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Which is better a VSD or Motor Controller?

Friday, May 4th, 2012

The answer to the above question is, they are complimentary products and both methods should be incorporated into an energy efficient system.  From January 2015 legislation is being introduced that introduces IE3 motors as the minimum acceptable efficiency for motors between 7.5 and 375kW. In addition to this the legislation gives dispensation for IE2 motors which will allow the use of IE2 motors when using a Variable Speed Drive (VSD) for control. When using a VSD on fixed speed applications the amount of energy used by the system is larger than when fitted with a fixed speed motor controller. It is true that if the application requires (or could benefit) from reducing the speed then the energy saving by fitting a VSD can be great.
For example slowing down an extraction fan in a restaurant when they are less busy can save energy as not much air is needed to be extracted. Now take a fixed speed application, for example a conveyor which must move a certain amount of material per hour, if a VSD is used and the motor is slowed down by half then it will take twice as long to move the amount of material required. This will increase the energy used by the system and thus the efficiency will be greatly reduced, so in this case a fixed speed motor controller will be the most efficient solution.

table
The point above is discussed at length by CAPIEL in their document Motor Regulation – Efficient System Design which can be downloaded here: http://capiel.eu/data/Journal_CAPIEL_MOTEUR-2_EN.pdf. In this document CAPIEL and CEMEP make the comment “An IE2 motor equipped with a variable speed drive is not equivalent to an IE3 motor equipped with a motor starter. Instead, it is always necessary to consider the speed and load requirements of the application in order to choose the best solution.”
It is crucial that the system designers understand the different motor starting and running solutions available and in most systems it is good practise to incorporate both technologies. For applications that require variable speed a VSD is the only solution, for applications that do not require or can’t be slowed down the most efficient starting solution is a fixed speed motor controller.  Referring again to the CAPIEL document “Does the regulation change the criteria to use a motor starter? No, because the selection of a motor starter should always be the result of a system analysis… many applications will achieve higher energy efficiency using a motor starter solution.”
Please see a table above that details some applications and the most efficient starting solution available:


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