How to find motor poles on motor with incremental encoder?

I have a surface magnet PMSM motor with an incremental quadrature encoder. I'm implementing a motor controller using field oriented control, and I need to establish the initial position of the rotor magnets. What's the best way to do this?

The hardware consists of three half-bridges, each driving one of the motor's phases. I have complete control over all six legs and can manipulate them at will. Is there a way to establish the rotor's electrical position by selectively energizing one or more of the motor phases?

The encoder is a 1000 line per mechanical turn encoder and the motor has four poles (two electrical rotations per mechanical rotation).
 
Just of the cuff..you know how many cycles it takes for one revolution...so you can work backwards to calculate ... solve for p...you have encoder you know what one rev is...you have control of the frequency...

https://www.engineeringtoolbox.com/synchronous-motor-frequency-speed-d_649.html

Also where is the magnetic field...in a asyn motor it moves around compared to your sync field i.e. slip.
No slip in perm magnetic motors...so surely it is inline with you applied field...so your dq resultant should give you this?...bearing in mind that there is a phase displacement in your current, volts and field readings...google phasor diagram for motor control and you will see a diagram indicating all the phase angles ...
And your position/angle is an intergration of the speed...this is highlited in most motor control text....
 
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