Finding number of pole pairs in a BLDC motor

Question

I have a 3phase CDROM BLDC motor for which I don't have any datasheet. How can I find out the number of pole pair in the motor?

Hint:-With a magnet, I could check that there are some N and S poles in the rotor inner layer but how can I know the exact number of pole pairs.?

Thanks, Charles Cowie. But I have another bldc motor which has 3 windings at 120 degrees apart. But its datasheet says is a 4 pole pair motor. Can you explain how it is possible.?

Answer 1

With a magnet, I could check that there are some N and S poles in the rotor inner layer

Best not to use a magnet (a Neodym magnet is strong enough to demagnetize a Ferrite magnet). Just use a screwdriver or other object made from ferrous metal. It will be attracted to each magnet pole in the rotor, so mark the first attraction point and move it around the circumference of the rotor while counting poles until you arrive back at the start.

I have another bldc motor which has 3 windings at 120 degrees apart. But its datasheet says is a 4 pole pair motor.

Depending on magnet configuration and winding pattern, The number of stator arms or slots may be higher or lower than the number of magnet poles. The chart below shows some example combinations (blue boxes are known good combinations, orange may work but were not tested). You can see that a 3 slot motor may have 2 or 4 magnet poles.

Answer 2

I have completely revised my answer considering the information in the question and the comment:

I think it has more than 10 pole pairs but not sure exact number. I know it because it takes more than 10 electrical commutation cycles to complete one mechanical revolution.

The photo seems to indicate this is a wye connected motor with the neutral point brought out for external connection. There appear to be three individual stator-winding conductors attached to three solder points at the bottom of the picture. Just to the left of the bottom, the ends of the three conductors appear to be twisted together and soldered to a fourth solder point. It is also obvious from the photo that this motor has salient-pole windings in the stator. The rotor magnets could also be considered to be salient poles.

The apparent construction is then a doubly-salient permanent-magnet motor (DSPM motor). A DPSM motor can have different numbers of poles on the stator and rotor. The stator could have 12 poles with the phases distributed alternately among the poles. It could also have 6 or 4 poles with 2 or 3 phases making up each phase. If the stator can be disconnected from the driver, a small DC voltage could be applied between each phase and neutral to determine which coils are magnetized by each phase and which are north and south. A diagram of the results can probably be used to determine the number of poles.

With a DSPM motor, the number of poles in the stator does not have to match the number of poles in the rotor. In that respect, a DSPM motor is similar to a stepping motor. There may also be a similarity to some switched reluctance motor designs.

To determine the number of poles in the rotor, count the number of poles by carefully moving a magnet around the inside circumference and noting attractions and repulsions. Take note of Bruce Abbott's advice about the possibility of demagnetizing with a strong magnet. You could just use a piece of steel, but you would have difficulty finding repulsive regions. Perhaps a very small magnet that is not Neodymium would be ok.

Answer 3

First column is the rotor pole number (permanent magnet), second column is the phase assignment for 3-pole stator, third column is for 6-pole, then 9-pole, 12-pole, and 15-pole.

     3       6         9            12             15
2   ABC   AcBaCb   AccBaaCbb   AccBBaaCCbbA   AAccBBBaaCCCbbA
4   ACB   ABCABC   AcaCbcBab   AcBaCbAcBaCb   AcBBaCbAAcBaCCb
6                  ABCABCABC      
8   ABC   ACBACB   AabBbcCca   ABCABCABCABC   ABabABCbcBCAcaC
10  ACB   AbCaBc   AacCcbBba   AabBCcaABbcC   ABCABCABCABCABC
12                 ACBACBACB      
14  ABC   AcBaCb   AbaBcbCac   ACcbBAacCBba   AaABbBbBCcCcCAa
16  ACB   ABCABC   AbbCaaBcc   ACBACBACBACB   AaACcCcCBbBbBAa
20  ABC   ACBACB   AccBaaCbb   AbCaBcAbCaBc   ACBACBACBACBACB
30                 ACBACBACB      

Again, in the table below the first column is the rotor pole number (permanent magnet), second column is the phase assignment for 18-pole stator, third column is for 36-pole.

              18                        36
2   AAcccBBBaaaCCCbbbA   AAAccccccBBBBBBaaaaaaCCCCCCbbbbbbAAA
4   AccBaaCbbAccBaaCbb   AAcccBBBaaaCCCbbbAAAcccBBBaaaCCCbbbA
6   AcBaCbAcBaCbAcBaCb   AccBBaaCCbbAAccBBaaCCbbAAccBBaaCCbbA
8   AcaCbcBabAcaCbcBab   AccBaaCbbAccBaaCbbAccBaaCbbAccBaaCbb
10  ABabcBCAcabABCbcaC   AcBaaCbAcBBaCbAccBaCbAAcBaCbbAcBaCCb
12  ABCABCABCABCABCABC   AcBaCbAcBaCbAcBaCbAcBaCbAcBaCbAcBaCb
14  ABbcaABCcabBCAabcC   AcBCbAcBabAcBaCAcBaCbcBaCbABaCbAcaCb
16  AabBbcCcaAabBbcCca   AcaCbcBabAcaCbcBabAcaCbcBabAcaCbcBab
20  AacCcbBbaAacCcbBba   ABabcBCAcabABCbcaCABabcBCAcabABCbcaC
30  AbCaBcAbCaBcAbCaBc   AabBCcaABbcCAabBCcaABbcCAabBCcaABbcC