4. A Y-connected 25 hp, 400 V, 60 Hz, 1760 rpm induction motor has the following parameters: R_{1}=0.25 \Omega \quad X_{1}=1.2 \Omega \begin{array}{llll} R_{2}=0.2 \Omega & X_{2}=1.1 \Omega & X_{m}=35

\Omega \end{array} The motor is connected to the rated supply and runs at the rated speed. Determine, a) The number of poles in the machine. b) The starting torque. c) The value of the external resistance required in each phase of the rotor such that the maximum torque occurs at starting.