330 ELECTRICAL ENGINEERING 



The resultant m.m.f. of the three phas&s is 



m = Vm^-\-m Y 2 = f nJo Vcos 2 6 + sin 2 = f ni/ 



and makes an angle 6 with the OX axis. 



The resultant m.m.f. is, therefore, constant in value being 

 equal to \ times the maximum m.m.f. of one phase and it revolves 

 at synchronous speed. 



This constant m.m.f. produces a field of constant strength 

 revolving at synchronous speed. The revolving field links suc- 

 cessively with the windings and generates e.m.fs. in them. The 

 flux linking with any phase is maximum when the current in that 

 phase is maximum and, therefore, the flux and current are in phase. 



If a four-pole stator, Fig. 310, had been chosen instead of the 

 two-pole stator, the m.m.fs. of the two-phase windings would have 

 been combined at 45 degrees instead of 90 degrees and the result- 

 ant m.m.f. and flux would not remain constant but would pulsate 

 four times during each revolution. The flux threading any phase 

 would, however, still vary according to a sine law and would be 

 in phase with the current in that phase. 



To reverse the direction of rotation of a two-phase induction 

 motor, it is necessary to reverse one phase only. 



To reverse a three-phase motor any two leads may be inter- 

 changed. 



203. The Rotor. The secondary or rotor is made in two 

 forms, (a) the wound rotor and (b) *the squirrel-cage rotor. The 

 wound rotor consists of a laminated iron core with slots carrying 

 the winding, which must have the same number of poles as the 

 stator winding but may have a different number of phases. It is 

 usually wound for three phases and the ends of the windings are 

 brought out to slip rings so that resistances may be inserted in the 

 windings for starting and the terminals short circuited under 

 running conditions. 



The squirrel-cage rotor winding consists of a number of heavy 

 copper bars short circuited at the two ends by two heavy brass 

 rings, Fig. 314. The construction is very rugged and there is 

 nothing to get out of order. 



When the rotor with its closed windings is placed in the revolving 

 magnetic field produced by the stator currents, the flux cuts across 

 the conductors on the rotor and generates e.m.fs. in them. Cur- 

 rents flow in the rotor equal to the e.m.fs. divided by the rotor 



