Metal Finishing Guide Book

2013

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troubleshooting, testing and analysis FREQUENTLY ASKED QUESTIONS ABOUT RECTIFIERS & CONTROLS BY PETER VAN GORP, AMERICAN PLATING POWER, FT. MYERS, FLA. Q: I have few questions about Pulse Reverse current rectifiers: 1- Should we expect to get the same ripple problems with PRC rectifier that we get from SCR DC ones? 2- Do the programmed currents for pulse and reverse rectifiers need to be readjusted from time to time? A: 1. The answer to this question depends on what you mean by Ripple. The Periodic Reverse Pulse Rectifier (RPP) rectifies the input voltage to create a so called DC-Link. This DC-Link voltage has a very low Ripple, and is then used to produce the output wave form. The result is both a Forward Current and Reverse Current with a very small Ripple. However, the overall output wave form goes both positive and negative, and thus can be considered a high Ripple waveform to your process. 2. The pre-programmed Current levels for both positive and negative pulses will not need any adjustment since the regulation circuits will continuously detect and correct any variations. Q: We are using cathodic electrodeposition, and we have an applied voltage of 350 Volt DC. We have a continuous problem as an arc at the contact point of the plating material (we use Bolymar Ecoat Material). Is this problem related only to the contact points? Or are there some other factors causing this problem? A: The only other cause I can think of is if you are using an SCR rectifier without output filtering; you'll measure 350 VDC RMS, but in reality the peak voltages of the rectified voltage are higher (350 x √2 = 495V !!!). You can verify this by looking at the output voltage with an oscilloscope. Q: We are currently facing a current drop issue during the plating process. The rectifier current will drop suddenly by 10-25% of its set value. A slight tap or "push" on the plating tank will result in the current returning to its set value. We have changed the rectifier cables, the cathodes. This is a lead-free plating process on a copper lead frame. What could be the issue and how do we solve it? A: Assuming you are running a voltage controlled process (constant output voltage), most likely you have a bad connection somewhere between the rectifier and the load. A bad connection will increase the resistance in the load circuit and at a constant voltage cause the current to go down (Ohms Law: Voltage = Current x Resistance). As you indicated, pushing the plating tank restores the connection – after which the current jumps back to the original value. In most cases, the bad connection can be found between the cables from the rectifier and the rack on which the load is mounted. I would check for some sort of build-up or dirt on all the contact points (saddles, etc.). Q: I am going to work on 200 amps for silver plating, but I don't know what rating rectifier I should order? Can you please explain how to decide rectifier rating (voltage/Amps) for different plating purposes if we know the current? In short, whether I should order 250 Amps/20 V rectifier or 250 Amps/10 V rectifier? 568

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