PMSM torque behavior and interpretation
For a simple motor with a two-pole armature, if \$\theta_A\$ is the armature angle, and \$\theta_S\$ is the stator's electrical angle, and a motor with a sinusoidal back-EMF, then the torque is going to be \$k_T I \sin \theta_A - \theta_S\$, where \$k_T\$ is the motor torque constant and \$I\$ is the armature current intensity.

So if the motor is turning a constant speed, and the excitation isn't at the same frequency, then the torque will be sinusoidally varying -- and it'll average out to zero.

Note that if the speeds are close, then the motor won't be going at a constant speed; it'll speed up and slow down, and may (after some alarming shuddering) settle into synchronous operation.