A common issue, especially with sensorless brushless DC motors
If your brushless DC motor judders, stutters or refuses to start at low speed, the cause is usually fixable — but first you need to know which problem you actually have.
Engineers and customers often call this behaviour “cogging”. In the video below, the unoptimised motor and controller stutter and jump at startup, rather like a car engine on a cold morning without enough choke. As the speed rises, the motor smooths out and the problem disappears.
That pattern causes real problems in the field. In the worst cases, startup becomes unreliable: the motor starts six times out of ten, and the other four it judders, stalls and gives up.
Three different problems, one word
“Cogging” gets used for three separate effects. They have different causes, so they need different fixes.
1. Cogging torque (a motor design issue). Strictly speaking, cogging torque is the detent torque caused by the rotor magnets being attracted to the stator teeth. You can feel it by turning an unpowered motor by hand: it clicks from one position to the next. It is set by the motor's construction — the slot and pole combination, magnet shape and whether the stator or magnets are skewed. A controller cannot remove it, although advanced controllers can compensate for it.
2. Torque ripple (a commutation issue). A trapezoidal (six-step) controller switches the motor phases in coarse steps. This produces a pulsing torque that is most noticeable at low speed, where the motor has time to respond to each step.
3. Sensorless startup and sync failure (a control issue). This is the most common cause of a motor that judders and then fails to start. A sensorless controller works out the rotor position from the back-EMF the motor generates. At standstill and very low speed there is little or no back-EMF to measure, so the controller has to start the motor blind and hand over to closed-loop control once it can “see” the rotor. If that startup sequence is poorly tuned, the motor loses sync, stutters and may stall.
What are the options for eliminating cogging?
For most sensorless applications, optimising the controller is the fastest and cheapest fix. The options below run roughly from lowest to highest cost.
1. Optimise the sensorless startup sequence. A sensorless controller typically starts the motor in three stages: it aligns the rotor to a known position, accelerates it open-loop, then hands over to closed-loop control once the back-EMF is strong enough to track. Each stage has settings — alignment time, ramp rate, handover speed — that need to suit your motor and load. Getting them right can take startup from unreliable to dependable without changing any hardware.
2. Tune the startup current. Too little current at startup is a common cause of failed starts, especially where the motor starts under load. More current gives the motor the extra torque it needs to break away. Too much, though, adds heat and torque ripple, so the aim is a tuned value rather than simply a higher one.
3. Use sinusoidal or field-oriented control (FOC). Trapezoidal controllers switch current in six coarse steps per electrical cycle. Sinusoidal and FOC controllers shape the current smoothly, which greatly reduces torque ripple at low speed. Sensorless FOC still faces the low-speed back-EMF problem, but techniques such as initial position detection and high-frequency injection can improve startup.
4. Add position sensors. A sensored motor knows where its rotor is from standstill, which removes the startup guesswork. Hall sensors give a position update every 60 electrical degrees and work well with trapezoidal control. For the smoothest low-speed motion, an encoder with FOC is the best option.
5. Change the motor. If the motor itself has high cogging torque, no controller setting will fully remove it. Motors with a fractional-slot design, or with a skewed stator or magnets, have much lower cogging torque. A higher pole count can also help low-speed control, though on its own it does not guarantee lower cogging torque.
How do I know which solution is best for my application?
The right fix depends on whether you need reliable startup or genuinely smooth low-speed motion.
If you need reliable startup, an optimised sensorless controller is usually the answer. Pumps, fans and blowers rarely need perfectly smooth motion at a few RPM, but they do need to start every time. Optimising a low-cost trapezoidal controller such as our ZDBL15 tailors it to your motor and to the demands of your application. Depending on volumes, we can often do this optimisation at no extra cost.
If you need smooth motion at very low speed, a sensored system with sinusoidal or FOC control is likely to be the better choice. Positioning, medical and laboratory equipment often fall into this category.
Cost matters too. Sensored motors, sinusoidal controllers and higher-specification motors usually add cost to each unit, while controller optimisation is typically a one-off engineering effort. For higher-volume products, that difference adds up quickly.
If in doubt – give us a call – we’re happy to help
The best fix for a juddering or unreliable motor depends on your motor, your load and your volumes. We design and optimise brushless motor controllers for OEMs every day, and we're happy to look at your application.
Talk to our engineers about your motor or explore our custom motor controller design service.