Notes on analog aspects to EL3751/ ELM3xxx
Beyond the general instructions relating to analog technology, the following instructions apply for the EL3751 and ELM3xxx (as far as applicable):
- The internal GND of of the analog terminal is connected to the connection point –Uv. Hence, when several terminals are wired, it has to be considered, that they must not exceed the permitted CommonMode voltage among each others.
- The "–Uv"-points must not be connected with each other or with other another potential, although it can be helpful to use it to correct system-specific negative influences.
- Voltage measurement at the high-side shunt
A high-side shunt is a shunt that is connected to the positive/upper potential, in which case the negative connection is generally used for switching, hence the term “negative switching”.
In principle, it is possible to use mV measurement at a shunt for current determination with the differential U inputs of the Beckhoff measurement devices. However, two important limitations must be considered: - Common mode voltage Vcm between the channels: For multi-channel terminals, Vcm, max (see technical data in this documentation) between the channels must not be exceeded. With a 24 V supply of the loads, it is therefore not possible to use a high-side shunt at 24 V potential on a channel and a low-side shunt at 0 V potential on another channel, because the resulting internal reference ground –Uv would assume a mean value such that Vcm is exceeded.
→ Therefore, only high-side shunts or only low-side shunts should be used at a terminal. - Dynamic processes through pulsed current: In general, the current is controlled through pulsing/PWM. Depending on the inductance in the load circuit, this can lead to sudden current changes and therefore voltage changes over the shunt. Vcm at the differential inputs changes accordingly. The channel (this therefore also applies to the single-channel EL3751) is LC-coupled to the internal reference ground –Uv, and the sudden increase in Vcm results in an increase in –Uv. During this transient (several ms), measurement errors may occur when exceeding Vcm, max.
→ PWM current measurement with a high-side shunt in 24 V networks is only possible in the 30 V measuring range.