MONOPHASE INDUCTION MOTOE. 163 



resistances in the rotor circuits, a parallel arrangement of a high 

 non-inductive resistance and a low resistance with high inductance 

 is placed in each rotor circuit. No switch is used, as the starting 

 device is always kept in the rotor circuits. 



The impedance of a parallel arrangement consisting of a non- 

 inductive resistance R\ t and a coil whose resistance is R& and 

 self-induction Z, is easily shown to be 



+ fl 



where p = 2ir times the frequency. 



This impedance approaches the value RI when p is large, and 



7? 7? 

 the value -p V V w ^ en -^ * s sma ^- 



Now, at the start the rotor currents have the full frequency of 

 the stator currents, so that the external rotor impedance is approxi- 

 mately R\ ; but when full speed is attained, their frequency is but 

 a small fraction (about 5 per cent.) of that of the stator currents, 



so that the external rotor impedance now approaches the value 

 p -p 

 l t>2 , which is so small that its inclusion in the rotor circuits 



makes no perceptible difference in the efficiency when working. 

 The results produced by this starting device are said to be 

 excellent. 



STARTING-TORQUE OF INDUCTION MOTORS IN GENERAL. 



111. From the foregoing it is seen that the only difference 

 between monophase and polyphase induction motors is that in the 

 former the stator is wound with two circuits, one of which is cut 

 out when speed is attained ; whereas in the latter all the stator 

 circuits are always in use. 



Any means of improving the starting-torque of a monophase 

 motor beyond this phase - splitting arrangement in the stator 

 windings is equally applicable to polyphase motors. Equation (15) 

 shows that to produce a good starting-torque in any induction 

 motor the rotor resistance and reactance should both be low, and 

 as nearly equal as possible. Any device by means of which this 

 is effected is equally applicable to monophase and polyphase 

 induction motors. Thus any improvement in the rotors, or rotor 

 windings, can be applied to all types of induction motors. 



