finishing equipment & plant engineering
DC POWER SUPPLIES
DYNAPOWER & RAPID POWER CORP., SOUTH BURLINGTON, VT.
www.dynapower.com
RECTIFIER OVERVIEW
Rectifiers were introduced to the surface-finishing industry over a half centuryago to replace rotating DC generators. Rectifiers have a major advantage in thatthey have few, if any, moving parts, which results in significant decreases inmaintenance and downtime. Today, rectifiers are one of the most reliable andefficient means of power conversion, and nearly all surface-finishing rotatinggenerators have been replaced.
A rectifier can be divided into three major components: a main power transformer,a regulating device to control the DC output, and a rectifying elementto convert the incoming AC to output DC. A rectifier also contains auxiliarycomponents, such as control electronics and cooling.
Main Power Transformer
The main power transformer receives line voltage and steps it down to a suitablebut unregulated AC voltage. To produce a transformer of the highestefficiency and reliability, three major design factors must be considered. First,all conductors must consist of electrolytically pure copper. Second, the corelaminates must be made from low-loss, high-quality transformer steel. Third,extremely high-quality, high-temperature insulating material must be utilized.If the quality of any of these areas is compromised, transformer efficiency and
longevity will be sacrificed.In a high-quality transformer, electrolytically pure copper is used to windthe transformer coils, with insulating material located between each conductor.Once wound, the coils are vacuum impregnated with a high-temperaturevarnish, and all terminals are then silver brazed. The coils are then placed ontothe core.The transformer core is constructed from low-loss, grain-oriented silicon
transformer steel. The steel is cut into the proper lengths and single stack laminatedto form the core structure. If a great deal of attention has not been paidto the construction of the core, there will be air gaps between the laminations.This will decrease the transformer’s ability to handle magnetic flux, resulting ina transformer with less efficiency.The majority of transformer power losses is the result of excessive temperatures.The only way to avoid this condition is through proper engineering. Thisincludes designing for low-current densities in the windings, low-flux densityin the transformer core, and of course, ensuring proper transformer assembly.Quality transformers are manufactured in this manner. Unfortunately,improper transformer design or construction is not always visible to the nakedeye. A conservatively designed quality transformer will look physically similarto a lesser quality transformer. Because the differences lie in the design andmaterials, the effect will only become apparent during operation. A higherquality transformer will run 10 to 15% cooler. A transformer operating at lower
temperatures will have a much higher efficiency and greater longevity. Although
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Fig. 1. Primary thyristor.
the manufacturing cost is higher on the more efficient unit, the payback forthe additional expense is relatively short. Most manufacturers will guaranteea well-designed transformerfor 5 years; however, such welldesigned transformers will typicallyoperate for a minimum of
15 years without problems.
Rectification and Regulation
The silicon diodes used in rectifiersare the simplest and mostreliable rectifying devices available.Silicon, when properly
treated with certain elements,allows current to flow in onedirection only. When a silicondiode is hermetically sealed, itbecomes completely imperviousto external conditions, makingit capable of withstanding theharsh environments commonlyfound in metal-finishing facilities.Another silicon device thatis instrumental of today’s rectifiersis the silicon-controlledrectifier, commonly known asa thyristor or silicon-controlledrectifier (SCR). The thyristor is basically a silicon diode tha
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Fig. 2. Secondary thyristor.
will conduct only in one direction and only when a signal is applied to a terminalon the thyristor known as a “gate.” In some instances, the thyristor functionsas a regulating element, whereas in others, it acts as both a rectifying and aregulating device.In the primary thyristor configuration, illustrated in Fig. 1, thyristors are connectedbetween the incoming voltage source and the transformer. In this design,
a thyristor operates at a relatively high voltage and low current. Generally, all thyristorshave a fixed forward voltage drop across them. This drop ranges from 1 to1.5 V. When the highest quality thyristors are used as primary elements, with aninput of 230 or 460 V, the efficiency of the thyristor network is greater than 99%.In the primary thyristor configuration, the thyristor is solely used to varythe AC supply voltage from zero through maximum. In order to make a fullyregulated controller, each phase of the three-phase input must have two thyristorsconnected back to back, as shown, and their gates must be symmetricallytriggered.The regulated voltage is then fed from the thyristors to the isolation transformer,which converts the incoming high voltage/low current to a lower voltageand a higher current. From the transformer, you now have the desired outputvoltage and current, but it is still in an AC form. It is here that the silicon diodesare utilized. The function of the diodes, as stated earlier, is to allow conductionof current in only one direction. When the diodes are used, as shown in Fig. 1,they will rectify the transformer output and provide DC.Another method is to place the thyristors on the secondary side of the transformer,as shown in Fig. 2. This is known as a secondary thyristor design. Inthis configuration, the thyristors perform both the regulation and rectificationoperations, and no diodes are required. Either design can provide the desiredDC output, and although each method has its advantages and disadvantages,the cost is usually the determining factor.The advantages of the primary method are as follows:Soft start—Because the controlling element is in the primary side of the transformer,it can control the inrush current to the transformer.Efficiency—It is slightly more efficient than some secondary designs.The advantages of the secondary method are as follows:Reliability—Fewer components mean greater reliability. It has greater voltagesafety margin on SCRs. It is less susceptible to line voltage transients.Reversing—It is able to achieve solid-state reversing.
