- Email: [email protected]
Common rectifier technologies for metal-finishing applications
Common Rectifier Technologies for Metal-Finishing Applications by Christopher
C. Smith
Rapid Power Technologies
Inc., Brookfield,
Conn.
ectifiers form an integral part of virtually all metal-finishing processes. Throughout the .years, new devices have been introduced, allowing- new technologies and flexibilities to be enjoyed by metal finishers; however, when cost and performance issues are considered, older technologies may still be best suited for certain applications. A direct current (DC) power supply or rectifier consists of three main components: the main stepdown/isolation main transformer, semiconductors, and controls. The main transformer converts the incoming high-voltage, low-current alternating current (AC) to low-voltage, high-current AC. The semiconductors [transistors, diodes and/or thyristors (SCRs)] convert the AC to DC. The controls allow the operator to vary the DC output to a desired level. There are two means of output control: unregulated and regulated. Unregulated controls are those that allow the operator to change the output voltage, but the rectifier does not monitor the output, nor automatically adjust for changes in the output caused by input voltage variations or load changes. Examples of unregulated rectifiers are tap-switch and variable-transformer designs. Regulated controls are those that continuously monitor the output voltage and current and automatically adjust the output to maintain the level selected by the operator. The SCR rectifier is the most common type of regulated rectifier.
R
TAP-SWITCH
RECTIFIERS
The most basic rectifier found in metal finishing is the tap-switch rectifier. The DC output of a tap-switch rectifier is varied by changing the position of the tap-switch knobs on the front of the rectifier. Standard designs feature three 8-position tap switches, allowing 22 different voltage outputs. The AC enters the rectifier and passes through the tap switches to various taps on the transformer. The transformer converts the high-voltage AC to low-voltage AC. Finally, diodes convert the AC to DC. The advantages of tap-switch rectifiers are low initial cost when compared with SCR rectifiers; 5% October 2001
ripple over the output range (with all tap switches set at same position); and they are ideal for electrocleaning and other noncritical metal-finishing applications. The disadvantages of tap-switch rectifiers are no automatic output regulation; no remote control capability; tap switches can degrade over time if not properly maintained; and only 22 steps of output voltage adjustment. A step above the tap-switch rectifier is the variable transformer controlled rectifier. The DC output of a variable-transformer rectifier is varied by changing the position of the control knob on the front of the rectifier, which changes the AC voltage applied to the main transformer. The AC enters the rectifier and passes through the variable transformer to the main transformer. The main transformer converts the high-voltage AC to low-voltage AC. As with the tap-switch rectifier, diodes convert the AC to DC. Where the tap-switch-type rectifier had discreet output steps, corresponding to the step positions of the tap switches, the variable-transformer design allows smooth, continuous adjustment of the output DC. A special version, called a motor-driven variabletransformer rectifier, uses a motor to change the position of the variable transformer. This configuration allows remote control of the rectifier output from a separate operator’s panel. The advantages of variable-transformer rectifiers are low to moderate initial cost when compared with SCR rectifiers; 5% ripple over the output range; stepless voltage adjustment; available with motor drive for remote control; and they are ideal for electrocleaning and other noncritical metal-finishing applications where more precise output adjustment is required. The disadvantages of variable-transformer rectifiers are no automatic output regulation; variabletransformer brushes can degrade over time if not properly maintained; and they are limited in power capacity. SCR RECTIFIERS
SCR rectifiers are the most common type of metalfinishing power supply available today. In general, 51
