Stator vs Rotor: How They Work in Motors and Generators

Take the cover off an AC induction motor and look inside. The outer assembly is fixed to the motor housing, while the inner assembly is attached to a shaft and spins. The outer part is the stator. The rotating part is the rotor.

But there’s an interesting twist. The stator doesn’t physically rotate, yet it can produce a rotating magnetic field. The rotor can also carry an electric current without being connected directly to the power supply.

Understanding the difference between these two components means separating three things: what physically moves, what produces the magnetic field, and where the electrical current flows.

01Stator vs rotor: the essential difference

The definitions are straightforward:

• Stator: The stationary part of an electric motor or generator. It is normally attached to the machine’s frame or housing.
• Rotor: The rotating part of an electric motor or generator. It is usually attached to a shaft that transfers mechanical energy.

In a conventional induction motor, the stator surrounds the rotor. A small gap between them, called the air gap, allows the rotor to turn without touching the stator.

The important differences are summarised below.

Notice that the electrical roles are not fixed.

In an induction motor, the stator receives AC power and induces current in the rotor. In an alternator, the rotor can carry the magnetic field while the stator contains the windings in which the output voltage is generated.

The names stator and rotor describe physical movement, not electrical function.

There is also an exception to the usual arrangement. Some outer-rotor motors have a stationary inner assembly and a rotating outer shell. The rotating component is still the rotor, even though it sits outside the stator.

02Why does a stationary stator produce a rotating magnetic field?

This is where things become more interesting.

Imagine looking directly into a three-phase AC induction motor. Around the inside of the stator are three sets of windings, connected to a three-phase AC supply.

The three alternating currents have the same frequency but are separated in phase by \(120^\circ\).

Each winding produces its own magnetic field. As the currents change, the strength and direction of these fields change too.

The fields combine to produce one resultant magnetic field that continuously rotates around the inside of the motor.

This is called a rotating magnetic field.

A useful analogy is a stadium wave.

Each spectator stays in their seat, but the pattern of raised arms travels around the stadium. The movement belongs to the pattern, not to the people.

The same idea applies here. The stator windings stay in place, but the combined magnetic field moves around the motor.

The analogy has a limit: a magnetic field isn’t a physical object being passed between coils. Its direction changes because the individual magnetic fields combine differently at each instant.

How fast does the magnetic field rotate?

The speed of the rotating magnetic field is called the synchronous speed.

It depends on the AC supply frequency and the number of magnetic poles in the stator.

The relationship is:

\[ n_s=\frac{120f}{P} \]

Where:

• \(n_s\) = synchronous speed, measured in revolutions per minute (rpm).
• \(f\) = AC supply frequency, measured in hertz (Hz).
• \(P\) = total number of magnetic poles in the stator.

The factor of 120 accounts for the conversion from electrical cycles per second to mechanical revolutions per minute, taking the number of pole pairs into account.

For an Australian 50 Hz supply, some common synchronous speeds are:

A four-pole motor therefore has a rotating magnetic field travelling at 1500 rpm when supplied at 50 Hz.

Importantly, the stator itself remains stationary.

03The experiment that established the rotating-field principle

How did scientists establish that an alternating electrical supply could produce a continuously rotating magnetic field?

In 1885, Italian physicist Galileo Ferraris constructed an early two-phase induction motor. His apparatus demonstrated that two alternating magnetic fields could combine to produce rotation.

The experiment addressed an important problem: an alternating current produces a changing magnetic field, but how could that field produce continuous rotation rather than simply alternate in strength?

Setting up Ferraris’s apparatus

Ferraris arranged two stationary coils at right angles to one another.

Between the coils, he placed a small copper cylinder mounted so that it could rotate freely on a pivot.

The two coils were supplied with alternating currents of the same frequency and approximately equal amplitude, but with a phase difference of \(90^\circ\).

The arrangement can be described spatially:

1. The first coil sits along one axis.
2. The second coil sits perpendicular to the first.
3. The copper cylinder sits between the two coils.
4. Both coils receive phase-shifted alternating currents.
5. The cylinder is free to rotate.

