In the control circuit of an electric motor, there are usually protective devices such as fuses, automatic air switches, thermal relays, or overcurrent relays, especially fuses. Generally. The rated current of a fuse is about twice that of an electric motor. However, for some equipment with large starting loads (such as punchers) and without star delta conversion starting devices, the fuse current of the fuse should be large, but if it is too large, it cannot provide protection; If the selected range is too small, it may burn out during motor start-up, resulting in phase loss and motor burnout. Therefore, many motor control circuits are equipped with phase failure protection devices.
There are many solutions for open phase protection circuits, which have been introduced multiple times in electronic newspapers. This article introduces a motor control circuit that can automatically cut off the power supply when the phase is disconnected. In this circuit, only the neutral wire and intermediate relay are added. It can effectively achieve the purpose of phase failure protection. Refer to the attached diagram.
In the figure, QF is the main power switch, usually using an automatic air switch, KM is the AC contactor, and RF is the thermal relay. When working, press the start button SB2, KQ will turn on and perform self-protection, KM will turn on and the motor will rotate. If the C-phase fuse burns out, KQ will be immediately released. Due to power failure, Km will also disconnect to provide protection. At this point, the HL1 and HL2 indicator lights turn off, indicating that there is no power in phase C.
For example, when the fuse of phase B or phase A is blown. Although KQ is still in a closed state, KM will also be released to protect the motor. At this time, only HL2 is in the closed state, while HL1 is still in the open state. Just check the fuses of phase B or phase A.
In addition, in some intermittent automatic control systems (such as pressure and flow), intermediate relays are installed to receive switch signals from control instruments, making the application of this phase loss protection method more concise.
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