
58 Electric Motors and Drives
drives, but these will not be discussed here). So with =
415 V for example, the maximum d.c. output voltage (equa-
tion 2.6) is 560 volts. After allowances have been made for
supply variations and impedance drops, we could not rely
on obtaining much more than 520-540 V, and it is usual for
the motors used with 6-pulse drives fed from 415 V, 3-phase
supplies to be rated in the range 440-500 V. (Often the field
winding will be supplied from single phase 240 V, and field
voltage ratings are then around 180-200 V, to allow a mar-
gin in hand from the theoretical maximum of 216 V referred
to earlier.)
Firing circuits ...