266 INDUCTION MOTORS. [Exp. 



rent is much greater than with the resistance in circuit. (This 

 may be done at full voltage, if the motor is not too large.) With- 

 out the starting resistance, the starting torque is less ; this can be 

 shown* by means of scales and a lever arm fastened securely to 

 the rotor, which is now stationary. 



24. Polyphase Motor Started with a Compensator. If a 

 starting compensator is provided, start the motor at the low start- 

 ing voltage and throw over to full running voltage after the motor 

 speeds up. The initial current would be too great to allow start- 

 ing at full voltage, except in case of small motors without load. 

 When started with a load, the current is greater and the accelera- 

 tion less than when started without load. 



25. Polyphase Motor Running on Single Phase. After the 

 motor is in operation as a polyphase motor, disconnect all primary 

 lines except the lines of one phasef and note that the motor will 

 continue to run as a single-phase motor. While thus running, 

 measure the various terminal voltages and note that the machine 

 is also acting as a polyphase generator. In a 2-phase machine the 

 voltage of the idle circuit will be found to be in quadrature with 

 the voltage of the motor circuit which is connected to the line, 

 showing that the machine is capable of delivering true 2-phase 

 currents. 



In a 3-phase machine, the three measured voltages will form a 

 triangle. In this manner polyphase currents can be obtained from 

 a single-phase system ( 3, Exp. 7~A). (A polyphase motor may 

 be started, as well as run, on a single-phase circuit by the methods 

 shown in Figs. 2 and 3, discussed later.) 



26. Single Phase Motor with Phase Splitter. To start with 



* When an accurate measurement of starting torque is desired, it should 

 be made with a Prony brake so adjusted that the motor just turns at a 

 low speed, thus avoiding the error due to friction at standstill. 



t(25a). If an ammeter, voltmeter and wattmeter are connected in 

 one circuit, it is instructive to take readings with the motor running single 

 phase and polyphase and to compare the values of power factor, power 

 component of current and quadrature component of current. 



