There have been some interesting design changes in electroplating rectifiers over the past 50 years. But probably none as significant as the application of spaceage electronics to help metal finishers control voltage and current more precisely. Today's rectifiers offer built-in cycle memory, automatic control of plating solutions and brighteners, and data capture of everything that goes on in the plating cycle. Still, with all of the electronic frills that have been added, the basic rectifier is still a rectifier.
By definition, a rectifier converts alternating current to direct current. But in the metal finishing business, a rectifier usually is a complete system that takes utility-supplied alternating current and produces direct current in a form suitable for use in electroplating.
The typical plating rectifier has three basic components: (1) The transformer section, which reduces utility voltage (usually from 120 or 480 volts to 12 volts); (2) the rectification section, where the low voltage is converted to DC; and (3) the control section, which determines how much current or voltage flows to the plating tank.
About Power
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| 1. SCR's back to back. One controls negative half-cycle, the other the positive half-cycle. |
Since virtually no utility today is supplying DC to its commercial customers, the power flowing into a typical plating plant is AC, which means that the polarity of the power alternates between positive and negative. AC comes in either one or three phases, at a frequency of 60 hertz in North America, 50 hz in most other places. (Hertz, by the way, is a unit of frequency that equals one cycle per second.)
The hertz frequency is equal to the rate at which a wave on an oscilloscope screen alternates from positive to negative when the oscilloscope is connected to the power source. Thus, 60 hz equals 60 cycles per second, the wave fluctuating from positive to negative and back 60 times.
Because alternating current is generally supplied at between 208 and 480 volts, three-phase (120 or 220 volts, one-phase), a transformer is used to produce a lower AC voltage.
About Transformers
The basic principle of a transformer is that when current passes through a conductor, such as wire, a magnetic field (or flux) results around the conductor. If the current is 60-cycle AC, the magnetic field expands and retracts sixty times per second with the voltage fluctuation.
A transformer's magnetic core is wound with wire. In a plating rectifier, though, the transformer usually has two sets of windings—the primary and the secondary. The primary conveys the input, supplied by the utility. The secondary handles the output, which goes to the rectification section of the rectifier for conversion to DC.
The magnetic field in the primary coil changes constantly from positive to negative. This induces a voltage in the secondary winding proportional to the ratio of wire turns on the primary to that on the secondary. For example, a transformer with 400 turns on the primary and 10 turns on the secondary has a ratio of 40 to 1. So if 480 volts are applied to the primary, the secondary voltage is 12.
A transformer is rated according to the power it delivers, expressed in volt amps (VA) or thousands of volt amps (kVA). Volt amps are calculated by multiplying the amperage (or current) by the voltage. Thus a 40:1 transformer rated at 1000 VA (or one kVA), has a primary winding current of approximately two amps (1000 divided by 480). Since the transformer rating applies to the secondary as well, the secondary current would be approximately 80 amps (1,000 divided by 12) or 40 times greater than that of the primary.
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| 2. SCR can rectify and perform control function simultaneously. |
A transformer is not mandatory in a plating rectifier. There are other ways to reduce full utility voltage (even down to zero, if necessary). But experience has shown that a transformer is usually the most reliable, safest and most economical way to lower voltage for plating.
It's reliable because there are no moving parts, the transformer can withstand very high overloads, and it will last in definitely (as long as it is properly cooled).
It's safe because it isolates the plating tank from the utility line and limits the voltage to the tank.
And it's economical because it allows the plant operator to control the power used in the process. Without a transformer, the plant would have to use the full power supplied by the utility, at a relatively high amperage—as much as 40 times more power then is normally required.
About Rectification
Modern plating rectifiers convert AC to DC by using semiconductors, which conduct current in one direction only. As the wave alternates from positive to negative, the semi-conductor conducts only the positive part, so that the output is positive. The conversion is accomplished because direct current flows only in one direction (either positive
or negative).
Semiconductors can be made from several different minerals and compounds, such as selenium, copper oxide and germanium. However, silicon has proved the most efficient and is the most widely used material these days.
Semiconductor designs offer several choices.
The use of diodes allows current to flow in only one direction when a voltage is impressed.
Thyristors (better known as SCRs), also allow current to move in only one direction, but must receive a control signal at the gate lead before they will do any conducting.
Triacs, which really are two thyristors connected back to back, are used in AC
circuits, under controlled conditions, and conduct in both directions. But they are used for
control and not for rectification, which is done with diodes.
About Control
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| 3. Simplest control is tap switch. |
There are several ways to control a rectifier so that it delivers the performance required by today's cost-conscious product finishers. Because each method has its advantages and disadvantages, plant management has to do a little soul searching and a lot of analysis when specifying a rectifier. The various claims and arguments advanced by different rectifier suppliers tend to make an already complicated decision even more complex when trying to arrive at a proper choice for a given application.
