Switching general-purpose inverter frequency conversion and power frequency line with relay - News - Global IC Trade Starts Here.

Photocoupler

2. Working Principle

The SA is a mode selection switch that allows the system to operate in either "power frequency" or "frequency conversion" mode. When the switch is set to "power frequency operation," pressing the start button SF1 activates the intermediate relay KA1, which then self-latches. This causes KM3 to engage, connecting the motor M to the power frequency supply, enabling it to run in "power frequency operation." To stop the motor, press ST1.

When the SA is switched to "frequency conversion operation," pressing SF1 again energizes KA1, which triggers KM2 to connect the motor M to the output of the inverter (UF). Simultaneously, KM1 also activates, and pressing SF2 engages the intermediate relay KA2. This connects the inverter's FWD and CM terminals, allowing the motor to start and transition into "frequency conversion operation." Once KA2 is activated, the ST1 button becomes disabled to prevent direct power cutoff from the inverter, ensuring a controlled shutdown.

During normal inverter operation, if a fault occurs and UF trips, the circuit between "30B~30A" opens, causing KM2 and KM1 to de-energize. This disconnects the inverter and the power supply from the motor. At the same time, "30B~30C" closes, activating the buzzer HA and the indicator HL for an audible and visual alarm. The time delay relay's KT coil is also energized. After a set delay, the contact closes, allowing KM3 to activate and automatically switch the motor back to "power frequency operation." At this point, the operator should manually set SA to "power frequency operation" to silence the alarm.

If the stop button ST2 is pressed while the inverter is running, the intermediate relay KA2 is de-energized, disconnecting the inverter’s FWD from the CM terminal. This causes the motor to decelerate gradually and eventually stop in a safe manner.

3. Application

This system enables seamless switching between frequency conversion and power frequency modes, offering flexibility and reliability in industrial applications. It ensures continuous operation even during inverter faults by automatically reverting to power frequency, minimizing downtime and improving overall system performance.