Brushless Motor Selection Guide: Commutation, Cogging and Key Specs to Confirm

What a brushless motor is, in hardware terms

A brushless motor is a permanent-magnet synchronous machine whose stator windings are switched by an electronic drive instead of a mechanical commutator, so the only parts that touch are the bearings.

The parts count inside the housing is lower than most first-time buyers expect. A laminated stator carries three-phase windings; the rotor carries surface-mounted or interior permanent magnets; two bearings locate the shaft; and on sensored versions three Hall-effect latches sit 120 electrical degrees apart. The copper-on-carbon sliding contact that wears out a brushed motor is gone, and with it the brush dust, the commutator film and the re-tensioning that go with them.

The trade is straightforward: the drive becomes part of the motor specification. Commutation timing, current limit, the speed loop and, on integrated units, the fieldbus interface all sit in the electronics. A winding that is correct on a six-step drive can be wrong on a field-oriented one, because the two assume different back-EMF shapes and different current waveforms.

• Commutation angle, which sets torque per amp at every speed and decides whether the motor runs hot or runs quiet.
• Current limit and stall protection, which replace the thermal fuse a brushed design would need.
• Speed and position loops, which determine whether the machine needs a controller cabinet or a single integrated board.

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Commutation: six-step or field-oriented control

Six-step commutation switches two of the three phases at a time and is the cheapest way to spin a brushless motor; field-oriented control resolves stator current into torque and flux components and pulls torque ripple down from roughly 10 to 15 percent of rated torque to 3 to 5 percent.

Trapezoidal designs are built for six-step drives. The back-EMF is flat-topped over 120 electrical degrees, so current can be blocked in the same window and the product of the two stays nearly constant. Sinusoidal designs are built for FOC, which needs a continuous position estimate and a current loop fast enough to track it. Mixing the two is where most prototype noise comes from.

Electrical frequency, not mechanical speed, is what the drive has to track. A 4-pole-pair motor at 4,000 rpm sees 267 Hz; a 14-pole-pair outrunner at the same shaft speed sees 933 Hz. At a 20 kHz PWM carrier the current loop only gets about 21 switching periods per electrical cycle at the high end, which is why high pole counts and low-inductance windings push drives toward faster processors or lower carrier frequencies.

In SI units the back-EMF constant and the torque constant are numerically identical. A motor with 0.05 V per radian per second of back-EMF returns 0.05 N.m per amp of quadrature current. That single number ties bus voltage to top speed and current to torque, and it is the first thing to check when a sample runs too fast or stalls too early.

Brushed or brushless: the decision rests on duty cycle

A brushless motor wins whenever the duty cycle is long, the housing is sealed, or the load needs torque at low speed; a brushed motor with replaceable brushes still wins on drive cost and on projects where extra electronics cannot be qualified in time.

Brush life decides most of these arguments. A 40 mm to 52 mm brushed motor with replaceable carbon brushes typically carries a rated brush life in the 1,000 to 3,000 hour range, and that figure assumes clean air, moderate ambient and no sustained low-speed running. A brushless motor of the same frame has no wear item at that interface, so the limiting factors move to bearing grease life and the electrolytic capacitors inside the drive.

The six numbers that decide the specification

Continuous torque at the operating speed sets the frame size; every other number in the datasheet is a consequence of that pair plus the duty cycle.

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Where brushless designs fail in the field

Bearing current, low-speed overheating and drive-side overvoltage from regenerative braking account for more brushless failures in the field than magnet or winding defects.

• Shaft voltage from PWM switching discharges through the bearings. A ceramic hybrid bearing or a grounded brush ring fixes it before the raceway shows fluting.
• Low-speed running with high torque leaves the fan ineffective and the end windings hot. Derate continuous torque below 20 percent of rated speed unless the drive has a temperature input.
• Regenerative braking into a battery or a shared DC bus can lift the rail above the capacitor rating. Confirm the brake chopper threshold, not just the motor rating.
• Long motor cables add capacitance and reflected wave stress. Above 30 m of unshielded cable, check the drive's dv/dt limit and the winding insulation grade.

Catalog part or custom winding

Off-the-shelf brushless motors cover the common 24 V and 3,000 rpm duty points; the moment the requirement includes a non-standard shaft, an IP65 seal or a specific connector, the winding and the mechanical interface are usually customized rather than the machine redesigned around a catalog part.

Two families carry most of that work. Low-cogging designs use slot and pole combinations that suppress detent torque, which matters when an encoder resolves a few arc minutes and the load is a lens or a syringe. Integrated-controller units move the inverter into the rear cap, which removes a cabinet, shortens the motor cable and eliminates the reflected-wave problem entirely.

Both routes need the same input from the buyer: a torque-speed point, a duty cycle, a bus voltage, an ambient and a mounting drawing. Suppliers that also machine housings and assemble harnesses in house can hold the tolerance stack from stator bore to connector keyway in one drawing set, which is where most customization projects lose time.

For a low-detent design, review the low-cogging brushless DC motor range; for a sealed package with the electronics on board, start with the brushless DC motors with integrated controller.

BL112 Low-Cogging 112 mm Brushless DC MotorA 112 mm low-cogging brushless DC motor rated 24 VDC and 600 W, suited to precision CNC and robotics applications needing smooth, low-vibration operation.View Product →

Practical questions that come up before a sample run

How do I size a brushless motor for a 24 V bus?

Start from continuous torque at the duty-point speed and add 20 percent. Then pick a back-EMF constant so the motor reaches top speed at no more than 85 percent of the minimum bus voltage, which on a 24 V nominal rail is about 20.4 V. With 0.05 V per radian per second, that caps speed near 400 rad/s, or 3,800 rpm.

Can a sensorless brushless motor start under full load?

No. Back-EMF is zero at standstill, so a sensorless drive must ramp the motor open-loop until the observer locks, typically above 10 percent of rated speed. For full torque at zero speed, specify three Hall latches or an incremental encoder.

What IP rating should a washdown machine use?

IP65 to IEC 60529 covers dust-tight enclosures with low-pressure water jets from any direction, which suits food-processing and medical cleaning. IP67 adds short immersion. In both cases the shaft seal and the cable gland are the first components to be qualified, not the housing.

Does cogging torque matter in a positioning axis?

Yes, when the axis moves slowly or the encoder resolves below one arc minute. Cogging is typically 1 to 5 percent of rated torque in a standard 12-slot, 8-pole design, and a low-cogging slot and pole combination reduces it without changing the drive or the frame size.