How is the range for motor current set on the DCS?

Current transformers are installed on the power supply cabinet, and the current is sent to the DCS via current transmitters

Your question is a bit confusing; I’ll explain it separately as you requested: 1. In the case of variable-frequency control, the control is achieved by changing the motor’s speed. Therefore, 4-20mA corresponds to an opening setting of 0-100% in the control system (whether it is for opening or closing the valve, it can be set to the corresponding 0-100% value in the DCS and inverted as needed). At this point, the corresponding parameters such as flow rate or temperature can be controlled via frequency conversion. 2. For ordinary motors that are not subjected to frequency conversion, the motor operates at a constant specified speed. Therefore, it is not used as a means of adjustment to change parameters at this time; so is the 4-20mA setting meaningful?

Set the DCS value to the same as the current range of the inverter. In contrast, inverters have models in which the current is converted to 4-20mA based on the range, and the DCS then converts this 4-20mA signal back into a current value.

The variable-frequency drive and conventional three-phase asynchronous motors output 4-20mA current signals to the DCS; speed control is not involved nor is it required. Utilize the 4-20mA output built into the frequency converter itself ； The protection devices of ordinary three-phase asynchronous motors do not provide a 4-20mA current signal output; instead, a secondary current multimeter is used to convert the primary current into a 4-20mA current signal for transmission to the DCS. These two types of motors require different settings in the instrument’s backend configuration. The first instrument sets its range based on the motor’s rated current (for example, 75A; the instrument’s range is set to 75). The second instrument, on the other hand, sets its range according to the multimeter’s range (for example, with an input of 100/5; the instrument’s range is set to 100).

In general, it is rare to use the magnitude of current directly (at least in my experience). Rather, the second setting method you mentioned has a corresponding relationship; for example, 50% of the current output corresponds to certain parameter values (temperature, flow rate, etc.).

Experience value: Over 3KW, current = 2*power； Current of 3KW and below = 2.5*power. Without input from electrical specialists or manufacturers, the DCS range can be set according to this.

The range corresponding to 4-20mA is generally twice the rated current of motor protectors, soft starters, and frequency converters, and this value can usually be modified on the frequency converter itself.

I have already discussed this with the instrumentation DCS engineer; for 4-20mA signals, the corresponding range is generally the rated current of the motor, whether it is for motor starters, soft starters, or frequency converters.

It should be calculated based on the CT transformation ratio and the current transmitter’s transformation ratio

The main purpose of using current feedback here is to determine whether the motor is running without load (due to an empty material feed) or is overloaded (due to a blocked pipeline), by detecting abnormal currents. That’s probably the author’s intention as well