One of the most common applications for SCR phase control systems is speed control of commutator motors — such as those used for food mixers, sewing machines, pottery wheels etc.
However one of the disadvantages of controlling motor speed by varying input power is that as the effective power input is reduced to slow down the motor — the torque available is reduced as well. This may be overcome by using a feedback signal to advance the firing angle in proportion to the load on the motor — thus increasing the power input if more torque is required. The circuit shownin Fig achieves this load` compensating function by deriving a feedback signal from the armature back-emf (produced .by the residual field of the motor). In this single phase motor speed control circuit, the SCR is triggered when the voltage on the wiper arm of potentiometer R2 rises` to a high enough value to forward bias diode D2