What is a pole pair?
In a brushless DC (BLDC) or permanent magnet synchronous motor (PMSM), the rotor carries permanent magnets arranged with alternating north and south poles around its circumference. A pole pair is one north pole and one south pole. The number of pole pairs, usually written P, is half the number of magnet poles: a 4-pole rotor has P = 2 and an 8-pole rotor has P = 4. The stator winding is arranged to produce a rotating field with the same number of poles.
Motor datasheets are not consistent about this. Some quote poles, some quote pole pairs, and some say "poles" when they mean pairs. It is always worth confirming which one you have before configuring a controller, for reasons covered below.
Electrical angle and mechanical angle
A motor controller does not drive the shaft angle directly. It drives the stator currents through electrical cycles, and each electrical cycle advances the rotor by one pole pair. A motor with P pole pairs therefore completes P electrical cycles for every mechanical revolution of the shaft:
Why pole pairs matter to the motor controller
Commutation and Hall sensors
With six-step (trapezoidal) commutation, the controller changes the phase switching pattern six times per electrical cycle, which is 6 × P times per shaft revolution. Three Hall sensors likewise produce six state changes per electrical cycle, so an 8-pole motor (P = 4) gives 24 Hall edges per revolution. That finer resolution is one reason Hall feedback works well on high-pole-count motors at low speed.
Electrical frequency and controller limits
Because electrical frequency rises with both speed and pole count, the pole count largely decides how hard the controller has to work. With field-oriented control (FOC), the current loop typically runs once per PWM period, and the controller needs enough PWM periods within each electrical cycle to shape the current accurately. As a rule of thumb, 10 to 20 or more PWM periods per electrical cycle is comfortable. Below that, current ripple, torque ripple and position-estimation error increase quickly.
For example, a motor with 8 pole pairs at 30,000 rpm runs at 4 kHz electrical. At a 20 kHz PWM frequency that leaves only five PWM periods per electrical cycle. The same speed on a one-pole-pair motor is 500 Hz, giving 40 PWM periods per cycle. This is why high-speed spindles, blowers and compressors usually use one or two pole pairs, and why they need controllers designed for high electrical frequencies.
Iron losses
Stator core losses rise with electrical frequency, with eddy-current loss increasing roughly with its square. At the same shaft speed, a higher-pole-count motor therefore tends to have higher iron losses. Thinner laminations and lower-loss electrical steels help, but this becomes a real design constraint at high speed.
Position sensors and getting the setting right
With FOC and an encoder, the controller calculates electrical angle from shaft angle using θe = P × θm. If the pole-pair setting is wrong, the calculated angle drifts away from the true rotor position during each revolution, and the motor vibrates, draws excessive current or will not run at all. In sensorless operation, the controller derives speed from electrical frequency, so entering the pole count (2P) where it expects pole pairs makes the motor run at twice the commanded speed. Checking this one parameter solves a surprising number of commissioning problems.
Electrical frequency calculator
Enter the motor's speed and pole pairs and the controller's PWM frequency to see the electrical frequency and how much headroom the controller has.
Pole pairs and electrical frequency
Headroom bands are a rule of thumb: 20 or more is comfortable, 10 to 20 is tight, and below 10 needs a controller designed for high electrical frequency.
Torque, back-EMF and motor design
For a PMSM, electromagnetic torque is Te = &frac32; × P × λpm × iq, where λpm is the magnet flux linkage and iq is the torque-producing current. For a given flux linkage, more pole pairs means more torque per amp, and both the torque constant (Kt) and back-EMF constant (Ke) scale with P.
In practice it is not free. As the pole count rises, each magnet covers a smaller arc, so the flux per pole falls and leakage between neighbouring magnets increases. The benefit is that less flux per pole allows thinner back iron in the rotor and stator. That is why high-pole-count motors tend to be short and large in diameter, as in outrunners, hub motors and robot joints, giving high torque at low speed without a gearbox. Low pole counts suit compact rotors that need to spin fast.
Pole and slot combinations
The pole count also has to suit the number of stator slots. Combinations are often written as slots and magnet poles, for example 12N14P for 12 slots and 14 poles. Note that the P in this notation means poles, not pole pairs, which is another common source of confusion: a 12N14P motor has 7 pole pairs.
Cogging torque, the detent you feel when turning an unpowered motor by hand, repeats a number of times per revolution equal to the least common multiple of the slot and pole counts. The higher that number, the smaller the cogging tends to be. A 9-slot, 6-pole motor has 18 cogging periods per revolution, while a 12-slot, 14-pole motor has 84 and usually runs far more smoothly at low speed.
Typical pole-pair counts by application
| Application | Typical pole pairs | Reason |
|---|---|---|
| High-speed spindles, blowers and compressors | 1–2 | Keeps electrical frequency manageable at tens of thousands of rpm |
| Pumps and fans | 2–4 | Balances speed, efficiency and controller demands |
| Industrial servo motors | 3–5 | Good torque density with smooth low-speed control |
| Drone and hobby outrunners | 6–14 | High torque at low speed for direct propeller drive |
| Hub motors, robot joints and gimbals | 7–25+ | Direct drive at low speed without a gearbox |
| Direct-drive wind generators | 50+ | Generating at blade speed without a gearbox |
Typical ranges only; individual designs vary.
