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Topic Title: Possible noise interference on analogue inputs?
Topic Summary: Fitting external inverter to motor causing seperate motor speed indaction to fluctuate hugely
Created On: 08 October 2013 09:46 AM
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 08 October 2013 09:46 AM
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Posts: 1
Joined: 08 October 2013


I recently fitted a Mitsibushi inverter to a machine as part of a trial, to slow a machine speed down instead of using star delta starting method.
When i start the machine, the speed indication on our HMI which is an output from the PLC keeps rising as the motor current on the HMI rises, this never happened when the motor was in star delta starting.
i've been informed this could be a noise problem and could be solved using a 0.1uF capacitor on the input for the speed indication.
Would this be correct?

Any help would be hugely appreciated.

 09 October 2013 01:29 PM
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Posts: 187
Joined: 25 February 2008

Certainly a 0.1uF capacitor can be used for noise suppression.
However I would try and ascertain it is from the inverter or associated cables - and if so attempt to reduce it at source - have a word with Mitsubishi or their agents.

I would be concerned as to why a PLC o/p is prone to interference, or is it the HMI.
Is the PLC o/p current or voltage, as a 4-20ma analogue signal is most unlikely to be affected;
If voltage you could try a 0.1uF capacitor.
Failing that it is possible you may have earth borne rather than signal borne interference.

Finally without wishing to state the obvious, is the speed actually increasing with increasing current ?

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