Chapter 6 Electromechanical Systems 137 Copyright Goodheart-Willcox Co., Inc. The rotor of the synchronous motor has a permanent magnet. There are 50 teeth cast into the form of the rotor. The front section of the rotor has one polarity while the back section has the opposite polarity. The difference in the number of stator teeth (40) and rotor teeth (50) means that only two teeth of each part can be properly aligned at the same time. Since the rotor’s sections have opposing polari- ties, the rotor can stop very quickly and can reverse direction without hesitation. Because of its gear confi the synchronous motor can start in one and figuration, one-half cycles of the applied ac frequency and can stop in 5° of mechanical rota- tion. Synchronous motors of this type draw the same amount of current when stalled as they do when operating. This is very important in automatic machine tool applications that involve heavy mechanical loads. Nearly all high-power servomechanisms use dc stepping motors, Figure 6-20. This is a type of motor in which the rotor has a permanent magnet. These motors are primarily used to change electrical pulses into rotary motion. They are more efficient and develop significantly fi more torque than synchronous servomotors. fi The shaft of a dc stepping motor rotates a specifi number of degrees with each fic pulse of electrical energy. The amount of rotary movement, or angular displace- ment, can be repeated precisely. The velocity, direction, and travel distance of a piece of equipment can be controlled by a dc stepping motor. Stepping motors are energized by a dc drive amplifier that is controlled by a computer system. The movement error is gener- fi ally less than 5% per step. The construction of a dc stepping motor is very similar to that of an ac synchronous motor. Some manufacturers make servomotors that can be operated as either ac synchronous or dc stepping. The stator coils are wound using bifilar construction, fi in which two separate wires are wound into the coil slots at the same time. The two wires are small, per- mitting twice as many turns as with a larger wire. This simplifi control circuitry fies and dc energy source requirements. Anaheim Automation, Inc. Figure 6-20. These stepping motors are used to power linear actuators.