VFD Basics
If the main circuit of VFD is connected to the power line, the peripheral control circuit must be connected to the relevant terminal, and the VFD startup mode parameter must be set to the external operation mode.
VFD controls the operation of motor in two common ways: switch control and relay control.
1. Bi directional rotation circuit controlled by switch
The positive rotation circuit controlled by the switch is shown in the figure below. It relies on the manual operation of the external switch SA of the VFD STF terminal to control the bidirectional rotation of the motor.
The working principle of the circuit is as follows:
1.1 Preparation for start-up
Press SB2 button to open contactor km coil. The normally open auxiliary contact and main contact of KM are closed, the normally open auxiliary contact is closed and locked, and the KM coil is electrically and self-locking. The main contact of KM is closed to connect the main power supply of VFD.
1.2 Control positive rotation
Press the external switch on the VFD STF terminal, SA, STF and SD terminals are connected, which is equivalent to the two-way control signal input at the STF terminal. The U, V and W ends of VFD output bidirectional power supply voltage to make the motor run forward. If the potentiometer r outside the terminal is adjusted, the VFD output will change and the motor speed will change accordingly.
1.3 Error protection of VFD
If abnormal operation or error occurs during VFD operation, the internal equivalent switch(often closed) between the B and C ends of the VFD will be disconnected, the KM coil of the contactor will be disconnected, and the KM main touch will be disconnected to cut off and protect the VFD input power.
1.4 Stop control
If the SA switch is disconnected and the STF and SD ports are disconnected when VFD is working normally, VFD will stop the power output and the motor will stop.
To cut off the main power supply of VFD input, press SB1 button, the KM coil of the contactor will lose power, and the KM main contact point will be disconnected, thus cutting off the power supply of VFD.
2. Two way rotating circuit controlled by relay
The following figure shows the positive rotation circuit controlled by the relay.
The working principle of the circuit is as follows:
2.1 Preparation for start-up
Press button SB2, the contactor km coil is opened, the km main touch and two normally disconnected auxiliary touch are closed, the km total touch is closed, and the VFD is connected to the power supply.
Normally, the disconnected KM auxiliary contact will be closed, the power supply of KM coil will be locked, and other KM auxiliary contacts will normally be closed to prepare power supply for intermediate A coil of relay K.
2.2 Bi directional rotation control
Press button Sb4, the relay KA coil opens, and the three normal contacts of KA close. A constant tangent point will close. The locking coil KA will be energized. A constant tangent point will close. Push button SB1 section, another constant contact is closed. STF and SD terminals will always be connected together.
This means that STF terminal inputs two-way control signal, and U, V, W terminals of VFD output two-way power supply voltage to make the motor rotate in both directions. If the potentiometer R connected to the terminal is adjusted, the output of the VFD will change and the motor speed will also change.
2.3 Protection against VFD is invalid
If the VFD fails, the internal equivalent switch between the B end and C end of the upper lung of the VFD will be disconnected, causing the KM coil of the contactor to lose power and the KM main contact to be disconnected, thus cutting off and protecting the VFD input power. In addition, the KA coil of the relay will also lose power. In addition, the three tangents of KA are usually separated.
2.4 Stop control
When VFD works normally, press Sb3 button, KA coil power is turned off, three frequently opened contacts of KA are disconnected, one of them is frequently opened, connection between STF and SI terminals is disconnected, VFD stops output power, and motor stops working
To cut off the power supply when VFD is running, you must first stop VFD, and then press SB1 button. At this time, the KM coil of the contactor is closed, the main touch of KM is disconnected, and the input power of VFD is cut off.
To stop VFD without executing the above steps, simply press SB1 directly to turn off the power of VFD.
This is because when VFD works normally, the KA breakpoint that is normally opened is merged with SB1, so disconnecting SB1 is invalid. This prevents SB1 from failing and cuts off the power supply when VFD is working.