horsepower motor may be less than 50%, but an integral motor may be over 90% efficient. Power loss in a motor is converted to heat energy. Motor cooling dissipates (gets rid of) the heat. Open-type motors are cooled by fans that draw air through openings in the motor frame and blow it over the windings. Sometimes, fan blades are attached to the rotor shaft inside the motor to blow air through the windings. See Figure 25‑7. For effective cooling, openings in the motor ends must be kept free of dirt and lint. Motors located inside hermetic and semiher- metic compressors are cooled by returning cool suction gas. Sometimes, additional cooling is provided by an externally mounted fan or liquid injection. 25.2 Single-Phase Motors Single-phase induction motors are typically used in small business and residential applica- tions. Single-phase motors do not start with the run (main) winding alone. To accomplish automatic starting, an additional field (stator) winding is required. 25.2.1 Start Winding Called the start winding, the additional winding establishes another magnetic field in the stator that is “out of step” with the main winding. As formula previously covered. This represents the speed of the stator’s magnetic field. The magnetic field of the rotor tries to keep up with the rotating magnetic field of the stator however, a motor’s actual speed is slower than its synchronous (stator field) speed. The rotor tends to lag behind the stator’s rotating field as the motor is loaded. The greater the load on the motor, the more the rotor lags behind the stator’s rotating field. Synchronous speed is nearly achieved when there is no load on the motor. With a motor running at full load, the rotor turns at about 4% to 5% below synchronous speed. The percentage of relative motion is called slip. See Figure 25‑6. Horsepower Motors are rated by the amount of torque (turning power) they can produce. Turning power is rated in horsepower. The traditional English measure of horsepower is the amount of power required to lift 550 pounds one foot in one second. In the United States, horsepower is calculated using the power needed by the motor to produce torque. Torque-producing power is rated in watts. One horsepower is equal to 746 watts. Thus, a five-horsepower motor uses 3730 watts (5 × 746 = 3730). When the wattage and voltage are known, Ohm’s Law makes it possible to determine the amount of amperage drawn under full load conditions. Motors rated less than one horsepower are called fractional horsepower motors. Motors of one or more horsepower are called integral horse- power motors. Efficiency Electric motors are not 100% efficient. Like any machine, some power loss occurs in the motor when it operates. The efficiency of a fractional Goodheart-Willcox Publisher Figure 25‑6. A comparison of the synchronous speed of various motors with the actual speed under load. Motor Speeds Number of Poles Synchronous Speed Actual Speed Two-pole motor Four-pole motor Six-pole motor 7200 ÷ 2 = 3600 rpm 7200 ÷ 4 = 1800 rpm 7200 ÷ 6 = 1200 rpm 3450 rpm 1725 rpm 1150 rpm Fan blade wheel Goodheart-Willcox Publisher Figure 25‑7. To provide cooling in an open-type motor, fan blades may be attached to the same shaft as the rotor. Chapter 25 Motors 489 Copyright Goodheart-Willcox Co., Inc.