PLATING
Direct Current Plating
Direct current electroplating covers a broad range of processes. These include,but are not limited to, chromium, nickel, copper, zinc, cadmium, silver, andgold. Whereas each of these processes vary somewhat in their particular voltageand current requirements, they all require some form of DC power to depositthe metal out of solution onto the part being plated.A typical DC plating power supply will have a three-phase input of either230 or 460 V AC. The output will be somewhere in the range of 6 to 18 V andbetween 50 and 10,000 A. These values will vary depending on whether still- orbarrel-plating methods are employed, the type of finish required, and the sizeof the parts being plated.Direct current plating power supplies are relatively straightforward. Theincoming AC is converted to DC by means of the main power transformer andeither a primary thyristor/secondary diode or secondary thyristor rectificationsystem. In modern systems, the output voltage and current are controlled by thephase angle of the thyristors. Most rectifiers today are equipped with both automaticvoltage control (AVC) and automatic current control (ACC) as standardequipment. In many cases, a variable ramp system is also provided to regulateautomatically the rate at which the output is increased from minimum to thedesired level.The ripple component of the output at full-rated power is nominally 5%rms of nameplate rating. This will increase as the thyristor’s phase angles arechanged to reduce the output. If particular processes demand continuous use ofa system phased back, either a properly sized unit should be utilized, or a ripplefilter should be installed to bring the ripple component to an acceptable level.Cooling can be by a number of different methods. Forced air and direct waterare the most common. Forced air is acceptable when the surrounding environmentis relatively clean and free of contaminants. In a forced-air system, air isdrawn in through a series of filtered openings in the rectifier enclosure, forcedpast the internal power-supply components, and exited through an opening,typically in the top of the supply. Air that contains corrosive materials can causeaccelerated deterioration inside the power supply, resulting in reduced life andefficiency.If a plating rectifier is situated in an aggressive atmosphere, direct watercooling should be considered. Direct water-cooling systems pass water througha series of cooling passages in the main power transformer and semiconductorheat sinks. Water-cooled systems are more compact than air-cooled designs,and multiple rectifier systems can be placed closer to each other than air-cooled
power supplies; however, water-cooled systems are sensitive to contaminationand minerals in the supply water, and in these cases, the power supplies mayrequire periodic maintenance to clean the water passages and filters.
Pulse Plating
Direct current plating deposits metal utilizing a continuous application ofenergy, pulse-plating systems provide the opportunity to modulate the voltageor current to achieve different results. The application of gold, silver, and copperwith pulse plating results in finer grain structures, higher surface densities, andlower electrical resistance. Additionally, plating times can be reduced by up to50%. These characteristics make pulse plating attractive, if not mandatory, inthe electronics industry.From an industrial standpoint, pulse plating has found a number of importantapplications. For example, when used in chromium plating, pulse plating willresult in a harder, more wear-resistant surface. In a nickel plating application,using pulse plating may eliminate the need to add organic compounds to controlstress and will result in a brighter finish with better thickness control and reducedplating times.Many plating profiles are available, including standard pulse, superimposedpulse, duplex pulse, pulsed pulse, and pulse on pulse. These waveforms can beobtained from a unipolar power supply. Other variations, possible when usinga bipolar pulsing rectifier, include pulse reverse, pulse reverse with off time,pulsed pulse reverse, and pulse-on-pulse reverse. Fig. 3 illustrates a few of themany different pulse waveforms available. The pulsing profile you use will bedetermined by the type of plating finish desired, the makeup of the plating bath,and the type of power supply available.There are three basic types of power supply technologies employed to achieve pulsed outputs. The most common design consists of a standard SCR phasecontrolledrectifier with a semiconductor switch on the output. Although thissystem can be successfully employed in almost all pulsing applications, thereare some drawbacks, mainly the inherent limitations associated with pulse riseand fall times.When faster pulsing speeds or square waves are required, linear power suppliesare a viable technology. A linear design consists of a fixed output powersupply, followed by a parallel combination of field-effect transistor (FET) orbipolar transistors, with the exact configuration determined by the outputvoltage levels required. This bank of transistors determines the final output bypulsing the fixed DC supplied to it.The efficiency of a linear supply is generally less than that of a SCR phasecontroldesign, due to the fact that the rectification section always provides fullpower to the regulator, which must then dissipate the energy difference betweenfull power and the desired output voltage.On the other hand, linear designs are capable of providing virtually perfectsquare wave pulses, due to the ability