A three$-$phase six$-$pole $60-$cycle $230-$volt wound$-$rotor motor has its stator connected in delta and ita rotor in star. There are $75$ per cent as many rotor conductors as stator conductors. Calculate the voltage and frequenoy between slip rings if normal voltage is applied to the stator when $($a$)$ the rotor in at rest; $(6)$ the rotor slip in $0\mathrm{.}04$; $($c$)$ the rotor is driven by another machine at $800$ rpm in a direction opposite from that of the revolving feld.