In order to keep the speed of the motor constant when loads are applied, a control application circuit is needed to apply the purpose. The H-Bridge circuit may control the speed and direction of the motor. The below diagram shows H-Bridge with 4 inputs and external power supply, the control application must allow the motor to be operated in forward and reverse directions.
motor will rotate in one direction while if Q4 and Q2 are on (Q1 and Q3
are off) and vice versa. If, transistors (Q3 and Q2) and/or (Q4 and Q1)
are on the power supply becomes shorted to ground. The transistors are
protected from large reverse voltages and currents by the diodes
(D1-D4), while Resistors R1-R4 limit the currents into the bases of the Darlington configurations.
This H-Bridge circuit, clamp diodes, and digital logic interface circuitry are all built into the L293D. Further details on The H-Bridge Circuit Control Design in DC Motor Application can be seen in the following datasheet article of pdf filetype (source: ece.unh.edu).