The current taken by an induction motor from a constant pressure line varies with the speed as in a direct current motor. When a load is thrown on, the speed is reduced correspondingly and as the self-induction or reactance is diminished, more current circulates in the squirrel cage winding, which in turn reacts on the field coils in a similar manner and more current flows in them from the line. In this manner the motor automatically takes current from the line proportional to the load and maintains a nearly constant speed.
The so-called constant speed motors require slight variations in speed to automatically take current from the line when the load varies.
Induction motors vary in speed from 5 to 10 per cent., while synchronous motors vary but a fraction of one per cent.
Single phase motors to render efficient service must be able, where requisite, to develop sufficient turning moment or torque to accelerate, from standstill, loads possessing large inertia or excessive static friction; for example, meat choppers and grinders, sugar or laundry centrifugals; heavy punch presses; group driven machines running from countershafts with possibly over taut belting, poor alignment, lubrication, etc.
CHAPTER LII
TRANSFORMERS
The developments in the field of electrical engineering which have rendered feasible the transmission of high pressure currents over long distances, together with the reliability and efficiency of modern generating units, have resulted in notable economies in the generation and distribution of electric current.
This has been accomplished largely by the use of distant water power or the centralization of the generating plants of a large territory in a single power station.