How Brushless DC Motors Produce Torque Through Electronic Commutation

Learn how permanent-magnet rotors interact with electronically switched stator coils to produce continuous torque. Understand how BLDC motors differ from brushed DC motors and AC induction motors.

November 5, 2026 20 min read

A brushed DC motor uses a split-ring commutator to reverse the current in its rotating coil every half-turn. But a brushless DC motor has no brushes and no rotating electrical contacts. How, then, does it keep turning in the same direction?

The answer is that the coils stay still while permanent magnets rotate with the rotor. An electronic controller switches the currents in the stationary coils, continually moving the magnetic field ahead of the rotor.

This process is called electronic commutation. Understanding it explains how a brushless DC (BLDC) motor produces continuous torque, how its direction and speed are controlled, and why it works differently from an AC induction motor.

01Start with the arrangement: magnets on the rotor, coils on the stator

A conventional permanent-magnet BLDC motor has two main parts.

Rotor: The rotating part, attached to the motor shaft. It carries permanent magnets with north and south poles.
Stator: The stationary part surrounding the rotor. It contains an iron core and several sets of copper windings.

The windings are commonly arranged into three electrical phases, labelled A, B, and C.

When current flows through a winding, it produces a magnetic field. Changing the current through the different phases changes the direction of the combined magnetic field inside the motor.

The rotor, meanwhile, already has its own magnetic field because it carries permanent magnets. It doesn’t need an electrical current flowing through it to produce this field.

Predict what happens: Suppose the controller sends current through two stator phases and then leaves those currents unchanged. Will the rotor keep spinning?

No. The rotor will experience a magnetic torque that turns it towards alignment with the stator’s magnetic field. Once the fields align, the turning torque falls to zero in the idealised model.

With a real load, the rotor may settle at a small angle to the stator field, allowing it to provide a holding torque. However, a stationary magnetic field cannot produce continuous rotation.

This gives us the central problem that electronic commutation solves: the stator magnetic field must keep changing direction if the rotor is to keep turning.

A useful mental picture is a compass needle inside a ring of electromagnets. The needle turns to follow the magnetic field. If you keep changing the field’s direction, the needle keeps following it.

The analogy has a limit: the rotor isn’t chasing a physical object. Magnetic torque acts directly on its permanent magnets, and the rotor can fall behind if the field advances too quickly.

02Where does the torque come from?

The underlying force comes from the motor effect.

A current-carrying conductor placed in a magnetic field experiences a force:

\[ F=BIL\sin\phi \]

where:

\(F\) is the magnetic force, measured in newtons (N).
\(B\) is magnetic flux density, measured in teslas (T).
\(I\) is current, measured in amperes (A).
\(L\) is the length of conductor inside the magnetic field, measured in metres (m).
\(\phi\) is the angle between the current direction and the magnetic field.

When a coil carries current, opposite sides of the coil experience forces in opposite directions. Because these forces act at different positions, they create a turning effect called torque.

In a BLDC motor, the stator currents produce a magnetic field that interacts with the permanent-magnet rotor. The rotor experiences a magnetic torque that tends to align its magnetic axis with the stator field.

The important relationship is:

A stronger stator magnetic field generally produces a greater torque.
Increasing the stator current generally strengthens that field.
The angle between the rotor magnetic axis and the stator field affects the torque.

The angle between the rotor magnetic axis and the stator field is sometimes called the torque angle.

When the rotor and stator fields are perfectly aligned, there is no turning torque in the simplified magnetic-dipole model. When they are offset, a torque acts to bring them into alignment.

The controller takes advantage of this. Rather than allowing the rotor to settle into alignment, it moves the stator field forward before the rotor catches up.

That’s how continuous rotation becomes possible.

03Electronic commutation: moving the magnetic field

In a brushed DC motor, a mechanical split-ring commutator reverses the current in the rotor coil at the appropriate point in its rotation.

A BLDC motor achieves the equivalent result electronically.

An electronic controller, usually containing a three-phase transistor bridge, switches the current through the stator windings. It uses information about rotor position to decide when to change the current paths.

