A-C. elevator motor drive

There are very few data available on the problems of using alternating current when applied directly to a motor on an elevator. It is apparent, also, that eventually there will be no d-c. power transmitted for elevator service. Because of these facts and the size of this industry it seems that such data should be available. A paper covering the entire field in detail would be excessively long and it is therefore the aim of this paper to cover the subject in a general way, giving such outstanding facts as are felt to be of most interest at this time. It is hoped also to correct a false impression that is sometimes found to exist — that an a-c. elevator is not practical for car speeds above 350 ft. per min. Without question, this understanding was correct six or seven years ago but it is desired to call attention to the fact that for the past five years many a-c. elevators have been installed with car speeds in excess of 500 ft. per min. and today some are operating as high as 700 ft. per min., and nothing has appeared to indicate that there is a limit of car speed other than for any other type of control. A brief outline of the necessary requirements of the elevator machine is given because as yet the development of a-c. elevators has depended upon the success of this unit. The desirable characteristics of the motor are given somewhat in detail, the important ones being positive speed control, elimination of exposed and sliding contacts, speed ratios of at least 6:1, a rotor of low kinetic energy, quiet under operation, allowing torque characteristic changes, smooth control of speed changes, liberal temperature range, high power factor, a maximum torque capacity and maximum practical starting torque per ampere. The desirable characteristics of the controller which permit high speed elevator operation with economical and reliable service and a minimum number of shut-downs, may be outlined as follows: Full magnetically operated but with a minimum number and types of magnets, types of magnets that guarantee against magnetic hum or chatter requiring no oil immersion and giving a constant pull. The controller parts in general should be as interchangeable as practical, with oilless bearings and a minimum of auxiliary parts and contacts. As a whole, the controller and its wiring must be simple and easily understood. The principles of control allowing the high-speed elevator operation are rapid but smooth acceleration and retardation, a forced slowing down of the elevator by the motor irrespective of the operator and allowing the simultaneous or overlapping braking action of the slowing down and stopping means. The brake magnet must be one guaranteeing against magnetic hum or chatter, giving a constant pull, and must be positive and rapid but not violent in action. The curves which were taken by power companies serve to show the high power factor and a minimum of line disturbance.