
224 Electric Motors and Drives
speed. These systems inevitably compare badly with d.c.
drives, though they manage to save face by being promoted
as 'energy-saving' drives.
CONTROL ARRANGEMENTS FOR INVERTER-FED
DRIVES
Because inverter-fed drives are relatively new, no one par-
ticular approach has yet emerged as the best control philos-
ophy in the way that it has for the d.c. drive (Chapter 4). For
speed control manufacturers offer options ranging in sophis-
tication from a basic open-loop scheme which is adequate
when precise speed holding is not essential, through closed-
loop schemes with tacho or encoder feedback, up to vector
control scheme ...