334 ALTERNATING CURRENTS 



137. Types of Rectifiers and Converters. 1. The Rectifying 

 Commutator. The rectifying commutator is a commutator driven 

 by a synchronous motor. The segments are so connected that 

 when the alternating current reverses, the connections to the 

 direct-current circuit are simultaneously reversed, as shown in 

 Fig. 307. A uni-directional current is thus obtained. As the 

 brushes cannot have zero width, it is difficult to commutate at 

 the point of zero current and the current and voltage are rarely 

 zero at the same time. Hence, such devices spark more or less, 

 and so are limited to small currents and voltages. 



FIG. 307. Commutating-type rectifier. 



2. The Vibrating Rectifier. The vibrating rectifier, Fig. 308, is 

 based on the same principle as the rectifying commutator, except 

 that the circuit connections are reversed by contacts which are 

 opened and closed, synchronously, by alternating-current mag- 

 nets and a polarized armature. This type of rectifier ordinarily 

 is designed for use on 110- volt, 60-cycle circuits. The circuit 

 voltage is reduced by means of a step-down transformer, the 

 secondary of which has a middle tap. This secondary excites 

 two series-connected, alternating-current magnets, which are so 

 connected that they both have the same polarity on correspond- 

 ing ends at every instant. The vibrator is a soft-iron bar mag- 

 net, pivoted below these alternating-current magnets, each of 

 its two ends being directly beneath one of the alternating- 

 current magnets. This bar magnet is excited by direct current 

 taken from battery terminals and has therefore a fixed polarity. 

 Assume that at some particular instant the right-hand end of 

 the transformer secondary is positive. By following through 

 the circuits in Fig. 308 it is seen that both the lower ends of the 

 alternating-current magnets are north poles. Also, the left-hand 

 end of the bar magnet is a north pole, and its other end is a 

 south pole. This left-hand end is therefore repelled downwards 



