Heating and Cooling Essentials 5e, Textbook Page 490 (508 of 800)

Direction of Rotation Rotation is the circulation motion of the motor shaft. The direction of rotation is determined by the start winding. Rotation to the right is clock- wise (CW). Rotation to the left is counterclock- wise (CCW). To reverse rotation, simply reverse the two connections to the start winding. On open-type motors, the electrical connections are located at the lead end of the motor, which is where the electrical wires (leads) are located. The rotor shaft exits the opposite end of the motor (the shaft end). See Figure 25‑9. For most types of motors, the direction of rotation (CW or CCW) is identifiable as viewed from the shaft end toward the lead end. Motors made by General Electric are an exception. Direction of rotation is determined by viewing from the lead end toward the shaft end in GE motors. Direction of rotation should always take into account which end of the motor is being viewed. Disconnecting the Start Winding The sole purpose of the start winding is to get the rotor moving (start the electric motor). Because of its high resistance, the start winding will burn out if it is not disconnected from the circuit once shown in Figure 25‑8, the start winding is placed at a 90° angle to the main winding. At start-up (when the main switch closes), current flows through both the start and run windings. The start winding has much smaller- diameter wire and more turns in the coil than does the run winding. The smaller diameter wire makes the start winding have high resistance. Magnetism builds up faster in the run winding because it has less resistance. These two magnetic fields, one behind the other, create a turning force on the rotor. S R S R N N S S S S R R Run winding (low resistance) Start winding (high resistance) Start Run 90° Goodheart-Willcox Publisher Figure 25‑8. The start winding in this two-pole motor is placed at a 90° angle to the run winding. The magnetic fields are out of step, which keeps the rotor turning. Electrical leads Shaft end Shaft Lead end Shaft End Lead End CCW CW CCW CW Goodheart-Willcox Publisher Figure 25‑9. When describing the rotation of a motor shaft, the end of the motor from which the rotation is viewed must be noted. 490 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc.

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Direction of Rotation Rotation is the circulation motion of the motor shaft. The direction of rotation is determined by the start winding. Rotation to the right is clock- wise (CW). Rotation to the left is counterclock- wise (CCW). To reverse rotation, simply reverse the two connections to the start winding. On open-type motors, the electrical connections are located at the lead end of the motor, which is where the electrical wires (leads) are located. The rotor shaft exits the opposite end of the motor (the shaft end). See Figure 25‑9. For most types of motors, the direction of rotation (CW or CCW) is identifiable as viewed from the shaft end toward the lead end. Motors made by General Electric are an exception. Direction of rotation is determined by viewing from the lead end toward the shaft end in GE motors. Direction of rotation should always take into account which end of the motor is being viewed. Disconnecting the Start Winding The sole purpose of the start winding is to get the rotor moving (start the electric motor). Because of its high resistance, the start winding will burn out if it is not disconnected from the circuit once shown in Figure 25‑8, the start winding is placed at a 90° angle to the main winding. At start-up (when the main switch closes), current flows through both the start and run windings. The start winding has much smaller- diameter wire and more turns in the coil than does the run winding. The smaller diameter wire makes the start winding have high resistance. Magnetism builds up faster in the run winding because it has less resistance. These two magnetic fields, one behind the other, create a turning force on the rotor. S R S R N N S S S S R R Run winding (low resistance) Start winding (high resistance) Start Run 90° Goodheart-Willcox Publisher Figure 25‑8. The start winding in this two-pole motor is placed at a 90° angle to the run winding. The magnetic fields are out of step, which keeps the rotor turning. Electrical leads Shaft end Shaft Lead end Shaft End Lead End CCW CW CCW CW Goodheart-Willcox Publisher Figure 25‑9. When describing the rotation of a motor shaft, the end of the motor from which the rotation is viewed must be noted. 490 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc.

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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.

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motor. The operating (“running”) amperage is called full load amperage (FLA). Induction motors usually operate at less than FLA because the motor rarely works at fully loaded condition. An overload occurs when the amperage flow exceeds the FLA rating on the motor data plate. Except for the temporary high start-up amperage, any amperage that exceeds the FLA rating is converted to heat energy. 25.2.2 Overload Protectors The induction motor is designed to convert electrical energy to mechanical energy. If the rotor does not turn normally, excess amperage flow will result. The motor will convert the excess amperage to heat energy, which will burn out the motor windings. The two most common causes of motor problems are: • •• A locked rotor (rotor unable to turn). • An overload (rotor turning too slowly). The motor overload protector (usually referred to simply as the overload) is a device that protects the motor windings from damage caused by over- heating due to overloaded conditions or poor ventilation. the motor reaches about 75% of normal speed. Burnout can happen very quickly, usually in less than one second. One of three methods is used to disconnect the start winding from the circuit, depending upon the type of motor. Open-type motors use a centrifugal switch. Small hermetic compressors use a current relay. Larger hermetic and semihermetic compressors use a potential relay. Current and potential relays are fully explained in Chapter 26. The centrifugal switch used to disconnect the start winding of an open-type motor is inside the motor frame. The switching device (current or potential relay) used by hermetic and semihermetic compressors is outside the motor or compressor’s metal housing. For hermetic and semihermetic compressors, relays must be outside of the compressor and motor, because of the arcing (sparking) that occurs when switch contacts open or close. Arcing would create damaging acids inside a hermetic or semihermetic compressor by causing a breakdown of refrigerant and compressor oil. The run winding and the start winding are two separate loads and must be connected in parallel. See Figure 25‑10. A switch is connected in series with the start winding and disconnects that winding after start-up. A main switch controls the power supply to the motor and is connected in series with both windings. The main switch can be installed at any remote location. Locked Rotor Amperage (LRA) At start-up, little or no back-emf is generated by the motor. Amperage flow is determined by the resistance of the motor windings. Starting current flow is typically five times higher than normal running amperage. The high current flow during start-up is called locked rotor amperage (LRA). As the motor picks up speed, back-emf is generated and rapidly increases the motor’s resis- tance to current flow. As noted, the start winding is disconnected at about 75% of normal speed. Consequently, amperage flow is substantially reduced. The motor operates on the run winding alone. Amperage flow is now determined by the resistance of the run winding combined with the back-emf generated by the motor. Full Load Amperage (FLA) Immediately after the start winding is discon- nected, the resistance of the run winding is increased by the back-emf generated by the Centrifugal switch (inside motor) Run winding Common Main switch Start winding L1 Goodheart-Willcox Publisher Figure 25‑10. The run and start windings are separate loads and connected in parallel. A centrifugal switch is connected in series with the start winding. Chapter 25 Motors 491 Copyright Goodheart-Willcox Co., Inc.

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