Brushless Motor

[nrg85]

Hy,

I'm working on a project with a Brushless DC Motor. The motor hasn't got hall or any else position sensor. This motor placed within a rotary compressor for air conditioner device, hence the load on rotor can vary within one revolution of rotor. I've made a 3 phase Voltage Source Inverter driver circuit with an Atmel 8 bit microcontroller. I wrote a software to drive the motor without feedback of rotor position. It works but the reason of varying load conditions the torque degree isn't constant.
The problem is that, above certain frequencies the motor being instable, gets resonate and consume very high currents.
I've figure out that some feedback is necessary to try the degree of torque hold within appropriate limits.
The circuit have three current sense resistor at the low side of bridges on output and measured by the microcontroller's AD through op-amps. I used these resistors for over-current protection only so far.
My questions is, can be the rotor position determined from this three current value?
Somebody have made a bldc motor driver without hall-sensors?
I can't use the Back-EMF phenomenon, because I use a MOSFET driver IC to drive the bridge FETs and one of them is always on.
Can somebody suggest me some application note about this or a manufacturer(s) for this type of inverters??

Thanks!

Regards..

 

[blacksea]

Look at Allegro controller/driver for BLDC Sensorless commutation:

http://www.allegromicro.com/en/Products/Part_Numbers/8904/index.asp

Such commutation is based on backEMF sensing because one of motor phases is open, as with six-step commutation. And you can use it.

 

[ijl]

Brushless motors without hall (or any other) sensors are very popular in radio controlled model airplanes. Motors and speed controllers are commercially available in the power range from some watts up to a few kW. The controller works in the way blacksea described: The voltage induced in the open phase is fed to the controller to tell the speed and position of the rotor. More info and several links at, for example,

http://www.wattflyer.com/forums/showthread.php?t=14542