Motor Speed Motor speed is determined by dividing the number of stator poles into the number of times the current changes direction (alternates) per minute. Because two alternations occur per cycle, 60-cycle current changes direction 120 times per second. Motors are rated in revolutions per minute (rpm), so alternations must be figured on a per-minute-basis. Therefore, the current alternates 7200 times per minute (120 × 60 = 7200). See Figure 25‑5. For every alternation of current flow, the rotor travels from one stator pole to the next. With a two-pole motor, the rotor makes one complete revolution for every two alternations (one cycle). The speed of a two-pole motor is 3600 rpm (7200 ÷ 2). The speed of a four-pole motor is 1800 rpm (7200 ÷ 4), so it requires four frequency changes (two cycles) for the rotor to make one complete revolution. A six-pole motor rotates at 1200 rpm (7200 ÷ 6), so it requires six frequency changes (three cycles) for the rotor to make one complete revolution. A motor’s synchronous speed is based on the electrical supply’s frequency and the number of poles in the motor winding, as shown in the rpm of wires, copper bars are inserted into slots formed in the surface of the core. The copper bars are joined at their ends by thick copper rings. The effect resembles a round cage, hence its name. The magnetic field established by the stator cuts across the closed loops in the rotor, inducing large currents. As a result of the induced currents, the rotor becomes a magnet and is turned by its interaction with the rotating magnetic field of the stator. 25.1.2 Induction Motor Characteristics Motors are designated by their operational characteristics. Three of the most important motor characteristics to consider are the following: • •• Motor speed. •• Horsepower. • Efficiency. Squirrel cage rotor Bearing Rotor shaft A B Welded at all joints Copper bars Laminations forming iron core Copper end ring Goodheart-Willcox Publisher Figure 25‑4. A—A squirrel cage rotor consists of a laminated iron core with copper bars. The core surrounds the motor shaft. B—Expanded view of construction. S N S N N N S S S S N S S S S N N N N N N N S S 2 Poles 7200 ÷ 2 = 3600 rpm 4 Poles 7200 ÷ 4 = 1800 rpm 6 Poles 7200 ÷ 6 = 1200 rpm Goodheart-Willcox Publisher Figure 25‑5. Since 60-cycle current alternates 7200 times per minute, the number of stator poles determines motor speed. With two poles, the rotor turns ½ revolution for each current alternation (3600 rpm). With four poles, the rotor turns ¼ revolution per alternation (1800 rpm). With six poles, motor speed drops to 1200 rpm as the rotor turns only 1/6 revolution per alternation. 488 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc.