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Trim Resistor used in Transformer Hot spot circuit

Trim Resistor used in Transformer Hot spot circuit

Trim Resistor used in Transformer Hot spot circuit

(OP)
Can someone explain what the purpose of the Trim Resistor is in the circuit for Transformer Hot spot measurement.

The circuit i saw has the B-phase winding CT connected to the Trim resistor and the Heater coil, so the heater coil and trim resistor are in parallel.

Thanks

RE: Trim Resistor used in Transformer Hot spot circuit

It just allows for adjustment of the amount of heating produced by the heating element that is attempting to replicate the heating inside the transformer.  This has to be adjusted depending on the relationship between the CT rating and the transformer ampere rating.  

For new equipment it is mostly handled electronically with a digitally-based thermal model and there is no need for this resistor.  

 

David Castor
www.cvoes.com

RE: Trim Resistor used in Transformer Hot spot circuit

Uquresh1 - one more note. Since the CT secondary acts like a "current source", a parallel connection is appropriate. The trim resistor will divert a portion of the CT secondary current away from the heater.

=====================================
(2B)+(2B)'  ?

RE: Trim Resistor used in Transformer Hot spot circuit

(OP)
Ok, so the trim resistor is splitting the CT current between itself and the heater coil to keep the heater coil from heating up too much with continuous current.

Thanks guys

RE: Trim Resistor used in Transformer Hot spot circuit

Sort of.  The heater is intended to generate heat that corresponds to the heat generated inside the transformer windings.  So it is a question of putting the correct amount of current through heater to provide a reasonable approximation of winding temp.  The resistor and the heating element form a current divider.   

David Castor
www.cvoes.com

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