338 ELEMENTS OF ELECTRICAL ENGINEERING. 



starts to rotate in a reverse direction when a is greater than b. 

 When the pointer rotates very slowly we have approximate syn- 

 chronism, when the pointer is in a certain position the alternator 

 A has the proper phase relation to the bus-bar voltage, and when 

 both of these conditions are realized the main switch which con- 

 nects alternator A to the bus-bars is closed. 



The potential transformer which is connected to the bus-bars 

 supplies current to the windings SS, Fig. 289, and the potential 

 transformer which is connected to the machine which is being 

 started supplies current to the two windings C and D of a 

 rotating armature A. The collector ring g is connected to one 

 of the terminals of the "machine-transformer" and thence to the 

 common junction e of the coils C and D, so that coil C is 

 connected between the terminals of the " machine-transformer " 

 with a high inductance L in series with it, and coil D is con- 

 nected between the terminals of the " machine -transformer " with 

 a large non-inductive resistance R connected in series with it. 

 The core A is mounted on a spindle which carries the pointer n. 

 Under these conditions the pointer n stands stationary when the 

 bus-bar voltage and the machine voltage are in exact synchronism, 

 and when the two voltages are not in synchronism the pointer n 

 rotates in one direction or the other at a speed which is equal to 

 the difference of the frequencies of the two voltages. 



152. The compensated voltmeter. When a voltmeter is con- 

 nected between the mains at a generating station, either directly 

 or through a potential transformer, it of course indicates the volt- 

 age between the mains at the station, which is generally greater 

 than the voltage between the mains at a distant receiving station. 

 It is frequently desirable to control the voltage between a pair 

 of feeders at a generating station so as to maintain a constant 

 voltage at the distant receiving station which is supplied with 

 current through the given pair of feeders, and, in order to do this, 

 it is necessary to arrange a voltmeter in such a way that it will 

 indicate the voltage across the mains at the distant receiving station. 

 A voltmeter so arranged is called a compensated voltmeter, and 



