364 THE INDUCTION MOTOR. 



150 vibrations per minute, which, with a frequency of 50, 

 would correspond to a maximum slip of 2'5 per cent. 



If the rotor is of the squirrel-cage type, an ammeter 

 cannot be inserted in its circuit. In this case a revolving 

 contact maker may be employed. Any form of contact 

 maker described in Chapter IV. might be employed, but a 

 small stud on the rim of the motor coupling will serve 

 the purpose. In this case one of the mains is connected 

 to the shaft of the motor and another to a light spring 

 (which is touched by the stud as it revolves) in series 

 with a dead-beat voltmeter. The voltmeter will then 

 show a full deflection each time the moment of contact 

 coincides with a maximum voltage in one direction. If 

 there are as many contact points as there are pairs of 

 poles, the deflections of the voltmeter will equal the slip. 

 If only one contact is employed, the deflections must be 

 multiplied by the number of pairs of poles. 



Another method of determining the slip when this is 

 not great is to employ an alternating arc lamp. This 

 lamp is supplied with current from the same source as 

 the motor, and the light given out by it will flicker with 

 the velocity of the alternations of this current. If white 

 sectors are painted on the motor pulley, or on a piece of 

 card attached to the motor shaft, these sectors will rotate 

 with a speed slightly less than that of the flickering of the 

 arc. At each period of maximum illumination the sectors 

 will consequently be slightly behind the position which 

 they occupied at the last time of illumination. The sectors 

 will thus appear to rotate slowly backwards, and will 

 make the same number of apparent revolutions as the 

 slip of the motor, if the number of segments is equal to the 

 number of pairs of poles. 



A very simple device which maybe employed to measure 

 the slip in an induction motor provided with external 

 starter is a small pivoted compass needle, provided with 

 stops to prevent the needle swinging through more than 

 a small angle. 



This needle is placed immediately above, or below, one 

 of the connections to the rotor starting resistance, this 

 wire being placed in such a direction that the needle 

 normally points along the wire, so that the alternating 

 currents induced in the rotor will deflect the needle. 

 Thus, as in the case of the ammeter just described, the 



