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.