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I actually have a 3 phase BLDC motor ( The datasheet is not available) with 3 hall sensors. I would like to measure the motor speed (rpm). Therefore I need to know the mechanical revolution or number of poles and the frequency.

How can I find out the number of poles?

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  • \$\begingroup\$ Because it is a 3-phase motor, the poles will be in multiples of 3. \$\endgroup\$
    – user105652
    May 4, 2016 at 20:47
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    \$\begingroup\$ @Sparky256 Number of phases and number of poles are not necessarily linked like that. You commonly get 4pole and 8pole 3phase motors. \$\endgroup\$ May 4, 2016 at 20:54
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    \$\begingroup\$ @Sparky256 Actually, the poles cannot be an odd number. Try to find a magnet with 3 poles.... north, south, and... where is that other pole going to go? It is the stator slots that will be in multiples of 3. \$\endgroup\$ May 4, 2016 at 23:21
  • \$\begingroup\$ @slightlynybbled. My Bad. That is what I was referring to. I have counted them and they are always in multiples of 3 for this type of motor. I do not think of a motor in terms of standard magnets. "Stator" would have been a better word to use. \$\endgroup\$
    – user105652
    May 5, 2016 at 1:07
  • \$\begingroup\$ @Sparky256 No problem at all, it happens. That is what SE is all about. Together, we are better! \$\endgroup\$ May 5, 2016 at 1:47

3 Answers 3

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The number of poles is the number of magnetic cycles in one revolution of the shaft. When setting up for driving a BLDC, it's useful to step thru the drive phases one by one manually. I usually mark a point of the shaft, then add a sticker or something on the stator to record where the shaft is.

I like to use a 12 phase drive. In that case, I go thru each of the 12 phases and mark where the shaft is. This not only tells you what order you need to generate the 12 phases in, but the number of poles falls out by seeing how many times the pattern repeats in a full revolution.

Another way to just get the number of poles is to connect one lead to the supply, one lead to ground, and leave the last lead floating. That drives the motor to a particular phase. By manually rotating the shaft, you can see how many places the rotor is pulled to in a rotation. That is the number of poles.

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Turn the motor shaft one revolution and analyze the output of the Hall-effect sensors.

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If you get the rotor out of it, or just if you can get close you can use a magnet to determine how many North and South poles the rotor has, this is the number of poles.

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