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ELECTRICAL MACHINERY 



current because 1 2 is a maximum at the time ti; suppose 

 the polarity is as shown, there being a N pole at 2 and a 

 S pole at 4. Then, at the time ti, the magnetic field is in 

 the condition shown on the circle marked ti in Fig. 218. 

 At a time t 2 (Fig. 217) the current /i is a maximum and 7 2 

 has a zero value; poles 1 and 3 are therefore magnetized 

 while poles 2 and 4 now have zero magnetism. This gives 

 a field as shown in the circle marked fe in Fig. 218. 



At a time fe windings 1 and 3 carry no current and 

 windings 2 and 4 carry a maximum current, but this current 



FIG. 218. Polarity of the Different Poles at Successive Instants. ' 



is in the opposite direction to what it was at the time t\. 

 Hence the magnetic field is as shown on the third circle 

 of Fig. 218, marked 3. At a time t poles 1 and 3 are 

 magnetized but in the opposite direction to what they were 

 at the time t 2 , and poles 2 and 4 have no magnetism. This 

 condition is shown on the circle U. At a time t$ the mag- 

 netic field is in the same direction as it was at the time ti, 

 and after U the previous cycle is repeated. 



Combination of Fields Equivalent to a Rotating Field. 

 This succession of conditions in the magnetic circuit is evi- 

 dently equivalent to a rotating magnetic field; this may be 