they offer the highest power in the smallest cabinet size, and can easily be air or liquid cooled. Most importantly, they offer precise, automatic regulation of the output voltage or current. The DC output of a SCR rectifier is varied by changing the position of control knobs, typically mounted in a small, remote-control enclosure. This remote can be mounted at the plating tank for ease of operator use. The controls electronically vary the DC output by determining when the SCRs allow current to pass through them. SCR remote controls are available in either standard analog or optional digital configurations. SCR rectifiers are available in two configurations: primary SCR-secondary diode, and secondary SCR. In the primary SCR configuration the AC enters the rectifier and passes through the SCRs and then to the main transformer. The main transformer converts the high-voltage AC to low-voltage AC. The diodes convert the AC to DC (rectification), and the SCRs control the level of the DC output (regulation). In the secondary SCR configuration the AC entering the rectifier is connected directly to the main transformer. Once again, the main transformer converts the high-voltage AC to low-voltage AC. Finally, the SCRs control the DC output (regulation) and also convert the AC to DC (rectification). The advantages of SCR rectifiers are many: automatic output regulation to 20.5% of rated output; remote control is standard on most configurations; interfacing with remote PLCs or computers is easily accomplished; available with many different cooling options (air, water, air-air, air-water, water-air, water-water); and they are available with numerous options including variable output ramp, membrane digital control panel; touch-screen digital-control panel, pulsed output, polarity reversing output, and automatic current density compensation. SCR rectifiers are ideal for applications that require highpower or precise, consistent output. The disadvantages of SCR rectifiers are few: output ripple increases as voltage is reduced (can be corrected with optional ripple filter) and higher initial cost (when compared with smaller capacity, unregulated rectifiers). SWITCH-MODE
RECTIFIERS
A relatively new technology was introduced to the metal-finishing industry about 5 years ago; the switch-mode rectifier. The switch-mode rectifier, commonly known as a “switcher,” uses transistors instead of SCRs to control the DC output. It also operates at higher frequencies (up to about 40 kHz1, whereas SCR rectifiers operate at line frequency (50
Hz or 60 Hz). Operating at such high frequencies allows switch-mode rectifiers to offer a high amount of power in a small package. Switch-mode rectifiers are available in air or water-cooled versions. The advantages of switch-mode rectifiers are automatic output regulation to -i-0.25% of rated output; less than 1% ripple over the output range; remote control is optional on most configurations; interfacing with remote PLCs or computers is easily accomplished; much smaller and lighter than all other technologies; and it is also available with numerous options including variable output ramp, membrane digital control panel, touch-screen digital-control panel, pulsed output, and polarity reversing output. Switch-mode rectifiers are ideal for applications that require precise, consistent output, or where floor space is at a premium. The disadvantages of switch-mode rectifiers are limited output voltage and current ratings. WHICH TYPE OF RECTIFIER IS BEST FOR YOU?
Before deciding which type is rating must be determined to applicable. If the rating is such that available (usually below 3,000 the application.
best, the DC output see which types are all of the types are A), the next issue is
Is the Plating Thickness Critical? If plating thickness is critical the tap-switch and variable-transformer rectifiers should be eliminated, since they do not offer automatic regulation. Automatic regulation is imperative for precise and consistent plating from batch to batch or part to part; therefore, an SCR or switch-mode rectifier should be selected. If plating thickness is not critical then a tapswitch or variable-transformer rectifier should suffice. Do You Need Remote Control? If so, then tap-switch and manual variable-transformer rectifiers should not be considered. Alternatives are motor-driven variable-transformer, SCR, or switch-mode technologies. Is Output Ripple a Concern? If plating chrome, acid copper, or nickel, or if electropolishing, chances are that ripple of 5% or less is necessary. With a tap-switch or variable-transformer rectifier, the ripple is inherently less than 5%, and with a switch-mode rectifier, it is less than 1%. With an SCR rectifier, the ripple is approximately 5% at full-rated output but increases as the output
voltage is reduced. So, if an SCR rectifier is needed for the remote control and automatic regulation features but ripple is a concern, a ripple-reduction filter will be needed to limit the output ripple to an acceptable level.
considerably more efficient than those of 15 or more years ago. Other operating costs are those associated with water usage and treatment if water-cooled rectifiers are employed.
Where Will the Rectifier be Located? The physical location of the rectifier, as well as the cooling requirements, will determine the type desired. If the rectifier will be located in an area with highly corrosive fumes or splashing liquids, air cooling is not recommended. Instead, the rectifier should be a sealed, water-cooled design, which is much less susceptible to damage from harsh environments.
