# position of hall sensor in bldc motor

#### milan87

Joined Mar 11, 2020
3
Hello all,

I'm trying to create bldc motor, and have some problem regarding position of hall sensors.
It is 12 pole winding on stator, and 8 pole magnets on rotor, and 3 hall sensors.
But I am not sure where to locate this 3 hall sensors...
i tried locate them on every 120 degrees, but i'm not sure if there is maybe some better positions.
If I use formula Θe = p/2 * Θm, p is number of pols on rotor, Θe is angel of field, Θm is mechanical angle of the shaft, am i right?
So it will be 30degree?
Can someone help me with this issue?

Thanks,
Milan

#### nsaspook

Joined Aug 27, 2009
8,662
The sensors provide the positional signals needed for accurate Trapezoidal commutation of the rotational magnetic field inside the motor.

https://www.digikey.com/en/blog/using-bldc-hall-sensors-as-position-encoders-part-1
The magic of 3 in BLDCs

As seen in Figure 3, the Hall sensors are centered in the coil faces. The center-to-center span between any two sensors is three coils, which results in 40 degrees of separation.

2 full coils + 2 half coils = 3 coil span

360 degrees / 27 coils * 3 coil span = 40 degrees

This configuration yields the same output values as if the sensors were physically 120 degrees apart. One third of the magnets will pass by each of the sensors resulting in 10 pulses from each sensor. Together, the sensors will deliver 30 pulses per 120 degrees or 90 pulses in one complete revolution.

9/27 (Coils) = 10/30 (Magnets) = 120/360 (degrees) = 30/90 (pulses) = 1/3 (of one rotation). Neat!
https://www.nxp.com/docs/en/application-note/AN4058.pdf
The brushless BLDC motor is also referred to as an electronically commuted motor. There are no brushes on the rotor and at certain rotor positions commutation is performed electronically. The stator magnetic circuit is usually made from magnetic steel sheets. Stator phase windings are inserted in the slots (distributed winding) as shown in Figure 2, or wound as one coil on the magnetic pole. Magnetization of the permanent magnets and their displacement on the rotor are chosen in such a way that the back-EMF (the voltage induced into the stator winding due to rotor movement) shape is trapezoidal. This allows a rectangular-shaped 3-phase voltage system (see Figure 3) to create a rotational field with low torque ripples.
The most common type of sensor used in BLDC motors is the Hall effect sensor. The Hall effect sensor is a sensing switch that outputs a logic level based on the detection of a magnetic field. Hall effect sensors are economical and because of the permanent magnets inside a BLDC motor are easy to install inside the motor. The motor in this application note comes with the Hall effect sensors pre-installed. Because of the six-step control scheme, there is no need for a high resolution output from the sensor. The only thing you need to know is if the rotor advanced 60°. This can be known with three Hall effect sensors (one for each phase) and the output combinations they generate. Figure 5 shows for every 60° there is a specific combination output from the three Hall effect sensors.
https://www.mathworks.com/help/mcb/gs/hall-sensor-sequence-calibration-bldc-motor.html

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#### milan87

Joined Mar 11, 2020
3
thanks on reply, but still i'm not sure how to calculate exact position of hall sensors...

Joined Feb 20, 2016
3,909
The HALL sensors go at the 120degree positions I think, not of the motor, but of a phase.

I could not find a good picture on line and it is to late and cold for me to got to the shed and get you one, but here is a Fischer and Paykel washing machine motor showing the rough locations.
I think the PCB houses the HALL sensors.

#### shortbus

Joined Sep 30, 2009
8,803
You only need hall sensors if you are going old school to drive it, like a MC33035 based drive. The modern way with using a ESC has a way of sending a small signal to the coils/phases and reading position from that.