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Energy saving with VSDs


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There are many claims and promises about the value of VSDs on reducing the energy consumed by machines.

While there is no doubt that there are machine inefficiencies that can be reduced by the use of VSDs, there are also many applications where the use of the VSD will increase the losses and running costs.

Any thoughts or experiences in this field are sought.

Best regards,

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  • 2 weeks later...

Hello homerjay


The actual figures are dependent on the technology used and how thinga are set up.

You have energy loss in the input rectifier stage, there will be a voltage drop across the rectifiers that will give a static loss. If AC or DC reactors are used, there will be additional losses in those, but the harmonic currents will be lower reducing losses in the rectifier, fuses and capacitors.

The output stage is the major contributor of the output losses. There are two levles of loss here. There is staic loss due to the voltage drop across the ouptu devices times the current through them, and there are switching losses that are dependent on the switch ON and switch OFF times of the switching elements, and the rate at which they are being switched.


There is an additional loss that is the energy required to run the electronics.


Best regards,

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  • 3 months later...

Hi Marke,


The only way I can see how a VSD can save energy is by reducing the motor speed, say set the max speed to 45Hz instead of 50Hz. Otherwise, I can not see where can the energy saving comes from? Furthermore, the efficiency of VSD could not be 100% right?


There is no free lunch in this world, you will not get something out of nothing! I believe you will have to sacrify something in order to gain the so-called "energy saving" (unless your application is not running at full speed all the time, then energy saving by VSD is possible via PID control)


Pls correct me if I am wrong! Cheers!

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  • 5 months later...

Hello Lakshimikantha


Welcome to the forum.

The energy saved is the reduction in mechanical energy. The voltage reduction does little in reality to reduce energy, but is necessary on a VSD to keep the flux constant and prevent increased losses.

Best regards,

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