Module 3.18 — AC Motors Construction

What is an AC motor?

An electrical machine that converts alternating-current electrical energy into mechanical energy.

What form of energy does an AC motor receive?

Electrical energy.

What form of energy does an AC motor produce?

Mechanical energy.

What are the three main AC motor types identified by BA?

Synchronous motors, induction motors and series or commutator motors.

Which AC motor type does BA describe as the most common on aircraft and in industry?

The induction motor.

Which AC motor type does BA describe as commonly used in domestic equipment?

The series or commutator motor.

What is the main operating principle of synchronous and induction AC motors?

AC applied to the stator produces a rotating magnetic field that interacts with the rotor.

What is the stator?

The stationary part of an electrical machine.

What is the rotor?

The rotating part of an electrical machine.

What is the main advantage of many AC motors over conventional DC motors?

Simpler construction with fewer brushes and commutators.

Do all AC motors have commutators?

No.

Do all AC motors have slip rings?

No.

Why can AC motors require less maintenance than DC commutator motors?

Many designs have no commutator or brush gear.

What electrical quantity strongly influences AC motor speed?

Supply frequency.

Why are AC motors useful on constant-frequency aircraft electrical systems?

Many types operate at approximately constant speed.

Do all AC motors operate at constant speed regardless of type?

No.

Which AC motor type does BA identify as having variable-speed characteristics similar to a DC series motor?

The AC series or commutator motor.

Can AC motors operate from single-phase supplies?

Yes.

Can AC motors operate from polyphase supplies?

Yes.

What does polyphase mean?

An AC supply containing two or more phase-displaced alternating quantities.

What is the most common polyphase supply discussed by BA?

Three-phase AC.

What does BA identify as the main starting disadvantage of a basic synchronous motor?

It is not self-starting.

Why may synchronous and induction motor characteristics be combined in one machine?

To provide induction starting and synchronous running.

What other AC motor types are included in BA 3.18?

Shaded-pole and hysteresis motors.

What is a rotating magnetic field?

A magnetic field whose resultant direction continuously rotates.

How can a rotating magnetic field be produced in a three-phase stator?

Supply suitably spaced stator windings with three-phase AC.

How many phase winding groups are used in BA's three-phase stator example?

Three.

What letters identify the three phases in BA Figure 1?

A, B and C.

What labels identify the opposite windings of each phase in BA Figure 1?

A1, B1 and C1.

How are the two windings of each phase connected in BA Figure 1?

In series.

How are the two windings of a single phase wound relative to one another?

To produce opposite magnetic poles.

What poles are produced by current flowing from A to A1 in BA Figure 1?

A becomes north and A1 becomes south.

What happens to magnetic pole polarity when current through a phase reverses?

North and south poles reverse.

What type of supply connection is illustrated in BA Figure 2?

Delta.

How many external terminals does the Delta-connected stator use?

Three.

What is the phase displacement between balanced three-phase currents?

120 electrical degrees.

How does the magnetic field of an individual stator phase vary?

It follows the magnitude and polarity of its alternating current.

What happens to an individual phase's magnetic field when its current reverses?

Its magnetic polarity reverses.

Why are the three stator magnetic fields displaced from one another?

Their phase currents are separated by 120°.

How is the resultant magnetic field obtained from the three phase fields?

By vector addition.

Does the resultant three-phase field remain fixed in one direction?

No, it rotates.

What does BA Figure 3 illustrate?

The continuously changing direction of the resultant magnetic field in a three-phase stator.

What is happening to phase A at Position 1 in BA Figure 3?

Its instantaneous input current is zero.

Which phases produce magnetic effects at Position 1 in BA Figure 3?

Phases B and C.

What determines the resultant magnetic axis at any instant?

The combined effects of the three phase currents.

What path does magnetic flux preferentially follow?

A path of low magnetic reluctance.

What is magnetic reluctance?

Opposition to magnetic flux.

What happens to the direction of the resultant field as the three-phase currents progress through a cycle?

It moves continuously around the stator.

How far does the resultant field rotate during one complete AC cycle in BA's two-pole illustration?

360°.

How many mechanical revolutions does the rotating field make per electrical cycle in BA's two-pole illustration?

One.

Can stationary windings create a rotating magnetic field?

Yes.

What makes the magnetic field rotate if the stator coils do not physically move?

The changing magnitudes and polarities of the phase currents.

How does the resultant magnetic field strength behave in BA's ideal balanced three-phase example?

