
D.C. Motor Drives 129
Armature voltage feedback and IR compensation
In low-power drives where precision speed-holding is not
essential, and cost must be kept to a minimum, the
tachogenerator is dispensed with and the armature voltage is
used as a 'speed feedback' instead. Performance is clearly not
as good as with tacho feedback, since whilst the steady-state
no-load speed is proportional to armature voltage, the speed
falls as the load (and hence armature currrent) increases.
We saw in Chapter 3 that the drop in speed with load was
attributable to the armature resistance volt-drop (IR), and
the drop in speed can therefore be compensated b