of the transistors to cycle on and off rapidly.A reversing linear system can also provide transition through zero outputwith no dead time.A relatively new configuration, when compared with SCR and linear designs,is the switch mode power supply, more commonly known as a switcher.Although an SCR phase-controlled power supply technically is a switcher, practicalconsiderations usually limit pulse repetition rates to 12 times line frequency.Functionally, a switcher will typically start by rectifying the incoming linedirectly. This raw DC will then be chopped by a variable pulse width modulator,feeding the primary of a high-frequency transformer. The high-frequencytransformer performs the desired voltage/current transformation. The outputfrom the secondary of the transformer is then rectified and filtered.Switchers have a number of advantages over the other designs. Because of thehigher frequencies, both transformer and filter inductor sizes and weights canbe reduced, resulting in a more compact unit.Additionally, switchers have efficiencies comparable to that of phase-controlsystems. This is due to the fact that the semiconductors are either fully on(saturated) or off, as opposed to the linear supplies, where the semiconductorsare biased in the active region.Table I illustrates the relative merits of each design when considering rippleefficiency, bandwidth, physical size, and initial cost. The configuration that ismost suited to your application will depend on factors such as those. Contactyour power-supply manufacturer for additional information.Table I. Pulse Technology Comparison
ANODIZING
Direct Current Anodizing
As in the case of electroplating, there is a wide variety of anodizing processescurrently in use. Electroplating deposits a metal layer onto asubstrate, whichmay be a metal itself or some nonmetallic material such as plastic. Anodizing,on the other hand, is the conversion of the surface layer of a metal to an oxide.The metal most commonly anodized is aluminum, but other metals, such asmagnesium and titanium, can also be successfully anodized.Aluminum will naturally form an oxide layer when exposed to oxygen, but thisis a relatively thin layer. Anodizing provides a much thicker coating. Anodizedfinishes exhibit a number of desirable properties. They are capable of being processed
further to modify the appearance of the aluminum. For example, coloredfinishes are easily obtained by such techniques as dyeing or color anodizing.Anodizing also improves the wearability of aluminum. An anodized finish ismuch more resistant to abrasion than the base metal. Anodizing is also extensivelyused in environments where corrosion is a problem.A number of anodizing processes are employed for aluminum. The mostcommon is the sulfuric acid anodizing process. This provides a coating typically0.1 to 1.0 mil. thick and lends itself to further color processing. Other conventionalaluminum anodizing processes are those utilizing chromic acid (foundin marine and aircraft applications) and phosphoric acid (used as a surfacepreparation for adhesive bonding and as a base for electroplating).
These conventional anodizing processes require a DC power supply similar innature to those found in electroplating, except that the voltages typically usedin conventional anodizing (18-50 V) are higher than those commonly found inplating (6-18 V). Otherwise, the design of the rectifiers for DC electroplatingand DC anodizing is basically the same.Hard-coat anodizing is often employed in applications where a more abrasiveor corrosion-resistant oxide layer than that obtained with conventionalanodizing is desired. Hard-coat anodizing processes typically demand voltagesbetween 50 and 150 V, and in many cases, pulse power supplies are utilized toobtain specific results. As in electroplating, the pulse rectifiers are very similarin design, options, and usage.
Color Anodizing
Many architectural aluminum anodizing applications require that color beapplied to the finished product. Colored finishes are obtained through the useof dyeing, integral, or electrolytic color processes.Dyeing is a simple process. A dye bath is composed of water and dyeing material,and the anodized aluminum is placed in the dye bath for some minutes.After removal from the dye bath, the aluminum is then rinsed and sealed in anormal manner.Integral color is a process by which the color is produced during the conventionalanodizing process. Organic acids are added to the anodizing bath, and theseacids produce a color, ranging from amber through black, in the aluminum oxide.Standard DC rectifiers are used, though at a voltage approximately three timesthat found in sulfuric acid anodizing.The electrolytic or two-step process begins by conventional sulfuric acidanodizing using DC power. The parts are then placed into a coloring solution consisting of salts of various metals such as tin, nickel, and cobalt, and AC poweris applied. The AC current causes the deposition of metallic particles in thepores of the anodic coating. By varying the relative amplitudes and times of thepositive and negative half cycles of the AC output, numerous colors and finishcharacteristics can be obtained. The electrolytic coloring processes have becomepopular as they require less energy than competing methods.An ideal power supply for the two-step process will provide the opportunityto adjust the voltage and on-and-off times of the positive and negative portionsof the output independently. This provides the maximum amount of flexibilityto generate the broad range of colors available through electrolytic coloring.
