Page:Electricity (1912) Kapp.djvu/230

226 a large current at a low power factor. At starting the rotor is at rest and only gradually gathers speed. Whilst running slowly the frequency with which the rotor bars are cut by the revolving field is great, and consequently the e.m.f. and the rotor current are also great. A great rotor current means a weakening of the flux originally produced by the stator current, but as this flux is mainly instrumental in balancing the e.m.f. impressed on the stator terminals, and since this e.m.f. is constant, no appreciable weakening can take place. The action of the machine is that it automatically admits more current through the stator to make up for the weakening effect produced by the excessive rotor current at starting. A sudden rush of current taken from the supply terminals is disturbing to the rest of the machinery supplied from the same system, and hence the use of squirrel-cage motors must in the interest of all consumers be restricted to small types. When motors of large power are required it is necessary to limit the excessive rush of current at starting, and this is done by using a rotor with proper winding, the terminals of which are connected to slip-rings on the shaft. On these slip-rings are placed brushes which are