Rectifier Voltage Explained

Why does the rectifier voltage climb after dipping parts into an electrocoat line?

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Q. We have an e-coat line and would like to know why our rectifier voltage, from the day we bought it, keeps climbing up during the cycle after we dip our parts in. I thought this to be normal, but our customer has questioned us. Is it possible that the voltage is not reaching the set point? Can you explain a normal voltage cycle? —R.S.

A. A rectifier can be set to control the coating cycle on voltage or on amperage. The choice of type of control depends on the film requirments, the size of the parts and the anode/rectifier capability of the equipment, among other factors. Typical electrocoat systems running multiple part sizes operate on voltage control.

Under a voltage control electrocoating cycle, the voltage will start at zero at the beginning of the cycle and ramp up to the voltage set point at a rate determined by the slope percentage of the rectifier. This slope, or rate of voltage ascent, is operator-set based on trials or experience with the type of parts. The greater the slope, the greater the rate of voltage ascent. During a normal cycle, the voltage will continue to climb from zero, according to the slope rate, until reaching voltage set point. The voltage is then maintained at set point for additional seconds until the cycle timer times out and the cycle ends.

If your cycle time is in the typical range of 2 to 5 min., there could be two  reasons why the voltage would always be climbing during the cycle and never reach the voltage set point. (Of course this is not considered a normal or typical coating cycle.)

1. The slope setting too low. The percentage setting is too slow and the cycle timer times out before voltage can reach set point.
2. A super-high coating load. If amperage draw during the cycle is excesssive due to extreme square foot load, then the voltage climb will slow down or even back up so that the product of voltage and amperage does not exceed the Kwh rating capacity of the rectifier.

When you run your system on amp control, the amperage will climb up to reach a certain amperage set point, just duplicating what voltage does when on voltage control.

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