It is approximately constant.

How does the resultant field speed depend on supply frequency?

Higher frequency produces higher rotating-field speed.

What happens to rotating-field speed if supply frequency increases at fixed pole count?

It increases.

What happens to rotating-field speed if supply frequency decreases at fixed pole count?

It decreases.

Why is the direction of the rotating magnetic field important?

It determines the direction in which the rotor is driven.

What happens to the field's direction as the three-phase supply progresses through its normal phase sequence?

It rotates in the corresponding sequence.

What is the main purpose of a rotating stator field?

To produce the magnetic interaction needed to turn the rotor.

What does BA Figure 4 illustrate?

A synchronous motor rotor following the rotating magnetic field.

What is a synchronous AC motor?

A motor whose rotor runs in step with the rotating stator magnetic field.

What does synchronous mean in motor operation?

The rotor rotates at the same speed as the stator's rotating magnetic field.

What construction is similar to that of a synchronous motor?

A rotating-field AC generator.

Why can a generator also operate as a motor?

Electromechanical energy conversion can operate in either direction.

Where is the AC supply connected in BA's synchronous motor?

To the stator windings.

What type of field is produced by the synchronous motor's stator?

A rotating magnetic field.

What type of magnetic field source can be fitted to a synchronous motor rotor?

A permanent magnet or DC-excited field winding.

What type of current energises the wound rotor of BA's synchronous motor?

DC.

Why is DC applied to a synchronous motor rotor winding?

To establish magnetic poles that can align with the rotating stator field.

How does an energised synchronous rotor behave magnetically?

Like a bar magnet.

What does a magnetic rotor tend to do in a rotating magnetic field?

Align itself with the field.

Why does a synchronous rotor rotate once synchronism is established?

Its magnetic poles remain locked to the rotating stator field.

What is magnetic locking in a synchronous motor?

The rotor and stator field maintaining a fixed relative magnetic position while rotating together.

What is synchronous speed?

The speed of the stator's rotating magnetic field.

What is the rotor speed of a synchronous motor after it has locked in?

Equal to synchronous speed.

What is the slip of an ideal synchronously running motor?

Zero.

Does a synchronous motor normally run slightly below synchronous speed when its load increases?

No, it remains synchronous while magnetic lock is maintained.

What determines synchronous motor speed according to BA?

The frequency of the applied AC supply and the motor's field configuration.

What happens to synchronous motor speed if supply frequency increases at fixed pole arrangement?

It increases.

What happens to synchronous motor speed if supply frequency decreases at fixed pole arrangement?

It decreases.

What makes a synchronous motor useful for constant-speed applications?

Its rotor follows the rotating field at synchronous speed.

What aircraft-related application characteristic does BA associate with synchronous motors?

Constant-speed drives.

What ventilation application does BA give for synchronous motors?

Ventilation fans.

Can a basic synchronous motor start directly from rest on its own in BA's arrangement?

No.

Why is a basic synchronous motor not self-starting?

The rotating field moves too quickly for the stationary rotor to establish sustained starting rotation.

What physical property prevents the stationary rotor from immediately following the rotating field?

Rotor inertia.

How is BA's basic synchronous motor brought up to speed?

By an auxiliary starting motor.

What kind of motor is commonly used as the auxiliary starter?

A small induction motor.

What happens as the synchronous rotor approaches rotating-field speed?

It can lock magnetically to the field.

What happens after the rotor has locked into synchronism?

It continues to rotate at synchronous speed.

What is meant by a single-speed synchronous motor?

A motor whose running speed is fixed by the supply frequency and pole arrangement.

How does a synchronous motor respond to a moderate increase in load while remaining locked?

It continues at synchronous speed.

What happens if mechanical load becomes excessive?

The rotor can lose synchronism.

What happens when the magnetic lock is broken in BA's basic synchronous motor?

The motor stops.

Why does BA associate basic synchronous motors with comparatively light mechanical loads?

Excessive loading can break synchronism.

What is the main difference between synchronous motor starting and running?

Starting requires assistance; once locked, it runs at synchronous speed.

What is an induction motor?

An AC motor whose rotor current is produced by electromagnetic induction from the stator field.

Which AC motor type is most commonly used on aircraft according to BA?

The induction motor.

What supply types can induction motors be designed for?

Single-phase, two-phase or three-phase AC.

What is a principal advantage of the basic three-phase induction motor?

It is simple, robust and relatively inexpensive.