This switching process is called commutation.

One common method is six-step commutation. Two phases carry current at a time, while the third phase is left unpowered, or floating.

The six switching states follow this sequence:

Step	Current enters	Current leaves	Floating phase
1	A	B	C
2	A	C	B
3	B	C	A
4	B	A	C
5	C	A	B
6	C	B	A

We can shorten these descriptions using positive and negative signs:

A+ means current enters phase A.
B- means current leaves phase B.
C floating means the controller isn’t actively driving phase C.

After step 6, the controller repeats the sequence.

Notice what the sequence achieves.

Each switching step changes the direction of the combined stator magnetic field. The field advances around the motor, and the permanent-magnet rotor follows it.

Six switching states complete one electrical cycle. In a real motor with multiple pole pairs, the relationship between electrical cycles and mechanical revolutions depends on the number of pole pairs.

The controller can influence motor operation in three important ways:

Changing the switching order changes the direction of rotation.

If the controller reverses the phase sequence, the stator field rotates in the opposite direction. The rotor follows, so the motor reverses.

Changing the switching frequency changes the motor’s speed.

If the controller advances through the switching states more quickly, the rotating magnetic field moves faster. The rotor follows at the corresponding synchronous speed, provided it remains in step with the field.

Changing the current changes the available torque.

Increasing the phase current generally strengthens the stator field and increases the torque the motor can produce. The controller must still respect current, temperature, and magnetic limits.

This is why a BLDC motor can be controlled without a mechanical commutator. The controller performs the switching electronically.

04How does the controller know when to switch?

The controller needs to know the rotor’s position. Otherwise, it wouldn’t know when to advance to the next commutation state.

There are two common approaches.

Hall-effect sensors

Some BLDC motors contain Hall-effect sensors positioned around the stator.

These sensors detect changes in the magnetic field as the rotor’s permanent-magnet poles move past them.

The sensors send position signals to the controller, which uses them to determine when to switch the stator currents.

Hall sensors provide rotor-position information even when the motor is stationary. This makes starting and low-speed operation easier.

Sensorless control using back EMF

Other BLDC motors estimate rotor position without Hall sensors.

They use a phenomenon called back electromotive force, or back EMF.

As the permanent magnets rotate past the stationary stator windings, the magnetic flux through those windings changes. According to electromagnetic induction, this changing flux induces a voltage in the windings.

The induced voltage is called back EMF because, during normal motoring operation, its polarity opposes the applied voltage.

The controller can measure the back EMF in the phase that is currently floating. It uses features of this voltage signal to estimate rotor position and determine when to commutate.

There is an important limitation.

At standstill, the rotor isn’t moving, so the magnetic flux through a stator winding isn’t changing because of rotor motion. Once any switching transient has passed, there is no rotation-induced back EMF to measure.

A sensorless controller therefore needs a starting strategy. It may initially align the rotor using a known current pattern, then advance the stator field using an open-loop switching sequence. Once the rotor is moving quickly enough to generate a measurable back EMF, the controller can use that signal to estimate rotor position.

This is one reason Hall-sensor motors can have an advantage during starting and very low-speed operation.

05BLDC motors compared with brushed DC and AC induction motors

These three motor types all convert electrical energy into mechanical energy, but they achieve this in different ways.

Brushed DC motor

A conventional brushed DC motor has a coil mounted on the rotor and a magnetic field produced by the stator.

Current flows through the rotating coil. The motor effect produces forces on opposite sides of the coil, creating torque.

The difficulty is that the coil would eventually rotate into a position where the torque reverses direction.

A split-ring commutator solves this problem.

The commutator is attached to the rotating shaft, while carbon brushes remain stationary and maintain electrical contact with the commutator segments.

Every half-turn, the split ring reverses the current in the rotor coil. This keeps the torque acting in the same rotational direction.

Brushless DC motor

A BLDC motor reverses this general arrangement.

The rotor carries permanent magnets, while the stator contains the current-carrying coils.

Instead of using brushes and a split-ring commutator, an electronic controller switches current between the stator phases.

