Page:Electricity (1912) Kapp.djvu/231

Rh connected with a starting resistance. As the motor gathers speed this resistance is gradually short-circuited. Thus at no time is there any excessive rush of current in the primary or stator winding. When the motor has attained full speed the whole of the starting resistance is cut out and the slip-rings are short-circuited. In this condition the speed at which the rotor winding is cut by the revolving field is only a few per cent. of the speed at starting. It is the difference between the speed of the revolving field and the speed of the rotor. This is technically termed the "slip" of the motor, and varies from about 6 per cent. in small motors to 2 per cent. in large motors. The heavier the mechanical load on the motor, the greater is the slip. A motor of 50 H.P. would have about 1½ per cent. slip at half load and about 3 per cent. at full load, so that practically, although it is non-synchronous, the speed may be considered as approximately constant. This constancy of speed under variable load is a desirable feature in most of the industrial uses of motive power, and together with the great simplicity of mechanical construction explains why these motors, originally grown out of a scientific discovery made by a professor of physics, have become so popular