The Diseases Of Electrical Machinery

 68 THE DIS EAS ES OF from mäins supplied at either a different voltage or a different frequency. Thus, suppose a four-pole in- duction motor, the no-load speed of which is 1500 revolutions per minute, at a frequency of 50 periods per second at 110 volts between lines. If put on a different Circuit, where the frequency is 40 periods per second, the voltage must likewise be reduced to 88, and then it will run at 1200 only, and will give only |th its previous output with the same current as before. As a rule induction motors cannot be used for higher frequencies, or higher voltages, than those for which they are designed, but may be used for lower fre- quencies, provided the voltage is proportionately lowered. Rotor.—That whiqh principally distinguishes in- duction motors from alternators is the rotating part of the machine, the rotor, While the generators here possess a complete magnet-wheel or system with a number of separate poles excited by a continuous current, in all the induction motors we have a simpler piece of machinery which is built either as a short- circuited (or squirrel-cage) rotor, or as a slip-ring rotor with starting resistance. In the latter case we find principally the same faults as with continuous-current machines, while in the first case faults are altogether rare. Short-Circuited Rotor.—The great mechanical simplicity of the rotor ensures almost entire freedom from faults. With early types, in which the single