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Aucom Emx3 Thermistor Input


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Hi All


What are the requirements for thermistor wiring for the EMX3? The manual has all the necessary information but does not elaborate in some areas. I have listed below my interpretation, would appreciate if you guys could provide some comment, thanks.


1) Our cable length from the starter to the motor is 500m. The thermistor's trip level is 3600 kOhm (pretty standard) - so for three PTC thermistors in series, each with a trip level of 250ohm (again pretty standard). So based on some elementary calculations we could run 500m of 1.5mm2 cable to the thermistors without trouble.


2) The EMX3 manual states 'the thermistor circuit should be run in screened cable'.


a) Is this to protect the thermistor reading? or to prevent noise being coupled into the starter control circuit?

B) Is screened cable really necessary? Radiated noise from the bypassed soft starter motor and cable should be very low, but the run is quite long, the motor and thermistor cables will be parallel probably with their sheaths touching. If it was a VSD, or even a non-bypassed starter, I would not argue, but this seems overkill in the case of a bypassed soft starter.

c) If we run in unscreened cable is there anything we can do to filter noise from the thermistor input? Is there galvanic isolation on the EMX3 thermistor input?


Thanks Guys,


Michael Belperio




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Hello Michael


A 500M cable is a long run for a thermistor input cable.

The thermistor input is a low voltage low current circuit, so it can be susceptable to noise induced from other circuits is they are close.


I would recommend that :

a) Use a screened cable with the screen earthed at on end only. I would recommend earthing the screen at the EMX3 end.

B) Keep the thermistor cable well separated from other cables if at all possible. This will reduce the induced voltage and/or currents from other circuits.


If you choose to use an unscreened cable, then try to use a twisted pair, well segregated from all other cables and metalwork.


If you suffer intermittent noise induced tripping, I would suggest that you connect a 100 uF 16V capacitor across the thermistor input at the starter. This should help to reduce any induced AC noise to the thermistor circuit. - Check the polarity with a multimeter


Best regards,


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