Does the Friction Coefficient "B" of AC Motor changes with the change of a load connected to an AC motor? OR it is always constant?
The coefficient of friction is the ratio of the force of friction between two bodies and the force pressing them together. The only friction in most ad motors is the friction in the shaft bearings. That is probably what the "B" refers to in the question. The coefficient of friction is normally considered to be a constant, but it can change over time due to inadequate or failing lubrication and normal wear. There could also be a small change due to a change in operating temperature.
The force due to friction is independent of speed. The force on the bearings is often only the weight of the motor's rotor, so the force due to friction may not change due to load. If the load is belt driven, the load will cause a side force on the bearings called "overhung load." That could cause a change in the force due to friction to change due to load changes. If the shaft supports a pump impeller or a fan, the load could cause a thrust force on the bearings that would change with speed and load.
In addition to the kinetic friction described above, the bearings have some static friction that only causes a force that acts at the transition between transition and motion. That would be pretty much constant except for changes in the effectiveness of lubrication.
In addition to bearing friction, motors have aerodynamic drag or "windage" that is a type of friction between the moving parts of the motor and air. Most motors have some windage that is deliberately added by incorporating a fan or rotor fins for cooling purposes.
AC motors with slip rings have brush friction in addition to bearing friction.