In a shunt DC motor, how can that be practically justified?
Field current reduction increases motor speed. That is, zero field current causes infinite speed.
How can that be practically justified, since if there is no field, no rotation will take place?
Interaction of armature current field produced in individual conductor(one side of coil), with pole magnetic field, produces high field strength on one side of conductor, and weak field strength, on other side of conductor, which produce force on conductor. Same way force will produced on other side of coil (conductor), but in opposite direction. Hence the forces produces couple acting on coil mounted on rotor, which makes rotor to rotate. Thus we can say that the high and weak field produced on both side of conductor produced torque, to rotate. With weak, pole field flux, torque produced will be less, the back EMF also, should reduced, and as applied voltage is same(V=Eb+IaRa), but Eb should increased, as (Ia) cannot increased above rated, or speed (Eb=OZNP/60A) must increased to increase Eb, to balanced the motor equation, a stage will come, after which speed cannot be increased(unstable), or torque is too weak to, increase speed up (drive) motor. If graph is extrapolate, one can say at zero (field) torque, the speed would be infinity, which practically not possible. Like a person on diet losing weight, and if the graph of Time/Weigh is plotted(negative slope) and extrapolate to X axis at zero weight( say, after this time, you will be dead), which is impossible.
This is because in real DC machines (and AC) the field winding always has remnant flux in the iron core (due to hysteresis). If one starts a DC shunt machine with no field it will either just blow the armature circuit fuse or if it is able to get running can reach high speeds which may destroy the armature windings. This has been asked elsewhere in RG and is why in practice field loss relays are used to disconnect the armature winding
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