What should the experiment show?

If each coil produced a magnetic field that simply increased and decreased along its own axis, the combined field could behave differently from a field that only oscillated backwards and forwards.

The prediction was that the two perpendicular fields, changing at different times, would combine to produce a magnetic field that continuously changed direction.

If this happened, the copper cylinder should begin rotating.

What was observed?

The copper cylinder rotated.

This supported the idea that a rotating magnetic field could produce rotational motion in a conducting object without physically rotating the coils or a permanent magnet.

The important observation was not simply that the copper cylinder moved. It was that the electrical arrangement could create a continuously rotating magnetic field.

Why did the cylinder rotate?

The changing magnetic field induced circulating currents within the copper cylinder.

These are called eddy currents.

The induced currents interacted with the magnetic field, producing magnetic forces that generated a torque on the cylinder.

This is an early example of electromagnetic induction being used to produce mechanical motion.

The historical experiment demonstrated the principle, but it was not yet the efficient industrial motor used today.

Later induction motor designs developed more effective closed conducting paths in the rotor and improved multi-phase stator arrangements. In particular, the development of three-phase systems allowed smoother rotating fields and practical motor designs.

The physical interpretation is the same as in modern induction motors: a rotating magnetic field induces currents in a conductor, and those currents interact with the field to produce torque.

Historical background: Karlsruhe Institute of Technology’s history of the electric motor and B. J. G. van der Kooij’s historical case study from TU Delft.

04How the rotating field induces current in the rotor

Now consider what happens inside a modern squirrel-cage induction motor.

The stator and rotor are separated by a small air gap. The stator windings are connected to the AC supply, but the rotor has no direct electrical connection to that supply.

So how does current appear in the rotor?

The answer is electromagnetic induction.

The process follows a clear sequence.

Step 1: The stator produces a rotating magnetic field.

Three-phase AC flows through the stator windings, creating a magnetic field that rotates around the motor.

Step 2: The rotating field moves relative to the rotor conductors.

The rotor contains conducting bars. As the magnetic field moves past these bars, the magnetic flux associated with the rotor circuits changes.

Step 3: An emf is induced in the rotor.

According to Faraday’s law, a changing magnetic flux induces an electromotive force (emf).

An emf is a voltage produced by an energy source or by electromagnetic induction.

Step 4: Current flows through the rotor.

In a squirrel-cage rotor, conducting bars are connected at both ends by conducting rings called end rings.

These connections form closed electrical paths, allowing the induced current to flow.

Step 5: The rotor develops torque.

The current-carrying rotor conductors experience magnetic forces in the stator’s magnetic field.

The forces combine to produce a torque that turns the rotor in the same direction as the rotating field.

The key point is that the rotor current is induced, not supplied directly through an external electrical connection.

This is an application of the same changing-flux principle used in mutual induction between solenoids.

There is one important condition, however.

The rotating magnetic field must move relative to the rotor to keep inducing an emf.

That brings us to slip.

05What is slip, and why does an induction motor need it?

An induction motor has two rotational speeds:

• Synchronous speed (\(n_s\)): the speed of the stator’s rotating magnetic field.
• Rotor speed (\(n_r\)): the actual rotational speed of the rotor and shaft.

These speeds are not normally equal when the motor is producing torque.

The difference between them is called slip.

For an induction motor operating in motoring mode:

\[ s=\frac{n_s-n_r}{n_s} \]

Where:

• \(s\) = slip, expressed as a decimal.
• \(n_s\) = synchronous speed in rpm.
• \(n_r\) = rotor speed in rpm.

To calculate percentage slip:

\[ s_{\%}=\frac{n_s-n_r}{n_s}\times100 \]

Why doesn’t the rotor reach synchronous speed?

Suppose the stator’s magnetic field rotates at 1500 rpm and the rotor turns at 1450 rpm.

The magnetic field moves 50 rpm faster than the rotor.

This relative motion means the magnetic flux through the rotor conductors continues changing. An emf is induced, current flows, and the rotor produces torque.

Now imagine the rotor reaches exactly 1500 rpm.

