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PRINCIPLES OF ELECTRICAL DESIGN 



If the machine is driven by a high-head water turbine, or by 

 an alternating current motor as in motor-generator sets the 

 speed would be somewhat higher, and the number of poles fewer, 

 than the average figures as given in the above table. 



When using this or any other table or data intended to assist 

 the designer with approximate values, it is necessary to exercise 

 judgment, or at least be guided by common sense. For instance 

 it may be necessary to depart from the values given in the table 

 in the case of machines direct-coupled to slow-running engines. 

 This is especially worth noting in the case of the smaller 

 machines, which may require more than four poles in order to 

 give the best results on very low speeds. 



