How do I initialize position Brushless AC .


Thread Starter


I want to know how many methods to initialize position Brushless AC motor when i control it by Vector control method.

Brandon Ellis

Are you referring to the various means of "homing" a vector axis, or simply commanding the vector drive to define a certain position to zero (or some other known value)? If homing is your goal, is your application linear or rotary motion?


Brandon S. Ellis
Sales Manager
Robotic Control Group
700 S. Illinois Ave.
Suite A104
Oak Ridge, TN 37830
Tel: (865) 425-0301, Ext. 160
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I presume you are asking how you establish a "phase reference", that is, how you know where you are in the phase commutation cycle so you can apply current in the phases to get the torque you desire.

Fundamentally, there are two methods. If you have an absolute sensor on the motor, you do a "phasing read", using the absolute position value to figure out where you are without any motion. If you do not have an absolute sensor, you must do a "phasing search", requiring some motion to either figure out where you are, or to drive you to a known position in the cycle.

A phasing read is usually done from a resolver, an absolute encoder, or "hall-effect" commutation sensors (some incremental encoders have optical "hall tracks" that mimic the output of magnetic hall sensors). Resolvers and absolute encoders are high-resolution devices that can give you accurate phase position immediately. Hall-type devices are low-resolution, breaking the cycle into 6 states, so at best they give you +/-30 degrees (electrical) -- a correction is usually required when doing true AC (sinusoidal) commutation; often this correction is done when the encoder index pulse is found.

There are many possible ways of doing a phasing search move. The most common is to drive the motor like a stepper motor, forcing a certain pattern of current into the phases, waiting for the motor to settle at the (known) position determined by these currents, then forcing this position value into the commutation position register.

Because the phasing search does not require an absolute sensor, it can save money, but it is not appropriate for all applications, particularly if there is a significant net load on the motor, as in a vertical axis.

Curt Wilson
Delta Tau Data Systems

Alex Ruderman

> I want to know how many methods to initialize position Brushless AC motor when i control it by Vector control method.< Conventional technique for AC (sinusoidal) brushless is to use Hall sensors to start as DC (trapezoidal) brushless, find zero encoder pulse (absolute position) and then switch to sin usoidal operation. In the absence of position Hall sensors, we can bring the rotor to predetermined position by feeding two motor phases. This random "magnetic alignment" rotor movement (180 el.deg. in worst case) is unacceptable for many applications hence the need for smart position initialization that eliminates long trial movement for an arbitrary rotor position. I have wrote a report about such a smart initialization (too long to be published overhere). The report comprises 5 pages of text, 4 drawings and 6 equations with the following contents: 1. Introduction 2. 3-Phase Sinusoidal Brushless Basics (Torque Generation, Current Loop etc) 3. Smart Initialization Principle 4. Rotor Movement Equation 5. Control Algorithm Convergence 6. Conclusions Contact me for details. -Alex [email protected]