# Trying to determine the internal construction of a BLDC Motor

The motor is an Electrocraft RPX52600V12. The datasheet indicates 14 poles, 12V, 20.5A, .6 Nm, 1 Ohm, and .6mH. It is 'Y' connected. There is no access to the common center connection. I don't know if it has concentrated or distributed windings. Measuring phase to phase when turned with a drill it produces an absolutely perfect sinusoidal waveform on an oscilloscope.

Q1: Does this mean it is constructed as if it were a PMSM with sinusoidal BEMF rather than a BLDC Motor with trapezoidal BEMF?

After reading the stack exchange topic under 1 I am still uncertain about the answer to the above question because

Rotating the shaft it cogs 24 times. One post I found here said:

"The # of cogs per revolution is equal to the least common multiple of the number of poles and the number of stator slots"

The least common denominator for 14 poles and 24 cogs is 1, an impractical number of slots.

Q2: Is it possible to know the number of stator slots?

-K

• How strongly does it 'cog'? – Bruce Abbott Dec 1 '20 at 21:03
• I'm not sure how to answer the question. It is very easy to turn the shaft between cogs using fingers. It is so easy that I must be careful not to cog twice when counting. The shaft has 24 very definite location it will pull to with no load, but it takes very little friction to prevent this. If spun using fingers it stops immediately, but a small amount of weight on the shaft could cause it to continue. – K-SCI Dec 1 '20 at 22:21
• That's what I wanted to know. – Bruce Abbott Dec 1 '20 at 22:57

Q1: Does this mean it is constructed as if it were a PMSM with sinusoidal BEMF rather than a BLDC Motor with trapezoidal BEMF?

It has probably been designed for low cogging rather than specifically for sinusoidal BEMF. I tested a dozen 14 pole outrunner motors designed for 6 step commutation (which favours trapezoid BEMF). Some have strong cogging, some are 'lumpy' with ill-defined positions, and some are weak. But all bar one produce close to sine wave BEMF even though that is not required for their intended application.

Rotating the shaft it cogs 24 times... The least common denominator for 14 poles and 24 cogs is 1, an impractical number of slots.

Q2: Is it possible to know the number of stator slots?

Probably not, though you might be able to narrow it down to a few possibilities. All my motors have 12 stator slots, and of those with well defined cogging all bar one has 12 cogging positions. However the odd one out has 36!

With distributed windings several adjacent stator slots can share a single phase, but cogging is determined solely by the alignment of magnets to stator arms (the windings play no part). In a 3 phase motor the number of individual windings should be a multiple of 3, so the stator slots should also be a multiple of 3. The number of magnet poles is a multiple of 2. 14 poles are commonly combined with 12 slots in outrunners, but an inrunner with distributed windings could have a higher multiple (eg. 24).

So the question is, does your motor have 24 cogging positions because it has 24 slots, or are the magnets aligning in 2 positions per slot? Without opening up the motor (or contacting the manufacturer) you probably can't tell, but for normal operation you shouldn't need to know.

• Bruce, everything you stated is easily understood. The motor is designed and sold as a BLDC Motor, rather than a PMSM, so the sinusoidal BEMF had me puzzled. This motor is unusual in having a low maximum speed of ~3600rpm. Does that suggest anything about the number of slots? – K-SCI Dec 2 '20 at 16:16
• The low rpm suggests a large number of slots, which it needs to match the number of poles. With 14 poles and 12 slots it would spin ~7 times slower than the equivalent 2 pole motor (same dimensions, same number of turns per slot etc.) – Bruce Abbott Dec 2 '20 at 23:00