INDUCTION MOTOR 69 As seen, in the constants at load, constant excitation, S, is prac- tically as good as varying unity power-factor excitation, $0, drops below it only at partial load, though even there it is very greatly superior to the induction-motor characteristic, /. It thus follows: By converting it into a synchronous motor, by passing a direct current through the rotor, a good induction motor is spoiled, but a poor induction motor, that is, one with very high exciting current, is greatly improved. ,_40 _ INDUCTION MOTOR So SYNCHRONOUS, UNITY POWER FACTOR 8 SYNCHRONOUS, CONSTANT EXCITATION X CSoSYNCHR.CONOAT.INDUCT.,UNITY P.P. A C8 8YNCHR.CONCAT.INDUCT.,CONSTANT EXCIT.-J20 + CC COMMUTAT.MACH.CONCAT.INDUCTION " * CONDENSER IN SECONDARY _ _J0 ±0 CONDENSER IN SECONDARY 180 140 150 160 170 180 190 200 40 60 60 70 80 90 l6o 1JO FIG. 28.—Comparison of apparent efficiency and speed curves of high- excitation induction motor with various forms of secondary excitation. 46. The reason for the unsatisfactory behavior of a good induc- tion motor, when operated as synchronous motor, is found in the excessive value of its synchronous impedance. Exciting admittance in the induction motor, and synchronous impedance in the synchronous motor, are corresponding quanti- ties, representing the magnetizing action of the armature cur- rents. In the induction motor, in which the magnetic field is produced by the magnetizing action of the armature currents, very high magnetizing action of the armature current is desirable, BO as to produce the magnetic field with as little magnetizing cur- rent as possible, as this current is lagging, and spoils the power- factor. In the synchronous motor, where the magnetic field is produced by the direct current in the field coils, the magnetizing action of the armature currents changes the resultant field excita- tion, and thus requires a corresponding change of the field current to overcome it, and the higher the armature reaction, the more