Chapter 6 Electromechanical Systems 135 Copyright Goodheart-Willcox Co., Inc. A servomotor is a component of a servo system that produces controlled shaft r displacements used to achieve a precise degree of rotary motion, Figure 6-17. Ser- 7 7 vomotors must respond accurately to error signals and be capable of reversing direction quickly. The amount of torque developed by a servomotor must be high. There are two types of servomotors: the synchronous motor and the stepping motor. A synchronous motor contains no brushes, commutators, or slip rings. It is r comprised of a rotor and a stator assembly, but there is no direct contact between the rotor and stator poles. An air gap must be carefully maintained in order for synchronous motors to operate. The speed of a synchronous motor is directly proportional to the ac frequency and the number of pairs of stator poles. Since the number of stator poles cannot be altered after the motor has been manufactured, frequency is the most significantfi factor in controlling speed. Speeds of 28, 72, and 200 rpm are typical. A speed of 72 rpm is commonly used in numerical control applications. Single-phase ac synchronous motors are commonly used in low-power appli- cations, Figure 6-18. This type of motor is normally limited to low-power applica- tions because it develops excessive amounts of heat during starting conditions. A typical low-power application of single-phase ac synchronous motors is for preci- sion timing circuits that require precise and constant speed. The stator layout of a two-phase synchronous motor with four poles per phase is illustrated in Figure 6-19. This motor can start, stop, and reverse quickly. In this example, there is room for 48 teeth around the inside of the stator. However, one tooth per pole must be eliminated to provide space for the windings. This leaves a total of 40 teeth. The four coils of each phase are connected in series to achieve the correct polarity. Controller Motor shaft Adept Figure 6-17. A typical servomotor is used for robot axis positioning. The controller provides signals to position the axis accurately and the motor shaft connects to the robot axis.