Document 3Q77Mvm728X9NXV6gxp34EpGy

880 CHAPTER 40 1953 Guide'. and'when equipped with a high1 slip rotor, may be used for adjustable varying speeds through line voltage control. The motors are ideally suited for fan duty. ' Speed-torque characteristics of single phase motors are shown in. Fig.3. Full voltage starting of single phase motors is general'practice, but since most of these motors; arc connected to secondary distribution-systems located in light, load density -areas, power companies'check-, carefully the starting currents in order to prevent objectionable voltage .dips.-. This:is particularly important for motors started, frequently, such as those con trolled by pressure or temperature sensitive devices- and applied to re frigerators, stokers, oil burners and water pumps. . The report of a joint committee of A IEC-EE1-NEMA * recommended three application rules taking into account the greater annoyances re sulting from frequent motor starting during lighting hours against infre- Table 6; Recommended Single Phase -Motor Ratings fob Full Voltage Starting HP Ratino Lockbd-Rotob Cubbbnt* at 25 C Amperes ; Gbnebal.U&b (Rules l A 2) . Special Conditions (with. Utility Permission) (Rule 3) :115 V: 230 V 115 V 230 V >- 115 V . 230 V. Ho a H H H H 1 IH 2 3 5 1 20 20 20 : 23 31 45 61. 70 10. . A-M 10 ,A-M- 10 A-M - 11.5 A-M 15.5 M- 22.5 M 30.6 35 40 50 70 100 A-M A-M A-M A-M A-M a-m: m M M M . A-M A-M A-M A-M A-M A-M A-M A-M A-M A-M A-M A-M A-M A-M A-M A-M A-M A-M A-M A-M A refers to automatically controlled.device^ , , M refers to manually controlled devices. 1 * These values of Locked-rotor current are the' same as the NEMA standard for single-phase Design M motora. .V, ' ' quent starting at any time, Table 6 shows .typical applications of these rules which are becoming widely accepted for full voltage starting. Rules 1 and 2 apply to general use. - Rule 1 limits the locked-rotor current to 20 amp at 115 volts, and 25 amp at 230 volte when automatically controlled (usually frequently started) motors are used. Rule 2 permits twice these values when manually con trolled (usually infrequent starting) motors are used. Rule 3 applies to special conditions where larger currents may be allowed above`Rules 1 and 2 upon approval of the electric company. 'Hermetically enclosed single-phase a^c motors are widely used in appliances such as domestic and commercial refrigerators, room air conditioners, beverage coolers, freezer chests, and water coolers. In such appliances, the motors are sealed inside the compressor, and the gas and oil circulate freely within and around the motor. Centrifugal starting switches are hot used on such motors because any, arcing in the gas and oil atmosphere would be detrimental, and also because the unite are permanently sealed and cannot be serviced or repaired in the field. In place of a centrifugal Switch, an external relay is used to start hermetically enclosed motors. * Refer to Glossary at end of chapter. Mhtors and Motor Controls 881 ' The motors for hermetically enclosed serviceJare-usually[:of. therSplitr phase type or of the capacitor-start (or two-value capacitor) type, j f; For starting the split-phase type of motor, two methods are generally acceptable. In one method a thermally^operated time. delay. is1 used. When the motor is started, a contact which is normally closed; applies power to the starting winding. A thermal element which (Controls ,these contacts is in series with the motor and carries line current. : Due to . the current flow through this element, it is heated until, aftera definite period of time, it is warmed sufficiently to open the contacts and'remove-power from the starting winding. The running current then-heats the element enough to keep the contacts open. The setting of the time for the starting contacts to open is determined by tests on the system components, i.e., the?relay, the motor, and the'compressor, and is based.on a prediction-of the amount of time delay required to bring the motor up to speed.' i ACROSS THE LINE STARTING REDUCED VOLTAGE STARTING ' Arrangements 2, 4 and 5 provide automatic push-button starting. Fig. 4. Recommended Controls fob Squirrel-Cage Motors The second method of starting split-phase motors utilizes a series-coil or Current-operated relay. This device is affected indirectly by motor speed. . In this type of connection, a relay coil carries the line current going to the motor. When the motor is started, the inrush current to the running winding, passing through the relay coil, causes the contacts to close, and applies power to the starting winding. As the motor comes up'to speed, the current decreases until at a definite calibrated value of current corre sponding to a pre-selected speed, the magnetic force of the. coil diminishes to a point which allows the contacts to open, removing'power from the starting winding. Capacitor-start and two-value capacitor hermetically sealed motors are usually started with a voltage-type relay. In this method of starting the relay coil is connected in parallel with the starting winding. When power is applied to the line, the relay does not operate because it is calibrated to operate on a higher voltage. As the motor comes up to speed, the voltage across the starting winding and relay coil increases in proportion to the motor speed. At a definite voltage corresponding to a pre-selected speed, the relay operates and opens its contacts, thereby opening the starting