Maintenance Costs All rectifiers require some degree of periodic maintenance to ensure proper and reliable operation. Because of the electromechanical nature of tap-switch and variable-transformer rectifiers, they experience somewhat higher maintenance costs than SCR and switch-mode power supplies. Finally, the overall value must be considered. The best value is not always the lowest initial cost. If a low-cost unit is purchased at the cost of compromising performance, that is certainly not a good value. By the same token, if the performance of a low-cost unit is sufficient to do the proper job, it would also be a poor value to purchase a higher-cost unit.
What About Costs? The actual overall cost than the amount paid costs include the initial and maintenance costs
of a rectifier is much more when it is purchased. Total cost as well as the operating for the life of the rectifier.
Initial Costs In the range of 50 to 500 A the variable-transformer design is usually the lowest price alternative. In the range of 500 to 5,000 A the tap-switch is cost effective. The motor-driven variable-transformer design is moderately priced in the 50- to 2,000-A range. Switch-mode rectifiers in the lo- to 1,000-A range are considered moderate to high cost,, and the SCR rectifier generally has the highest initial cost of all types. Operating Costs Operating costs are primarily comprised of utility costs, typically tho‘se charges levied by power companies for energy demand and usage. Utility costs are primarily a function of the power delivered and the efficiency of the rectifier. Today’s rectifiers are
IN STOCK-READY
TO SHIP-5
YEAR COMPRESSOR
CONCLUSION
There are a number of different rectifier technologies still widely used. When considering a new rectifier, a careful review of the application and detailed discussion with a qualified rectifier vendor will ensure the best possible equipment is selected. BIOGRAPHY
Christopher C. Smith earned a BSE from Florida Technological University (University of Central Florida) in 1973. He has been involved in power supply applications for semiconductor fabrication equipment and metal-finishing processes since 1981. He joined Rapid Power Technologies in 1996. NW
WARRANTY.
ZARSKY WATER CHILLERS 800-473-9178 * 713-462-2500 FAX 713-462-2544 E-mail: [email protected] www.waterchiller.s.com
F.O.B. Houston, TX
50 ton 60 ton 80 ton
II!!!iw
$251670 $29,418 $36,205
l
ALL MAJOR CREDIT
PLANS AVAILABLE
Circle 066 on reader card or go to www.thru.to/webconnect October 2001
53
C. Smith
Rapid Power Technologies
Inc., Brookfield,
Conn.
ectifiers form an integral part of virtually all metal-finishing processes. Throughout the .years, new devices have been introduced, allowing- new technologies and flexibilities to be enjoyed by metal finishers; however, when cost and performance issues are considered, older technologies may still be best suited for certain applications. A direct current (DC) power supply or rectifier consists of three main components: the main stepdown/isolation main transformer, semiconductors, and controls. The main transformer converts the incoming high-voltage, low-current alternating current (AC) to low-voltage, high-current AC. The semiconductors [transistors, diodes and/or thyristors (SCRs)] convert the AC to DC. The controls allow the operator to vary the DC output to a desired level. There are two means of output control: unregulated and regulated. Unregulated controls are those that allow the operator to change the output voltage, but the rectifier does not monitor the output, nor automatically adjust for changes in the output caused by input voltage variations or load changes. Examples of unregulated rectifiers are tap-switch and variable-transformer designs. Regulated controls are those that continuously monitor the output voltage and current and automatically adjust the output to maintain the level selected by the operator. The SCR rectifier is the most common type of regulated rectifier.
