Cogging Torque vs Torque Ripple: What Is the Difference

Introduction

Turn an unpowered BLDC motor by hand and you feel it click into a series of soft detents. That stepping is cogging torque, and many people confuse it with torque ripple.

Cogging torque is the position-dependent torque that comes from the rotor magnets lining up with the stator slots. It is there even with no current in the windings. Torque ripple is the broader variation in the motor’s output torque while it is being driven. Cogging is one cause of torque ripple, not a synonym for it.

Cogging vs Ripple in One Line

Here is the fast version before we go deeper.

Cogging is a property of the motor’s construction. Torque ripple is the total output you measure when the motor runs. Keep that split in mind and the rest is straightforward.

What Is Cogging Torque? (the focus)

Cogging torque is a torque that depends only on rotor angle. It exists in most permanent-magnet motors, BLDC and PMSM alike, and it does not need any drive current to appear.

Why It Happens: Slots, Magnets and Reluctance

The stator of a typical motor is not a smooth ring. It has slots that hold the windings, and iron teeth between them. The rotor carries permanent magnets. Magnetic flux always tries to take the path of least reluctance, which means the magnets prefer the positions where they face the most iron.

As the rotor turns, the magnets move from one preferred position to the next. The motor resists leaving each one and snaps toward the next. That resistance is cogging torque. It rises and falls many times per revolution.

The number of cogging cycles per turn depends on the slot and pole combination. As a rule, the cycles relate to the least common multiple of the slot count and the pole count. A higher count means smaller, more frequent bumps, which usually feel smoother.

How You Feel and Measure It

The easy test is by hand. With the motor unpowered, rotate the shaft slowly. The detents you feel are cogging. Engineers call these stable points detent positions.

To measure cogging properly, you drive the shaft very slowly with no current in the motor and record the torque against angle. Low speed matters here, because at low speed there is little inertia or back EMF to hide the effect. The result is a small periodic torque, often a few percent of the motor’s rated torque.

Cogging shows up most at low speed. At higher speed the rotor’s inertia smooths it out, so you rarely feel it once the motor spins fast.

Where Cogging Hurts

Cogging is harmless in many applications. A fan or a pump never notices it. It becomes a real problem where smooth, slow motion matters.

• Slow positioning, where the shaft must creep to an exact angle and the detents fight fine control.
• Camera gimbals, where any bump turns into visible jitter in the footage.
• Precision robotics and direct-drive joints, where there is no gearbox to mask the effect.

In those cases a small cogging torque is enough to spoil the motion. That is why low-cogging motors exist and cost more.

What Is Torque Ripple? (the contrast)

Torque ripple is the variation in the motor’s output torque while it is running. Cogging is one input to it, but not the only one. Ripple also comes from current harmonics, from imperfect commutation, from back-EMF shape, and from small errors in the current control.

So cogging is a building block, and torque ripple is the full picture you see at the shaft. A motor can have low cogging and still show ripple if the drive feeds it distorted current.

This article stays focused on cogging. For the full treatment of torque ripple, its causes, its effects, and the ways to reduce it, see our dedicated guide on torque ripple.

Cogging vs Ripple vs Detent Torque

Three terms get mixed up, so it helps to separate them.

• Cogging torque is the position-dependent torque from magnets and slots, with no current applied.
• Detent torque is, in most contexts, another name for the same effect. You will often see “detent torque” used for the same hand-felt stepping, especially with stepper and BLDC motors. Treat cogging and detent as the same thing unless a datasheet defines them apart.
• Torque ripple is the total variation in driven output torque, with cogging as one contributor.

The key split is current. Cogging and detent torque exist with no current. Torque ripple only exists when the motor is driven.

How to Reduce Cogging Torque

You can attack cogging from two sides: how the motor is built, and how it is driven.

Motor Design (skewing, slot/pole combo, magnet shaping)

Most of the fight against cogging happens in the motor factory, not in the driver.

• Skewing, where the stator slots or the magnets are twisted slightly along the shaft, so the detents blend instead of lining up.
• A well chosen slot and pole combination, which raises the cogging frequency and lowers its amplitude.
• Magnet shaping, such as curved or stepped magnet edges, to soften the flux transitions.

These choices are baked into the motor. The driver cannot change them. If your application needs very low cogging, you pick a motor designed for it.

Control-Side Mitigation

A motor controller cannot remove cogging, because cogging is mechanical. It can reduce the ripple you actually feel by feeding clean, well-shaped current and by keeping the motion smooth.

Field Oriented Control (FOC) is the main tool here. FOC shapes the stator current as a smooth rotating vector, which keeps the torque-producing current steady and avoids adding ripple of its own. Good current control plus a high-quality position feedback signal, from an encoder or hall sensors, lets the loop hold a clean torque. For the torque side of control, see our overview of torque control.

A smooth motion profile helps too. Gentle acceleration through the low-speed range, where cogging is worst, generally keeps the motion fluid.

How SOLO Helps Smooth Motion

SOLO controllers run FOC with clean current control and feedback filtering, which keeps motion smooth even at low speed where cogging is most visible. You can try it on any of our motor controllers.

Conclusion

Cogging torque is the position-dependent stepping you feel when you turn an unpowered permanent-magnet motor by hand. It comes from the rotor magnets aligning with the stator slots, and it is fixed by the motor’s design. Torque ripple is the wider variation in driven output torque, and cogging is just one of its causes. A motor controller cannot delete cogging, but a clean FOC drive reduces the ripple you actually feel, especially at low speed. If smooth low-speed motion matters to your project, pairing a low-cogging motor with a controller that runs FOC natively, like a SOLO motor controller, is the practical path.

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