One issue, for example, is where the control SCRs should be placed—at the primary or the secondary. Since SCRs today are the most commonly used control, this issue tends to surface more often than others. Some rectifier manufacturers favor primary, others secondary. And some offer either approach.
What it boils down to, as in most design controversies, is which will work more efficiently in a given situation. Which type will waste the least power? In some cases, such as a 500-amp unit in a given application, secondary might work best. In the case of a 10,000-amp unit in another application, primary might be more efficient.
The choice is not an easy one and detailed knowledge of the rectifier and the application are required to decide intelligently.
Platers know how to finish metal. Rectifier manufacturers know how to make rectifiers. There should be a frank exchange of information to assure that the right rectifier is being selected for a given need.
Design issues aside, the following are the more widely used control methods in today's plating rectifier.
About SCRs
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| 4. Full-Range tap switch control is more versatile. |
What used to be called the silicon controlled rectifier (SCR), and now is called the thyristor, has been around for some 30 years now. The SCR makes an excellent rectifier, since it can control very high currents. No wonder SCRs eventually replaced vacuum tubes.
Initially SCRs were expensive, and because they were used inappropriately at times, there seemed to be some question about their reliability. But through the years SCR manufacturers improved the state of their art and engineers learned a lot more about how to use them. Today, when used within its rated capacity, the SCR is less costly and has proven to be very reliable and useful.
Until the mid-1970s, SCRs came in relatively low current ranges, and therefore they were not a very practical substitute for diodes on the secondary side of the rectifier transformer. However, since current on the primary side is much lower, SCRs soon found a home in rectifier manufacture on the primary side.
To accommodate the AC of the primary, the SCR must control both the negative and positive half-cycles. This is done by connecting two SCRs back to back on each phase (Fig. 1). One controls the negative half-cycle, the other the positive half-cycle. This not only works very well, but enables the manufacturer to produce a more compact power supply.
Because SCRs have become increasingly less expensive, they are being used much more frequently. As to where to locate them, primary or secondary, there are advantages and drawbacks in either case.
The advantage of locating SCRs on the primary side, in some situations, is greater operating efficiency and possibly even lower manufacturing cost. The drawback, however, may be more susceptibility to line noise transients, especially where incoming power is subject to fluctuations resulting from utility switching, starting up motors, and lightning. As for locating them on the secondary side, in some cases the SCRs could replace diodes and perform rectification instead of just control (Fig. 2). The drawback might be the need for heavier-duty SCRs, which can handle 12,000-amp loads readily.
There is one slight problem with SCRs in general. They produce a DC output distorted by "ripple," or the residue of the fluctuating AC sine wave. Since many plating applications require a very smooth DC wave form, it becomes necessary to filter output to remove the ripple. This is done rather easily.
Other Control Approaches
Probably the simplest rectifier control is the tap switch (Fig 3). The principle is best understood with an example. Take a transformer with 400 turns on the primary, 10 on the secondary, and an output voltage of 12. If you wind an additional 400 turns on the primary and add a switch to move the incoming power load from the 400-turn section to the 800-turn section, you raise the turns ratio from 40:1 to 80:1. Now the output voltage is six volts instead of 12. If you add another 400 turns, making a total of 1200, the turns ratio is now 120:1. Switching to that section produces a four-volt output, and so on.
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| For motor driven, motor is added to the above. Motor may be immersed in sealed tank with insulating fluid. For AVS or ACC, a servo-amp is added. |
| 5. Variable Autotransformer permits continuously variable control. |
That is exactly how a tap-switch control works. Because the primary is extended by adding more and more turns, this approach is called an "extended primary." With three-phase input, this type of control is usually the most economical for plating. Typically, this rectifier design has three tap switches, a three-phase transformer and varying numbers of diodes, depending on current requirements, as well as the standard rectifier components, such as fans, meters, protection circuits and starting circuits.
A tap-switch unit produces less than five pct ripple at any output voltage, as long as all the taps are in the same position. This type of rectifier has proven useful in such non-critical applications as electrocleaning and barrel plating; also where plating thickness is not all that important.
Because output is not regulated, and is therefore vulnerable to voltage fluctuation, the tap switch is not recommended for plating operations where thickness is critical. Two identical jobs, running with the same settings and length of time, could wind up with different coating thicknesses.
There are other drawbacks to tap-switch units. The unit has to be located close to the tank and the output voltage can't be controlled smoothly—only in discrete steps. For a little extra cost, though, a full-range tap-switch control can be ordered and will control down to zero volts. It uses an extra single-wound transformer called an autotransformer (Fig. 4). The taps are set at different points, from full to zero. The tap switch connects to the different points and the tap switch output feeds to the main transformer, which is a simple two-winding step-down unit rather than an extended-primary type.
Another control option is the variable autotransformer (Fig. 5). It works on the same principle as the full-range tap-switch unit, but it has no taps. Instead, a section of the autotransformer winding is exposed by removing the insulation.
