Discussion on the issue of current interlock for the mixing motor
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A few days ago, an experienced expert came for an inspection and pointed out that there was no interlock mechanism set for the current of our mixing motor; this reactor mainly starts its mixing function when epoxy is fed in. I raised an objection at the time; the expert explained that since the kettle doesn’t have observation holes, it’s impossible to determine whether the mixing blades have fallen off or if there are other abnormalities, but these issues can be detected through electrical currents. Therefore, they suggested that we add them. The leader has agreed as well, but I still haven’t figured out this requirement. It is necessary to set both a lower and an upper current limit. The lower limit isn’t a problem, as there’s already an interlock mechanism that prevents feeding when the stirring feedback is not active; therefore, the lower limit doesn’t affect overall operation. However, the upper limit poses more difficulties. When the motor starts, there is definitely a sudden spike in current that then gradually decreases, and this spike can interfere with normal operation. Does anyone have any good suggestions? Using a delay for interlocking also seems a bit impractical
Regarding the interlock issue related to the upper limit of the stirring motor current, there is indeed a sudden increase in current during startup, which is a normal phenomenon when the motor starts. It is possible to consider setting a start-up delay period, during which the current is allowed to exceed the set upper limit in order to avoid false alarms. A practical approach is to set a start-up delay timer in the control system; for example, 20–30 seconds can be designated (the exact time should be adjusted based on the motor’s characteristics and actual conditions). During this period, monitoring of the high-current limit is not carried out, or the high-current threshold is set higher, and normal monitoring of the high-current limit resumes once this time period has passed. This can effectively prevent frequent shutdowns caused by high starting current. It is also necessary to ensure that the interlock logic is correct in order to avoid safety hazards. .
Your control logic is way too simple; you need to learn to use the ST language. Of course, it’s possible to work with ladder diagrams as well, but it’s quite difficult. You can trigger it at a later time. In Zhejiang University’s Zhongkong system, the typical execution cycle for commands is 0.5 seconds. You can create a UINT variable, for example named JS. The current value of the motor driving this stirring mechanism can be represented by P0101_Current.PV, while the upper limit is set at P0101_Current.HH. By checking whether the PV value exceeds this upper limit, if it does, timing begins; once 10 seconds have passed, an alarm is triggered indicating that the upper limit has been exceeded. If the instantaneous current value is less than the upper upper limit, reset JS. It’s simple; a ladder diagram can also be created for it. Complex logic can be very confusing when represented using ladder diagrams.
Haha, thanks to the forum member. I know how to work with logic. I just think it’s not reasonable to use delays for interlocking – could there be any risks associated with that?
Can it also be determined when interlock instruments should be added? Isn’t it determined by the equipment and processes? Including the interlock values and interlock logic, all of that is provided by them; we are just the ones who carry out the instructions. Don’t do anything improper, or you’ll definitely face trouble from them~! There was nothing wrong originally, but implementing interlocking actually disrupted production, and the management will give you trouble as well~!
A switch can be added to the interface; when it is turned on, interlocking is enabled, while when it is turned off, it is ignored.
If the mixing motor is driven by an inverter, the inverter will reduce the current during startup to enable a smooth start, preventing overcurrent. Yours is basically driven by an inverter, because in that case the current just needs to be drawn from the inverter. If it is driven directly by a power supply, then a transmitter for measuring alternating current must also be connected. If it is driven by an inverter, there is no issue of excessive starting current. With the motor status feedback indicating that it is running, determine whether the motor current is greater than the upper current limit or less than the lower current limit. If that is the case, trigger the alarm after a certain delay. In fact, if the motor is overloaded, the thermal relay will trip; of course, it will do so a few minutes after the current becomes too high.
Thank you to the forum member; your answer cleared up my doubts. We started it directly, and this will have some impact
It’s fine to check the current only 30 seconds after the motor starts