为什么要用补偿盒对SM进行外部冷端补偿及怎样补偿.docVIP

External Compensation by Means of a Compensating Box on the Analog Modules SM 331(6ES7 331-7KFxx-0AB0 and 6ES7 331-7KBxx-0AB0) QUESTION:Why do I require a compensating box for connecting thermocouples to an analog module SM 331(6ES7 331-7KF02-0AB0 and 6ES7 331-7KBx2-0AB0) and what should I watch out for in the connection? ANSWER:Compensating boxes are used for performing temperature measurements with thermocouples on analog modules. You require a compensating box whenever you transfer from the thermocouples to a copper wire via a terminal strip. The thermo-electromotive force occurring at this place (reference junction) is measured there with the compensating box and equalized. The influence of fluctuations in temperature at the reference junction can be compensated for by an equalizing circuit. The compensating box (external compensation) is used as an equalizing circuit.The compensating box contains a bridge circuit which is supplied by a mains-operated voltage source. A voltage of opposite polarity is added to the measured voltage and to the thermo-electromotive force, which occurs at the reference junction, thus compensating for the thermo-electromotive forces at the reference junction.The compensating box must be located at the reference junction for the thermocouples and must have a voltage-free supply.Compensating externally by connecting the compensating box to the COMP ports on the module can only take place for one particular type of thermocouple. All the channels which work with external compensation must use the same type. Compensating boxes must be used with a reference temperature of 0°C for the purposes of the compensation of the analog modules. Note:We recommend that you use a SIEMENS compensating box as the reference junction. The order data depends on the thermocouple you use. You can identify it from the table below. Fig. 1: Order data for the compensating box The picture below shows the scheme for connecting a compensating box to