Document 8VvYMkXMaqYao3Xd3jvJ0vJjm

1002 CHAPTER 39 1958 Guide : The report of a joint committee of AIEC-EEI-NEMA recommended three application rules taking into' account the greater annoyances re sulting from frequent motor starting during lighting hours against infre quent starting at any time. Table 6 shows typical applications of these rules which axe 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 volts 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 Fig. 4. Speed-Torque Characteristics of Single Phase Motors 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 appli ances 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 not used on such motors because any arcing in the gas and oil atmosphere would be detrimental, and also because the units 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. Hermetically enclosed motors for refrigeration compressors will; in the future, have the horsepower rating eliminated. The full load current is to be the operating current in amperes, when the compressor is deliver- Motorsand Motor Controls 1003 ing rated output. The motors for hermetically enclosed service are usually of the split-phase type or of the capacitor-start (or two-value capacitor) type For starting the split-phase type of motor, two methods are generally acceptable. In one method a thermally-operated time delay is 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, after a 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.. Table 6. Recommended Single Phase Motor Ratings for Full Voltage Starting HP Rating Lockei>-Rotob Cubbent* at 25 C Amperes General. Use (Rules 1 & 2) Special. Conditions (with Utxlitt Permission) (Rule 3) 115 V 230 V 115 V 230 V 115 V 230 V Ho 20 10 A-M A-M A-M A-M 20 10 A-M A-M A-M A-M H 20 10 A-M A-M A-M A-M H 23 11.5 A-M A-M A-M A-M H 31 15.6 M A-M A-M A-M H 45 22.5 M A-M A-M A-M ,H 51 30.5 M A-M A-M 1 70 35 M A-M A-M 1*4 40 M A-M 3 5 50 70 . 100 M A-M A-M A-M A refera to automatically controlled devices. M refers to manually controlled devices. These values of Locked-rotor current are the same as the NBMA standard for single-phase Design M tne 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. 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 windmg) passing through the relay coil, causes the contacts to close, and applies power to the starting winding. As the motor comes up to speed, tUe current decreases until at a definite calibrated value of current corre sponding to a pre-selected speed, the magnetic force of the coil.diminishes a point which allows the contacts to open, removing power from the starting winding. Capacitor-start and two-value capacitor hermetically sealed motors are started with a voltage-type relay. In this method of starting the ay cod is connected in parallel with the starting winding. When power one tle<^ to *`ne' t*le relay does not operate because it is calibrated to awoR a ^ voltage. As the motor comes up to speed, the voltage mot start,lnS winding and relay coil increases in proportion to the or speed. At a definite voltage corresponding to a pre-selected speed, relay operates and opens its contacts, thereby opening the starting