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 — thus allowing gate current to flow. As the back emf tends to reverse bias D2, the firing point of the SCR depends largely upon the back emf and this in turn is a function of speed. lf the motor is loaded, the speed reduces, thus also reducing the back emf — hence D2 becomes forward biased earlier in the cycle (triggering the SCR earlier in the cycle), and thereby supplying the moto; with more power to offset the effect of the loading. ' The component values shown are suitable for most fractional horse- power motors — for optimum results it will be' necessary to adjust com- ponent values to suit the motor used, The circuit described above will provide stepless speed control over a wide range of motor speed — but tends to cause jerky operation at low speeds.