Discussion on line disturbances caused by special squirrel-cage and wound-rotor motors when starting elevators and hoists (Lincoln) Cleveland, Ohio, March 18, 1915. (see proceedings for March, 1915)
|Publisher:||IEEE - Institute of Electrical and Electronics Engineers, Inc.|
|Publication Date:||1 November 1915|
|Page(s):||2,847 - 2,850|
H. D. James: I was connected for five years with a large elevator company and wish to state that this company had very good information on the efficiency of its machines. This was based on several hundred tests showing the torque required to start the elevator and the power required to operate same. Undoubtedly Mr. Lincoln is referring to smaller companies that have not accumulated much of this information. Mr. Lincoln states in his conclusions - "If the standard series relay type of controller is used on the wound-rotor motor the maximum torque must be exerted on the first step for sometimes the maximum load has to be lifted." The torque referred to is the torque at the circumference of the hoisting drum and the amount of torque exerted by the motor will depend upon the gear ratio between the motor and the drum. An elevator is sold to lift a specified number of pounds at a specified speed. Let us assume that the load is 3000 lbs. and it is to be lifted at the rate of 100 ft. per min. The motor must be geared to the drum to give this speed of car with the load specified. In selecting the gearing the synchronous speed of the motor can not be used as an induction motor runs at a speed less than synchronism, depending upon its full load slip. If we assume that a cage motor is used having 30 per cent full load slip and compare it with a slip ring motor having 4 per cent slip, it is readily seen that there is a gear ratio in favor of the slip ring motor of approximately 40 per cent. The slip ring motor will be required to exert 40 per cent less torque than the squirrel cage motor to start the same load. The current taken in starting on the first notch of the controller is the current corresponding to the torque required to start this load. It is not the current corresponding to the same torque at the motor shaft. Mr. Lincoln points out that the squirrel cage motor can be designed to exert a given torque with less amperes input than the slip ring motor. This is true as the squirrel cage motor has a better interlinking of induction between the primary and secondary than the slip ring motor. This comparison, however, is not a true one. It has been shown above that the slip ring motor runs at a higher speed than the squirrel cage motor and, therefore, has a different gear ratio which favors the slip ring motor at starting. In order to compare the two motors the torque required to start a given load should be determined for each type of motor and the current input at this torque compared. An analysis of this kind will show that the secondary resistance of a squirrel cage motor can be increased with profit up to a certain point, beyond which the advantages of increased resistance are more than counterbalanced by the increased slip of the motor. The gear ratio to give full speed will increase the torque required from the motor faster than the resistance will improve this torque. Where the slip ring motor and the squirrel cage motor are both proportioned to give the best results when referred to the speed of the car, it will be found that the amperes input to start the load will not differ materially in the two designs.