130 COMMERCIAL ELECTRICAL TESTING 



1-2-3, then phase 1 is on the current coil of wattmeter 

 R, connected at A, and the pressure coil is connected 

 across 1 and 2. Likewise with the other meter S, the 

 current coil of which is on phase 3 at B, with its pres- 

 sure coil between 2 and 3. If the voltage is too high for 

 direct use on wattmeters, multipliers (non-inductive coils 

 of known resistance) or potential transformers must 

 be connected between the meter and the volt lines at 

 the table. 



On motors of less than 20 h.p. the lines to the pri- 

 maries of the potential transformers must be attached to 

 the generator side of the lines coming to the top of the 

 dynamometer board. If placed on the motor side of top 

 of the board, or on the motor terminal block, the excita- 

 tion current of the potential transformer passes through 

 the wattmeters. Although this current is small, with a 

 small motor it may be an appreciable percentage of the 

 excitation current. Hence an error is ^ caused and an 

 abrupt break made in the excitation curves every time 

 the ratio of the potential transformer is changed. On 

 large motors the excitation current of the potential 

 transformer is so small in comparison with the motor 

 current that the incidental errors are negligible. The 

 above does not apply to multipliers because they are 

 non-inductive. 



On large motors the volt leads should always be 

 attached at the terminal block in order to eliminate the 

 line drop in switches and leads from the table to the 

 motor. The current leads to the wattmeters should be 

 twisted together throughout their length and come 

 direct from the terminal to the meter without loops or 

 sharp turns. All connections must be kept tight and 

 clean. 



The air gap should be taken before the test is started. 

 On voltages above 500 volts all instruments must have 

 any static charge thereon discharged and a small fuse 

 connected between the current terminal and the nearest 

 volt terminal of the wattmeter. Do not ground the 

 secondaries of the transformer. 



As soon as the machine is wired and ready to start, the 

 switches on the dynamometer board should be closed. 



