48 COMMERCIAL ELECTRICAL TESTING 



In calculating the rise of temperature by resistance 

 the following formula is used: 



Let Rtz = hot resistance of copper measured at the 



temperature / 2 

 Rti = cold resistance of copper measured at tem- 



perature ti 

 RO = resistance of copper at deg. C. 



When using this formula it is assumed that 0.0042 is 

 the temperature coefficient of copper at deg. C. The 

 rise obtained from this formula should be corrected 

 by one-half of one per cent for each degree C. that the 

 final room temperature differs from 25 deg. C. This 

 correction is added if the temperature is below 25 deg. C. 

 and subtracted if above. The temperature of the 

 winding itself must therefore be very carefully observed, 

 as well as that of the room, when the hot and cold 

 resistances are taken. 



It is often necessary to make a heat run on an alter- 

 nating current machine at a specified power-factor. 

 To do this, in the case of a generator, the machine 

 is loaded on water boxes connected in parallel with a 

 synchronous motor. The motor merely floats on the 

 line, its field being adjusted to give the desired power- 

 factor. Instead of loading the generator on water boxes, 

 the motor is often belt or direct connected to a direct 

 current generator which feeds back onto the shop 

 circuit. 



Synchronous motors are run under load at a certain 

 power-factor by being driven from an alternating current 

 source of power and loaded on a direct current gener- 

 ator. When power-factor runs are made, generators 

 should always be run with lagging and synchronous 

 motors with leading current, unless otherwise specified. 



In addition to an ammeter and voltmeter, wattmeters 

 should always be inserted in the armature circuit of 

 the machine tested, in order to check up the power- 

 factor of the circuit. 



Equivalent load heat runs are frequently made at 

 a given power-factor. In the case of an open circuit 



