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Emx3 Thermistor Input - Electrical Isolation Between Windings And Starter Control Circuit


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G'Day All,


We are fitting soft starters to some old motors (20 years old) - these motors have thermistors but they have never been used - hence we are unsure about the integrity of the insulation between the thermistor and the windings. The issue being, we are taking these old motors and likely increasing the thermal demands on them which may destroy the insulation and potentially put mains voltage on the thermistor and destroy the soft starter.


Also... there is a line in the EMX manual which states "The thermistor circuit must be electrically isolated from earth and all other power and control circuits"


So how does one do this? Is there galvanic isolation in the EMX3 thermistor input which accomplishes this function?


I understand that an external thermistor relay would accomplish this, but that would make the EMX3 thermistor input redundant




Michael Belperio



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Hello Michael


The motor will have been built with the thermistors electrically isolated from the windings and from earth. The warning is primarily to warn you not to connect the thermistors to a common earth return, but to run a two core that is not connected to anything else.


Old motors is good for your application, they will tend to produce more torque at 500% current than modern motors. A little lower in running efficiency, but much higher in starting efficiency!!


Best regards,


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Cheers mate


That makes sense


I have heard horror stories about insulation failure causing drive failure and some of our customer's specify separate thermistor relays for this case, but I now understand that the advice in the manual is not designed to cater for any insulation failures.


Oh and Let's hope those old motors keep on keeping on!


Thanks so much for your advice. The people over at the eng-tips forum told me you were the man! Cheers mate

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