
132 Electric Motors and Drives
DEVELOPMENTS IN INDUSTRIAL D.C. DRIVES
Some drives now feature digital speed feedback, in which a
pulse train generated from a shaft-mounted encoder is
compared (using a phase-locked-loop) with a reference pulse
train whose frequency corresponds to the desired speed. The
reference frequency can easily be made accurate and drift-
free;
and noise in the encoder signal is easily rejected, so that
very precise speed holding can be guaranteed. This is especi-
ally important when a number of independent motors must
all be driven at identical speed. Phase-locked loops are also
used in the firing-pulse synchronizing circuits ...