MEASUREMENT OF FRICTION LOSS 211 



in the motor. The disc is illuminated as usual by a source of light 

 connected to the same mains as those supplying the motor. If the 

 position of the disc is such that its diameter is equal to that of the 

 cone, then disc and cone will revolve at the same speed, and if this 

 speed were that of synchronism, the disc would appear to be 

 stationary. This position gives the zero of the scale. Since the 

 speed of the motor is, however, below that of synchronism, the disc 

 will appear to rotate if placed at the zero of the scale. But its speed 

 may be increased by traversing it towards the thicker end of the 

 cone, until it appears stationary ; the method is thus a zero method. 

 The value of the slip corresponding to a given displacement of the 

 disc from the zero position is easily calculated from the angle of the 

 cone in constructing the direct-reading scale. 



123. Methods applicable to Slip-ring Rotors 



The method just described is applicable to motors having either 

 wound rotors provided with slip-rings or permanently short-circuited 

 rotors. With wound rotors, many other methods for measuring 

 small slips are available. Thus, a sensitive dead-beat moving-coil 

 galvanometer or milli-ammeter may be connected as a shunt across 

 one of the connecting leads between the slip-rings and the starting 

 resistance, and the number of throws to the same side of zero 

 determined per second. This number gives the frequency 11^ of the 



rotor currents, and if n\ is known, s = - . Instead of the milli- 



n\ 



ammeter, a telephone receiver may be used ; the frequency ti^ is then 

 given by half the number of clicks per second. 



124. Measurement of Friction Loss 



We have already, in considering the circle diagram, explained 

 one method of finding the friction loss in a motor. Another very 

 convenient method, which may be used as a check on the former, is 

 to take one set of wattmeter readings when the motor is running 

 light at the normal p.d. with its rotor short-circuited as usual, and 

 another set when the rotor has been suddenly open-circuited, the 

 second set of readings being obtained as quickly as possible after 

 open-circuiting the rotor. The difference of power in the two cases 

 practically represents the frictional loss. 



