Why is direct current used in the control circuit of high-voltage motors?
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I work with DCS systems, and I’ve encountered this issue in projects: controlling the start and stop of motors using a DCS system. In such cases, the DCS acts as a relay with normally open contacts; these contacts are connected to the relays in the electrical control circuit, which in turn controls the main circuit of the motor. However, some of the electrical control circuits are operated using direct current, which places higher demands on the relay contacts in the DCS system. I would like to ask: why is it necessary to use direct current for controlling those electrical control circuits? (Is it really possible to use direct current for that purpose?) I heard that it’s because the motors in question are high-voltage motors; so why does the control circuit for high-voltage motors need to use direct current? Please ask an expert to help answer
DC control offers high reliability; basically, any device that requires a reliable control circuit uses DC control. Therefore, virtually all control circuits for high-voltage motors are operated by DC panels. If you are encountering this issue for the first time, it means this is your first time implementing DCS for high-voltage motors?
It seems that in control circuits for high-voltage electrical equipment, DC power supplies are generally used; for example, the control circuits of high-voltage distribution cabinets also rely on DC power. This is mainly because DC power supplies are reliable, and they are equipped with UPS backup battery packs as well. In the event of a power outage in the mains supply, the UPS can continue to supply power to the control circuit, carrying out various power conversion and switching operations such as automatic backup switching and reclosing ； Or in the event of a fire or discharge in the main circuit, since the control circuit is powered by a DC source, it is not affected by alternating current and can still shut off the power supply promptly and normally. In summary, the main purpose of using a DC power supply for control is to ensure safety and reliability.
You need to check what voltage the control power supply for the motor contactor coil is. If 220V AC is used, you can add an intermediate relay; it operates in the secondary circuit during startup, and is connected in series to the control circuit when stopping. It's very simple
I work in instrumentation as well; configuring DCS projects is no problem for me, and I’m also capable of working on electrical systems. First, you need to understand the secondary circuits and interlocks in electrical systems, and be able to draw simple diagrams of those secondary circuits – once that’s done, it becomes quite straightforward.
DC power offers high reliability; the DC circuits of high-voltage motors are generally powered by DC distribution panels.
I basically agree with the opinions expressed by those above. Additionally, the high-voltage switches trip frequently; in particular, the closing mechanism requires a significant amount of energy to close, and the power supplied by the AC source is not sufficient to meet the energy demands during closing. At that time, there was no power supply available for closing the circuit; closing was done manually by hand. It’s time-consuming and laborious, and it’s not safe either. This issue is overcome once the high-voltage operating power supply is powered by a DC panel. Since modern high-voltage switchgear is equipped with a closing energy storage device. The actual capacity required by the DC panel has been significantly reduced; in other words, the capacity of modern DC panels can be made smaller while still meeting the operational requirements. It’s mainly a matter of money.
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