In modern industrial automatic control systems, inverters serve as core speed-regulating equipment for motors. Their stability and reliability directly determine the continuous operation of production lines. However, inverters are prone to various faults during long-term continuous operation. Realizing automatic switching from inverter operation to mains operation upon faults has become a key technology to guarantee production continuity. This paper elaborates on the implementation principle, hardware configuration and programming key points of this automatic switching scheme.

1. System Working Principle and Core Requirements

The core objective of the inverter-mains automatic switching system is to complete power mode switching within milliseconds once an inverter fault is detected, ensuring uninterrupted motor operation. Three key technical problems need to be solved in this process: rapid and accurate detection of fault signals including over-current, over-voltage, overheating and other common faults; phase synchronization control during switching to avoid current impact caused by direct switching; and real-time system status monitoring and alarm functions.

2. Hardware System Architecture Design

2.1 Main Circuit Topology

The system adopts a dual-power design, consisting of an inverter power supply branch and a mains power supply branch. The inverter output connects to the motor via the KM1 contactor, and the mains power connects to the motor via the KM2 contactor. Mechanical and electrical interlocks are installed between the two contactors to prevent simultaneous closure. For key component selection: contactors shall be selected with 1-2 higher current ratings under AC-3 utilization category to ensure on-load switching reliability; the rated current of fast fuses shall be 1.5-2 times the motor rated current; the precision of voltage detection modules shall be within ±1%.

2.2 Signal Detection Unit

(1) Fault detection circuit: The inverter fault relay outputs (TA-TB-TC) are connected to the PLC DI module, with parallel hardwiring to the switching control loop for dual protection.

(2) Phase detection module: A zero-voltage comparator circuit monitors the mains power phase in real time and outputs square wave signals to the PLC high-speed counter to provide timing references for synchronous switching.

(3) Current transformer: Open-type CT is installed on the main motor circuit to detect current changes during switching and feed back data to the PLC for dynamic adjustment.

2.3 Control Unit Configuration

PLCs with high-speed counting functions (e.g., Siemens S7-1200 and Mitsubishi FX5U series) are recommended for the control system.

3. Control Logic and Program Design

3.1 Fault Detection and Processing Flow

The system performs self-inspection after power-on to confirm the normal status of the inverter and mains power before formal operation.

3.2 Phase Synchronization Control Algorithm

Precise phase synchronization is the core of avoiding current impact. The switching advance time is determined by actual measurement of contactor action time, generally 8-15ms. For high-power motors (>55kW), additional phase angle difference detection is recommended to ensure the phase difference between mains voltage and motor back EMF is within ±15° during switching.

3.3 System Protection Mechanism

Multiple protection strategies are designed for the system:

Mechanical interlock: The mechanical interlock device of KM1 and KM2 physically prevents simultaneous closure.

Electrical interlock: Interlock logic is programmed for the two contactor outputs in the PLC program.

Switching timeout monitoring: Emergency stop is triggered if the switching process takes more than 300ms after a fault occurs.

Current impact protection: The power supply is cut off immediately if the current continuously exceeds 2 times the rated value within 200ms after switching.