The result is continuous rotation without the sliding electrical contacts used in a brushed motor.

AC induction motor

An AC induction motor uses a different rotor mechanism.

Its stator contains windings supplied with alternating current. These currents produce a rotating magnetic field.

The rotor is usually a conducting squirrel-cage structure made from metal bars connected by conducting end rings.

As the stator’s rotating magnetic field moves relative to the rotor, it induces an electromotive force in the rotor conductors. This produces currents in the rotor.

The interaction between the rotor currents and the stator magnetic field generates torque.

The key differences are summarised below.

Feature	Brushed DC	Permanent-magnet BLDC	AC induction
Rotor	Current-carrying coil	Permanent magnets	Conducting rotor, usually squirrel cage
Stator	Permanent magnets or field windings	Current-carrying windings	AC windings
Commutation	Mechanical brushes and split ring	Electronic switching	Rotating field produced by AC
Rotor current required for normal torque?	Yes	No	Yes, induced
Main torque mechanism	Motor effect on rotor conductors	Interaction between rotor magnets and stator field	Interaction between induced rotor currents and stator field

The important distinction: induction in a BLDC motor

There is a subtle point here that is particularly important in HSC Physics.

A conventional permanent-magnet BLDC motor does involve electromagnetic induction, but you must identify exactly where it occurs.

As the rotor magnets rotate, they induce a back EMF in the stationary stator windings.

That is electromagnetic induction.

However, the BLDC rotor does not require an induced current to produce its driving torque. Its permanent magnets already provide a magnetic field.

The normal driving torque comes from the interaction between the stator magnetic field, produced by the supplied phase currents, and the permanent-magnet rotor field.

An AC induction motor works differently. Its rotor currents are induced by the changing magnetic flux, and those induced currents are essential to its torque production.

Real BLDC motors can experience small parasitic eddy currents in conductive rotor components, but these are unwanted losses rather than the intended torque-producing mechanism.

Keep this distinction clear:

BLDC motor: Back EMF is induced in the stator windings. The rotor’s permanent magnets interact with the stator field to produce torque.
AC induction motor: An EMF and current are induced in the rotor conductors. Their interaction with the stator field produces torque.

Both motors can have rotating stator magnetic fields. That doesn’t make their rotor mechanisms the same.

06Worked examples

Worked example: Calculate the force on a stator conductor

A straight section of a BLDC motor’s stator winding has a length of \(0.080\ \mathrm{m}\) inside a magnetic field of flux density \(0.30\ \mathrm{T}\). It carries a current of \(2.5\ \mathrm{A}\), perpendicular to the magnetic field.

Calculate the magnetic force on the conductor.

Step 1

\[ F=BIL\sin\phi \]

The conductor is perpendicular to the field, so \(\phi=90^\circ\).

Step 2

\[ \begin{aligned} F&=(0.30)(2.5)(0.080)\sin90^\circ\\ &=0.060\ \mathrm{N} \end{aligned} \]

Step 3

The magnetic force on the conductor is:

\[ \boxed{F=0.060\ \mathrm{N}} \]

This is the force on one active conductor section. In an actual motor, the forces acting on multiple conductors and their positions determine the overall turning effect.

Worked example: Calculate BLDC torque and mechanical power

A BLDC motor has a torque constant of \(0.040\ \mathrm{N\,m\,A^{-1}}\), defined relative to the motor current measured by its controller.

The motor draws a current of \(3.0\ \mathrm{A}\) and rotates at \(1800\ \mathrm{rpm}\).

Calculate:

a) The torque produced by the motor.

b) Its mechanical output power.

Step 1

For the specified motor operating conditions, use:

\[ \tau=k_t I \]

where:

\(\tau\) is torque in newton metres.
\(k_t\) is the torque constant in \(\mathrm{N\,m\,A^{-1}}\).
\(I\) is motor current in amperes.

Substitute:

\[ \begin{aligned} \tau&=0.040\times3.0\\ &=0.12\ \mathrm{N\,m} \end{aligned} \]

The motor produces \(0.12\ \mathrm{N\,m}\) of torque.

