# Whats is the angle precision of a 3-phase BLDC motor?

I have a small BLDC motor and I would like to use it in a project of mine.

Is there any relation between the minimum angle (precision) I can move the motor and its number of poles?

How could I calculate the minimum step I can move the motor, considering that it has 12 poles?

• A BLDC motor in angle operation behaves rather a lot like a stepper motor. The controller will determine your interpolated angle precision. Typical microstepping is 16 to 256 of a full step (up to 200 full steps per rotation equivalent to 50 poles on a 4 phase motor) for stepper motors, standard microstepping products for BLDC motors are a bit less common or DIY so the answer is that it depends. The camera gimbal people have solved this problem a few times, check them out. Commented Nov 19, 2016 at 22:27
• It looks like twelve poles. Has to be a multiple of three, doesn't it? Or does the rotor have fourteen teeth? Commented Nov 20, 2016 at 8:19
• I edited the question. The motor has 14 permanent magnets on the outer part (rotor) and 12 electro-magnets on the stator, connected to the 3 phases. Commented Nov 20, 2016 at 9:50

The motor itself has virtually infinite resolution (minimum angle that it can be moved). Practical resolution is limited by the controller, which has a limited number of PWM steps.

Having more poles is better because the controller's resolution is relative to each pole. The minimum number of poles is two, which produces a full revolution from 1 cycle of 3 phase drive. Your motor has 14 poles so it has 7 'electrical revolutions' per mechanical revolution, making its effective resolution 7 times higher.

Precision is limited by random mechanical effects such as bearing friction and slop. Accuracy is affected by nonlinearities in the motor's electromechanical response. If run open-loop the rotor will not move by precisely equal amounts per step. So while you might get eg. 360 steps per revolution, they won't all be exactly 1° apart. Torque ripple also occurs as the motor rotates, so any load will cause the rotor to be pulled off position by varying amounts.

Gimbal motors are normally run closed-loop with gyros and accelerometers providing feedback, which compensates for torque loading and nonlinearities in the motor.

Let's assume that you have a motor that looks like this...

(ref)

... with 14 magnets on the rotor and 12 magnets on the stator. This magnet uses a delta-winding.

Your motor will rotate 1/42nd of a complete revolution for each step of the controller.

Let's assume that the controller only has 6 drive states. After the motor controller cycles through all 6 of these states, the rotor will have rotated 1/7th of a complete revolution (51.4 degrees).

To complete a full revolution, the motor controller will need to cycle through 6*7=42 states in total.

If you limit yourself to just using the motor controller's 6 states, then you'll be able to step the motor to 42 different positions. Therefore the angle precision is 360/42 = 8.57 degrees.