BIPOLAR DIAGRAM 165 



frequency due to the non-uniform crank effort of a single cylinder 

 steam-engine which drives the supply alternator. 



If the impedance (including generator, motor and line) be very 

 low, i.e., if the air-gap be long and the coupling stiff, the motor 

 speed will follow closely the frequency pulsation. There is thus a 

 pulsation strain on the coupling accompanied by a large pulsation of 

 power and current. If the motor is large enough as compared with 

 the alternator, the stiff coupling will have the effect of adding fly- 

 wheel capacity to the alternator and will tend to reduce the frequency 

 pulsation, but at the expense of heavy current and power pulsations. 

 If, on the other hand, the impedance be large, the frequency 

 pulsations will be partly absorbed in the soft coupling, the motor 

 will not follow closely the frequency pulsations, and there will 

 be much less pulsation of power and of current. There is, how- 

 ever, an obvious limit to the desirable softness of coupling, namely, 

 that beyond which there is danger of a breakdown of the motor 

 because of the soft coupling. 



There is also another objection to the soft coupling, which will 

 appear from an inspection of the E.M.F. diagram (Fig. 27). If the 

 reactance be high, the coupling angle will be relatively large for a 

 given power, and there will be a large variation in the power factor 

 of the motor from no load to full load under constant excitation, or a 

 frequent adjustment of excitation will be demanded in order to 

 maintain a constant power factor. 



When polyphase synchronous motors or synchronous converters 

 are started by the induction motor action of the damping coils, or 

 by hysteresis and eddy current torque, it is desirable to have as short 

 an air-gap as possible in order to keep the necessary starting current 

 within limits. 



The choice of air-gap is thus a compromise and depends some- 

 what upon the particular service demanded of the motor in question, 

 and upon the nature of the system on which it operates. The gap 

 should preferably be not as long as demanded by good regulation in 

 the synchronous alternator, nor as short as demanded by high-power 

 factor in the induction motor or induction generator. On this point 

 there is considerable difference in practice, particularly in the case 

 of synchronous converters. 