The rotor and magnetic field would be travelling at the same speed and in the same direction.

In the ideal induction motor model, there would be no relative motion between the rotating field and the rotor conductors. The fundamental rotating field would no longer produce a changing flux through the rotor circuits.

Consequently:

• No fundamental induced rotor emf.
• No fundamental induced rotor current.
• No electromagnetic torque from that rotating field.

The rotor would no longer be able to produce the torque needed to drive its mechanical load.

This is why an induction motor must operate slightly below synchronous speed when producing torque.

What happens when the motor carries more load?

Suppose a motor is operating at 1470 rpm with a synchronous speed of 1500 rpm.

Now increase the mechanical load.

The rotor slows slightly. The stator’s rotating magnetic field continues at 1500 rpm because the supply frequency and number of poles have not changed.

The difference between the two speeds increases.

This increases slip, which increases the relative motion between the field and rotor conductors. The induced rotor emf and current respond, allowing the motor to develop more torque.

The motor settles at a new operating speed where its electromagnetic torque balances the increased load torque.

The rotor doesn’t need to slow dramatically. A relatively small change in speed can produce a meaningful change in slip.

Does negative slip mean there is no induced current?

No. This is an important distinction.

If an external mechanical drive forces the rotor to turn faster than synchronous speed, the rotor still experiences relative motion with respect to the rotating magnetic field.

The slip becomes negative.

The machine can then operate as an induction generator, converting mechanical energy into electrical energy.

The electromagnetic torque acts against the rotor’s motion, and electrical energy can be returned to the supply.

For this generating operation, the connected electrical system must also provide the magnetising reactive power required by the induction machine.

The general rotor current frequency relationship is:

\[ f_r=|s|f \]

Where:

• \(f_r\) = frequency of the induced rotor current, in Hz.
• \(s\) = signed slip.
• \(f\) = stator supply frequency, in Hz.

The absolute value is used because frequency is a positive magnitude. The sign of slip tells us about the relative direction of motion and the operating mode.

For a more detailed explanation of the calculation, see How to Calculate Slip from Rotor Speed and Synchronous Speed.

06Do all motors use their stator and rotor in the same way?

No. This is probably the most important distinction to remember after learning the definitions.

The stator and rotor are physical components. Their electrical functions depend on the type of machine.

Consider four examples.

1. AC induction motor

The stator receives three-phase AC and produces a rotating magnetic field.

The rotor receives no direct electrical supply in a squirrel-cage design. Its current is induced by the rotating magnetic field.

The rotor must operate below synchronous speed to produce torque in motoring mode.

2. Brushed DC motor

The stator produces a stationary magnetic field using permanent magnets or electromagnets.

The rotor contains the armature winding, which receives electrical current through brushes and a split-ring commutator.

The current-carrying armature experiences magnetic forces, producing torque.

Unlike an induction motor, the rotor current is supplied directly through an electrical connection.

3. Synchronous AC motor

The stator produces a rotating magnetic field using AC.

The rotor contains permanent magnets or an electromagnet supplied with DC excitation.

The rotor locks into synchronism with the rotating stator field and operates at synchronous speed.

It does not require the steady induction-motor slip described earlier to produce its torque.

4. Synchronous alternator

An alternator converts mechanical energy into electrical energy.

In a common synchronous alternator design, the rotor contains an electromagnet supplied with DC excitation. This rotating magnetic field passes through the stationary stator windings.

The changing magnetic flux induces an emf in the stator windings, producing electrical output.

Notice the difference from an induction motor:

• In the induction motor, the stator receives AC and the rotor develops induced current.
• In the alternator, the rotor carries the magnetic field and the stator produces the electrical output.

The stator and rotor have not changed their definitions. Their electrical responsibilities have changed.

Don’t confuse the armature with the rotor

You may also encounter the terms armature and field winding.

These describe electrical functions rather than physical movement.

• The field winding produces the main magnetic field.
• The armature is the winding associated with the machine’s useful induced emf or torque-producing current, depending on the machine design.

An armature can therefore be located in the rotor or the stator.

