SOLUTION

• This is known as field weakening or constant power mode of operation.
• When the field winding current reduced the Field’s magnetic strength reduced which in turn reduced the back EMF and at the same time the rotor current increases
• Due to reduced magnetic field pole intensity rotor starts rotating faster till it develops the same Back EMF which should be equal to its original value.
• So in simple words when shunt field current I_f decreases, φ decreases, and the speed rises as speed is inversely proportional to flux.