R
TAP-SWITCH
RECTIFIERS
The most basic rectifier found in metal finishing is the tap-switch rectifier. The DC output of a tap-switch rectifier is varied by changing the position of the tap-switch knobs on the front of the rectifier. Standard designs feature three 8-position tap switches, allowing 22 different voltage outputs. The AC enters the rectifier and passes through the tap switches to various taps on the transformer. The transformer converts the high-voltage AC to low-voltage AC. Finally, diodes convert the AC to DC. The advantages of tap-switch rectifiers are low initial cost when compared with SCR rectifiers; 5% October 2001
ripple over the output range (with all tap switches set at same position); and they are ideal for electrocleaning and other noncritical metal-finishing applications. The disadvantages of tap-switch rectifiers are no automatic output regulation; no remote control capability; tap switches can degrade over time if not properly maintained; and only 22 steps of output voltage adjustment. A step above the tap-switch rectifier is the variable transformer controlled rectifier. The DC output of a variable-transformer rectifier is varied by changing the position of the control knob on the front of the rectifier, which changes the AC voltage applied to the main transformer. The AC enters the rectifier and passes through the variable transformer to the main transformer. The main transformer converts the high-voltage AC to low-voltage AC. As with the tap-switch rectifier, diodes convert the AC to DC. Where the tap-switch-type rectifier had discreet output steps, corresponding to the step positions of the tap switches, the variable-transformer design allows smooth, continuous adjustment of the output DC. A special version, called a motor-driven variabletransformer rectifier, uses a motor to change the position of the variable transformer. This configuration allows remote control of the rectifier output from a separate operator’s panel. The advantages of variable-transformer rectifiers are low to moderate initial cost when compared with SCR rectifiers; 5% ripple over the output range; stepless voltage adjustment; available with motor drive for remote control; and they are ideal for electrocleaning and other noncritical metal-finishing applications where more precise output adjustment is required. The disadvantages of variable-transformer rectifiers are no automatic output regulation; variabletransformer brushes can degrade over time if not properly maintained; and they are limited in power capacity. SCR RECTIFIERS
SCR rectifiers are the most common type of metalfinishing power supply available today. In general, 51
they offer the highest power in the smallest cabinet size, and can easily be air or liquid cooled. Most importantly, they offer precise, automatic regulation of the output voltage or current. The DC output of a SCR rectifier is varied by changing the position of control knobs, typically mounted in a small, remote-control enclosure. This remote can be mounted at the plating tank for ease of operator use. The controls electronically vary the DC output by determining when the SCRs allow current to pass through them. SCR remote controls are available in either standard analog or optional digital configurations. SCR rectifiers are available in two configurations: primary SCR-secondary diode, and secondary SCR. In the primary SCR configuration the AC enters the rectifier and passes through the SCRs and then to the main transformer. The main transformer converts the high-voltage AC to low-voltage AC. The diodes convert the AC to DC (rectification), and the SCRs control the level of the DC output (regulation). In the secondary SCR configuration the AC entering the rectifier is connected directly to the main transformer. Once again, the main transformer converts the high-voltage AC to low-voltage AC. Finally, the SCRs control the DC output (regulation) and also convert the AC to DC (rectification). The advantages of SCR rectifiers are many: automatic output regulation to 20.5% of rated output; remote control is standard on most configurations; interfacing with remote PLCs or computers is easily accomplished; available with many different cooling options (air, water, air-air, air-water, water-air, water-water); and they are available with numerous options including variable output ramp, membrane digital control panel; touch-screen digital-control panel, pulsed output, polarity reversing output, and automatic current density compensation. SCR rectifiers are ideal for applications that require highpower or precise, consistent output. The disadvantages of SCR rectifiers are few: output ripple increases as voltage is reduced (can be corrected with optional ripple filter) and higher initial cost (when compared with smaller capacity, unregulated rectifiers). SWITCH-MODE
RECTIFIERS
A relatively new technology was introduced to the metal-finishing industry about 5 years ago; the switch-mode rectifier. The switch-mode rectifier, commonly known as a “switcher,” uses transistors instead of SCRs to control the DC output. It also operates at higher frequencies (up to about 40 kHz1, whereas SCR rectifiers operate at line frequency (50
Hz or 60 Hz). Operating at such high frequencies allows switch-mode rectifiers to offer a high amount of power in a small package. Switch-mode rectifiers are available in air or water-cooled versions. The advantages of switch-mode rectifiers are automatic output regulation to -i-0.25% of rated output; less than 1% ripple over the output range; remote control is optional on most configurations; interfacing with remote PLCs or computers is easily accomplished; much smaller and lighter than all other technologies; and it is also available with numerous options including variable output ramp, membrane digital control panel, touch-screen digital-control panel, pulsed output, and polarity reversing output. Switch-mode rectifiers are ideal for applications that require precise, consistent output, or where floor space is at a premium. The disadvantages of switch-mode rectifiers are limited output voltage and current ratings. WHICH TYPE OF RECTIFIER IS BEST FOR YOU?
