Variable Frequency Drive Selection Criteria

This article is the 5th and last part in our series Using VFDs in Refrigeration and Cold Storage Applications.

When selecting a Variable Frequency Drive for refrigeration and cold storage applications there are a number of important selection criteria that need to be taken into consideration.

• Harmonics
  All VFDs generate harmonics. Care should be taken to select a VFD which has built-in harmonic filters. These filters are commonly known as ”DC Link Chokes”. They reduce the harmonics generated by the VFD to acceptable standards.
• Peak voltages generated by VFDs
  A VFD subjects the motor to a higher peak voltage than when it is operating on mains. Therefore the design peak voltage of a motor has to be taken into account. For example, we come across 650V grade motors in some countries which have an input voltage of 380-440V. A motor like that has a design peak voltage of 1100 Volts. The VFD selected should not generate higher voltages than that, with full length cable between the VFD and motor.
• Standard motor capability
  Some VFD manufacturers recommend inverter duty grade motors. These motors are expensive and need to be specially ordered. Instead ask for a VFD which accepts standard motors. These VFDs do not cause additional heating in the motor, provide a full output voltage that is nearly sinusoidal and have lower peak voltages.
• Auto–manual operation
  Look for a VFD which supports auto and manual operation. This way you can be sure that if the control system or sensors / transmitters connected to the VFD malfunction, you still have the possibility to control the VFD in manual mode and continue the operation of the plant.
• Motor cable length
  It is normal practice to install VFDs in a control room. As a result the distance between the VFDs and the motors they control can be quite long. It such a case it is essential that the VFDs support long cable lengths. Look for a VFD which allows for a motor cable length of at least 100 meter.
• Cascade control
  When operating multiple compressor packs, it is advisable to speed control one compressor and cascade the other compressors as per the load requirement. Look for a VFD with a cascade controller for compressor applications that supports this type of operation. This will be very economical and improve the system reliability at the same time, as you need to install only one VFD per compressor pack with an on-board cascade controller that eliminates the need for external controllers.
• Refrigerant charts
  Look for VFDs that have built-in refrigerant pressure – temperature charts. This allows operators to easily program temperature set points in degC or degF in the VFD after selecting the refrigerant that is used in the system . This simplifies the commissioning process and minimizes human errors.
• PID control
  VFDs should have built-in PID controllers to control the speed in such a way that the set point for temperature or pressure is accurately maintained.
• Sleep mode operation
  Sometimes condenser fans are naturally efficient. If ambient temperatures are low (night time & winter) the load is low. During such conditions there is no need for the fan to run. The VFD should be able to detect such conditions and automatically switch off to save energy and reduce wear and tear.
• Programmable minimum and maximum speeds
  The VFD should allow setting of minimum and maximum speeds. In screw compressor applications for example, it may be necessary to program a minimum speed of 50% if the compressor does not allow lower speeds.
• Overload capacity
  VFDs for reciprocating compressors should have at least 160% over load capacity, whereas VFDs for screw compressors should have at least 110% overload capacity. These VFDs should be of constant torque type. VFDs for condenser and evaporator fans should have at least 110% overload capacity and have to be of variable torque type.
• IP protection rating
  If VFDs are to be installed outside, these VFDs should have at least IP 66 protection and should be able to operate in temperatures up to 45-50 degC. VFDs with IP 54 protection shall be preferred for other applications, as these can be installed without the need of an additional enclosure and are generally resistant to dust and moisture – which are common issues in most refrigeration plants.
• Display of parameters
  Select a VFD which is able to display information in English or in your local language. This saves your operators from the trouble of referring to manuals, each time a message is displayed on the VFD.
• Meters
  Look for a VFD which can display the most essential variables like kW, kWh, running hours, voltage, amperes and most importantly the parameters that are being monitored by the VFD like temperature or pressure. This information is vital to your installation and when connected to a computer, can provide log details of the data being monitored.
• Energy optimization
  VFDs for evaporator and condenser fans should have an energy optimization function. With such a function, you typically get an additional 5-10% energy savings.
• Real time clock
  Always look for a VFD with the possibility of a built-in real-time clock. This enables you to program your on/off times, defrost schedules, maintenance schedules, night set back times etc.

Of course all or most of these criteria may also be of importance for other applications than refrigeration and cold storage.

For more information about using variable frequency drives in refrigeration and cold storage applications also read the previous posts in this series:
Part One: Introduction
Part Two: Variable frequency drives in refrigeration systems
Part Three: Screw compressor VFD application
Part Four: Using VFDs to control evaporator fans

Article by Deepinder Singh Chani