If the rotor is spinning at 3,600 rpm, one half of the rotor faces from a north pole to another north pole in one half revolution to another north pole at one revolution, 60 times per second. The other rotor half faces only south poles.
Stator current is low. Rotor current is even lower, and again there is an absence of torque.
To overcome friction and windage, even an unloaded motor must have some loss of synchronous speed, or slip.
This results in stator or magnetic poles not aligned with rotor magnetic poles, which oppose the change, slowing, that produced them.
Welte Electric and Refrigeration
Publication date: 07/23/2001