A carbon brush rides along the exposed area and senses voltage. The control afforded is much smoother than with a step-type tap switch. Also, by ganging three autotransformers in one assembly for a 3-phase unit, a single knob can be used to make control adjustments more easily. This not only prevents imbalance between phases but reduces ripple. This type of rectifier design is competitive cost-wise with the tap-switch control, but generally is not considered cost effective above 1500 amps or so.
Another consideration with variable autotransformer units is manual versus motor-driven. Manual means a lot of operator attention. Motor-driven can be controlled remotely, normally with pushbuttons to increase or decrease output voltage. Adding a servo-drive amplifier and electric circuits automates voltage stabilization, constant-current and average current density or a combination of the three. By interfacing the automated unit to a computer, total automation can be achieved along with precise, programmable, interactive current or voltage control.
One disadvantage in variable autotransformer units, though, is their exposed copper windings. A highly corrosive plating environment usually is better off with sealed windings from the standpoint of maintenance and wear. Even in less demanding environments, heavy use causes excessive brush wear.
To get around the problems associated with corrosive environments or excessive brush wear, some autotransformer units are available with motor and gears immersed in a sealed tank of insulating fluid. The fluid lubricates bearings and is an excellent heat sink, allowing the variable autotransformer to operate at a higher rating. Since the unit is sealed it is protected from corrosion.
Another type of control is the electromechanical saturable-core reactor (Fig. 6). This design was a lot more popular in bygone days than at present and doesn't warrant much discussion.
Still another control alternative that is used from time to time is the transistor type. It's generally used on small units that require automatic control. That's because transistors have very limited current-carrying capacity. Transistorized units are popular today in lab applications and for precious-metal plating of jewelry and electronic connectors. Their principal advantage is very low ripple (as little as 0.5 pct).
Switch Mode Power Supplies
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| 6. Saturable core reactor allows automatic constant control of voltage, current, or both. |
The newest power supply technology to the electroplating industry is the switching transistor-type power supply. The technology was first developed to provide small power supplies for computers, and was later expanded for use in the semiconductor industry for testing purposes. Most recently, these power supplies began to appear in plating shops. They contained certain features which made them ideal for certain plating operations; namely small size and very low ripple.
The way these features are achieved is as follows: (see Figure 7).
Line voltage is connected to an EMI (Electromagnetic Interference) filter. It is then rectified and filtered to provide a DC voltage. This voltage is applied to an inverter to change it to a high-frequency AC voltage (approximately 1,000 times the line frequency). This high-frequency AC is then stepped down to the level required for plating, and due to the high frequency, the transformer becomes very small and lightweight.
The low voltage is then rectified and filtered to give a high current, low voltage output. Again, due to the high frequency, the filter assembly becomes very small and lightweight, and the output ripple becomes extremely small, usually in the order of less than
100 millivolts.
A problem existed though. Since the supplies were originally designed for "clean room" areas, operating them in a plating atmosphere was out of the question. They had to be put in separate clean areas (defeating one of the advantages of a small supply), or put in another sealed enclosure, and provide means of cooling them. This would usually require air conditioning, water cooling, or special clean air ducting into the cabinet. All of these methods were very cumbersome and costly.
The next generation of switch mode power supplies is now here. These are ones which are totally sealed without the use of external cooling mediums, and are designed specifically for the electroplating market. They contain all of the desirable features of the switch mode supplies as well as being able to operate in the plating atmosphere.
This new technology is presently available up to 1,000 amps at 12 volts DC.
Special Applications
Many applications occasionally call for variations in common rectifier designs. Pulse plating, for example, requires a DC output that is segmented into rectangular pulses. Power supplies for electrolytic coloring of aluminum produce AC, not DC, yet these units are referred to as "rectifiers."
Other applications require AC to be superimposed onto the DC waveform, deliberately producing a ripple effect.
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| 7. Switchmode transistor |
Some operations require DC with extremely high voltages, as in electrostatic painting. Still other special needs include polarity reversal, dual proportional outputs and special automatic control systems.
Most rectifiers can be retrofitted with special options for particular needs, such as ramp control for hard coat anodizing; ampere-hour metering for precise thickness control; and accessories to produce the proper current for automatic operation of solution-feed pumps.
One of the more dramatic developments in controlling power-supply systems today is the software program that allows a personal computer to control larger plating operations.
Using Computer-Controlled Systems
The more progressive rectifier suppliers today are offering digital interfacing systems that can totally automate a plating line when used with computer-controlled hoists. A single computer can control a group of rectifiers from a remote location and not only automate the process but provide data that helps management monitor power usage, track other factors that affect operating costs, and measure plant productivity—the kind of data plant managers need to improve process performance.
Undoubtedly new rectifier designs will come along to meet the needs of metal finishers as competitive forces change or as research produces new processes. But for the most part don't expect any radical changes for the time being. PFD
