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Peter van Gorp
American Plating Power, LLC

Peter van Gorp

Rectifiers, Rectifier Controls

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Q: Hi Peter, I have few questions about Pulse Reverse current rectifiers: 1- Should we expect to get the same ripple problems with PRC rectifier that we gets from SCR DC ones? 2- Do the programmed currents for pulse and reverse rectifiers need to be readjusted from time to time?

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.

Date asked: 21 February 2013

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?

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.

Date asked: 23 November 2012

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? Thanks.

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.).

Date asked: 21 January 2010

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?

Basically, the output voltage of the rectifier depends on the total resistance of the load (including leads, saddles, etc.). On average, for your application, 15VDC should be sufficient (example: a 15V/250A Switch Mode Power Supply).

Date asked: 01 August 2009

Q: We run our commercial anodizing plant using a thyristor-controlled rectifier supplied by a Spanish company called Marmavin. Since a recent 'upgrade' we are now unable to achieve any of the electrolytic colors we were previously producing regularly. The first prognosis was that our thyristors were unbalanced (?), however, as these have now both been replaced with no improvement it seems our software provider is at a loss to pinpoint the problem. They have tried to put the blame on the cabling feeding the rectifier, although as this hasn't changed since the upgrade was done we cannot see how this can be the case.

The only possible cause we can find is that we are struggling to achieve the correct power output of 10 volts that is required to produce the colors. We do not seem to be able to get above 8.7 volts, is it possible some sort of 'restrictor' has been fitted or can you suggest any other possible cause?

You are experiencing what many users are with a similar upgrade.

An upgrade to a thyristor control system for an electrolytic coloring system gives, in most cases, a lower quality, resulting in inconsistency and ‘patchy’ coloring.

We recommend using a variable ratio controlled power supply providing a balanced sine-wave to the to-be-colored load.

Date asked: 01 August 2009

Q: We are going to install an anodizing process. What kind of material should we use for the cooling coil if I'm going to use freon to cool (the freon presure is 200 psi)?

You should consider, instead of indirect cooling by cooling pipe, a direct cooling using a water-to-acid heat exchanger. For pipes you can use titanium or lead, however, a solder spot can be a weak part of your cooling system.

Date asked: 01 August 2009

Q: How much ripple should you have in a rectifier used for chromic anodize? I have always been told that about 5%, or less, is really good.

We haven’t seen any performance differences between high and low ripple for the anodic coating produced in chromic solutions.

See: Chromic Acid Anodizing Standard, MIL-A-8625 F TYPE.

Date asked: 01 August 2009

Q: What is the best way to stop rectifiers condensating when switched off, for a period of time, due to a supply failure?

No water should be running through the cooling system of the rectifier when the system is not in use. In colder climates one should avoid pipes freezing up.

Contact your rectifier supplier for specific advice.

Date asked: 01 August 2009

Q: What if any effect on type II anodizing do voltages below 5V have on the coating? We ramp our voltages, but the SCR on our rectifier does not allow a response below 5 V, so we lose any effect of the ramp from 0–5 V.

Depending on the material thickness, you might get burning marks caused by the input current rush due to the offset voltage of 5 V. Non-anodized materials act like a short circuit in the tank.

It could also trip the rectifier if the load size is big enough and trips the main fuses.

I recommend you contact the rectifier manufacturer for service to correct the cause of the offset voltage.

Date asked: 01 August 2009

Q: We are piston ring platers. I want to know about the ramp limitations/specifications for hard chrome plating of piston rings for both compression & oil rings. I'm finding problems of double plating due to frequent power trippings especially in India. How many types of rectifiers are available in the industry prersently & which one is the best for hard chromium plating over piston rings of cast-iron make?

Of course it is not possible to avoid the power trips, so to guarantee the quality of your products after power outages you’ll have to etch the parts again and re-plate.

The best hard chrome rectifiers offer a very efficient DC ripple smoothing filter, and for a number of applications, an etching cycle as well.

Date asked: 01 August 2009

Q: How well would it work to use a small programmable pulse rectifier to modify the output of a larger (75-volt, 500-amp) rectifier to generate a pulse output for anodizing?

I intend to connect them in parallel.

To start with, NEVER connect rectifiers in parallel unless the rectifiers are specially designed to be connected that way, and always check first with the manufacturer. In your case, do not connect them in parallel, or it will be a very short, smoky event.

I assume that you are trying to have the large rectifier ‘follow’ the pulse pattern generated by the pulse rectifier. This is possible by using the output signal of the pulse generator as a reference signal for the large rectifier. First, you need to know what maximum reference voltage or current level the rectifier accepts on the voltage and current inputs (typically 0–5 V, 4–20 mA or 0–10 V) and then make this value the maximum output value of the pulse generator/rectifier. A limiting factor, however, will be the rate of change that the rectifier can output. A standard rectifier will always need a certain time to get to a certain output level, so if your pulse generator outputs relatively short pulse times, the output of the rectifier will not be able to exactly follow the reference signal, resulting in a distorted output waveform.

