The instrumentation team from next door has come to ask for advice on an issue where the chiller cannot be started remotely; we hope for your guidance
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One of the small ice makers in our factory has been converted from being controlled by a PLC to being controlled by a DCS. On the main power supply circuit that enables the ice maker to start, there is an interlock shutdown contact Y22 (controlled by a DO output from the DCS). Once the remote control button on the field operation panel is pressed, the DCS outputs a start signal via a DO output to activate the ice maker (contact Y21). There is also a touch-sensitive Y3 (DCS DO output) that allows for startup. Problem: 1. After switching to the local mode at the control panel, the oil pump is started locally; once the oil pressure reaches the normal level, the button to start the chiller is pressed, but the oil pump shuts down and the chiller fails to start. 2. By directly short-circuiting the Y3 contact on-site, the chiller can be started normally. 3. When the control panel is set to remote mode, the DCS sends a command to start the chiller (by short-circuiting the Y3 contact on-site); however, the chiller fails to start, and a noise from the electrical contactor can be heard. It can be concluded that when starting remotely, both Y22 and Y21 have their contacts controlled by the DCS, the length of the circuit cables is 2*2*250 (i.e., 1000 meters), and a noise from the contactors is heard during startup, but the chiller still does not start ； If started locally, only contact Y22 will be controlled by the DCS; that is, with a circuit cable length of 500 meters, the ice maker can start. Therefore, it is determined that the load is too high; the electrical power supply is an 110VAC transformer installed on-site (reused). So what is the solution? Can increasing the rated capacity of the transformer solve the problem? Our electrical design team says that increasing capacity won’t solve the problem; only reducing the wire resistance will work. The electrical design team states the following regarding the issue raised by the instrumentation department about insufficient transformer capacity: The rated capacity of the transformer on site is 1.2 KVA, with a rated current of around 10 A. After communicating with the construction team regarding the entire commissioning process, it was found that when the local control switch was set to DCS control, the secondary voltage of the transformer was 110V, the voltage across the compressor coils was 85V, and the current in the control circuit was around 2A. When the switch was switched to the local position, the oil pump motor and the compressor motor could start properly, which indicates that the transformer was not overloaded and there was no issue with its capacity. There are two measures to reduce line voltage drop: one is to reduce the line impedance, i.e., the voltage on the secondary side of the transformer, and the other is to increase the voltage. Since there is no adjustment knob switch on the secondary side of the on-site transformer, this method does not work. Therefore, it is still necessary to consider reducing line impedance; intermediate relays should be installed on-site in the electrical system to isolate the control signals sent by the instruments before connecting them to the electrical control circuit ；
There’s too much verbal description; it confuses the listeners! ! If it really is as you described, then using a relay for isolation is the best solution! Additionally, DCS signal transmission over a distance of 1000 meters is no problem; please check the loop cable to see if there are any connectors, if the wire gauge is too small, or if the current-carrying capacity at some point in the control loop is insufficient, which could affect the entire control loop. This one has a high probability
There’s too much verbal description; it confuses the listeners! ! If it really is as you described, then using a relay for isolation is the best solution! Additionally, DCS signal transmission over a distance of 1000 meters is no problem; please check the loop cable to see if there are any connectors, if the wire gauge is too small, or if the current-carrying capacity at some point in the control loop is insufficient, which could affect the entire control loop. This one has a high probability
In short, when the DCS control circuit has 500 meters of cable (1*2*1.5), it fails to start, but it can start when there is only 250 meters of cable.
According to you, it’s likely that the circuit is too long, resulting in excessive voltage drop, which in turn leads to insufficient voltage at the coil and prevents the contactor or relay from being activated
Generally speaking, it is not reasonable to directly control AC contactors remotely; it is recommended to use an intermediate relay as an intermediary
Add two intermediate relays, to be installed directly at the chiller. Use two of them: one for allowing startup, and the other for initiating the DCS; these 24V DC intermediate relays will be used as two DO points. I never connect 220V signals to the DCS