Types of DC Servo Motors
It was discussed in Chap. 2 that there are various types of DC servo motors. They are available from sizes of fraction of horsepower to several hundreds of horse- power. 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 ac- celerate 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.
Some servo motors instead of winding on the stator have permanent magnets. There are two types of permanent magnets of ceramic and rare earth types. The rare earth permanent magnet generates almost ten times more than magnetic fields and is used when a large power to weight ratio is required. The problem with DC motors is brushes that often require periodical inspection to ensure satisfactory operation.
To remove the problem associated with brushes recently, with the advancement in electronics circuitry they are made brushless and the armature is made of per- manent magnet and the power windings are moved to the stator. The power to the stator is commuted with electronic circuitry. In this arrangement, a larger power to mass ratio is achieved.
The types of servo motors available in the market are summarized in Fig. 4.1. There are some small motors known as printed and moving coil DC servo motors. They are suitable for applications when large power is not required. They are de- signed to have very low armature inertia for fast response. These motors are still in the stage of design and only they are available in very small sizes of motors and will not be discussed in this book.
