How are BLAC motors wound and what makes them different from BLDC motors?

[KLoux]

Hello,

I've recently read a couple of things about brushless motors that have spurred quite a few questions. Googling has solved some of them, but I'm hoping that you can help with the rest

Apparently there are two types of permanent magnet brushless motors - DC and AC. This is what I've gathered so far:

- BLDC motors have a trapzoidal back-EMF waveform (phase-to-phase), whereas BLAC motors have a sinusoidal back-EMF waveform
- Either type can be driven with a trapezoidal-type or sinusoid-type commutation scheme, although using the correct scheme for the motor increases efficiency
- BLDC motors can use three hall-effect sensors to time the commutation, but for a sinusoidal commutation scheme, much better rotor position resolution is needed

My question is how do you wind a BLAC motor? I've been reading about home-built BLDC motors made from CD-ROM motors or from parts you can buy (www.gobrushless.com) - seems to be popular with the RC airplane crowd - but no one uses BLAC motors, probably because you need better feedback and more complicated controls to use them. The (short) Wikipedia entry for BLAC motors says "the stator windings of a brushless AC motor are sinusoidally distributed windings, while those of a brushless DC motor are salient field coils."

In my head "sinusoidally distributed windings" looks something like each phase getting wound around almost every tooth, but maybe the number of winds around each tooth changes? And each phase has maximum wind counts occurring 120 electrical degrees from the previous phase, with the number of electrical degrees being determined by the magnets?

I would appreciate any references you can recommend, online or in print.

Thanks for your help!

-Kerry

 

[anorlunda]

BLAC is a seldom used term. It means an AC synchronous motor with permanent magnet field. It can have salient poles like BLDC or it can be a round rotor with sinusoidaly distributed windings.