Date asked: 01 August 2009

Q: We plate out copper in a reclaim system for our plate pickling tanks. We are considering the purchase of a new rectifier and I wanted to know how to spec a rectifier for the ripple percentage and how ripple effects plating. Our reclaim tank is comprised of a series of lead and copper plates. The pickling solution is 10/12% sulfuric at 110 degrees. The material we pickle is copper or copper/nickel alloy.

For pickling, the output ripple of the rectifier does not really matter. If you know the maximum voltage and current your process needs, you can select a rectifier with a maximum output of around those values. Using a standard thyristor-controlled rectifier or a tap switch rectifier will give you the lowest ripple (5%) and the highest efficiency.

Date asked: 01 August 2009

Q: We have a cathodic electrocoating paint bath in which two different lines are passing. The conveyor speeds are the same. We are using three sets of electrodes. One at each side & a third one at the middle of the tank. The component loading on both the lines may or may not be similar. For this configuration, should we go with one rectifier or two different rectifiers of the same capacity & why? What is the best alternative?

The best alternative here would be two rectifiers. With two rectifiers you can individually control the process voltage/current for each line, enabling you to individually control the coating thickness depending on the total surface area per line.

Date asked: 01 August 2009

Q: Several years ago we installed variable speed frequency drives to control the rpm rates of the motors on our two plating lines. Since that time there has existed an apparent energy field which radiates throughout the building (11,000 square feet). For example, when I pull up in front of the building to park, my am radio station is overtaken with static, and second the eddymag measuring units on the shop floor continually loose their set points and accuracy during the day and need to be recalibrated every 2 hrs.

We were advised and did install filters that were supposed to work but have not. I have asked many but seldom gotten a straight answer?

From my experience with VFDs I can only think of the ‘Carrier Frequency’ setting of your drives as a possible source. I’ve seen situations in the past where the Carrier Frequency setting was set relatively low, resulting in a high level of noise on the motor lines.

Date asked: 01 August 2009

Q: Reverse pulse plating in a sulfamate Ni electroforming bath. We are not using such. What are the advantages in time savings, etc?

Reverse pulse plating in a sulfamate Ni bath does bring some advantages.

  1. Slight improved throwing power: a standard DC process gives about 10% throwing power, reverse pulse can improve this to 25%. I even heard some customers claiming 50%, but that seems high. Some users benefit from this in their electroforming process: they can skip some steps of mechanical machining during their 900-hour process, due to less whisker forming and improved material distribution.
  2. The crystal structure can be influenced, and therefore the hardness. Hardness between 150 and 500 V can be obtained.
Plating time savings is minor, but with thick-layer electroforming there are savings possible due to less machining steps.

Date asked: 01 August 2009

Q: What are the advantages and disadvantages of using a tap switch rectifier for type II anodizing?

There are no advantages only disadvantages. The main disadvantages are:

  1. No control

    You can only set the output voltage. The current will first shoot up and will then drop, which results in longer process times in comparison to a controlled system.

    So your manufacturing cost are higher because you pay more for energy due to the longer process time. If your coat is 0.6 mil instead of 0.5 mil, your energy cost are 20% higher than you have calculated. For a small 3000-A rectifier these are 0.7 U.S. $ per load. Which totals up to 1.4 U.S. $ an hour for just one tank. So for just one tank per day it is about 17 U.S. $. It sums to about 4,500 U.S. $ a year for just one tank.

    Of course it gets worse if your deviation between the requested and achieved oxide thickness is even higher.
  2. More manpower

    To set the rectifier correctly somebody needs to stay next to the rectifier to ramp it up and to re-adjust the taps. Of course this also results in additional costs.
  3. Quality

    Manual operation results in poor repeatability.
  4. Maintenance-tap switches need to be maintained and spare parts are very expensive
Advantage of a controlled system:

Money savings from the first load—No or less over anodizing, less manpower, less maintenance.

Date asked: 01 August 2009

Q: What does output Ripple of a rectifier mean and how can I measure it?

The output Ripple of a rectifier is a fluctuation of the output DC Voltage of that rectifier. Technically speaking it is an AC (Alternating Current) component superposed on a DC (Direct Current) voltage. Different types of rectifiers will behave differently as far as the output Ripple is concerned depending on the application and settings. When we talk about Ripple in percentages we are talking about the amount of fluctuation of the output voltage. This Ripple can be calculated by dividing the RMS voltage of the AC component by the average value of the DC output voltage, then multiplying the result times 100%.

Date asked: 01 August 2009

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