What is a pole pair?
In a brushless DC (BLDC) or permanent magnet synchronous motor (PMSM), the rotor carries permanent magnets arranged with alternating north and south poles around its circumference. A pole pair is one north pole and one south pole. The number of pole pairs, usually written P, is half the number of magnet poles: a 4-pole rotor has P = 2 and an 8-pole rotor has P = 4. The stator winding is arranged to produce a rotating field with the same number of poles.
Motor datasheets are not consistent about this. Some quote poles, some quote pole pairs, and some say "poles" when they mean pairs. It is always worth confirming which one you have before configuring a controller, for reasons covered below.
Electrical angle and mechanical angle
A motor controller does not drive the shaft angle directly. It drives the stator currents through electrical cycles, and each electrical cycle advances the rotor by one pole pair. A motor with P pole pairs therefore completes P electrical cycles for every mechanical revolution of the shaft:
Why pole pairs matter to the motor controller
Commutation and Hall sensors
With six-step (trapezoidal) commutation, the controller changes the phase switching pattern six times per electrical cycle, which is 6 × P times per shaft revolution. Three Hall sensors likewise produce six state changes per electrical cycle, so an 8-pole motor (P = 4) gives 24 Hall edges per revolution. That finer resolution is one reason Hall feedback works well on high-pole-count motors at low speed.
Electrical frequency and controller limits
Because electrical frequency rises with both speed and pole count, the pole count largely decides how hard the controller has to work. With field-oriented control (FOC), the current loop typically runs once per PWM period, and the controller needs enough PWM periods within each electrical cycle to shape the current accurately. As a rule of thumb, 10 to 20 or more PWM periods per electrical cycle is comfortable. Below that, current ripple, torque ripple and position-estimation error increase quickly.
For example, a motor with 8 pole pairs at 30,000 rpm runs at 4 kHz electrical. At a 20 kHz PWM frequency that leaves only five PWM periods per electrical cycle. The same speed on a one-pole-pair motor is 500 Hz, giving 40 PWM periods per cycle. This is why high-speed spindles, blowers and compressors usually use one or two pole pairs, and why they need controllers designed for high electrical frequencies.
Iron losses
Stator core losses rise with electrical frequency, with eddy-current loss increasing roughly with its square. At the same shaft speed, a higher-pole-count motor therefore tends to have higher iron losses. Thinner laminations and lower-loss electrical steels help, but this becomes a real design constraint at high speed.
Position sensors and getting the setting right
With FOC and an encoder, the controller calculates electrical angle from shaft angle using θe = P × θm. If the pole-pair setting is wrong, the calculated angle drifts away from the true rotor position during each revolution, and the motor vibrates, draws excessive current or will not run at all. In sensorless operation, the controller derives speed from electrical frequency, so entering the pole count (2P) where it expects pole pairs makes the motor run at twice the commanded speed. Checking this one parameter solves a surprising number of commissioning problems.
Electrical frequency calculator
Enter the motor's speed and pole pairs and the controller's PWM frequency to see the electrical frequency and how much headroom the controller has.
Pole pairs and electrical frequency
Headroom bands are a rule of thumb: 20 or more is comfortable, 10 to 20 is tight, and below 10 needs a controller designed for high electrical frequency.
Torque, back-EMF and motor design
For a PMSM, electromagnetic torque is Te = &frac32; × P × λpm × iq, where λpm is the magnet flux linkage and iq is the torque-producing current. For a given flux linkage, more pole pairs means more torque per amp, and both the torque constant (Kt) and back-EMF constant (Ke) scale with P.
In practice it is not free. As the pole count rises, each magnet covers a smaller arc, so the flux per pole falls and leakage between neighbouring magnets increases. The benefit is that less flux per pole allows thinner back iron in the rotor and stator. That is why high-pole-count motors tend to be short and large in diameter, as in outrunners, hub motors and robot joints, giving high torque at low speed without a gearbox. Low pole counts suit compact rotors that need to spin fast.
Pole and slot combinations
The pole count also has to suit the number of stator slots. Combinations are often written as slots and magnet poles, for example 12N14P for 12 slots and 14 poles. Note that the P in this notation means poles, not pole pairs, which is another common source of confusion: a 12N14P motor has 7 pole pairs.
Cogging torque, the detent you feel when turning an unpowered motor by hand, repeats a number of times per revolution equal to the least common multiple of the slot and pole counts. The higher that number, the smaller the cogging tends to be. A 9-slot, 6-pole motor has 18 cogging periods per revolution, while a 12-slot, 14-pole motor has 84 and usually runs far more smoothly at low speed.
Typical pole-pair counts by application
| Application | Typical pole pairs | Reason |
|---|---|---|
| High-speed spindles, blowers and compressors | 1–2 | Keeps electrical frequency manageable at tens of thousands of rpm |
| Pumps and fans | 2–4 | Balances speed, efficiency and controller demands |
| Industrial servo motors | 3–5 | Good torque density with smooth low-speed control |
| Drone and hobby outrunners | 6–14 | High torque at low speed for direct propeller drive |
| Hub motors, robot joints and gimbals | 7–25+ | Direct drive at low speed without a gearbox |
| Direct-drive wind generators | 50+ | Generating at blade speed without a gearbox |
Typical ranges only; individual designs vary.