Step 2

Mechanical power depends on angular velocity, measured in radians per second.

The conversion is:

\[ \omega=\frac{2\pi n}{60} \]

where \(n\) is rotational speed in revolutions per minute.

Substitute:

\[ \begin{aligned} \omega&=\frac{2\pi(1800)}{60}\\ &=188.5\ \mathrm{rad\,s^{-1}} \end{aligned} \]

Step 3

The relationship between torque, angular velocity, and mechanical power is:

\[ P_{\mathrm{mech}}=\tau\omega \]

Substitute:

\[ \begin{aligned} P_{\mathrm{mech}}&=0.12\times188.5\\ &=22.6\ \mathrm{W} \end{aligned} \]

Therefore:

\[ \boxed{P_{\mathrm{mech}}=22.6\ \mathrm{W}} \]

The motor delivers approximately \(22.6\ \mathrm{W}\) of mechanical power at its shaft.

Its electrical input power will be higher because some energy is lost through winding resistance, magnetic losses, switching, and friction.

Notice the difference between torque and power. Torque describes the turning effect, while mechanical power also depends on how quickly the motor rotates.

07Questions and solutions

Question 1

A BLDC motor has a torque constant of \(0.030\ \mathrm{N\,m\,A^{-1}}\), defined using the current measured by its controller.

a) Calculate the torque produced when the motor current is \(2.0\ \mathrm{A}\).

b) Calculate the current required to produce a torque of \(0.12\ \mathrm{N\,m}\).

Assume the motor operates in a range where torque is proportional to current.

Solution 1

The answers are:

a) \(0.060\ \mathrm{N\,m}\)
b) \(4.0\ \mathrm{A}\)

The torque constant relates the motor current to the torque produced.

Part a

Use:

\[ \tau=k_t I \]

Substitute:

\[ \begin{aligned} \tau&=0.030\times2.0\\ &=0.060\ \mathrm{N\,m} \end{aligned} \]

Part b

Rearrange the equation to make current the subject:

\[ I=\frac{\tau}{k_t} \]

Substitute:

\[ \begin{aligned} I&=\frac{0.12}{0.030}\\ &=4.0\ \mathrm{A} \end{aligned} \]

The motor requires a current of \(4.0\ \mathrm{A}\) to produce the specified torque.

The important assumption is that the torque constant remains approximately constant. In a real motor, current limits, heating, and magnetic saturation can affect its performance.

Question 2

A simplified two-pole BLDC motor has a rotor magnetic axis positioned \(25^\circ\) clockwise from the stator magnetic-field axis.

The controller selects the switching state B+ C- and then accidentally holds this state instead of advancing to the next commutation step.

Assume there is no external load and that damping is present.

Describe:

a) The initial motion of the rotor.

b) What happens after the rotor settles.

c) Whether the motor can continue rotating at a constant speed while the stator field remains fixed.

Solution 2

The rotor initially turns anticlockwise towards the stator magnetic-field axis. It eventually settles into alignment and cannot continue rotating while the stator field remains fixed.

Part a

The rotor permanent magnet experiences a magnetic torque that tends to align its magnetic axis with the stator field.

Because its axis is initially \(25^\circ\) clockwise from the field axis, the rotor turns anticlockwise towards alignment.

Part b

As the rotor approaches alignment, the angle between the magnetic axes decreases.

Consequently, the turning torque decreases. With damping present, the rotor may briefly overshoot its equilibrium position before settling into alignment.

Once aligned, the idealised magnetic torque becomes zero.

Part c

The motor cannot maintain continuous rotation.

The controller has stopped advancing the stator field. There is therefore no continuously moving magnetic field for the rotor to follow.

This is precisely why commutation must occur at the correct times. Keeping the same current pattern indefinitely produces a stationary magnetic field, not a rotating one.

Question 3

A BLDC motor uses sensorless control. Its controller estimates rotor position by measuring the back EMF in the phase that is currently floating.

During startup, the rotor is stationary. After the initial switching transient has passed, the controller measures no back EMF in the floating phase.