COMPUTERIZATION
In the 1970s many metal finishers investigated modifications that would berequired to upgrade their rectifiers to computer control. At that time, however,the price and risk of automation was too high for most companies, forcing themto continue using manual control.Today, the importance of incorporating some degree of automation intothe metal-finishing processes is becoming more evident. For example, smallerfirms find themselves at a disadvantage when competing against larger, moreautomated companies, especially for jobs where the finished parts require precisecoating thickness and consistent finish qualities. Additionally, certain platingapplications require multiple layer applications to achieve the desired coatingthickness and surface quality. These multilayer processes demand extremelyaccurate and repeatable coatings.The major advantage of computer over manual control of a rectifier is thecomputer’s ability to repeat a particular operation or procedure time aftertime. Computers can perform a variety of different functions when integratedwith rectifiers. The computer can simultaneously monitor a number of outputcurrents and voltages, detailing them on a video-display terminal. It can alsomaintain those voltages and currents within designated parameters, therebycompensating for varying input voltage or load changes. The computer caneasily regulate pulsing and reversing power supplies. The computer replaces theswitches, meters, and potentiometers typically required for manual operation;yet a manual override is included in case of malfunction.The advantages of a simple computer package are easily seen. The first majorimprovement is in the consistency of a finished product. Due to the preciseapplication of power, the coating is exact from piece to piece, and this cansignificantly reduce rework and reject rates. Furthermore, a computer’s precisioncontrol of cycle times and rectifier operation can reduce power consumption,resulting in lower electricity bills. Finally, the computer can calculate andtransfer exact amounts of chemicals to finishing tanks, minimizing associatedmaterial costs and reducing waste and sludge-disposal expenditures.A computerized system should be custom designed for the specific application,regardless of the size of the finishing operation or the degree of automationdesired. Customization is the key to successful systems integration. The systemshould, however, be designed and constructed using standard components. Thisprocedure provides a system that exactly matches the needs of the user whileminimizing the initial cost.A computer control system typically consists of a number of basic componentgroups. The illustration in Fig. 4 shows the structure of a multiple rectifiercomputer control system. A review of each of the basic groups provides a betterunderstanding of how the system works as a whole.
The Rectifier
For a rectifier to be controlled by a computer, there must be a means for thecomputer to communicate with the rectifier. The rectifier must then be capableof modifying its operation to satisfy the requests of the computer. Typical commandssent from the computer to the rectifier include output voltage, outputcurrent, ramp timer, ramp rate, power on/off, and cycle start/stop.Additionally, information might be sent from the rectifier to the computer,for example, power status, output voltage, output current, interlock status, andcooling system operation.In some instances these signals will be transferred directly between the computerand the rectifier. In other cases there may be an intermediary computerthat processes some or all of the information. A third situation may arise inwhich there is a single board computer located in the rectifier itself that has thesingular role of operating the rectifier based on data from the control computer.Virtually any rectifier utilizing solid-state electronics to control the output canbe adapted to computer automation.
The Host Personal Computer
The host personal computer (PC) is the center of the automated system. It is typicallyconfigured around a PC compatible and can be enhanced by a wide variety ofperipherals. The host computer is the “brains” of the system, providing the input/output, storage, and communications capabilities needed for optimum operation.
Input Devices
In most cases a keyboard is used to enter information into the computer. Itallows an operator to change process data, load parameter profiles, or commenceor terminate plating cycles, along with other functions determined by the user.Most host PCs will include a floppy disk drive. Floppy disks may contain datasuch as profile information, system software updates, and security codes. Thefloppy disk can be programmed by a supervisor on a PC in his/her office, andthe disk can then be taken to the host PC and the data transferred.Another type of input device is a bar-code reader. A bar code consists of aseries of alternating black and white vertical bars that contain informationdefined by the user. A bar-code scanner is passed across the bar code to readit. The spacing and width of the bars determine the data contained therein.Information such as part number, process identification, vendor, and customerare typical examples of data that can be contained in a bar-code format.
Output Devices
A monitor to verify data being entered from one of the input devices is necessarywith any computerized system. Once a process is running, the monitorcan display a number of different screens. These screens can include processstatus, alarm conditions, rectifier operation, and virtually any other informationdesired by the user. It is quite possible for the computer to monitor,display, and control nonrectifier operations, such as bath heaters/coolers,bath agitators, and chemical feeders.A printer may be desired to obtain a hard copy of any of the data recordedor operations performed by the computer. This information can be used in anumber of different ways, from statistical process control to process tracking.
