Speed control of DC motor

The main function of a motor is to drive a load at required speed. Any mechanical load has its specific speed torque charecteristics. Motor has to supply more torque than load requirement to produce fast accelaration. When motor torque equals to load torque, speed of load and motor stabilised. DC motor has advantage of full range of speed control from 0 to max speed with full load torque at all speed. The rated rpm of DC motor is known as base speed of motor.Basically speed of motor is directly proportional to applied voltage to armature circuit and inversely proportional to field current. These principles are used as a basis for speed control. Various methos of speed control is as follows:-
1. Armature voltage control:- In this method supply given to armature is controlled from 0 to rated armature voltage by mean a mg set or with the help of thyristor converters. ward leonard method uses a mg set. small motor can also  be control with auto transformer with bridge rectifier. With this method we can control speed up to rated speed. After that speed can be further increased and  controlled by field control method.Ward leonard method is shown in fig in which motor m2 armature is connected with armature of generator G. voltage is controlled with field winding of generator.

2. Field control method:- In this system series field regulator is used for the control of field current. As field current reduces  motor speed increases and vice versa.Field diverter resistances are also used.