What Is Cogging Torque and Why Does It Matter?

Turn the shaft of a permanent-magnet (PM) motor by hand and you will feel a jerky, stepped resistance even with the power switched off. That notchy click as the rotor snaps between positions is cogging torque, also called detent torque or no-current torque. It sits at the heart of torque ripple, motor vibration and noise, and low-speed positioning error — and it is one of the most important and most frequently misunderstood characteristics in electric motor design.

What Is Cogging Torque?

Cogging torque is the torque produced in a permanent-magnet motor by the interaction between the rotor magnets and the slotted stator core when no current is flowing. The magnets naturally try to align with the positions where the magnetic flux path has the lowest reluctance, so the rotor clicks through a repeating pattern of stable and unstable positions, producing an oscillating, periodic torque.

Three defining facts about cogging torque:

• It is present at zero current. Cogging torque does not depend on current, voltage, or load; it appears whenever magnets face the stator teeth.
• It is periodic with rotor position. The pattern repeats every few degrees of rotation, following the slot and pole geometry.
• Its average is zero. Over a full revolution the peaks cancel out, so it does not drive the shaft but makes the rotor cog and hesitate.

What Causes Cogging Torque?

Three ingredients combine to create cogging torque in any permanent-magnet motor:

1. Permanent magnets on the rotor provide the flux source.
2. A slotted stator core creates a magnetic path whose reluctance changes as the rotor moves.
3. The slot and pole geometry determines how many teeth face how many magnets, and how wide each one is.

As each magnet edge passes a stator tooth, the magnetic circuit becomes easier or harder, and the rotor experiences a brief pull or push. The result is a periodic detent torque that shows up as a ripple in the motor output.

Why Does Cogging Torque Matter?

On paper a torque that averages to zero sounds harmless. In practice it is the root cause of several field problems.

1. Torque Ripple

Cogging torque adds a periodic disturbance on top of the useful electromagnetic torque, producing torque ripple that makes output uneven at low speeds, exactly where precision matters most.

2. Vibration and Acoustic Noise

Because the rotor is repeatedly pulled toward and pushed away from alignment, the motor vibrates at the cogging frequency. That vibration becomes audible noise, a serious concern in EVs, appliances, medical devices, and camera gimbals.

3. Poor Low-Speed Smoothness and Positioning

In servo motors, robotics, and direct-drive systems, cogging torque degrades positioning accuracy. The controller must fight the detent steps to hold or move the load smoothly, wasting energy and causing jerky motion.

4. Degraded Feel and Control Quality

For power steering, e-bikes, drones, and handheld tools, cogging torque shows up as a rough, gritty feel that engineers work hard to minimise.

How Is Cogging Torque Measured?

Cogging torque is measured by rotating the unpowered motor at very low speed with a high-resolution torque transducer and recording torque against rotor angle. The key metrics are:

• Peak-to-peak cogging torque — the amplitude of the detent ripple.
• Cogging frequency — the number of steps per revolution, set by the slot/pole combination.
• Cogging torque vs. rated torque — expressed as a percentage to judge acceptability.

How to Reduce Cogging Torque

Engineers have several proven levers for lowering detent torque. The best choice depends on the motor type and manufacturing budget.

1. Skew the Rotor or the Stator

Skewing angles the magnets or slots along the axial length so the torque peaks from different axial slices cancel each other out, dramatically smoothing the waveform.

2. Choose a Better Slot/Pole Combination

A slot/pole combination with a higher LCM produces more, smaller steps. Fractional-slot windings, such as 9 slots / 8 poles instead of 12 / 8, reduce both cogging torque and torque ripple in PMSM and BLDC designs.

3. Shape or Notch the Magnets

Optimising the magnet arc, rounding magnet corners, or adding small notches softens the sharp flux transitions that generate detent torque.

4. Use Dummy Slots or Auxiliary Teeth

Adding narrow dummy slots or teeth shifts the effective slot pitch and increases the cogging frequency, reducing amplitude without changing the winding.

5. Optimise the Air Gap and Magnet Width

A slightly larger air gap, and tuning magnet width against slot pitch, reduce the reluctance variation that drives cogging torque, at the cost of a little torque density.

Frequently Asked Questions

Conclusion

Cogging torque is the hidden stepped resistance in permanent-magnet motors, and it quietly shapes how smooth, quiet, and precise a motor feels. Understanding what it is, why it causes torque ripple and vibration, and how to reduce it through skewing, slot/pole optimisation, and magnet design is essential for anyone selecting or designing a BLDC motor or PMSM.

If you are choosing a motor for a low-speed, low-noise, or high-precision robot joint motor application, ask your supplier for the cogging torque specification and how it was measured. For more selection guidance, see our Robot Joint Motor Technical Guides.

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