A cumulative compound motor has a varying load upon it which

A cumulative compound motor has a varying load upon it which requires a variation in armature current from 50 amp to 100 amp. If the series-field current causes the air-gap flux to change by 3 percent for each 10 amp of armature current, find the ratio of torques developed for the two values of armature current.

Solution

Torque of DC motor is proportional to flux produced and also the armature current.

As per the given data

Flux is increased by 3% of the rise in armature current

Let is the rated flux at 10 Amps of current

Then the flux at 50A is 15% increased to 1.15

T *Ia

T₁/T₂= ₁*Ia₁/( ₂*Ia₂)

Ratio of torques = *10/(1.15* *50) = 0.173