The direction of the developed torque may be determined from an end view of the conductors and magnet poles. The direction of flux due to the known direction of current was determined by the right – hand rule and the direction of the mechanical force on each conductor, due to the interaction of the magnetic fields, was determined by the flux bunching effect.
TD = BPIAKM
BP = Flux density in air gap produced by shunt field poles (tesla)
IA = Armature current (ampere)
KM = Constant
Constant KM depends on the design of the motor and include the number of turns, effective length of armature conductors, number of poles, type of internal circuitry and units used.
The torque developed by a DC motor is proportional to the flux density in the air gap and the current in the armature.
Torque in AC
The torque developed by AC motors has two components: A Reluctance – torque component and a magnet – torque component. The reluctance – torque component is due to the normal characteristic of magnetic materials in a magnetic field to align themselves so that the reluctance of the magnetic circuit is minimum
The magnet – torque component is due to the magnetic attraction between the field poles (magnets) on the rotor and the corresponding opposite poles of the rotating stator flux.
It is also justified for salient – pole motors operating from 50 percent rated load to above 100 percent rated load, with power factors ranging from unity to leading, the reluctance torque for such loads is significantly smaller than the magnet torque.