VFD ACCESSORY USE AND SELECTION
Variable Frequency Drives (VFD) are out-of-the box motor speed controllers. These devices scale from small for low horsepower needs to cabinet sized for high horsepower motors. They are often programmable and support a large variety of features and functionality.
VFD’s have accessories available that perform specific functions such as to provide protection to electrical disturbances, improve operation capability, enable different communication protocols and add different types of physical connectivity.
VFD Accessory compatibility is not uniform. One drive may support an accessory while others may not. Compatibility differs based on the design of the drive and default built-in features. Most of the time the specific functionality you need can be incorporated.
Operator displays are also known as Operator Interface Terminal (OIT) or simply keypad. HITACHI VFDs have a standard keypad installed at the VFD’s front panel. It allows local VFD configuration and control. VFD configuration includes definition of VFD operating parameters and communication settings while control permits motor START/STOP, FORWARD/REVERSE, and inching operation. Speed control is achieved by entering the desired value via the digital switches.
Certain keypads have embedded memory that permits copying of VFD configuration from one unit to another. This contributes to faster engineering and commissioning of the unit as well as maintenance and troubleshooting.
Replacement operator interfaces may also include a rotary potentiometer option for easier speed control adjustment. There are also specific keypad versions that allow remote mounting to the enclosure’s front door to prevent opening of enclosure to access the VFD. Enclosures are usually designedfor specific VFD panel designs where a disconnect switch is mounted to the enclosure door. Other optional operator display includes a NEMA4X rated enclosure, ideal for outdoor use in harsh environments.
Operator Display communication cables
Communication cable accessories for operator interface permits operation of the VFD unit via a remotely mounted operator interface. It may also be used to allow flush mounting of the operator display to the panel door for safety and quick access. Flush mounting of the interface is typically done when a disconnect switch is installed.
An operator interface that is locally mounted to the VFD unit requires monitoring and entry of user actions by opening the enclosure to enable access to the drive. In some applications, this is not allowed especially in hazardous locations as this action may pose potential risks and hazards. Other installation may require inclusion of disconnect switch that will require turning OFF of the VFD before opening the enclosure to gain access. If VFD modification or testing is required, both situations will require VFD stoppage. The practical solution to this restriction is by remotely mounting the keypad, which may be accomplished if a operator display communication cable is utilized.
Excess energy is generated when a VFD stops a motor. This excess energy must be dissipated or re-used to avoid voltage surge to the VFD’s DC bus circuit. This is where braking units are useful.
Generally, there are Two (2) types of braking units. First, simply a braking unit that dissipates the excess energy as heat with the use of a braking resistor. Second, a regenerative braking unit that allows the excess energy to be re-used by other VFDs in the network. Instead of wasting this energy as heat, the energy is regenerated and supplied back to the DC bus, thus called regenerative.
A braking unit may comprise of One (1) or more braking resistors. Resistors may be connected in parallel to increase braking Torque performance.
Like braking units, regenerative converters work when a VFD is commanded to stop the motor. As previously stated, when a VFD stops the motor, the excess energy produced by the motor needs to be consumed otherwise, it may damage the DC bus of the VFD. While breaking resistors consume this excess energy as heat, regenerative converters allow this excess energy to be brought back as reusable power which may be used to power other drives connected in the same DC bus. This increases operational efficiency as well as contributes to environmental compliance.
AC Line Reactors
AC line reactors are inductors, essentially. This component may be installed in the input side and the output side of the inverter. A line reactor installed in the input side is called an input reactor while a reactor installed at the output side is termed as load reactors.
When acting as an input reactor, this component suppress harmonics created on the power supply input lines. As non-linear loads such as a VFD produce abrupt short pulses causing distortion to the current waveform, this distorted current is pushed back to the power system which may cause rapid rise of current and repeatedly trip the inverter. It also has a high potential of destroying the inverter power module and other parts of the VFD as well.
An input line reactor limits current-spikes or abnormal deviation of Current in the input line. By preventing this event, Harmonic induced errors or VFD problems are minimized thus, increasing the availability and reliability of the unit.
When used as an output reactor, this component reduces motor vibration caused by the switching waveforms produced by the inverter. Conditioning these waveforms approximates commercial power quality, eliminates noise and current spikes.
Filters, prevent noise or disturbances from affecting the VFD. Noises may be propagated from input and output supply lines as well as generated by the VFD circuitry itself. Depending on the type of noise, an appropriate filter may be used for Hitachi drives.
Radiated noise from the main power wires in the inverter input side may be reduced by a Capacitive filter. This filter may only be applied in the input side of the VFD.
EMI Filters reduces conducted noise on the power supply line generated by the inverter. EMI filters are connected to the inverter primary. For compliance to EMC Class A directive (Europe) and C-TICK (Australia), EMI filters may be used. Most EMI filters exhibit leakage current therefore, connection to ground is essential.
A Ferrite Core, when used with Hitachi drives complies with EMC Class B standard. Typically, EMI filters are installed between an EMI filter and inverter.
Input / Output RFI Filters (Zero Phase) are filters that may be installed in the input and output side of a VFD. This type of filter contributes in reducing radiated noise from the inverter wiring caused by nearby radio frequency generating equipment.
DC filters for regenerative converters eliminates a pulsating DC or a ripple, which is a component of AC power. As regenerative converters basically recycle power and return this recycled power to the DC bus, ripples may happen. DC filters cleans out these pulses in DC signal and to help produce a clean DC signal.
DC Link Chokes
DC Link Chokes functions similarly to an AC line reactor. This component suppresses harmonics and help stabilize DC bus voltage and Current. This component is typically installed between the input rectifier and the DC bus capacitors, contributing to a ripple free DC bus waveform and improved AC input waveform.
Expansion card comes in different varieties. One type of expansion card provides an optional communication capability to the drive, in addition to the existing or embedded communication method. Most VFD support MODBUS RTU communication using RS485 communication standard as the native communication interface.
Expansion cards may include EtherCAT option slot in cassette board, Ethernet Communication Interface Modbus/TCP Protocol, DeviceNet, Profibus-DP option slot in cassette board, Ethernet/IP Option Board and other protocols. Form factors of these cards may differ as well as installation, depending on the model of the VFD. These expansion cards improve networking capability and controllability of the drive.
Expansion cards are not limited to communication cards. Equally important cards include an Encoder Feedback Option, allowing encoder interface for motion and position control of the VFD aside from speed control.
Terminal boards may be used to replace or enhance the embedded Input and output interface of the VFD. Not all VFDs are designed with replaceable terminal boards. One may need to check this specific feature if supported by the VFD.
I/O interface board of the VFD is used to connect the drive with field input and output devices through hardwiring. This allows remote operation of the VFD. If the integrated I/O interface is not sufficient for the intended operation, terminal boards may be used. If existing I/O terminals are defective, terminal boards may be used for replacement.
External Channel Interface
External channel interface is used if field voltage signals are not compatible to the VFD module’s voltage category. This component is typically installed outside the VFD’s housing to convert I/O signals voltages.
Programming software supports offline and online configuration of the VFD. Aside from the mentioned basic functionalities, a programming software serves as commissioning tool of the VFD which also allows configuration of the VFD operational parameters and communication settings.
Communication cables are used mainly to download the configuration created from the programming software to the VFD. One end of the cable attached to the configuration station’s communication port while the other end to the VFD’s communication interface.
Conduit boxes are cable management accessories used to protect VFD wires and cables from dust, dirt, grease and other field contaminants. These conduit boxes are designed with 1 or more cable entry points, to provide adequate separation of power lines and I/O wiring.