Data Storage
A means to store the operating system, control programming, process profiles,and operating data must be provided. The most economical data storage deviceis a hard disk, which should be located in the host computer. By using a harddisk a process profile can be retrieved almost instantaneously simply by callingup a code number or name. By using profiles from the computer to control themetal-finishing operation, as opposed to setting parameters manually by turningknobs and pushing buttons, consistency is maintained.Some method of backing up the data on the hard disk is mandatory. If, forexample, there is a power disruption or a failure of the computer, informationwill most likely be lost. If a regular backup is performed many hours ofreprogramming may be avoided by simply restoring the data from the backupdevice to the computer. Although floppy disks are commonly used for backup, astreaming tape system, which utilizes a removable tape cassette, is a much betteralternative, as all the data from a hard disk can usually be stored on one tape.
The Interface Controller
The interface controller acts as the translator between the computer and the rectifier.It receives commands from the computer and converts those commandsto a language the rectifier can understand. The rectifier transmits informationto the interface controller, which sends it to the computer. Both inputs to, andoutputs from, the interface controller come in digital signals over interfacecables. The interface controller may be situated in the computer itself, or it maybe a separate system located adjacent to the computer.
The Interface
To keep the equipment as standard as possible, the popular choices for interfacesare the RS-232 and RS-422. Each requires only a pair of shielded, twistedwires to transmit information. This significantly reduces the number of wiresneeded for a multiple-rectifier system, as the twisted pair simply connects fromthe interface controller to each rectifier in a sequential fashion. In other words,the same pair of wires goes to the first rectifier to the second to the third, andso on. This eliminates the many wires that are commonly found connectingremotely located control panels to rectifiers.
The Software
The software should consist of standard control packages modified to meet theuser’s specific requirements. A language such as Quick Basic, used on the hostcomputer, will provide the necessary operating speed for the host, along with theability to modify or upgrade the program easily at any point. Faster languages,such as assembly code, may be required for a microcomputer located on therectifier to control output waveforms adequately.
A Main Frame
A link between a main frame and the host computer is always a possibility,increasing the overall capability of the system. Such a link might be the first steptoward complete factory automation. Use of a main frame provides a means fordata from all parts of the finishing operation to be accumulated, correlated, anddisseminated to various departments.For many smaller and middle-sized operations, computer automation is
becoming financially feasible. Benefits include reductions in rework and rejectrates, in downtime, and in chemical costs. Additional savings could be realizedby the reduced power usage of a computer-controlled operation. In the nearfuture computer automation may very well be the key factor in whether certainmetal-finishing operations are profitable.
RECOMMENDED TEST EQUIPMENT
Aside from the usual hand tools usually found in a well-equipped industrial toolbox, the following are recommended tools for power supply troubleshooting:
1. A clamp-on AC ammeter
2. A digital volt-ohm meter (DVM)
3. A battery-operated oscilloscope
There are several options to consider when purchasing these instruments fortesting in an industrial environment. The clamp-on ammeter should be an ACdevice, as it will be used at currents up to 1,000 A AC. All exposed metal partsmust be sufficiently insulated to ensure safe use around 600 V AC equipment.An analog-type clamp-on ammeter is preferred over most digital ammeter types,unless the digital unit is sufficiently filtered to prevent display jitter when measuringincoming line AC. When buying a digital ammeter, one should test theinstrument on an operating power supply before making the final purchasedecision.The digital volt-ohm meter best suited for power-supply testing is batteryoperated and durably packaged so that it will stand up in an industrial environment.A heavy-duty rubber-covered case is best. To be the most useful, theDVM should have “true rms reading” capabilities. Make sure that the test leadsare equipped with heavy plastic leads and rated for 5,000 V DC service. TheDVM should have at least the following ranges: voltage of 10 mV to 1,000 VAC and DC, current of 1 to 10 mA AC and DC, and resistance of 0.1 ohm to10 megohm. Some additional features to look for are autoranging and/or adiode testing range, which measures the forward voltage of a diode rectifier. Analarm on some DVM instruments is a convenient means to measure continuityin cables and wire harnesses.The oscilloscope should be a high-quality, battery-operated portable instrument.Some models incorporate a built-in digital display, which allows one toobserve the power-supply output waveform while reading the DC operatingpoint and the AC ripple content at the output bus. Although an oscilloscopeis not always necessary, you will find it a convenient tool when making a quickcheck on an operating power supply to see if any further testing is necessary.Of these three electronic tools, the clamp-on ammeter is the first one youwill most likely use to measure the three-phase line current. The measurementpoint should be just after the main contactor, near the transformer inputterminals. This measurement can be performed at no load to determine themagnetizing current of the main transformer, which should be about 5% offull load rated line current. With a load on the DC output bus of the rectifier,the balance of the AC line current can be measured, and the three line currentsshould be within 10% of each other.The next instrument you may use is the DVM. It will allow you to verify thethree-phase, line-to-line input voltages at the thyristor regulator section justahead of the main transformer. If you then measure the line-to-line voltages onthe transformer side of the thyristors, you can determine if the thyristor regulatorpart of the system is feeding balanced voltages to the main transformer.The oscilloscope is valuable when performing fast maintenance checks on anumber of power supplies. The scope should be connected to the back of theoutput DC panel voltmeter. As the voltage control on the panel is increased,a waveform will appear that has six peaks and valleys for each cycle of the linefrequency. Each period is 16.6 milliseconds long. If any of the six major peaksis missing or the valleys are too wide, there is a serious problem in the powercircuit that must be investigated further.