This is why memorising that the rotor always contains the armature, or that the stator always produces the magnetic field, will lead to mistakes.

07What should you remember for HSC Physics?

The most reliable way to compare a stator and rotor is to separate their mechanical and electrical roles.

Syllabus context (November 2026): The Physics Stage 6 Syllabus (2017) remains in use in 2026. The Physics 11-12 Syllabus (2025) is scheduled for implementation from 2027, with its first HSC examination in 2028. This article explains machine operation as an application of electromagnetic induction; it does not claim that every design detail or advanced slip calculation is a separately prescribed syllabus point. Check the syllabus version and course outline relevant to your cohort. NSW Physics syllabus implementation information.

08Questions and solutions

Question 1

A motor contains an outer laminated iron assembly that is bolted to the housing and an inner conducting cylinder attached to a rotating shaft.

Identify the stator and rotor.

A student argues that because the stator is stationary, its magnetic field must also remain stationary. Explain why this reasoning is incorrect.

Solution 1

The outer laminated iron assembly is the stator, and the inner shaft-mounted cylinder is the rotor.

The distinction is based on physical movement relative to the motor housing.

However, a stationary stator can produce a rotating magnetic field.

In a three-phase induction motor, the stator contains three sets of windings supplied with alternating currents separated by \(120^\circ\) in phase. Their individual magnetic fields combine to create a resultant magnetic field that continuously rotates around the motor.

Therefore, the stator remains stationary while its magnetic field rotates.

The student’s mistake is confusing the movement of the physical windings with the movement of the magnetic field they produce.

Question 2

A four-pole induction motor operates from a 50 Hz supply and has a percentage slip of 3.2%.

Calculate:

a) The synchronous speed of the rotating magnetic field.

b) The actual rotor speed.

c) Explain why the rotor does not rotate at synchronous speed while producing torque.

Solution 2

The synchronous speed is 1500 rpm, and the actual rotor speed is 1452 rpm.

Part a: Calculate synchronous speed.

Use:

\[ n_s=\frac{120f}{P} \]

Substitute:

\[ n_s=\frac{120\times50}{4} \]

\[ \boxed{n_s=1500\ \mathrm{rpm}} \]

Part b: Calculate rotor speed.

Rearrange the percentage slip relationship:

\[ s=\frac{n_s-n_r}{n_s} \]

\[ n_r=n_s(1-s) \]

The slip is 3.2%, or 0.032 as a decimal.

Substitute:

\[ n_r=1500(1-0.032) \]

\[ n_r=1500(0.968) \]

\[ \boxed{n_r=1452\ \mathrm{rpm}} \]

Part c: Explain why the rotor must be slower.

The rotor must rotate more slowly than the stator’s magnetic field to maintain relative motion between the field and the rotor conductors.

This relative motion causes a changing magnetic flux through the rotor circuits, inducing an emf and current.

The induced current interacts with the stator’s magnetic field to produce torque.

If the rotor reached synchronous speed, the fundamental induced rotor emf and current would become zero in the ideal model. The motor would therefore lose its induction torque.

Question 3

A four-pole induction motor is connected to a 50 Hz supply.

An external mechanical drive forces its rotor to rotate at exactly 1500 rpm while the stator remains energised.

A mechanical load is then attached to the shaft.

Assuming the ideal induction motor model:

a) Calculate the slip.

b) Explain what happens to the fundamental induced emf and current in the rotor.

c) Can the induction motor provide the torque needed to maintain the load at exactly 1500 rpm?

d) What happens if the external drive allows the rotor to slow slightly?

Solution 3

At exactly 1500 rpm, the slip is zero, the fundamental induced rotor emf and current are zero, and the induction motor cannot produce steady induction torque.

Part a: Calculate slip.

The synchronous speed is:

\[ n_s=\frac{120\times50}{4} \]

\[ n_s=1500\ \mathrm{rpm} \]

The rotor speed is also 1500 rpm.

Therefore:

\[ s=\frac{1500-1500}{1500} \]

\[ \boxed{s=0} \]

Part b: Explain what happens to induction.

The rotating magnetic field and rotor are moving at the same speed and in the same direction.

