Forum Diagram

In the Given ckt a Single phase controlled Rectifier is shown  which is used to control the DC motor speed. This ckt is consisting of four SCR and four Diodes. The DC voltage is supplied to the motor winding through four diodes while the DC voltage supplies to the armature through four Diodes.

In This ckt the motor is controlled through armature voltage, while the field voltage is at a constant value.

In the ckt the SCR 1 & SCR 4 are triggered at same time in first cycle, while in next half cycle SCR 2 & SCR 3 are conduct.

The motor armature receives a continuous or Discontinuous voltage depends on the firing angle of the SCR, Power factor.   

In Discontinuous armature current , SCR 1 & SCR 4 are fire at angle α due to which the current flow path is form α to β , as shown in the Fig:

The motor armature receives voltages form α to β, while form β to π + α the motor can’t receive current.

Through this way the speed of a DC motor is controlled.