KSQF in Water Control System Application
Background ＞
In the control system of the water supply pump in the community, the water customer uses the contactor to switch the water pump to the frequency conversion or common frequency circuit to meet the change of water demand in the community.
In daily use, because the water demand is high and low, the contactor needs to act continuously. Because the water pump motor is an inductive load, when the contactor is frequently pulled in and released, the continuous arc pulling between contacts may lead to the contactor contacts stick and the failure of normal disconnection. At this time, the frequency conversion and common frequency circuits are connected at the same time, resulting in the failure of the control system.
In order to solve this problem, the water customer seeks better electrical switching elements. Solid state relay has become a more suitable choice than the contactor because of its long endurance life and no arc during on & off. However, in consideration of cost, the frequency conversion circuit uses solid-state relay, while the common frequency circuit continues to use the contactor.
Picture 1 Relay wiring diagram of frequency conversion control cabinet of water pump in a community
However, in the actual use, the water customer feedbacks that the conventional three-phase solid-state relay KSQF480D80R will be abnormal. When the frequency conversion circuit is switched to the common frequency circuit (the solid-state relay of the frequency conversion circuit is disconnected and the contactor of the common frequency circuit is pulled in), the front-end frequency converter will give an overload alarm. Therefore, we conducted field confirmation at the customer's site and conducted simulation experiments in Kudom's internal laboratory according to the actual working conditions.
Abnormal cause analysis ＞
Firstly, the test circuit of conventional KSQF480D80R product is built according to the customer's wiring mode (as shown in Picture 2). The simulation test conditions are as follows:
1. The load is 750W three-phase AC motor, and the load voltage is 380VAC
2. Solid state relay does not apply control signal status
3. The working frequency of the contactor is 5S on and 5S off
Using the oscilloscope to collect the waveform, the acquisition positions are A and B in Picture 1. The waveform diagram is shown in Pictures 3 and 4.
Picture 2 Product Test Wiring Diagram
Picture 3 Collected Waveform
Picture 4 Partial Enlarged View of Picture 2 (Red Square Part)
As we could see from Picture 3 and 4, when the solid-state relay is not connected and the contactor is connected, there is a high dv/dt at both ends of the thyristor of the solid-state relay, which leads to the mis-energization of the solid-state relay. Then the voltage of the common frequency circuit (grid voltage) and the voltage of the frequency conversion circuit will be superimposed on the load end of the frequency converter at the same time, and the frequency converter will give an overload alarm.
Solution ＞
We have replaced the components with weak anti dv/dt in the product, and improved the anti electromagnetic interference ability through Kudom's unique input control circuit design method, so that the solid-state relay will not be mis-energized.
Solution Verification ＞
The replacement of devices with stronger anti-interference ability and combined with special design methods have been verified on other Kudom products, and the anti-electromagnetic interference ability is much stronger than that of conventional products, meeting IEC61000-4 standard:
Burst immunity level | IEC61000-4-4 | 2kV/100kHz |
| Surge immunity level | IEC61000-4-5 | 2kV/common mould, 1kV/different mould |
| Electrostatic discharge immunity level | IEC61000-4-2 | 4kV/contact discharge, 4kV/air discharge |
At the same time, according to the test conditions described at the beginning, we conducted continuous test for more than 7 days, and the contactor was turned on and off more than 60K times, and no abnormality was found.
Based on the working conditions of similar products in worse electromagnetic interference environment and the requirements for electrical life and performance in this application, the improved products can meet the performance and quality requirements of this application.
