VFD drive working principle
The VFD drives are usually provided with the three-phase voltage supply to power the motor. The VFD drive consist of three main parts, namely VFD controller, inverter and feedback device.
3-level VFD controller
The VFD controller works according to the desired rpm value that is being given by the computer. First VFD controller is received by the computer and VFD controller's feedback device detects it. The VFD then works on this information to control the speed of the motor.
The vfc (variable frequency controller) converts low voltage dc input into corresponding output for v/f conversion, amplifies the magnitude of ac power according to output, then feeds to v/f converter.
VFD inverter configuration
The VFD inverter is used to convert input power into three phase output ac. This VFD inverter consists of six IGBTs (integrated gate bipolar transistors) in the case of 6-pulse v/f configuration and twelve pulse v/f configuration VFD inverter.
The VFD inverter has three main components: input, output and control terminals. There are three power devices in VFD inverter, namely IGBTs (insulated-gate bipolar transistors), thyristors and SCRs (silicon controlled rectifiers). These IGBTs and thyristors are connected in the case VFD inverter as per three-phase v/f configuration.
The VFD inverter is designed such that it converts low voltage dc input into corresponding ac output with v/f conversion and amplifies the magnitude of ac power according to v/f setting. If we want to increase the speed of a motor, then we have to increase the v/f ratio.
5 VFD drive operation
There are five VFD drive operations namely, VFD fault finding calculation, VFD start up procedure, VFD positioning control, VFD speed control and VFD troubleshooting tips.
VFD Fault-finding calculation
VFD faulty components can be easily calculated by VFD fault calculation formula. VFD drive v/f magnitude will be proportional to v/f ratio of the VFD drive. We can find v/f relation at any speed by this calculation formula. Let X = input voltage, Y = output voltage and Z = load current.
VFD Wiring
VFD Startup Procedure
To start v/f VFD drive, VFD start up procedure should be followed. VFD controller sends switching sequence from VFD inverter to VFD motor. Switching sequences vary according to v/f ratio of the VFD drive.
VFD Speed Control
V/f control is a type of VFD speed control system in which voltage and frequency are controlled by VFD controller and VFD inverter respectively. It is a feedback control system in which v/f controller calculates the error and then corrects it according to VFD start up procedure.
VFD Troubleshooting Tips
If v/f VFD drive is not working properly, v/f VFD drive fault will be shown on VFD v/f controller's display. v/f VFD drive fault can be found using VFD fault finding calculation formula. v/f VFD drive also has some VFD controls like VFD speed control, VFD positioning control and v/f VFD drive troubleshooting tips.
Examples of VFD applications
They are used in many types of industries, mostly the mechanical and electrical industry where they are found in servo systems, VFD-based control systems, VFD-fed inverters for traction applications (like trains), VFD's that regulate frequency for power plants, VFD's that run compressors or pumps at their best efficiency, VFD's that run the spindle on CNC machine tools for improved precision, VFD-fed inverters for welding power supplies (one of the biggest VFD applications), VFD-fed control systems that provide better speed regulation, VFD-fed welding machines and VFD's in robotics where they provide torque and speed control.