IEEE 251-1984 pdf free.IEEE Guide: Test Procedures forDirect-Current Tachometer Generators.
IEEE 251  Direct-readingindicatinginstruments,either analog or digital type, maybe used for some tests where the specifiedtolerances are large compared with the instrument accuracy. When an indicating instrumentis used，it should be selected so that readingswill be well up on the scale, preferably at least75% of fullscale value. For many of the tests,the permissible deviations in the tachometergenerator voltage are of the same order of mag-nitude as the precision of the best indicatinginstruments. For these tests a null-balancepotentiometer type direct-current voltmeter,having an accuracy at least 4:1 better than themaximum voltage deviation to be measured,shall be used.
Ammeters.Direct-reading indicating instru-ments，either analog or digital type,are ordin-arily sufficiently accurate for reading loadcurrent.The instrument should be so selectedthat readings will be well up on the scale,preferably at least 75% of full-scale reading.The accuracy requirements for measuring fieldcurrent ofseparatelyexcited tachometergenerators are the same as those for measuringoutput voltage and similar instruments shallbe used.
Tachometer Generator Drive. For most ofthe tests，the only satisfactory drive is a synchronous motor.The motor should have aratingseveral times the maximum torquerequirement of the tachometer generator andshould be well damped to minimize the effectsof voltage variations in the power supply andtorque variations in the tachometer generator.Speeds other than the synchronous speed ofthe motor should be obtained by precisiongearing or by belts and pulleys. If belts andpulleys are used，the output shaft shall havemarkings so that the drive can be checkedstroboscopically to make sure that the speedratio is exact and there is no slippage. Thestroboscope shall be connected to the samefrequency-controlled power source as the synchronous motor.
Speeds other than the synchronous speed ofthe drive motor may also be obtained by usinga mechanical adjustable-speed drive betweenthe motor and the tachometer generator. Whenthis is done an electronic counter，having anaccuracy at least 4:1 better than the spe-cified maximum percentage voltage deviationto be measured,shall be used to measuretachometer generator speed.
Power Supply for Tachometer Drive.The preferred power supply for the drivemotor is a power amplifier controlled by atemperature-stabilized tuning fork. In any caseit shall have a frequency stability, in percent,at least 4:1 better than the specified toler-ance of the characteristic to be measured. Theoutput of the power amplifier should have asuitably small wave form distortion. In particu-lar the output should be checked to make surethat there is no beat frequency between thecontrolled frequency，and thepower-linefrequency. A tuning-fork frequency at least5%different from the line frequency and itslower harmonics is suggested to minimize theeffect of beat frequency.IEEE 251 pdf download.