When does the motor need to send a current signal to the DCS? What about the ammeter?
Thread Content
When specifying the electrical control interlock conditions for electrical and instrumentation systems, I’m not quite clear about the “current signal” and “ammeter” options in the table. Regarding ammeters: Colleagues say that ammeters are usually installed on-site for motors with a power rating of over 37 KW, variable-frequency motors, or very important motors; those of other types generally do not have such ammeters. When an ammeter is added, it generally also sends a current signal to the control room. But it can also have only a current signal, without an ammeter – why is that? Which is more important, the current signal or the ammeter? Which one is more necessary to install?
A current signal is essential; since it’s connected to the DCS, the level of automation will be high. Moreover, even in ordinary systems, a current signal is required, so that the control room can monitor the operation of the equipment in real time. The ammeter is merely intended to facilitate on-site operation and observation by maintenance personnel, and it is generally not necessary. We generally don’t use ammeters for large compressors and leachers of several hundred kilowatts; they are only used in very important areas such as cryogenic units. In simple terms, the current signal and ammeter are equivalent to the relationship between a pressure transmitter and a pressure gauge
According to the standards, ammeters must be installed for all devices with a rated power of ≥37 kW
The usual approach is that for motors with variable frequency control, signals can be sent directly from the inverter’s communication interface to the DCS, using serial communication protocols such as RS485/MODBUS/RTU. A single communication cable can be used to transmit a range of data including current, voltage, frequency, and PID adjustment values, which is very convenient. In the case of conventional motors, since on-site or panel-mounted ammeters are available, the 0–5A signals received by these ammeters must come from current transformers. A current transmitter is placed at the output of the current transformer to convert the 0–5A current signal into standard signals of 4–20mA DC/1–5V DC, allowing the DCS system to receive and display this information.
The current signal and the ammeter are two different things; the ammeter is meant for those on site to use, to check whether the current is normal once the motor starts up The current signal is intended for the DCS control room; of course, it can also be recorded to analyze operational conditions.