Which alternator type does BA say is not normally used on aircraft?

Rotating-armature.

What is the main advantage of stationary alternator output windings?

They can carry substantial load current without rotating output connections.

What is another advantage of stationary output windings?

They can be cooled more effectively.

Why can stationary output conductors be made larger?

They are not subjected to rotor centrifugal forces.

What advantage does a rotating-field alternator have regarding slip rings?

Only the smaller field excitation current needs to pass through them.

How does rotating-field excitation current normally compare with output current?

It is much smaller.

Why are smaller slip rings possible in a rotating-field alternator?

They carry field excitation current rather than full load current.

What is the main purpose of an alternator's field?

To provide magnetic flux linking the generating windings.

What is the main purpose of an alternator's output winding?

To develop and supply the generated electrical output.

What does BA Figure 1 compare?

A rotating coil in a stationary field and a rotating field with a stationary coil.

What is common to both arrangements in BA Figure 1?

Relative movement between magnetic flux and conductor.

What output waveform is shown for both BA Figure 1 arrangements?

The same alternating waveform.

Why can a stationary coil generate an EMF?

A rotating magnetic field changes the flux linking it.

Which rule does BA use to determine induced-current direction?

Fleming's right-hand rule.

What does the thumb indicate in Fleming's right-hand rule?

Relative conductor motion.

What does the first finger indicate in Fleming's right-hand rule?

Magnetic field direction.

What does the second finger indicate in Fleming's right-hand rule?

Induced conventional current direction.

What happens to induced current direction if relative conductor movement reverses?

It reverses.

What happens to induced current direction if magnetic field polarity reverses?

It reverses.

What happens if both the field and relative movement reverse?

Induced current direction remains unchanged.

What does BA Figure 2 illustrate?

A rotating-armature alternator.

Where are the generating windings in a rotating-armature alternator?

On the rotor.

Where is the main magnetic field in a rotating-armature alternator?

On the stationary part.

What component collects generated output from a rotating armature?

Slip rings and brushes.

Why must rotating-armature slip rings be sized for substantial current?

They carry the main generator load current.

What is a disadvantage of carrying generator output through slip rings?

High-current rotating contacts increase construction and maintenance demands.

What does BA Figure 3 illustrate?

A rotating-field alternator.

Where are the generating windings in a rotating-field alternator?

On the stator.

Where are the field windings in a rotating-field alternator?

On the rotor.

What cuts the stationary stator windings in a rotating-field alternator?

The rotating magnetic field.

What is induced in the stator when the rotor field rotates?

Alternating EMF.

What does a conventional rotating-field alternator's slip-ring assembly carry?

DC field excitation.

Does the magnetic polarity of an excited rotor winding reverse merely because the rotor rotates?

No; its magnetic poles remain fixed relative to the rotor.

What creates the alternating output in a rotating-field alternator?

Alternating flux linkage experienced by the stationary windings.

What two magnetic-field sources are mentioned for rotating-field alternators?

Permanent magnets and DC-excited windings.

What is one limitation of a simple permanent-magnet field in BA's basic alternator discussion?

The basic example produces relatively small output.

What type of field system does BA describe for most aircraft AC generators?

DC-excited electromagnetic windings.

Why is DC supplied to the rotor field winding?

To establish a magnetic field of fixed polarity on the rotor.

What is excitation current?

Current used to establish a generator's magnetic field.

Why does the BA basic wound-field alternator require a source of excitation?

Its field must be energised before normal output can be generated.

What is a PMG?

Permanent Magnet Generator.

What function can an integral PMG perform?

Provide an initial source of electrical power for excitation and control.

Why is a PMG useful in the generator system?

It can generate power from permanent magnetic flux without relying on the main generator output.

What does the field strength affect in an alternator?

The magnitude of generated voltage.

How does stronger field excitation affect generated EMF at constant speed before saturation?

It increases generated EMF.

How does weaker field excitation affect generated EMF at constant speed?

It decreases generated EMF.

How does increasing rotor speed affect generated voltage under comparable field conditions?

It increases generated EMF.

How does decreasing rotor speed affect generated voltage under comparable field conditions?

It decreases generated EMF.

How does increasing the number of active linked conductor turns affect generated voltage?

It increases generated voltage.

How does reducing the number of active linked conductor turns affect generated voltage?

It decreases generated voltage.

What three methods of increasing generated voltage does BA summarise?

Increase speed, increase active winding turns or increase magnetic field strength.

What electrical quantity is mainly determined by rotor rotational speed and pole count?

Frequency.

What happens to generated frequency when rotor speed increases?

Frequency increases.

What happens to generated frequency when rotor speed decreases?

Frequency decreases.

What is the relationship between frequency and rotational speed for fixed pole count?

Direct proportionality.

What happens to frequency if rotor speed doubles at constant pole count?

It doubles.

What happens to frequency if rotor speed halves at constant pole count?

It halves.

Can increasing field excitation independently double the generated frequency?

No; frequency depends on speed and pole count.

Can field excitation change output voltage without changing generator frequency?

Yes, under otherwise unchanged operating conditions.

What is rectification?

Conversion of alternating electrical output to unidirectional output.

Why may an alternator require rectification?

To provide DC for battery charging.

What component is commonly used to rectify AC?

A semiconductor diode.

What is the basic electrical property of a rectifier diode?

It readily conducts in one direction and restricts the opposite direction.

What hydraulic component does BA compare with a rectifier?

A non-return valve.

What is forward bias in a rectifier diode?

Polarity that permits conduction.

What is reverse bias in a rectifier diode?

Polarity that normally prevents conduction.

What were early metal rectifiers constructed from?

Stacks of plates using copper oxide or selenium.

What cooling arrangement was required for many early rectifiers?

Good airflow around the rectifier stacks.

What environmental conditions made early rectifiers difficult to protect?

Water, salt and dirt.

What advantage does a modern semiconductor rectifier have over early metal rectifiers?

It is smaller and can be incorporated into the generator assembly.

What can happen when excessive forward current passes through a rectifier diode?

Overheating and failure.

What can happen if excessive reverse voltage is applied to a diode?

Reverse breakdown and possible destruction.

What can excessive operating temperature do to a semiconductor diode?

Cause thermal damage or failure.

Can a diode be internally damaged without obvious external appearance changes?

Yes.

What is half-wave rectification?

Rectification using only one half of each AC cycle.

What type of output is produced by a simple half-wave rectifier?

Pulsating DC.

What happens to the unwanted half-cycle in a half-wave rectifier?

It is blocked.

Why is half-wave rectification less efficient at using the AC waveform?

Only one half-cycle contributes to useful output.

What is full-wave rectification?

Rectification that uses both halves of an AC waveform to produce unidirectional output.

What rectifier arrangement does BA identify as common in alternators?

A full-wave bridge.

What direction does current take through the load of an ideal bridge rectifier?

The same direction during both half-cycles.

What does BA Figure 5 illustrate?

A single-diode half-wave rectifier.