# BLDC motor polus pairs of count

#### Peet19

##### Member
Hi!
I would like to ask what is the ratio of rotor and stator pole pairs of BLDC motors in general?
eg: stator 14, rotor 12.
...or stator 30, rotor 27, etc.

Solution
IIRC there is one less pole pair on the rotor than the stator, the number of 'teeth' on the stator decides the step angle or steps/rev

Three

#### Peet19

##### Member
What three? I do not understand.

##### Well-Known Member
The motor pole count can be found by shorting the 3 stator connections and counting the 'bumps' when turning 1 revolution.
There is the same amount of stator poles.
These can be seen by back-feeding (spinning) the motor, the three sine wave pole count can be seen on a 'scope.

#### Papabravo

##### Well-Known Member
What three? I do not understand.
Many BLDC motors have 3-phase drive. If they had more pole pairs that would not work.

##### Well-Known Member
BLDC motors have only two windings powered at any one time.
The AC 3ph PM with rotor are created the same way, just the means of commutation is different.
Each have three stator connections.
The PDF shows the wave form when back driven using a 'scope.

#### Attachments

• commutation.pdf
136.6 KB · Views: 19

#### rjenkinsgb

##### Well-Known Member
I would like to ask what is the ratio of rotor and stator pole pairs of BLDC motors in general?

"BLDC" is a very generic term and there are many, very different types, from massive ones such as in large robots and electric vehicles down to the tiny and electrically very crude ones used in a lot of small cooling fans and such (PC fans etc).

High torque servo type motors are usually have three stator pole pairs and a two pole magnet rotor.

Multi-pole ones such as used for drones etc. come in numerous pole combinations, there is no one set standard.
Direct link as it's not obvious:

The majority in that are 12 or 14 pole.

Ideally, look at the manufacturers data for the motor.

Otherwise, with an open frame outrunner motor you can probably count the coils and the magnets attached to the inner surface of the rotor.

Or to make the motor "notch" to you can feel steps to count them, you can apply a low voltage across two of the three connections (so the current is just a fraction of the motor rating). That will give detectable positions.
Just shorting wires does allow you to feel the poles, but you cannot count them reliably on high pole count motors like that, as there is only resistance at speed.

This is an article on working out pole counts for unknown motors:

[Sorry Max, but anything only driving two windings is using a cut-down or economy setup - most serious systems run all three phases; even the basic PIC BLDC stuff Microchip provide libraries for and like on the dev board is full three phase PWM.

Multi-pole Outrunner on the left - 19? pole if I remember correctly - and a small servo style, three pole pair stator & two pole rotor.

##### Well-Known Member
[Sorry Max, but anything only driving two windings is using a cut-down or economy setup - most serious systems run all three phases; even the basic PIC BLDC stuff Microchip provide libraries for and like on the dev board is full three phase PWM.
Not in my experience, BLDC generally have more torque than it 3ph counterpart for the same frame size, the motors are identical, as I have converted both to the other format. BLDC is a DC brushed motor turned inside out.
Most of my work, has been in the CNC motion control arena, including custom systems using Galil motion cards.
As I have related before that when BLDC motors first appeared, a sales rep that made a visit warned that they could not be ran satisfactorily at low RPM due to severe 'cogging', which was quite true, but under PID Motion control, they were as smooth as any high pole brushed DC servo motor..
BLDC and 3ph PM motors differ only in the means of commutation.

BLDC 4 pole example.

#### Attachments

• 4-pole-bldc-motor021804.gif
136.4 KB · Views: 11
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#### Peet19

##### Member
Thank you all for the replies.
I understand motor control. I would have been interested to know how much the number of teeth of the stator is less than the magnets of the rotor.
How do manufacturers calculate this? E.g.: if I want to make a 12-tooth stepper motor on the stator (6 pairs of coils), how many magnets do I need for the rotor?
Ex: stator 6 teeth
rotor 8 magnets.
or, stator 12 teeth, rotor 15 magnets.

##### Well-Known Member
IIRC there is one less pole pair on the rotor than the stator, the number of 'teeth' on the stator decides the step angle or steps/rev

#### Peet19

##### Member
I understand, thanks.That's what I wanted to know. Thanks again.

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