VFD Basics - How VFD Controls the Motor Running?

Computers & TechnologyTechnology

  • Author Jane Yu
  • Published April 30, 2021
  • Word count 922

When the main circuit of the VFD is connected with the power line, in order to control the motor, it is necessary to connect the surrounding control circuit to the relevant terminals and set the starting mode parameters of the VFD to the external operation mode.

There are 2 common methods that a VFD controls the operation of the motor: switch control and relay control.

1.Forward-Rotation Circuit Controlled by Switch

The forward-rotation circuit controlled by the switch is shown in the figure below. It relies on manual operation of the external switch SA at the STF terminal of the VFD to control the forward rotation of the motor.

The working principle of the circuit is described as follows:

1.1 Start Preparation

Press the button SB2 and the contactor km coil is energized. The usually open auxiliary contact and main contact of KM are closed, and the usually open auxiliary contact is closed and locked, and the KM coil is electrified and self-locking. The main contact of KM is closed to connect the main power supply for the VFD.

1.2 Forward-Rotation Control

Press the external switch of the STF terminal of VFD, and the SA, STF and SD terminals are connected, which is equivalent to the STF terminal inputting forward control signal. The U, V and W terminals of the VFD output the forward power supply voltage to drive the motor to run forward. Adjust the potentiometer r outside the terminal, the VFD output power will change, and the motor speed will change accordingly.

1.3 Error Protection for the VFD

If there is any abnormal operation or errors during the operation of the VFD, the internally equivalent switches between the B and C terminals of the VFD which are normally closed will be disconnected, and the KM coil of the contactor will lose power, and the KM main contact will be disconnected to cut off the input power supply of the VFD and protect it.

1.4 Stop Control

When the VFD works normally, if you disconnect the switch SA, and disconnect the STF and SD terminals, the VFD will stop the output power and the motor will stop. 

 If you want to cut off the main power supply of the VFD input, press the button SB1, and the KM coil of the contactor will lose power, the KM main contact will be disconnected, and the power supply for the VFD is cut off.

2.Forward-Rotation Circuit Controlled by Relay

The forward-rotation circuit controlled by the relay is shown in the figure below.

The working principle of the circuit is described below.

2.1 Start Preparation

Press the button SB2, the contactor km coil will be powered on, and the KM main contact and two auxiliary contacts which are usually open will be closed, making the KM main contact closed too, and the VFD will be connected to the power supply. 

 At this moment, one KM auxiliary contact which is usually open will be closed, and the locking KM coil is energized, and the other KM auxiliary contact that is usually open will be closed to prepare for the power supply for the median A coil of the relay K.

2.2 Forward-Rotation Control

Press the button Sb4, the relay KA coil will be powered on, and three contacts of KA that are usually open will be closed, and one usually open contact will be closed, and locking coil KA is power on, one usually open contact will be closed, and button SB1 is short-circuited, and another usually open contact is closed, till this moment, the STF and SD terminals are connected. 

 Here, it means that the STF terminal inputs the forward control signal, the U, V, W terminals of the VFD output forward power supply voltage, making the motor rotate forward. When you adjust the potentiometer R connected to the terminal, the output power of the VFD will change, and the motor speed will change accordingly.

2.3 Error Protection for the VFD

If the VFD runs into errors, the internal equivalent switch between the B and C terminals of the VFD that are normally closed will be disconnected, and the KM coil of the contactor will lose power, and the KM main contact will be disconnected, which will cut off the input power of the VFD and protect it. At the same time, the KA coil of the relay will lose power too, and the three contacts of KA which are usually open will be disconnected.

2.4 Stop Control

When the VFD works normally, press the button Sb3, KA coil loses power, three contacts of KA that are usually open will be disconnected, one of which is usually open, so that the connection of STF and SI terminals is cut off, the VFD stops outputting power, and the motor will stop running. 

 When the VFD is running, if you want to cut off the power supply, it is necessary to stop the VFD first, and then press the button SB1. At this moment, the KM coil of the contactor loses power, and the main contact of KM is disconnected, and the input power of the VFD is cut off.

 If you haven't taken the steps above to stop the VFD, and just press SB1 directly, you can't cut off the power supply of the VFD.

 This is because when the VFD works correctly, the contact of KA which is usually open has short-circuited to SB1, so it is invalid to disconnect SB1. In this way, it can prevent the SB1 from operating by mistake and cut off the power supply when the VFD is working.

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