How can brushless DC motor has constant torque?
Perhaps you have misunderstood the working principle of BLDC and PMSM. The working principle is similar to DC motor. We hava an armature current and excitation current (or permanent magnet), it is important that you undarstand that both magnetic fields are right angle, as the torque between two fileds are T=T_max*sin(phi).
IN BLDC is the same, the magnet never aproaches to final angle, because the stator is switched, so that there is always almost right angle - the same is for DC, there is a brush commutator.