# BLDC motor control



## tylerwatts (Feb 9, 2012)

Hi folks. Been thinking about a post (I can't find it now...) about a supposed fancy motor and controller combo. Anyway, without article it is difficult so let me explain. 

When thinking of AC motor control we induce a current in the rotor and 'pull'on the magnetic field generated by that current to turn the rotor. Now in a BLDC motor we simply don't induce the current as the magnets already produce a magnetic field. 

But why can't we pull the magnet as it approaches the phase in it's rotation and as the magnet passes the phase we reverse the polarity to push the magnet. Essentially one side could be pulling and the adjacent pushing such that 2 phase's are acting on the magnet.

This would require a complicated controller of course and I'm working out in my head how it would need to switch to control the polarity and timing to work but yet see why it wouldn't work with solid state switching.

What thoughts do others have please? I know there are very knowledgeable people on the forum and would love to hear your thoughts.


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## Siwastaja (Aug 1, 2012)

That's exactly how it works. Both polarities are used, if that's what you are after. Three-phase system makes polarity change easy by enabling bipolar driving with just 3 half-bridges (total 6 switches).

The drive is not that "complex" at all. Of course you modulate sine wave instead of driving the coils fully on and off to prevent jerking. This is exactly the same as "microstepping" in stepper motors; you approximate in-between positions between the electromagnet locations by varying the currents to two adjacent magnets, which will give you smooth torque.

If possible, try to avoid term "BLDC" as it can be very misleading. These are AC motors.


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## tylerwatts (Feb 9, 2012)

Thanks Siwastaja. I refer then to PMAC motors... 

Are you saying the motors/controllers already work in this manner? Ok that is good then, bit disappointed I didn't just revolutionise PMAC motors and control. Do Induction AC motors work the same then?


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## Siwastaja (Aug 1, 2012)

Yes they already work like that. Induction motors work too. The difference is only in the higher level control algorithm; choosing the frequency and voltage and timing it correctly.

Both create rotating magnetic field in the stator windings by applying bipolar voltages to the windings.

A two-phase motor, which is the simplest properly working AC motor in theory, needs "special" bipolar drive system, namely two H bridges (8 switches total), and a neutral wire. 

But 3-phase has balanced currents so that no neutral wire is needed and bipolar driving of windings can be achieved with just three half bridges (6 switches total). It's very simple. Create a PWM modulated by a sine wave so that when sin() = 1, upper transistor is 100% on, when sin() = -1, lower transistor is 100% on, and when sin() = 0, upper and lower transistors alternate 50%-50%. Make sure both are not on at the same time, and add some deadtime. Multiple this system by three, having three sinewaves 120 degrees apart. You have your drive.


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## tylerwatts (Feb 9, 2012)

Ah ok excellent thank you.

I'm wondering whether an internal, axial or out runner orientation is best. Each has compromises but I think axial can offer best of both. Just toying with the best way to configure the motor for optimum torque but being able to maintain good rpm for a wide power band. 

I have been looking at Enstroj's revised webpage on the EMRAX motor and it seems very good. I am only confused why they limit it to 4000rpm. I would want twice that to allow single or 2 speed drives. 

I think I need to draft up my idea for motor layout as it is difficult to explain.


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## tylerwatts (Feb 9, 2012)

Folks would it's be feasible to take an induction motor stator and twice the inner rotor and build an outer PM rotor (out runner) with no change to the stator? My aim is higher torque. I'd even toy with the idea of having under and outer rotor magnets (bearing in mind the weight) for the extra torque or potential efficiency.

What are your thoughts? I have some AC24LS motors lying amount so thought to use one of them. Mainly for a motorcycle build, but to prove a concept and support recycling motors and building custom motors with the right performance for designed applications.


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## major (Apr 4, 2008)

tylerwatts said:


> Folks would it's be feasible to take an induction motor stator and twice the inner rotor and build an outer PM rotor (out runner) with no change to the stator?


The stator produces no magnetic field outside its outer diameter, so no, it won't work.


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## tylerwatts (Feb 9, 2012)

Thanks Major. Why is that please? Is it shielded some how? Ripperton mentioned his hub motors had the magnetic field 'poking' out the iron and I guess I thought it was normal if there was no shielding on his motor. Am I wrong? 

Is there a way to recycle an electric motor like this to make another motor from the iron/stator? I guess an inrunner motor would be fine... but I wanted the extra torque.


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## major (Apr 4, 2008)

tylerwatts said:


> Thanks Major. Why is that please? Is it shielded some how? Ripperton mentioned his hub motors had the magnetic field 'poking' out the iron and I guess I thought it was normal if there was no shielding on his motor. Am I wrong?


Yes, you're wrong. The standard motor stator is designed to produce flux across the air gap which is between the rotor OD and stator ID. The stator provides a low reluctance path for flux between poles. Flux present outside the stator steel and rotor and air gap is undesirable and avoided in a proper motor design. 

You should take a study on the magnetic path involved in motors. Magnetic circuits are a well known science and resources commonly available to explain it.


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