This is a motor controller circuit that  control the speed  and direction of series-wound Motors. This circuit employs silicon controlled rectifiers (SCR) Q1-Q4, that are triggered in diagonal pairs. The switch St is used to control which pair is turned on because either coupling transformer T2  or coupling transformer T1 is connected to a pulsing circuit by this switch. Here is the  schematic diagram of the circuit:

Direction And Speed Control For Series Wound Motors circuit schematic

The direction of rotation is controlled by the field current that can be reversed by SCRs Q1 and Q4’s conduction or conduction of SCRs Q2 and Q3. C1 store the energy that will be used to fire the SCR. the SCRs  is driven by pulse through either transformer T2 or T1. [Circuit’s schematic diagram source:]