A VFD (variable frequency drive) is a electrical device that controls the speed of an electric motor by varying its input voltage. VFDs are used to control pumps, fans, conveyor belts and other types of machinery in industrial plants.
What is a VFD Drive?
The VFD (variable frequency drive) is a type of motor control that converts direct current (dc) into variable-frequency alternating current (ac). These VFD's not only convert the power, but also boost and regulate it before sending it to the ac motor. The VFD does this by varying the field on the stator and rotating its angle.
Image Source: https://americas.fujielectric.com/products/vfd-inverters-ac-drives/frenic-mega/
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.
VFD manufacturers
Some VFD manufacturers are:
ABB, Fuji Frenic, Emerson, Siemens, Allen Bradley and Mitsubishi
When should you install a VFD?
You should consider installing VFD's when you want to get the most out of your ac motor. For example, VFD's are often used on older ac motors because they get better performance from VFD's than they did with original equipment. VFDs are also installed when it is necessary to automate an ac process or improve its speed regulation.
In industry VFD's are installed for a variety of reasons. VFDs allow users to choose the speed that they want their motors to run at, and VFD's also allow process manufacturers to improve energy efficiency because VFD's can match the motor speed with an optimum electrical frequency.
Along with all this, VFD's maintain the same ac frequency as the power supply, which means it does not have to be converted from 50 or 60 Hertz frequency to a VFD's output frequency through an inverter. Therefore, VFD's reduce power consumption and help process manufacturers improve energy efficiency.
Some VFD's can also isolate electrical networks so that if one network has an issue, VFD's can disconnect it from the other networks and protect them. By doing this VFD manufacturers also help manufacturers increase safety.
Can a VFD be installed on any motor?
The VFD is a variable speed drive and it can be used on most ac motors. However, there are certain types of ac motors that VFD's cannot be used on. VFDs cannot be used on motors that need high starting torques. VFD's also cannot be used to run induction motors or ac induction motors because the VFD produces a variable frequency output which does not vary smoothly. The VFD can only control synchronous ac motors, and VFD's should never be used on dc motors.
Can I use a VFD on a single-phase motor?
Most VFD manufacturers recommend VFD's to be used on three-phase motors. VFDs can also be used on single-phase motors, but VFD's are not the best choice when it comes to speed control of single-phase ac motor. VFDs can work with single-phase supplies if they are protected properly.
Does a VFD replace a motor starter?
Most VFD manufacturers recommend VFDs to be installed near the motor in order to get improved control. VFDs are often used when it is necessary to isolate ac motor starters so that if one starter fails, VFDf will not affect all other motors on the same power distribution system.
Where do you mount a VFD?
VFDs should be mounted vertically with their cover on the top so that VFD fans or filters can easily access them for maintenance purposes.
How do VFDs work?
VFD's are used to control ac motors. The VFD's input voltage (ac power supply) is reduced by the VFD transformer, rectified and filtered into dc voltage. The VFD then converts the dc voltage into adjustable frequency ac output voltage that matches the speed of the motor to be controlled. Proportional speed control is achieved by varying the VFD output frequency. VFDs can produce up to 1000 pulses per revolution of the ac motor's rotor, and VFD's provide much smoother speed regulation compared to v/f control where a v/f converter is used to regulate the voltage going into an ac motor so that its torque matches load requirements. VFD's also allow the v/f converter to be bypassed so that it can protect motors from overload conditions, v/f converters do not provide this capability.
What are VFD advantages?
VFDs have many advantages over v/f control motor speed regulation, benefits of vfd include: smoother motor speed regulation, less weight, lower cost and better safety. VFDs also have additional advantages which include: VFDs can be used on more kinds of ac motors, VFD's can track a load step faster than v/f control systems, VFDs do not produce any harmonics that may cause problems with downstream equipment, VFD's have built-in overload protection, VFD's can be connected to a security system for remote start and VFDs do not require an external v/f converter.
What are VFD disadvantages?
VFDs have a few disadvantages compared to v/f control systems. The first disadvantage is that VFDs cannot provide very high starting torques because the VFD's maximum torque is limited to what the VFD motor can produce. v/f control systems are still better at high torques because v/f converters use a variable frequency square wave voltage output, but VFDs do not have this drawback because v/f conversion circuitry can be used in parallel with VFD's simple variable frequency ac output. VFD's also cannot provide v/f conversion when there is no high frequency ac supply available. V/f control systems are better in these cases because v/f converters run on dc power. Motor starting torque can be increased by adding v/f converters to v/f control systems, but the v/f converter's dc power input must match the VFD's v/f converter.
Proper VFD installation
When VFD's are being installed, it is important to take safety precautions. According to IndustLabs, a VFD control system company, VFDs should only be installed by qualified VFD field technicians who have obtained the correct license. Before installation, it is recommended that you consult your VFD manufacturer for specific safety guidelines regarding VFD installation and maintenance.
AUTHOR’S BIO:
Uziel Salgado is a co-founder at IndustLabs. He loves learning new things, researching and curating valuable content to save you time and help you achieve your goals.
I have been building and integrating Industrial Automation solutions for many industries for over 15 years. I have worked with Food & Beverage, Oil & Gas, Automotive Manufacturers, and just about any other industry you can think of.
Beyond services and automation I can provide quality Electrical Control Panels. I specialize in keeping lead times low and providing UL quality control panels even if the certification is not needed.
Also, I do not stop at just providing a box or an automated manufacturing process, I go beyond by integrating ours or your systems into our highly salable Internet of Things (IoT) platform so you can turn data into business insights. I also partner with the leading Industrial Artificial Intelligence company to provide greater insights into the data with anomaly detection and predicting machine failures.