Starting characteristics and control of polyphase squirrel-gage induction motors

In the application of squirrel-cage induction motors to such severe service as frequent start and stop or frequent plugging operation, it is desirable to know how much loss the motor will be required to dissipate. Plugging being understood as changing the motor from any speed in one direction to any speed in the opposite direction, by reversing the rotation of the field only. The subject of starting loss is also of interest from the control standpoint; such as the application of auto-transformers for starting purposes; or where an external resistance is inserted in the primary as is done in the control of elevator motors. Consideration is given, to the part that the primary and secondary resistance and the total reactance play in the determination and the manipulation of the starting losses; and, as it is true that in many applications the time spent in accelerating the rotor from rest is useless from the production standpoint, consideration is given to the value of secondary resistance that will give minimum starting time for a given field strength. This involves a method for the determination of the time taken to attain a given speed in general. There are many cases where a motor has already been designed and tested, and is about to be applied on a given job where the cycle of operation is known and includes either starting and stopping or plugging. In such an event, instead of working with the various test values to get the constants of the machine, it is much more convenient and accurate to work from such values as starting torque, maximum torque, slip at full load, locked current and primary resistance. It will be shown how the proceeding short-cut method can for most cases be made more simple without sacrificing the accuracy of the result to any appreciable extent. When a motor is plugged the problem is complicated by the difficulty that eddy currents flow in the rotor which give a slight increased effect to the torque at negative speeds. Although this paper does not show how to predetermine its amount, it does show how to handle the losses and the time of reversal, if the test speed-torque values are known or can be estimated from another machine.