The coefficient of adhesion is Low in case of dc traction and high in ac traction.
The adhesion force is the force required to pull two solid surfaces apart when they are in contact with each other, and it is influenced by factors such as the coefficient of friction, weight on the wheel, and the condition of the surface.
DC locomotives typically require more weight on the driving wheels to achieve the required tractive effort due to the light weight of their motors.
AC locomotives, on the other hand, can achieve the required tractive effort with less weight on the driving wheels due to the higher weight of their electric equipment.
The required level of adhesion depends on the type of vehicle and its acceleration rate.
Rapid transit cars with every axle motored typically require about 5% adhesion for each 1 mph/s of acceleration rate.
DC locomotives can typically attain 28-30% adhesion, while AC locomotives can achieve 33-35% adhesion on a clean, straight rail.