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Good day all. I'm trying to understand how a variable reactance transformer works. I have a vacuum furnace that uses the VRT for heating element control and there are 4 high current diodes up in the power supply and I'm trying to understand they keep blowing. These VRT's are controlled with a Variable transformer (rheostat)and what is happening is, the 125VAC output from the SCR to these variable transformer's becomes 260VAC and destroys the control board for the SCR Pack. Hopefully this is enough information. I would like to know about the VRT's because without knowing what to expect I don't know where to look. Thanks much for your time.


VRT is probably the same thing as a Magnetic Aplifier, which is also known as a Transductor - not Transducer.

The control windings usually have a high number of winding turns and, therefore, a high voltage is induced in those windings. A series reactor is usually used to reduce that induced voltage so it won't kill the control board.

Check to see if there is any problem with that series reactor, with capacitors used to filter out the induced voltage and other components in the circuit.

Question: Has there been any changes made in the equipmet lately? Is it possible that the control windings have been reconnected with the wrong polarity? Check voltages with an oscilloscope - they shall cancel.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.


Yes, there is a reactor inside the power supply cabinet. I don't have any prints available that show me how or why these diodes are there. I'm assuming they are used as a blocker, right? I'm not sure what the reactor does let alone how to test it.


You need someone that knows about that technology, then. Or change the system to something more modern. Magnetic amplifiers are not common these days.

The diodes are there for two reasons: To rectify and to self-magnetize the cores. Sometimes the outputs are combined to produce AC output and sometimes the DC is kept.

What power range do you have? Amps? Volts? Single phase or three phases?

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

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