SYNCHRONOUS GENERATORS 321 



Where troubles from excess cross currents are found, it can 

 usually be found due to a too close regulating machine, having a 

 too high short-circuit ratio in combination with insufficient fly- 

 wheel capacity. 



In considering the function of flywheel effect, a sharp distinc- 

 tion should be made between momentary speed changes or speed 

 fluctuations and slow changes or adjustments due to the speed- 

 load characteristic of the water wheel and governor, or what is 

 properly called speed regulation. All prime movers that operate 

 together to supply, power to a common load must operate at a 

 lower speed when loaded than when unloaded, in order that the 

 several prime movers will properly divide the load. It is also well 

 to differentiate between the function of flywheel effect in water- 

 wheel-driven generators and in reciprocating engine-driven gen- 

 erators. In the former the single purpose is to restrain speed 

 changes during the necessarily long period of adjustment of input 

 to output. In the latter the most important function is to pre- 

 vent the excessive changes in angular velocity during a single 

 revolution that would, otherwise, be caused by the varying torque 

 delivered by the engine cylinders. While with engine-driven units 

 flywheel effect is important from the standpoint of steady parallel 

 operation, this is not the case with water-wheel installations. 

 With the latter the flywheel effect influences the speed only with 

 sudden changes in load, and during the short time interval during 

 which the hydraulic conditions are changing to meet the new-load 

 conditions. 



The division of the load was entirely dependent on the angular 

 displacement between the bus-bar and induced generator e.m.f.'s 

 caused by the turning movements of the prime movers. It is, 

 therefore, evident that the speed regulation of the prime movers 

 must be the same, i.e., they must drop in speed from no load to 

 full load by the same percentage and in the same manner. If this 

 is not the case, the alternator connected to the prime mover of 

 closer speed regulation will take more than its share of the load 

 under heavy loads and less under light loads, and a too close speed 

 regulation is, therefore, not desirable for parallel operation of 

 alternators. To illustrate this further: Assume prime movers of 

 different speed regulation as shown in Fig. 188. When operating 

 in parallel it has previously been proven that, if an irregular speed 

 exists, a transfer of energy will take place between the alternators, 



