A $220$ V, $3$-phase, $4$-pole, $50$ Hz inductor motor of wound rotor type is supplied at rated voltage and frequency. The stator resistance, magnetizing reactance, and core loss are negligible. The maximum torque produced by the rotor is $225\%$ of full load torque and it occurs at $15\%$ slip. The actual rotor resistance is $0.03 \Omega/\text{phase}$. The value of external resistance (in Ohm) which must be inserted in a rotor phase if the maximum torque is to occur at start is _________.