What is a Variable Frequency Drive?

The purpose of this section is to describe how these components work together as an integrated unit. The basic concept behind Variable Frequency Drive or VFD technology was first developed by General Electric around 1950. Since then it has been widely adopted for use with induction motors because they are inexpensive, reliable, and easy to control. A typical application would be on a forklift truck where speed can be controlled easily from 0–100% without any mechanical or electrical changes.

 

Controller

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The most basic rectifier converter for the VSI drive is configured as a three-phase, six-pulse, full-wave diode bridge. Permanent magnet synchronous motors have quite a limited field-weakening speed range due to the constant magnet flux linkage. Basic programming of the microprocessor is provided as user-inaccessible firmware. User programming of the display, variable, and function block parameters is provided to control, protect, and monitor the VFD, motor, and driven equipment.

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A typical VFD controller includes a number of functional blocks including:

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• Digital interface module

• Analog front end module

• Processor module

• Memory module

• Communication module

• Driver circuit

• Protection circuit

• Fault detection circuit

• Diagnostic test unit

• Calibration unit

• Software development kit

• Communications port

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VFD Controller

Operator interface

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A typical VFD controller has several components including:

 

• Power supply circuitry

• Microprocessor 

• Memory

• I/O ports

• User controls such as knobs, dials, etc.

• Communication interfaces

• Mechanical parts

• Cooling fans

• Heat sinks

• Wiring harnesses

• Mounting hardware

• Software drivers

• Firmware

• Displays

• Motors

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Speed Control

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Networked involves transmitting the intended speed over a communication protocol such as Modbus Modbus TCP EtherNet/IP, or via a keypad using Display Serial Interface while hardwired involves a pure electrical means of communication. 4-20mA, 0-10VDC, or using the internal 24VDC power supply with a potentiometer. The most common method is to use an analog input on the drive itself that allows you to set the desired output voltage in order to achieve your desired RPM. This type of system requires no additional hardware other than the actual motor controller. The advantage here is that it's very easy to implement but has several disadvantages including limited resolution and accuracy due to the nature of analog inputs.  

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Programming a VFD

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The first step in setting up your new VFD is to determine which type you have and then download its appropriate program from the manufacturer’s website. Once downloaded it must be installed onto an available USB port on your computer. The next step is to connect the power supply to the VFD and turn it on. If everything goes well there should be no error messages displayed by the device. You may now proceed with configuring the settings using either the built-in keyboard or the touchpad that comes with the

 

Variable Frequency Inverter

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Starting and Software Behavior

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The starting sequence for most drives is as follows:

 

• Input voltage goes high

• Start pulse is sent out by the controller

• If no response is received within a specified time period then the drive assumes that there was a failure in the system and shuts down

• After this point, the drive waits for another start command before restarting again.

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Drive operation

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In starting a motor, a VFD initially applies a low frequency and voltage, thus avoiding high inrush current associated with direct-on-line starting. The basic function of a VFD is to convert an alternating input signal into a continuous output waveform having a fixed amplitude and varying frequency. In general terms, this means converting sinusoidal signals into square waves. However, there are many different types of VFDs that differ mainly in their control methods and power conversion techniques.

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VFDs have been used for decades in industrial settings to provide electrical power to motors.  

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Energy savings

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Such energy cost savings are especially pronounced in variable-torque centrifugal fan and pump applications, where the load's torque and power vary with the square and cube, respectively, of the speed. By using VFDs instead of fixed-speed induction motors, up to 50% or more of the energy consumed by these motors may be reduced. The potential savings range from 5% to 80%, depending upon application type and size. In addition, there are other benefits associated with the use of VFDs including improved reliability, increased system life expectancy, lower maintenance costs, and higher availability.

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           Marshal Electrical & Metal Products Co. Ltd. is the Best Electrical Supplier of Variable Frequency Drive in the Philippines