THE INDUCTION MOTOR. 



armature was 312. The resistance of the field was 0.015 ohm; of each 

 of the three phases of the armature, 0.08 ohm. 

 TESTS OF lo-HP MOTOR. 



107. It is instructive to calculate the armature loss in watts for the 

 greatest load. We find this to be 55* . 0.24 = 730 watts, corresponding 

 according to (15), to a slip of 2.1 per cent. The discrepancy between 

 this and the measured value of 2.66 per cent is doubtless mainly due 

 to the difficulty of measuring a small slip. 



CALCULATION OF THE INDUCTANCE OF THE SINGLE-PHASE MOTOR 



108. We must now turn to the calculation of the counter e. m. f. in- 

 duced by the oscillating field upon itself. We shall consider two 

 cases, viz., first, the case in which the field-coil is spread over two- 

 thirds of the pole-pitch; secondly, the case in which the field-coil 

 covers the whole pole-pitch. 



First We saw in the second chapter of this article that the e. m. . 

 induced by a field * in a coil of the width b is two-thirds as large 

 as the e. m. f. which would be induced by the same field in a coil 

 which is not distributed but lodged in only one groove. For the lat- 

 ter case we have, 



e = 2.22 ~ . * . * . io' 8 volts. 



Fig. 30 shows the shape of the field for a coil the width of which 

 is equal to two-thirds of the pole-pitch. We call the number of ac- 



60 



