To VSD or not to VFD … that is the question
By S Naicker, H Bennie, B Gruszka, Applied Energy Solutions
Electricity+Control, January 2014 (pages 42 - 44)
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Of numerous solutions available, the VFD (Variable Frequency Drive) or the VSD (Variable Speed Drive) has triumphed as a tried and tested solution in increasing energy efficiency without compromising production and operations.
A variable speed drive converts the ac (alternating current) supply power at a fixed-frequency (50 Hz) into dc (direct current) using an integrated rectifier. Power electronics integrated into the variable speed drive convert the dc signal into a sinusoidal output with a controlled frequency and voltage. Systems that require control of flow or pressure are most suited for variable speed drive use.
The most commonly used applications are pumps and fans that operate in industrial processes or as part of heating, ventilation and air conditioning systems (HVAC). Such systems are governed by the Affinity Laws, where consumed power is proportional to the cube of the flow or speed of the motor. In applications where motor speed and load are constant a variable speed drive will not always provide suitable savings of energy usage. However, if a load increases over a long period of time a variable speed drive may be a suitable solution to curb energy usage.
In a facility where variable loading and flow exist, the installation of a variable speed drive is likely to result in energy savings and process improvement. In addition to the installation of a variable speed drive it is important to ensure correct load matching and the correct match between variable speed drive and motor. If possible, older motors should be replaced with modern, energy efficient ones. Simply replacing a motor with an energy efficient version without careful consideration can result in an increase of energy consumption.
Variable speed drives should be considered as part of a facility’s energy efficiency programme, which should form part of an overall energy management policy and strategy. Initiatives such as these require careful consideration prior to implementation, and they require constant monitoring and evaluation to ensure that savings are maintained.
Energy management is a constantly evolving process that is as dynamic as the facility itself. Continuous monitoring and targeting, re-strategising and the application of new technologies should be applied in order to yield the best results. When in doubt, an energy management expert should be consulted to oversee the process and ensure success in the endeavour to reduce energy and costs.
- Variable speed drives increase energy efficiency without compromising production and operations.
- Variable speed drives should be part of a facility’s energy efficiency programme.
- Energy management is a constantly evolving process.