BASIC TROUBLESHOOTINGThis section briefly describes some basic diagnostics to determine why a powersupply is not operating properly. Before starting any diagnostic test on a powersupply, you should obtain a copy of the electrical schematic drawings for theparticular equipment you are working on. On these drawings, you should be ableto identify the basic functional areas that make up virtually any rectifier. Thefour basic building blocks of a power supply are the following:
1. Electrical controls
2. AC power circuits
3. DC power circuits
4. Electronic controls
CAUTION: Only qualified personnel should attempt to service power supplyequipment. Dangerous and lethal voltages may be present.
The electrical controls provide simple low-power functions for the powersupply. You will notice such items as push buttons (stop, start), pilot lights,relays, timers, limit switches, flow switches, thermal switches, thermal overlayrelays (heaters), and other 120 V AC protective devices. These items are typicallydrawn in the familiar ladder diagram format. Diagnostics in this area will usuallyrequire the DVM to measure continuity or the presence of control voltagesat various components.To check for proper voltages at the low-power components, find the commonon the ladder diagram and attach the voltmeter to it in the actual circuit. With thecontrol power energized, you will be able to check the AC controls on the ladderdiagram and measure for the presence of an AC voltage at the corresponding pointin the actual circuit. This method is most useful when there is a loss of controlcircuit voltage that prevents a portion of the controls from working properly.When the missing voltage returns at a particular point in the circuit, this indicatesyou have just moved past the defective component, such as a contact, a terminal,an interlock, or a thermal switch. The faulty component can then be repaired orreplaced. You may find there is more than one bad part; so be sure to test all of thelow-power components.The AC power circuit is the portion of the power supply located between theAC input power terminals and the regulation thyristors at the primary of thethree-phase powertransformer (assuming a primary thyristor/secondary diode configuration).The components representing this AC power section are usually found near thecenter of the electrical schematic.The clamp-on ammeter is the diagnostic tool used in the AC power circuit.Place the ammeter around one of the incoming AC conductors. Operate thepower supply with no load and check that the magnetizing current of the maintransformer is no more than 5% of the full load rated line current, which is usuallyindicated on the electrical schematic. If this reading is correct, the next stepis to measure the line current with a load of parts in the process tank that willrequire full output of the power supply. Measure all three incoming lines andverify that the currents are balanced to within 10% from one phase to the next.If an imbalance is detected, there could be a fuse blown or a thyristor shorted,or the gate signal to some of the thyristors may be improper.To determine which of the above is the problem, use the DVM on a high ACvoltage range and measure the line-to-line AC voltages. Extreme care should beexercised when making line voltage measurements to prevent any metal partsfrom coming in contact with the live conductors. At the same time, protectiveeye wear should be used. Measure the line-to-line voltages at each of the thyristors,after the thyristor fuses. If all voltages are okay, no fuses are blown, and allcontactors and safety switches are working, next measure the line-to-line voltageat the output of the thyristors near the connection to the primary of the mainpower transformer. If these voltages are relatively balanced but reduced in value,the thyristor regulator is in proper working condition.If after testing both the electrical controls and the AC power sections youfind that everything is normal (i.e., no defective fuses or thyristors, all electricalcontrols functioning) except for unbalanced line currents, there may be a problemwith the main power transformer or the diode section on the low-voltagesecondary side of the transformer.The DC power section typically consists of diodes, output bus connections,and metering for output voltage and current (in a secondary thyristor configuration,you would find thyristors in place of diodes). Testing in this section ofthe power supply consists of locating shorted or open diodes and verifyingmetering calibrations. Because of the high currents that flow in the low-voltagediode busing, a loose connection will cause a great deal of heat to be generated,which will cause a discoloration of the copper bus bars. By physically inspectingthe DC power section in detail, some of these connection problems may belocated and repaired simply by cleaning. The clamp-on ammeter may be usefulfor moderate-sized diodes that are supplied with a flexible cable connectionfrom one side of the case.Diodes that are supplied with a flexible connection at one end of the casecan be checked with the clamp-on ammeter. Measure the current at each diodeby placing the clamp-on ammeter around the flexible lead. A diode that is openwill draw no current, whereas a diode that is shorted will draw excessive current.In either case, the diode should be replaced. As these diodes are removed, theDVM may be used on the diode range to verify that the diode being removed is,in fact, bad. A defective diode will read either open