Document Exodk7B9Le0qV0ZZZEGN9EaYR

982 CHAPTER 39 1957 Guide t 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 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 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 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- Refer to Glossary at end of chapter. Motors and Motor Controls 983 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 fob Full Voltage Starting HP Rating Lockxd-Rotob Current* at 25 C Amperes General Use (Rules 14 2) Special Conditions (with Unurr Permission) (Rule 3) 115 V 230 V 115 V 230 V 115 V 230 V Mo H W H H H H 1 IH 2 3 5 20 20 20 23 31 45 61 70 10 10 10 11.5 15.5 22.5 30.5 35 40 50 70 100 A-M A-M A-M A-M H M 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 devices. * t*k 618 manu&Uy controlled devices. >tOTB 6 Valu0S * Locked'rotr current are the same as the NEMA standard for single-phase Design M the relay, the motor, and the compressor, and is based on a prediction of tne 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 m this type of connection, a relay coil carries the line current going to tne motor. When the motor is started, the inrush current to the running ending, passing through the relay coil, causes the contacts to close, and applies power to the starting winding. As the motor comes up to speed, ine current decreases until at a definite calibrated value of current corre sponding to a pre-selected speed, the magnetic force of the coil diminish^ starting1^' a^ows the contacts to open, removing power from the Capacitor-start and two-value capacitor hermetically sealed motors are JraUy ^frted with a voltage-type relay. In this method of starting the is a u18 conne.cted in parallel with the starting winding. When power onof t to t*le *ine. the relay does not operate because it is calibrated to aern D a ^her voltage. As the motor comes up to speed, the voltage mot^8 the jtarting winding and relay coil increases in proportion to the the r,8peed- At a definite voltage corresponding to a pre-selected speed, relay operates and opens its contacts, thereby opening the starting