158 ELEMENTS OF ELECTRICAL ENGINEERING. 



ever, from Figs. 127 and 128 that the higher voltage machine 

 can operate as a motor and take power from the lower voltage 

 machine acting as a generator, and two things concerning the 

 motor action of the higher voltage machine are at once apparent 

 from Figs. 127 and 128, namely, (a) The larger voltage machine 

 cannot take in as a motor as much power as the lower voltage 

 machine, and (b) the RP loss in the circuit is very large when 

 the higher voltage machine acts as a motor. It is therefore un- 

 desirable to operate asa synchronous motor an alternator which 

 has a voltage higher than the generator or supply mains from 

 which it takes current, except when the above mentioned disad- 

 vantages are off-set by an advantage depending on the following 

 peculiarity. 



An over-excited synchronous motor takes from its source of 

 supply a current which is ahead of the supply voltage in phase, 

 that is to say, an over-excited synchronous motor acts like a con- 

 denser ; and inasmuch as the reactance (i/o)C) of a condenser 

 is considered as a negative reactance, so also an over-excited 

 synchronous motor is equivalent to a negative reactance. The 

 possible use of a condenser (or of an over-excited synchronous 

 motor) for supplying the wattless current taken by an induc- 

 tive receiving circuit is discussed in Art. 40, and in fact the 

 over-excited synchronous motor is sometimes used for this pur- 

 pose. In this connection it is important not to confuse the in- 

 ductive reactance a>L of the synchronous motor armature, which 

 is what is represented by the letter X* in this chapter, with what 

 is called the negative reactance of the over-excited machine in 

 operation. The negative reactance of a condenser circuit is equal 

 to the wattless component of the impressed voltage divided by 



*The letter X in this chapter represents the value of wZ, where L is the entire 

 inductance of the circuit including machines A and B. If a synchronous motor is 

 operated from supply mains and if A represents the voltage across the supply mains 

 then there is no need to consider anything back of the point to which A refers. Al' 

 of the discussion of this chapter applies to this case if R is taken as the resistance of 

 the synchronous motor and its connecting wires, and if X is taken as w times the 

 inductance of the synchronous motor armature and its connecting wires. 



