Copyright Goodheart-Willcox Co., Inc. 252 Essential Electrical Skills for HVACR: Theory and Labs bearing brush brushless dc motor capacitor-start, capacitor-run (CSCR) capacitor-start, induction-run (CSIR) centrifugal switch commutator continuous voltage current magnetic relay (CMR) dc motors dropout voltage dual-voltage motor electronic starting relay electronically commutated motor (ECM) end bell hermetic compressor horsepower locked rotor amperage (LRA) magnetic pole mechanical load permanent split capacitor (PSC) pickup voltage potential relay PTC relay resistance-start, induction-run (RSIR) revolutions per minute (RPM) rotating magnetic field rotor run winding self-start servo motor shaded-pole motor slip slip ring snap disc split-phase motor squirrel cage rotor start winding stator stepper motor synchronous speed torque variable frequency drive (VFD) Technical Terms Introduction Electric motors convert electrical energy into mechanical energy, which is needed to drive various equipment that require physical movement. In the HVACR industry, motors drive components, such as compressors, fans, pumps, circulators, dampers, and valves. Because these applications require different amounts of mechanical energy to operate correctly, there are different motor designs to satisfy specific requirements. This chapter will cover the motor construction and motor controls that make it possible for the motor to deliver the required mechanical energy to motors in various applications. Two common factors in a motor’s design are its horsepower rating and applied electric power. Motors are rated by horsepower, which is the measure of mechanical energy. One horsepower equals 746 W, or 33,000 ft-lb of work per minute (550 ft-lb of work per second). Single-phase and three-phase ac are the main types of electric power that will be discussed in this chapter. While all motors are not identical, they operate on electromagnetic principles to generate rotational force called torque. Torque is applied to a mechanical load, such as a fan, to perform useful work. A mechanical load can be any object that requires a physical force to move it. The heavier the object is, the more torque that is required. Motors generate torque by using the repelling and attracting forces of a mag- netic field. 13.1 Basic Motor Construction An ac induction motor is widely used in HVACR appli- cations. It is the simplest motor design based on its con- struction and operation. Its construction consists of a stator, rotor, housing, and end bells. See Figure 13-1. The stator is the stationary part of the motor and made of windings, also called field windings. The coils Bearing End bell Rotor Stator’s field windings Courtesy of A.O. Smith Figure 13-1. Cutaway view of an induction motor.