There is no relative motion between the fundamental rotating field and the rotor conductors.

Consequently, the magnetic flux through the rotor circuits does not change due to that fundamental field.

No fundamental rotor emf or current is induced.

Part c: Can the motor maintain the load?

No.

Without induced rotor current, the fundamental rotating field cannot produce induction torque.

The external mechanical drive would need to provide the torque required to maintain the shaft speed against the load.

Part d: What happens if the rotor slows slightly?

Suppose the rotor slows to 1490 rpm.

The stator field continues rotating at 1500 rpm, creating a relative speed difference of 10 rpm.

This produces positive slip:

\[ s=\frac{1500-1490}{1500} \]

\[ s=0.00667 \]

The changing magnetic flux induces an emf and current in the rotor again.

The rotor can now develop induction torque.

This is the important result: the induction motor needs a small speed difference to generate torque, not an exact match between rotor speed and synchronous speed.

Question 4

Two electrical machines have stationary outer assemblies and rotating inner assemblies.

Their electrical arrangements are different.

Machine A:

• The stationary winding assembly is connected to a three-phase AC supply.
• The rotating assembly contains conducting bars connected by end rings.
• The rotor has no direct electrical connection to the supply.

Machine B:

• The rotating assembly contains an electromagnet supplied with DC.
• The stationary assembly contains output windings connected to an external electrical load.
• The rotor is turned by an external mechanical drive.

For each machine:

a) Identify the type of electrical machine.

b) Explain which component receives the electrical supply.

c) Explain where the induced current is produced.

d) Explain why the rotor does not necessarily carry the useful electrical output current.

Solution 4

Machine A is an AC induction motor, while Machine B is a synchronous alternator.

Their stator and rotor components have different electrical responsibilities.

Machine A: AC induction motor

The stator receives the three-phase AC supply.

The alternating currents produce a rotating magnetic field around the stator’s air gap.

This rotating field induces an emf in the conducting bars of the rotor. The rotor bars and end rings form closed conducting paths, allowing current to flow.

The induced rotor current interacts with the stator’s magnetic field to produce torque.

The rotor does not need a direct electrical connection to the supply.

Machine B: Synchronous alternator

The rotor receives DC excitation to produce an electromagnet.

The external mechanical drive rotates this magnetic field inside the stationary stator windings.

As the rotor turns, the magnetic flux through the stator windings changes, inducing an emf in them.

When the stator windings are connected to a load, electrical current flows through the output circuit.

The useful electrical output therefore comes from the stator, not the rotor.

Why are the current paths different?

The terms stator and rotor only identify which component is stationary and which rotates.

They do not determine where current must be supplied or where an emf must be induced.

In Machine A, the rotor develops induced current.

In Machine B, the rotor carries the excitation field, while the stator produces the electrical output.

Question 5

A four-pole induction machine is connected to a balanced three-phase, 50 Hz electrical grid.

Its stator produces a rotating magnetic field in a clockwise direction.

The rotor shaft is connected to a four-quadrant dynamometer, which can either supply mechanical power to the shaft or absorb power from it.

The dynamometer holds the rotor at three different speeds:

• Case A: 1470 rpm clockwise.
• Case B: 1500 rpm clockwise.
• Case C: 1530 rpm clockwise.

Assume:

• The grid supplies the magnetising reactive power required by the induction machine.
• The rotor and magnetic field remain in steady operation at each specified speed.
• The fundamental rotating magnetic field is the only source of induced rotor current.
• Mechanical and electrical losses are negligible.
• Positive rotational speed is defined as clockwise, in the direction of the rotating stator field.

For each case:

a) Calculate the signed slip.

b) Calculate the frequency of the induced rotor current.

c) Determine the direction of the electromagnetic torque acting on the rotor.

d) Determine whether real electrical energy flows from the grid to the shaft or from the shaft to the grid.

A student makes the following claim:

“When the rotor moves faster than the stator’s magnetic field, the field can no longer induce current in it. Therefore, the rotor current must become zero as soon as the rotor passes synchronous speed.”

Evaluate this claim.

