A $220$ V, $15$ kW, $1000$ rpm shunt motor with armature resistance of $0.25 \: \Omega$, has a rated line current of $68$ A and a rated field current of $2.2$ A. The change on field flux required to obtain a speed of $1600$ rpm while drawing a line current of $52.8$ A and a field current of $1.8$ A is
- $18.18 \%$ increase
- $18.18 \%$ decrease
- $36.36 \%$ increase
- $36.36 \%$ decrease