Chapter 6 Electromechanical Systems 131 Copyright Goodheart-Willcox Co., Inc. power source. There are three types of single-phase ac motors: universal motors, induction motors, and synchronous motors. A universal motor can be powered by either an ac power source or a dc power r source and is built like a series-wound dc motor, Figure 6-13. It has concentrated fi eld fi windings and speed and torque characteristics similar to those of series-wound dc motors. Universal motors are used mainly for portable tools and small equipment. The single-phase induction motor has a solid rotor, called a r squirrel cage rotor (Figure 6-14). The top and bottom of large-diameter copper conductors are sol- dered to connecting plates. When current fl ows in the stator windings, a current fl is induced in the rotor. The stator polarity changes in step with the applied ac frequency. This develops a rotating magnetic field around the stator. The rotor fi becomes polarized and rotates in step with the stator’s magnetic field. However, fi due to inertia, the rotor must be set into motion by some auxiliary starting method. The speed of an ac induction motor is based on the speed of the rotating mag- netic fi eld and the number of stator poles. The speed of the rotating magnetic field fi fi (rpm) is developed by the frequency of ac voltage (Hz) and the number of poles in stator windings. The stator speed is also referred to as synchronous speed. A two- pole motor operating with a 60 Hz source has a synchronous speed of 3600 rpm. For 60 Hz operation, the following synchronous speeds may be obtained: • Two-pole, 3600 rpm. • Four-pole, 1800 rpm. • Six-pole, 1200 rpm. • Eight-pole, 900 rpm. • Ten-pole, 720 rpm. • Twelve-pole, 600 rpm. + – DC source (Schematic) Series field Armature + – DC source Shunt field Long shunt Series field Armature + – DC source Shunt field Short shunt Low-resistance, series-field windings High-resistance, shunt-field windings (Pictorial) Goodheart-Willcox Publisher Figure 6-12. The compound-wound dc motor combines the best features of the series- wound and shunt-wound motors.