Why a single -phase induction motor not an inherently self -starting machine?

When a single phase ac supply is given to the stator winding of single phase induction motor, it produces its flux of magnitude, φm. According to the double field revolving theory, this alternating flux, φm is divided into two components of magnitude φm /2. Each of these components will rotate in opposite direction, with the synchronous speed, Ns. 

1. t starting, both the forward and backward components of flux are exactly opposite to each other. Also both of these components of flux are equal in magnitude. So, they cancel each other and hence the net torque experienced by the rotor at starting is zero. So, the single phase induction motors are not self starting motors.