CHAPTER XV 

 POLYPHASE INDUCTION MOTORS 



1. Lay off on two axes, making 60 degrees with each other, the 

 successive sets of field values due to a two-phase system of currents 

 in coils at right angles to the above axes. Combine these and 

 hence obtain a polar curve showing the strength of the revolving 

 field. Note also the angular velocity of the field at different 

 points. (10 rain.) 



2. Lay off on three axes, making 120 degrees with each other, 

 the successive sets of field values due to a three-phase system of 

 currents in three coils at right angles to these axes. Combine the 

 three fields of each set and thus obtain a polar curve showing the 

 strength of the revolving field. Note also the angular velocity at 

 different points. (15 rain.) 



3. A 12-pole (per phase) induction motor is running on a 60- 

 period circuit; what is the frequency of the current in the second- 

 ary, when it is running at 600, 500, 300 and r.p.m.? (4 ram.) 



4. Construct a table showing synchronous speeds in r.p.m. 

 for 2-, 4-, 6-, 8-, 10- and 12-pole motors at frequencies of 100, 60, 

 40 and 25. (9 ram.) 



6. Draw a series of diagrams showing a cross-section perpen- 

 dicular to the shaft, of the rings and of the field or primary con- 

 ductors of a 6-pole, 2-phase motor, and show by dotted lines the 

 positions of the field corresponding to 6 = 0, 45, 90 and 135 

 degrees in i = I sin 0. Draw the coils and field for one-half of 

 the ring only. Letter the phases A and B and indicate the suc- 

 cessive position of a particular pole by marking with an X. (15 

 rain.) 



6. Repeat problem 5 with a 4-pole, 3-phase motor and corre- 

 sponding to 6 = 0, 30, 90 and 180 degrees. (20 win.) 



7. The stator and rotor of a 440-volt induction motor are both 

 Y-connected. The stator current is 18 and the rotor current 108. 

 If the slip is 4%, what e.m.f. is set up in the rotor circuit between 

 neutral and one slip ring? (3 rain.) 



8. A 300-h.p., 440-volt, 50-cycle, 12-pole motor has a delta- 



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