An engineer suggests increasing the DC supply voltage to make the back EMF measurable, even if the rotor remains stationary.

Explain why this suggestion is incorrect.

Describe a suitable startup method that would allow the controller to begin using back EMF to estimate rotor position.

Solution 3

Increasing the supply voltage alone cannot produce a rotation-induced back EMF if the rotor remains stationary.

The key principle is electromagnetic induction: an induced EMF requires a change in magnetic flux through the winding.

Faraday’s law is:

\[ \mathcal E=-N\frac{\Delta\Phi}{\Delta t} \]

where:

\(\mathcal E\) is induced EMF in volts.
\(N\) is the number of turns in the winding.
\(\Delta\Phi\) is the change in magnetic flux through one turn.
\(\Delta t\) is the time interval over which the change occurs.

If the rotor is stationary, its permanent magnets aren’t moving relative to the stator windings.

After the initial switching transient, the flux through the floating phase is therefore constant, giving:

\[ \Delta\Phi=0 \]

and hence:

\[ \mathcal E=0 \]

Increasing the supply voltage may increase the current in the actively driven phases and help produce enough torque to start the rotor. However, it doesn’t create a rotation-dependent back EMF while the rotor remains stationary.

A suitable startup strategy is:

Apply a known current pattern to align the rotor with a known stator field.
Begin switching the stator phases in the correct sequence.
Gradually increase the switching frequency to accelerate the rotor.
Once the rotor moves quickly enough to generate a measurable back EMF, use the floating-phase voltage to estimate rotor position.
Transition to sensorless closed-loop commutation.

Alternatively, Hall-effect sensors can provide rotor-position information from the beginning, including at standstill.

The key distinction is that the controller needs rotor motion to generate the back EMF used in ordinary sensorless control.

Question 4

Two electric motors have identical three-phase stators producing rotating magnetic fields.

Motor P has a permanent-magnet rotor.
Motor I has a conducting squirrel-cage rotor.

Both motors are operating under load.

a) Explain how each motor produces torque.

b) What happens to the torque-producing mechanism of each motor if its rotor reaches exactly the synchronous speed of the stator magnetic field?

c) A student argues that both motors must rely on induced rotor currents because both have rotating stator magnetic fields. Evaluate this argument.

Solution 4

Motor P can produce torque at synchronous speed through permanent-magnet and stator-field interaction. Motor I cannot produce induction torque at exactly synchronous speed because its rotor currents disappear when there is no relative motion between the rotor and the rotating magnetic field.

Part a: Motor P

The permanent magnets on the rotor provide an existing magnetic field.

The stator currents produce a rotating magnetic field. The interaction between these fields produces torque on the rotor.

When the motor is operating under load, the rotor magnetic axis maintains a non-zero torque angle relative to the stator field.

Motor I

The stator’s rotating magnetic field moves relative to the conducting rotor bars.

This changing magnetic flux induces an EMF in the rotor conductors, producing rotor currents.

The interaction between these induced currents and the stator magnetic field generates torque.

Part b: What happens at synchronous speed?

For Motor P, the rotor can rotate at the same speed as the stator field.

The rotor and stator fields rotate together, maintaining a constant torque angle. Because the permanent magnets already provide the rotor magnetic field, induced rotor currents aren’t necessary for torque production.

For Motor I, the situation is different.

The rotor must move more slowly than the rotating stator field to maintain relative motion between them.

This difference in speed is called slip.

It can be expressed as:

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

where:

\(s\) is slip.
\(n_s\) is synchronous speed.
\(n_r\) is rotor speed.

At synchronous speed:

\[ n_r=n_s \]

Therefore:

\[ s=0 \]

There is no relative motion between the rotating stator field and the rotor conductors. Consequently, there is no induced rotor EMF or rotor current from this mechanism, and the induction motor cannot produce its normal induction torque.

Part c: Evaluating the student’s argument

The argument is incorrect because the two motors have different rotor constructions and torque mechanisms.

A rotating stator magnetic field does not automatically mean that induced rotor currents are responsible for torque.

