Abstract :
The subject of “Alternating-current motors for elevator service” was discussed at a meeting of the Pittsburg Section held on February 14, 1911. The discussion opened with a paper on the subject by Mr. W. H. Patterson, of the Westinghouse Electric and Manufacturing Company. Squirrel cage motors used for this service are designed with high resistance end rings so as to develop maximum torque at starting. They will develop about twice full-load torque and take about 2.5 times full-load current when connected directly across the line. The control is by means of a simple reverse switch, thereby replacing the more expensive controller and rheostat required by a phase wound motor. It is important that the torque be maximum at the start, for though the efficiency may be slightly improved by decreasing the rotor resistance, the motor would not then start as heavy leads. Moreover, as the required torque decreases after starting, an increased torque of the motor would cause a very sudden and disagreeable acceleration. Performance curves were shown. It was stated that the best results were obtained when the slip is about 20 per cent. The full load power factor was shown to be about 80 per cent, and full load efficiency about 70 per cent. While the apparent efficiency is low, it should not be compared with that of constant speed motors, as in this service efficiency is unimportant compared with the other characteristics. The efficiency of the elevator machine itself approximates 50 per cent. The power factor at starting varies from 70 per cent to 80 per cent. Performance curves of the wound rotor type were shown. These have somewhat better starting conditions, developing twice full-load torque with about twice full-load current. The conditions governing the size of motor required were discussed. It was shown that the nominal rating is of slight importance compared to the starting torque developed. The service conditions must then be considered to make sure that - he operation is not continuous for long enough periods to cause overheating. Squirrel cage motors are made in sizes up to 18 h.p., this being the practical limit, due to starting current. They are used for freight elevators up to speeds of 100 feet per minute; and in passenger service up to 150 feet per minute. The wound secondary type is satisfactory for elevators of all capacities up to 250 feet per minute. Elevators operating at higher speeds are practically always two-speed machines, a feature which eliminates the alternating-current motor, since with any given resistance in the rotor circuit the speed will vary with the load. Another feature limiting the use of alternating-current motors is the fact that with them dynamic braking can only be accomplished by the use of a small motor-generator set to supply direct-current excitation to the primary. However, alternating-current motors have been in use in elevator service for several years and have thoroughly demonstrated their adaptability for conditions within their range.
