The CCR transformer
The step between the thyristor stage and the series circuit: it isolates the field cable, builds the voltage the loop needs and sets how much headroom the CCR has. Change the supply, rating, load and tap and watch the ratio and currents respond.
The CCR transformer sits between the thyristor stage and the series circuit. It isolates the field cable from the supply and steps the voltage up to what the loop needs to carry the set current.
The voltage required is the load power divided by the current, so it falls with the step and rises with the load. A secondary tap sets the most the CCR can develop. If the loop needs more than the tap gives, the CCR cannot hold the current. Never open a live series circuit: the secondary voltage would climb far above normal.
Frequently Asked Questions
Q1What does the CCR transformer do?
It couples power from the thyristor stage to the series circuit, steps the voltage to the level the loop needs, and electrically isolates the field cable from the supply.
Q2Why is the output isolated from the supply?
The series loop floats with no direct connection to the supply, so a supply fault does not reach the field cable. A single earth fault on an isolated loop does not by itself stop the lights, and earth-fault monitoring can detect it.
Q3Is the transformer what holds the current constant?
In a thyristor CCR, no. The control loop adjusts the thyristor stage and the transformer gives the voltage ratio and isolation. Some older designs let the transformer itself help regulate, for example through variable leakage.
Q4What are the taps on the secondary for?
They set the maximum voltage the CCR can develop so it suits the circuit it feeds. If the loop needs more voltage than the tap provides, the CCR cannot hold the set current at the top step.
Q5Why must a live series circuit never be opened?
The CCR keeps driving current, so an open loop lets the secondary voltage rise far above its normal value, which is a danger to people and insulation. Open-circuit protection should trip, but the rule is to de-energise and lock out first, then follow site procedure.
Q6What is checked on the transformer in maintenance?
Overheating or odour, discolouration, tight connections and mounting, and the insulation resistance trend over time.
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