In Motor P, the permanent magnets provide the rotor field. In Motor I, the rotor field is produced by induced currents.

The distinction is the source of the rotor’s magnetic field, not simply whether the stator field rotates.

Question 5

A permanent-magnet BLDC motor is disconnected from its electronic controller while its rotor is still spinning.

An external flywheel keeps the rotor turning at a constant speed.

Consider one electrically isolated stator phase winding. The winding is tested under two conditions:

Condition A: Its terminals are left open.
Condition B: Its terminals are connected together through a low-resistance wire.

Assume the rotor magnets remain unchanged, and neglect iron losses, cogging, rotor eddy currents, and mechanical friction.

Answer the following:

a) Under Condition A, is an EMF induced in the stator winding? Does current flow? Is there an electromagnetic braking torque from this winding?

b) Under Condition B, what happens to the current in the stator winding, and in which direction does the resulting torque act on the rotor?

c) Does the induced current flow in the rotor? Explain why this experiment does not mean that normal BLDC motoring relies on induced rotor currents.

d) If the rotor speed is halved while the magnetic flux pattern remains unchanged, what happens to the amplitude of the open-circuit induced EMF?

Solution 5

Under Condition A, an EMF is induced but no significant current flows, so there is no electromagnetic braking torque in the idealised model.

Under Condition B, current flows through the stator winding and produces a braking torque opposing the rotor’s motion.

This question tests an important distinction between induced EMF, induced current, and electromagnetic torque.

Part a: Open-circuit winding

Yes, an EMF is induced.

The rotor magnets continue moving past the stationary stator winding. The magnetic flux through the winding therefore changes, producing an induced EMF according to Faraday’s law.

However, the circuit is open.

There is no complete conducting path for charge to flow, so the induced current is zero.

Without an induced current, the winding cannot produce the current-generated magnetic field responsible for electromagnetic braking.

Therefore, under the stated ideal assumptions:

Induced EMF: present.
Induced current: zero.
Electromagnetic braking torque from this winding: zero.

This is an important result. An induced EMF does not automatically mean an induced current must flow.

Part b: Short-circuit winding

When the winding terminals are connected, the circuit becomes complete.

The induced EMF drives a current through the stator winding.

According to Lenz’s law, the magnetic field produced by the induced current opposes the change in magnetic flux responsible for producing it.

In this arrangement, the resulting electromagnetic torque opposes the rotor’s motion.

The motor therefore experiences electromagnetic braking.

If the rotor were not being driven by the external flywheel, it would slow down. If the flywheel maintains constant speed, it must supply additional mechanical energy to overcome this braking torque.

That mechanical energy is converted into electrical energy and ultimately dissipated as heat in the winding’s resistance.

Part c: Where does the current flow?

The induced current flows in the stator winding, not in the permanent-magnet rotor.

The rotor provides the changing magnetic flux, while the stationary stator winding is where the EMF and current are produced.

This is electromagnetic induction occurring in the stator.

It doesn’t change the normal operating principle of a BLDC motor. During powered motoring, the controller supplies current to the stator windings, producing a magnetic field that interacts with the permanent-magnet rotor.

The short-circuit experiment instead demonstrates generator action: mechanical rotation induces an EMF and current in the stator, and the resulting magnetic interaction opposes the motion.

Part d: Halving the rotor speed

The amplitude of the open-circuit induced EMF is halved.

Faraday’s law shows that induced EMF depends on the rate of change of magnetic flux.

If the magnetic flux pattern remains unchanged, reducing the rotor speed by half also reduces the rate at which the rotor magnets pass the stator winding by half.

Therefore:

\[ \boxed{\mathcal E_{\mathrm{new}}= \frac{1}{2}\mathcal E_{\mathrm{original}}} \]

The induced voltage becomes half its original amplitude.

This also explains why back EMF becomes harder to measure at low motor speeds.

The next useful step is to connect the back-EMF waveform to electrical frequency and the number of rotor pole pairs. Together, these ideas allow you to predict motor speed and understand exactly when an electronic controller needs to switch the stator currents.