because single phase motor is not a self starting,and its 
starting torque is small,and it will rotate 90 Deg phase 
shift only,For 360 Degree rotation starting winding is need 
due to high starting torque

single phase supply given to single phase IM (having only 
one wdg)produces only alternating flux which can only 
induce emf but cannot make the rotor rotate.Auxiliary wdg 
is provided to split the phase.so the motor starts as two 
phase induction motor.suppply given to two phases will 
produce rotating magnetic fields so the motor becomes self 
starting.

in single phase motor strting torque is zero because average
value of the 1phase motor is zero.so zero flux