Outer rotor (outrunner) motor control circuit 1

[Vinylforever]

I am an electronic ignoramus. I wish to use a motor such as this

http://www.hobby-lobby.com/brushless-axi2204.htm

for a hifi turntable. These motors are primarily aimed at the radio control aircraft hobby market, so the only controllers I can find are for use with R/C systems. Can anyone suggest a source of motor controllers for this sort of motor which allow accurate speed control, running from the AC mains rather than batteries?

 

[benta]

The suggestions above are ingenious. However, we've moved away from BLDCs to synchronous motors, which you might use a BLDC for in this scenario.
My comment on sine-wave drive and the need for sensors come from the fact, that with sine-wave drive you have no meaningful back EMF and can not use it for commutation. Sensorless BLDC drive only works with trapezoidal drive, which again leads to torque ripple that is unwanted in your application.

The working mode for the sensorless controllers that I know is as follows:

1: DC exitation. This means, that the motor is brought to a know state by applying DC to the coils and thus rotating the rotor to a known place.
2: acceleration. A 3-phase (others are possible) frequency ramping drive voltage is applied to the motor, accelerating it.
3: At a certain point, the back EMF is large enough to be used for commutation. The motor is now in normal running mode.

Now, in this application, you could replace (3) with just forgetting the commutation and regard the motor as a synchronous one. It might work. However, just applying the frequency ramp and not bring the motor in a known state first is questionable, so a bit of extra logic is required for the circuit.

ncbuska: if we are looking at a BLDC as a synchronous motor in this case, why do a PLL? As I said, the back EMF would be meaningless, and the generated frequency for the motor would suffice. The feedback from the motor would just be the generated drive signals, and they are known already from the drive circuit.

Benta.

 

[nbucska]

Benta:
You are of course right: sine-wave drive doesn't generate
back-EMF and trapezoidal drive has varying acceleration
within a cycle.

It is again a matter of design to select a compromise
between the two extremes where the back-EMF is detectable
and the force variation is still acceptable.

The closer we come to the sine wave, the noisier will
be the back-EMF sensor. The PLL filters out this
jitter.



<nbucska@pc33peripherals.com> omit 33 Use subj: ENG-TIPS
Plesae read FAQ240-1032

 

[benta]

nbucska:
Right, but I still question the PLL. If the motor is running synchronously, there is no need. If the motor is out of synchronism, you don't know where you are anyway.
The premise here is to run a BLDC as a synchronous motor, and it that case the drive controls the rpm. I don't see the need for an extra control loop that is unable to do anything.

Regards,

Benta.

 

[nbucska]

If the back-EMF sensor has large jitter, it modulates the
commutated waveform. The Pll is a relative easy way to filter this noise out.

<nbucska@pc33peripherals.com> omit 33 Use subj: ENG-TIPS
Plesae read FAQ240-1032