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Use Of Chock In Vsd

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#1 ndeepak


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Posted 02 March 2008 - 02:50 PM

i usually find three phase chock coils at input of vsd
some new type comes with inbuit chock but when opened it there is no physical chock in it.
first of all why is chock needed and how it is eliminated in second version?

#2 marke


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Posted 02 March 2008 - 06:07 PM

Hello deepak

Small VFDs typically do not have a choke due to the weight and cost.
The input stage comprises a full wave rectifier which rectifies the input AC voltage and converts it to DC which is then applied to the power capacitors for smoothing.
If the capacitors are connected directly to the rectifier output, and there are no AC line reactors, then there is a very high peak charging current on the crest of the waveform only. - all the current flow on just a few degrees of each cycle. This causes very high harmonic currents and losses in the supply.

If a DC bus choke is used, or appropriate AC line reactors are used, the current flows for a longer time. (maximum 120 degrees per half cycle) This reduces the peak current and reduces the harmonics to mainly fifth and seventh.

The only reason to leave out the choke, is price price and size.

Best regards,

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