Before deciding which type is rating must be determined to applicable. If the rating is such that available (usually below 3,000 the application.
best, the DC output see which types are all of the types are A), the next issue is
Is the Plating Thickness Critical? If plating thickness is critical the tap-switch and variable-transformer rectifiers should be eliminated, since they do not offer automatic regulation. Automatic regulation is imperative for precise and consistent plating from batch to batch or part to part; therefore, an SCR or switch-mode rectifier should be selected. If plating thickness is not critical then a tapswitch or variable-transformer rectifier should suffice. Do You Need Remote Control? If so, then tap-switch and manual variable-transformer rectifiers should not be considered. Alternatives are motor-driven variable-transformer, SCR, or switch-mode technologies. Is Output Ripple a Concern? If plating chrome, acid copper, or nickel, or if electropolishing, chances are that ripple of 5% or less is necessary. With a tap-switch or variable-transformer rectifier, the ripple is inherently less than 5%, and with a switch-mode rectifier, it is less than 1%. With an SCR rectifier, the ripple is approximately 5% at full-rated output but increases as the output
voltage is reduced. So, if an SCR rectifier is needed for the remote control and automatic regulation features but ripple is a concern, a ripple-reduction filter will be needed to limit the output ripple to an acceptable level.
considerably more efficient than those of 15 or more years ago. Other operating costs are those associated with water usage and treatment if water-cooled rectifiers are employed.
Where Will the Rectifier be Located? The physical location of the rectifier, as well as the cooling requirements, will determine the type desired. If the rectifier will be located in an area with highly corrosive fumes or splashing liquids, air cooling is not recommended. Instead, the rectifier should be a sealed, water-cooled design, which is much less susceptible to damage from harsh environments.
Maintenance Costs All rectifiers require some degree of periodic maintenance to ensure proper and reliable operation. Because of the electromechanical nature of tap-switch and variable-transformer rectifiers, they experience somewhat higher maintenance costs than SCR and switch-mode power supplies. Finally, the overall value must be considered. The best value is not always the lowest initial cost. If a low-cost unit is purchased at the cost of compromising performance, that is certainly not a good value. By the same token, if the performance of a low-cost unit is sufficient to do the proper job, it would also be a poor value to purchase a higher-cost unit.
What About Costs? The actual overall cost than the amount paid costs include the initial and maintenance costs
of a rectifier is much more when it is purchased. Total cost as well as the operating for the life of the rectifier.
Initial Costs In the range of 50 to 500 A the variable-transformer design is usually the lowest price alternative. In the range of 500 to 5,000 A the tap-switch is cost effective. The motor-driven variable-transformer design is moderately priced in the 50- to 2,000-A range. Switch-mode rectifiers in the lo- to 1,000-A range are considered moderate to high cost,, and the SCR rectifier generally has the highest initial cost of all types. Operating Costs Operating costs are primarily comprised of utility costs, typically tho‘se charges levied by power companies for energy demand and usage. Utility costs are primarily a function of the power delivered and the efficiency of the rectifier. Today’s rectifiers are
IN STOCK-READY
TO SHIP-5
YEAR COMPRESSOR
CONCLUSION
There are a number of different rectifier technologies still widely used. When considering a new rectifier, a careful review of the application and detailed discussion with a qualified rectifier vendor will ensure the best possible equipment is selected. BIOGRAPHY
Christopher C. Smith earned a BSE from Florida Technological University (University of Central Florida) in 1973. He has been involved in power supply applications for semiconductor fabrication equipment and metal-finishing processes since 1981. He joined Rapid Power Technologies in 1996. NW
WARRANTY.
ZARSKY WATER CHILLERS 800-473-9178 * 713-462-2500 FAX 713-462-2544 E-mail: [email protected] www.waterchiller.s.com
F.O.B. Houston, TX
50 ton 60 ton 80 ton
II!!!iw
$251670 $29,418 $36,205
l
ALL MAJOR CREDIT
PLANS AVAILABLE
Circle 066 on reader card or go to www.thru.to/webconnect October 2001
53