Why Does My BLDC Motor Cog at Low Speed?
You're running a BLDC motor at low speed and it's jerky, noisy, or just plain rough. It feels like the motor is stepping through discrete positions rather than spinning smoothly. You've checked the wiring, the controller settings look right, and the motor runs fine at higher speeds.
What you're dealing with is cogging torque - and it's one of the most common frustrations engineers face with BLDC motors at low speed.
This article explains what causes cogging, why it's worse at low speed, and what you can actually do about it.
What Is Cogging Torque?
Cogging torque is the magnetic attraction between the rotor's permanent magnets and the stator teeth. As the rotor turns, the magnets try to align with the stator slots. That creates a series of "detents" or "cogging" points that the motor must overcome to keep spinning.
In technical terms, it's also called detent torque or no-current torque. It exists even when the motor isn't energized - spin a BLDC motor by hand and you'll feel it: the rotor "snaps" into certain positions rather than rotating freely.
Why it's worse at low speed: At higher speeds, the rotor's inertia smooths out these torque variations. At low speed, the inertia isn't enough to mask the effect. Each cogging event becomes a distinct jerk, vibration, or audible noise. In fan applications, cogging torque is often the dominating noise source at low-speed operation.
Why Your Motor Cogs at Low Speed
1. Motor Design - The Root Cause
The fundamental cause is the interaction between rotor magnets and stator slots. Some designs are inherently worse.
High pole-count motors have more interaction points and more potential for cogging. Poor slot/pole combinations and simple rectangular magnets without shaping also contribute. Motors designed without considering these trade-offs will cog more.
2. Control Method - How You Drive It Matters
Your control strategy can either mask or amplify cogging.
Six-step commutation is the worst offender. The discrete commutation events add torque ripple on top of the cogging torque. Sensorless operation adds another difficulty - at low speed, back-EMF amplitude is too small for accurate position detection, so the controller can't compensate effectively. Poorly tuned PID loops also make it worse.
3. Load Conditions - Light Loads Expose the Problem
Cogging torque is most noticeable under light load. When the motor is lightly loaded, the cogging torque represents a larger percentage of the total torque. With heavy load, the motor's output torque dominates and the cogging becomes a smaller fraction.
How to Fix It - Design Solutions
If you're specifying a new motor or working with a manufacturer, these design-level fixes can reduce cogging before it becomes a problem.
Technique | How It Works | Effectiveness |
Select the right slot/pole combination | Fractional-slot windings naturally produce lower cogging torque | High |
Reduce the slot opening | Narrower slot openings reduce magnetic variation | Moderate |
Shape the magnets | Sinusoidal magnetization or eccentric poles reduce torque peaks | High (up to 66% reduction) |
Skew the magnets or slots | Distributes cogging over wider angle, reducing peak amplitude | High (30° skew effective) |
Use auxiliary slots | Doubles cogging frequency, reducing output torque ripple | Moderate (17% reduction) |
Optimize the air gap | Asymmetric air gaps reduce cogging while ensuring self-starting | High (up to 66% reduction) |
How to Fix It - Control Solutions
If you're working with an existing motor, these control-side fixes can reduce or eliminate the noticeable effects of cogging.
Switch to Field-Oriented Control (FOC). This is the single most effective control change. FOC uses sinusoidal current waveforms that produce smooth torque. Unlike six-step commutation, FOC maintains stable torque even at low speeds. If your controller supports FOC and you're still using trapezoidal commutation, switch - the difference is dramatic.
Upgrade position feedback. Low-resolution Hall sensors don't provide enough position data for smooth low-speed operation. Upgrading to an encoder gives the controller continuous position feedback, allowing it to apply cogging compensation - injecting correction currents to cancel the cogging torque.
Increase PWM and loop frequency. Higher PWM frequency and faster control loops allow quicker response to torque variations.
Tune the speed loop at the actual operating speed. Default parameters are often tuned for medium speed. Low-speed operation may require different gains. Start with a smooth initial ramp and lower acceleration.
For sensorless systems, use high-frequency injection methods. Some controllers use signal injection for position detection at low speed, eliminating the back-EMF limitation.
What to Check First - A Field Engineer's Checklist
1. What control method are you using?
Six-step → Switch to FOC. Biggest single improvement.
2. What's your position feedback?
Hall sensors only → Consider upgrading to an encoder.
No feedback → Add it. Sensorless has fundamental limits at low speed.
3. Is the motor designed for low-speed operation?
High pole count without cogging reduction → Design compromise. Some cogging may be unavoidable.
4. Is the load very light?
Motor oversized → Running at low torque output exposes cogging. Consider a smaller motor or adding load.
5. Are the PID parameters tuned for low speed?
Default parameters often need adjustment. Tune at the actual operating speed.
FAQ
Q: When should I contact the motor manufacturer about cogging issues?
A: If you've tried control fixes - FOC, better feedback, PID tuning - and still have problems, it's likely a design issue. Ask the manufacturer for cogging torque specs, slot/pole combination, and whether skewing or magnet shaping was used. A reputable supplier will have this data.
Q: Is cogging torque a sign of a defective motor?
A: No. All permanent magnet motors have some cogging. It's only a problem if it affects your application. If the motor meets its published specs, it's not defective — it's just not the right fit for low-speed operation.
Q: Can cogging damage my motor or equipment?
A: Not directly. But the vibration it causes can stress bearings and mechanical components over time. If you feel significant vibration at low speed, address it for reliability reasons — not just for smoothness.
The Bottom Line
Cogging torque is a fundamental characteristic of permanent magnet motors. It's caused by magnetic attraction between the rotor magnets and stator teeth. At low speed, inertia can't smooth it out, so you feel every jerk.
You can fix it on the control side - switch to FOC, improve position feedback, tune the loops.
You can fix it on the design side - choose motors with optimized slot/pole ratios, magnet shaping, or skewing.
You can't eliminate it entirely. But you can reduce it to the point where it doesn't matter for your application.
Need help selecting a motor or tuning your control for low-speed operation? Our engineering team can help you diagnose cogging issues and recommend the right motor and controller combination.
Contact us: info@volcanomotor.com