or shorted in both directions.The DVM may be also used to determine possible metering circuit defects.To check the power-supply voltmeter, measure the voltage across the outputterminals of the rectifier and the terminals at the back of the panel voltmeter.Compare these readings with that of the panel voltmeter. They should all agree.Current is typically determined by measuring the voltage drop across a precisionresistor placed at the output terminals known as a shunt. This voltagedrop at full output will typically be 50 mV. This low-level voltage signal has tobe multiplied by a factor before comparing it to the actual meter reading.The oscilloscope is useful in locating problems where complete diode circuitbranches have burned open and left a missing section in the wave shape; however,this may also be a symptom of thyristor problems on the primary of themain transformer. If the AC ripple component of the output is important tothe process, then an oscilloscope with a built-in true rms feature can be used toview the ripple waveform, as well as determine the AC to DC ratio of the rippleusing the AC and DC coupling of the scope.The electronics are the most complex part of the power supply. Electroniccircuits are usually indicated on schematics by boxes with terminal numbers andfunctions labeled along the edges. The DVM is commonly used in the testing ofthese electronic circuits to measure signal and control voltages. Although thereare many different types of electronic circuits, two are found in every power supplyand must function correctly for proper power supply operation. These arethe drive circuit and the firing circuit. In some cases, these will be on one circuitboard, whereas at other times, they will be on separate boards.The drive circuit is an analog amplifier circuit. It receives current and voltagereference signals from the operators ACC and AVC potentiometers. Thesecontrol signals will typically range between 0 and 2.5 V DC, depending on theposition of the operator controls. To check a typical drive circuit initially, verifythat there is 120 V AC on the power terminals and that there are reference voltageson the ACC and AVC input terminals. You should then have a voltage atthe output terminals. If no signal is available at these output terminals, the drivecircuit may be defective or seriously out of adjustment. Remove and furthertest the drive circuit using the test procedures found in your operators manual.The firing circuit accepts the output signals of the drive circuit and producessynchronized gate pulses that fire the thyristors in the AC power circuit, whichin turn regulates the voltage to the primary of the main power transformer. Totest this circuit, ensure there is a signal of more than 2V DC at the input fromthe firing circuit. Then measure the signals at the gate outputs to the thyristorswith the DVM. They should typically be about 1 V DC. Perform these measurementswith great care against shorting any of the leads to ground or to anotherpair of terminals, as there may be line voltages of up to 600 V AC between theseterminals and ground. As with the drive circuit, if any signals are missing orincorrect, remove the board and bench repair using the procedures outlined inthe operators handbook
BASIC REPAIRS
Once a defective component has been located, it should be replaced with a partof comparable quality and ratings. It is especially important when replacing temperaturesensors that the replacement have the same temperature rating as the original.Caution: Before attempting replacement of any component, ensure that thepower is removed from the rectifier and that the capacitors are discharged.
Electrical and Electronics Components
Replacement of electrical components, such as push buttons, thermal switches,relays, and switches, as well as electronic PC boards, is relatively straightforward.Carefully mark all connections to the defective device before removal, replacewith the correct item, and reattach the wires. It is also advisable to check the restof the rectifier for clean and correct connections at this time.
Thyristors and Diodes
Thyristors are typically found in modular, stud-mount, and flat-pack configurations,whereas diodes are usually the stud-mount or flat-pack style. Thereplacement procedures for stud-mount and flat-pack thyristors and diodesare virtually identical, with the difference being that thyristors will have twoadditional small leads to be attached.The modular thyristor is the smallest of the three types and is typically foundin lower power systems. The module contains two thyristors and has terminalsfor connecting gate and input/output leads. Mounting holes in the base allowattachment to the bus bar. To replace a modular thyristor, perform the followingsteps:
1. Note where the gate and input/output leads are attached.
2. Mark the leads and remove the thyristor.
3. Clean the bus bar surface and the new thyristor surface.
4. Apply heat sink compound sparingly to both surfaces.
5. Fasten the new thyristor to the bus bar.
6. Reattach the leads. Replacement is now complete.Stud-mount thyristors and diodes are no more difficult to replace. Studmountdevices can be mounted on either air- or water-cooled heat sinks and aretypically found with or in. diameter studs. Replacement of stud-mount devicesis the same for both air- and water-cooled systems, following the steps below:
1. Mark and remove the two signal leads from the terminal blocks (thyristor).One of these is the gate lead, and the other is the cathode signallead.
