THE INDUCTION MOTOR. 385 



The no-load component, spent in exciting the magnetic 

 circuit, is usually not more than quarter to one-third of 

 the full-load current of the motor. Consequently, the 

 primary current will be nearly proportional to the 

 secondary current when the motor is loaded, and appears 

 as a nearly straight line in Fig. 186, within the usual 

 working range, bending to the right and becoming hori- 

 zontal for very small values of the slip. 



In Fig. 177 (page 357) were drawn the curves of primary 

 and secondary current and torque on a slip base for the 

 ideal case of a motor without magnetic leakage. 



If a motor could be constructed without leakage, it 

 would have a very strong turning moment, and would 

 operate without resistance in the rotor circuit with a 

 definite slip and speed for each value of the load, but would, 

 at starting and at low speeds, take an excessively heavy 

 current. 



It must be remembered that the total flux produced 

 by the primary current is constant, as in the case of a 

 transformer. The relation between primary flux and 

 primary voltage is given by the same formula as that on 

 page 126, for the transformer 



When Z = maximum flux produced. 

 n = periodicity. 

 N l = turns in winding. 

 e 1 = primary voltage. 



The flux consequently depends only on the construction 

 of the motor and the voltage applied, and is independent 

 of the load.* 



In a three-phase motor it may be shown that current 

 in each phase winding must produce a flux equal to 

 two-thirds of the total strength of the rotating field. 

 The value of the no-load current, or magnetising current, 

 will be given by applying the same formula as that given 

 for the magnetising current in a transformer, account 

 being taken of the fact that the current in each phase is 

 that required to produce two-thirds of the rotating flux. 

 For the same reasons as exist with the transformer, a short 

 magnetic circuit, with as small an air-gap as possible, 

 and a low saturation are employed. 



"This is neglecting the loss of voltage in the resistance of the primary 

 winding = c\ Ri 



