Electrical DC Servo Motors

It was discussed in Chapter 2 that there are various types of DC servo motors. They are available from sizes of fraction of horsepower to several hundreds of horsepower. The DC motors have two separate windings of one on the stator and the other on the armature. Depending on the design, they can be wired in series, shunt, or separately excited form. In servo motors applications, they are often designed so that the winding on stator are energized separately and the power to the armature is connected by brushes. There are several windings on the armature so that a smooth output torque is achieved. The winding on the stator is of low power nature and it is often used to generate a constant magnetic field. When the power is connected to the armature a large current flows in the winding, which develops a large initial current. A large electromotive force is developed which generates a torque to accelerate the armature. The initial current is very large and a current limiter must be designed in the power unit. As the armature accelerates, a back emf is developed which reduces the current. The operational of all DC servo motors are similar.