SYNCHRONOUS MOTORS 121 



machines, one-half for two-phase machines, and one-third 

 for six-phase machines. The scale should be divided 

 into four equal parts, each division line being numbered. 

 On each one of these scale divisions, the center line of the 

 marked pole should be placed and the motor started. 

 Thus five tests are made to insure that the motor will 

 not stick in any position. (See Fig. 36.) 



With the pole A moved to position No. 1 and the 

 machine at rest, sufficient current should be sent through 

 the armature to give a reasonable reading of amperes 

 and volts on the various phases, and induced volts on the 

 field. The induced volts field should be read by a poten- 

 tial transformer and alternating current voltmeter. 

 These readings are taken to determine which phase 

 gives maximum readings of current and voltage. 



The voltmeter and ammeter should be placed in this 

 phase and the armature current increased until the 

 motor starts. Volts armature, amperes armature and 

 induced volts field should be simultaneously read. 

 The starting voltage is now held constant until the 

 motor comes to synchronism, and the time required to 

 reach this point recorded. The machine attains syn- 

 chronism when the induced volts on the field fall to zero. 

 The machine is then shut down and the tests are repeated 

 for each of the other positions. 



If a motor shows a tendency to remain at half speed, 

 the alternating current voltage should be increased until 

 the motor breaks from half speed and comes up to syn- 

 chronism, the voltage required to accomplish this being 

 held until full speed is reached and then recorded. 



If the test is required to be made with a compensator, 

 the motor should be set with its field in the position where 

 greatest starting current is taken and allowed to rest in 

 that position for at least six hours until the oil is well 

 pressed out of the bearings. This is done in order to 

 obtain the worst starting conditions likely to occur in 

 normal operation. Connections are then made to the 

 lowest tap of the compensator, and with normal voltage 

 held on the line the starting switch of the compensator 

 is closed. If the motor fails to start, the voltage must at 

 once be switched off and connections made with the 



