THE INDUCTION MOTOR, 371 



Connections. Connect the stator windings of the 

 motor to the source of alternating current, inserting 

 in one main an ammeter and the series coil of a 

 wattmeter. Connect the shunt coil of the wattmeter 

 to a two-way switch so that it may be connected 

 alternately to the two mains not containing the 

 series coil. Connect also a voltmeter across one pair 

 of mains to read the supply voltage. Connect the rotor 

 windings to the usual starting switch after inserting an 

 ammeter of low resistance in one of the phases. The 

 starting switch and resistance may be dispensed with, 

 and the rotor may be short-circuited directly through the 

 ammeter if the motor is always started at a reduced 

 voltage. If the motor is of the squirrel-cage type, of 

 course, no starter is to be employed. 



Means must be provided for varying the primary 

 voltage through a considerable range. This may be done 

 either by inserting a variable three-phase resistance in 

 the primary circuit, or by varying the voltage of the 

 alternator supplying the circuit by regulation of its shunt 

 regulator. In either case the periodicity of the circuit is 

 to be kept constant. 



In carrying out the experiment it will probably be 

 found necessary to add a reversing switch for the shunt 

 coil of the wattmeter, as indicated in the left-hand 

 lower corner of Fig. 181. This necessity is owing 

 to the fact that in throwing over the shunt coil 

 from one main to the other the reading of the 

 wattmeter will be found to be reversed in some cases, 

 although not in all, since the relative direction of the 

 readings will depend upon the value of the power-factor. 

 If the reversing switch has to be thrown over between 

 the readings on the two mains one value observed must 

 be considered negative, and must be subtracted from the 

 other in order to give the total watts supplied. In .other 

 cases the turn of the readings gives the total watts. If a 

 three-phase wattmeter is employed, it reads the power 

 of the whole circuit directly without any throw-over 

 switches. 



In many cases it will be found advisable to read the 

 current and watts in all three phases, to avoid errors 

 due to unsvmmetrical winding. By means of throw-over 



