$$(P)$$ single phase fully controlled $$ac-dc$$ converter
$$(Q)$$ Voltage commutated $$dc-ac$$ chopper ($$E$$ = input $$dc$$ voltage)
$$(R)$$ Phase voltage of a star connected balanced three-phase load fed from a three-phase inverter with $${180^ \circ }$$ conduction (input $$dc$$ voltage=$$E$$)
$$(S)$$ Line voltage of a six stepped inverter with input dc voltage $$E$$
$$(T)$$ Three-phase diode bridge rectifier.