Solution 5

The machine operates as a motor below synchronous speed, has zero fundamental induction torque at synchronous speed, and operates as a generator above synchronous speed.

Most importantly, the rotor current does not disappear when slip becomes negative.

Step 1: Calculate synchronous speed.

The machine has four poles and a supply frequency of 50 Hz.

\[ n_s=\frac{120f}{P} \]

\[ n_s=\frac{120\times50}{4} \]

\[ \boxed{n_s=1500\ \mathrm{rpm}} \]

The rotating magnetic field therefore travels clockwise at 1500 rpm in all three cases.

Step 2: Calculate the signed slip for each case.

Use:

\[ s=\frac{n_s-n_r}{n_s} \]

For Case A, the rotor speed is 1470 rpm.

\[ s_A=\frac{1500-1470}{1500} \]

\[ \boxed{s_A=+0.020} \]

For Case B, the rotor speed is 1500 rpm.

\[ s_B=\frac{1500-1500}{1500} \]

\[ \boxed{s_B=0} \]

For Case C, the rotor speed is 1530 rpm.

\[ s_C=\frac{1500-1530}{1500} \]

\[ \boxed{s_C=-0.020} \]

The sign of slip matters.

• Positive slip means the rotor is slower than the rotating field.
• Zero slip means both have the same speed.
• Negative slip means the rotor is faster than the rotating field.

Step 3: Calculate rotor current frequency.

Use:

\[ f_r=|s|f \]

For Case A:

\[ f_{rA}=0.020\times50 \]

\[ \boxed{f_{rA}=1.0\ \mathrm{Hz}} \]

For Case B:

\[ f_{rB}=0\times50 \]

\[ \boxed{f_{rB}=0\ \mathrm{Hz}} \]

For Case C:

\[ f_{rC}=0.020\times50 \]

\[ \boxed{f_{rC}=1.0\ \mathrm{Hz}} \]

Notice something interesting.

Cases A and C have the same rotor current frequency, even though the rotor moves in opposite relative directions with respect to the magnetic field.

This happens because frequency depends on the magnitude of relative motion, while the sign of slip indicates its direction.

Step 4: Determine the direction of electromagnetic torque.

In Case A, the magnetic field moves faster than the rotor.

The induced rotor current interacts with the magnetic field to produce a clockwise torque.

The machine therefore operates as a motor.

In Case B, the rotor and magnetic field have the same speed.

There is no relative motion, so the fundamental induced rotor current and induction torque are zero.

In Case C, the rotor moves faster than the magnetic field.

The relative motion has reversed. The induced current still exists, but its relationship with the magnetic field produces an electromagnetic torque in the anticlockwise direction.

This torque opposes the rotor’s clockwise rotation.

The machine now operates as a generator.

Step 5: Determine the direction of energy transfer.

In Case A, electrical energy from the grid is converted into mechanical energy at the shaft. The dynamometer absorbs the mechanical output.

In Case B, there is no induction torque in the ideal model, so no real power is converted. The machine can still exchange reactive power with the grid to maintain its magnetic field.

In Case C, the dynamometer must supply mechanical energy to overcome the electromagnetic braking torque. The induction machine converts this mechanical energy into electrical energy and returns it to the grid.

Step 6: Evaluate the student’s claim.

The student’s claim is incorrect.

The rotor still experiences relative motion with respect to the rotating magnetic field when it moves faster than synchronous speed.

The direction of relative motion has simply reversed.

This means that an emf and current are still induced in the rotor. Their phase relationship changes, causing the electromagnetic torque to reverse.

Only at zero slip does the ideal fundamental rotating-field model predict zero induced rotor emf and current.

This is the important distinction: negative slip does not mean the absence of induction. It means the machine has moved into a different operating mode.

The same stator and rotor can therefore operate as either an induction motor or an induction generator, depending on their relative rotational speeds.

The next useful connection is understanding how the forces on individual squirrel-cage rotor bars combine to produce torque. Once you can distinguish the stationary stator, rotating magnetic field, induced rotor current, and slip, the complete operation of an AC induction motor becomes much easier to explain.