2. Remove the large braided cable.
3. Remove the nut and washers, and remove the device from the heat sink.
4. Clean the bus bar and new thyristor surfaces.
5. Spread a small amount of thermal compound on the new thyristor,taking care not to get any compound on the thyristor threads.
6. Insert the stud in the heat sink, reassemble the flat washer and the starwasher, and then tighten the retaining nut.
7. Attach all leads to the proper locations, being sure that all connectionsare clean and tight.
Flat-pack thyristors and diodes, sometimes referred to as “hockey pucks,” areused in higher power rectifiers. They range from 2 through 4 in. in diameter. Aswith the stud-mount devices, the only difference between a flat-pack thyristorand diode is the presence of gate and cathode leads on the device.A flat-pack device is secured between two current-carrying bus bars by aclamping mechanism. Some clamps have indicators built in, whereas othersdo not. When replacing a device secured with a gauged clamp, note the readingbefore removing the device.The other type of clamps used are either 5,000- or 10,000-lb clamps. Thesesystems consist of a pair of clamping bars, connected by two studs, betweenwhich is sandwiched the bus bars, a Belville washer system, and the semiconductordevice. Replacement of thyristors or diodes utilizing these types of clampsrequires the use of measuring devices. The following steps should be taken toreplace a flat-pack thyristor or diode (refer to Fig. 8).
1. Note the clamping arrangement being used. If a gauge is present on thelamp, record the indication. Mark and remove the gauge and cathodeleads if replacing a thyristor.
2. Uniformly and slowly loosen the nuts on the clamp studs. Remove theBelville washer assembly and the device. Note that the Belville washeris made up of four parts: a centering section, a flat washer, and twoconcave washers.
3. Clean the surfaces of both bus bars and the new thyristor or diode.Clean both clamping bars, and check that the insulated surfaces of theclamp have not been damaged.
4. Apply heat sink compound sparingly to both surfaces of the device andto the bus bars.
5. Place the new flat pack in the clamping mechanism, ensuring that thedevice is oriented properly. Check the other devices to verify this. Thereare typically roll pins in the bus bars that align with depressions in thedevice. Make sure the roll pins do not damage the flat-pack surfaces.
6. Reassemble the Belville washers as shown, making sure the two concavewashers are back to back. Now place the washers in the clamp.
7. Finger tighten the clamp nuts, ensuring all parts are situated properly,and tighten the nuts with a wrench one-quarter additional turn. Check
that approximately the same number of threads are visible beyond thenuts on each stud.
8. Using a depth gauge, measure through the center of the hole in the busbar and Belville washer system. Note this reading.
9. Tighten each nut one-half turn, and recheck with the depth gauge.Continue this tightening procedure until the difference from the originalreading is 0.048 ± 0.004 in. for a 10,000 lb clamp, and 0.026 ± 0.002in. for a 5,000 lb clamp.
10. Reattach the gate and cathode thyristor leads.
PREVENTIVE MAINTENANCE
Nothing is more important to rectifier reliability and longevity than a consistentprogram of preventive maintenance. The efforts expended in taking periodiccare of any equipment, especially those operated in the aggressive environmentstypically found in metal-finishing processes, will be returned many times over.The following provides a brief outline of the minimum maintenance thatshould be performed every month and every 6 months. The program you implementshould take into consideration the number of rectifiers, how many shifts,what type of processes, and the duty cycles of your particular operation.
Monthly
1. Ensure that all doors and panels are on the rectifiers and that the area aroundthe rectifier is free and clear of items that would hinder proper airflow oroperation.
2. On air-cooled systems, wash or replace the air filters. Refrain fromusing inexpensive cardboard framed filters, as the thin metal facingcan quickly deteriorate and be drawn into the rectifier. Also, check thatthe fan blades are secured to the fan motor shafts and that they runwithout vibration.
3. On water-cooled systems, remove and clean or replace the inlet waterstrainer. Check all water lines for signs of leaks or contamination accumulations.If contamination is evident, determine the source and correctif possible.
4. Check panel gaskets and repair or replace as necessary.
5. Check components such as pilot lights, switches, push buttons, etc., forproper operation and replace as required.
EVERY 6 MONTHS
1. Check writing and bus connections for tightness and cleanliness. Repairas required.
2. Clean semiconductors and heat sinks. Dirty and corroded heat sinkscan significantly increase the operating temperatures of the semiconductors and reduce the life of the rectifier
