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How Variable Freq. Drives Work

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How Variable-Frequency Drives Operate

A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.[1][2][3] A variable frequency drive is a specific type of adjustable-speed drive. Variable-frequency drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, microdrives or inverter drives. Since the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives.

Variable-frequency drives are widely used. In ventilation systems for large buildings, variable-frequency motors on fans save energy by allowing the volume of air moved to match the system demand. They are also used on pumps, elevator, conveyor and machine tool drives.

VFD types

All VFDs use their output devices (IGBTs, transistors, thyristors) only as switches, turning them only on or off. Using a linear device such as a transistor in its linear mode is impractical for a VFD drive, since the power dissipated in the drive devices would be about as much as the power delivered to the load.

Drives can be classified as:

  • Constant voltage
  • Constant current
  • Cycloconverter

In a constant voltage converter, the intermediate DC link voltage remains approximately constant during each output cycle. In constant current drives, a large inductor is placed between the input rectifier and the output bridge, so the current delivered is nearly constant. A cycloconverter has no input rectifier or DC link and instead connects each output terminal to the appropriate input phase.

The most common type of packaged VF drive is the constant-voltage type, using pulse width modulation to control both the frequency and effective voltage applied to the motor load.

VFD system description

VFD system

A variable frequency drive system generally consists of an AC motor, a controller and an operator interface.[4][5]

VFD motor

The motor used in a VFD system is usually a three-phase induction motor. Some types of single-phase motors can be used, but three-phase motors are usually preferred. Various types of synchronous motors offer advantages in some situations, but induction motors are suitable for most purposes and are generally the most economical choice. Motors that are designed for fixed-speed operation are often used. Certain enhancements to the standard motor designs offer higher reliability and better VFD performance, such as MG-31 rated motors.[6]

VFD controller

Variable frequency drive controllers are solid state electronic power conversion devices. The usual design first converts AC input power to DC intermediate power using a rectifier or converter bridge. The rectifier is usually a three-phase, full-wave-diode bridge. The DC intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching circuit. The inverter circuit is probably the most important section of the VFD, changing DC energy into three channels of AC energy that can be used by an AC motor. These units provide improved power factor, less harmonic distortion, and low sensitivity to the incoming phase sequencing than older phase controlled converter VFD’s. Since incoming power is converted to DC, many units will accept single-phase as well as three-phase input power (acting as a phase converter as well as a speed controller); however the unit must be derated when using single phase input as only part of the rectifier bridge is carrying the connected load.[7]

As new types of semiconductor switches have been introduced, these have promptly been applied to inverter circuits at all voltage and current ratings for which suitable devices are available. Introduced in the 1980s, the insulated-gate bipolar transistor (IGBT) became the device used in most VFD inverter circuits in the first decade of the 21st century.[8][9][10]

AC motor characteristics require the applied voltage to be proportionally adjusted whenever the frequency is changed in order to deliver the rated torque. For example, if a motor is designed to operate at 460 volts at 60 Hz, the applied voltage must be reduced to 230 volts when the frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to a constant value (460/60 = 7.67 V/Hz in this case). For optimum performance, some further voltage adjustment may be necessary especially at low speeds, but constant volts per hertz is the general rule. This ratio can be changed in order to change the torque delivered by the motor.[11]

In addition to this simple volts per hertz control more advanced control methods such as vector control and direct torque control (DTC) exist. These methods adjust the motor voltage in such a way that the magnetic flux and mechanical torque of the motor can be precisely controlled.

The usual method used to achieve variable motor voltage is pulse-width modulation (PWM). With PWM voltage control, the inverter switches are used to construct a quasi-sinusoidal output waveform by a series of narrow voltage pulses with pseudosinusoidal varying pulse durations.[8][12]

Operation of the motors above rated name plate speed (base speed) is possible, but is limited to conditions that do not require more power than nameplate rating of the motor. This is sometimes called “field weakening” and, for AC motors, means operating at less than rated volts/hertz and above rated name plate speed. Permanent magnet synchronous motors have quite limited field weakening speed range due to the constant magnet flux linkage. Wound rotor synchronous motors and induction motors have much wider speed range. For example, a 100 hp, 460 V, 60 Hz, 1775 RPM (4 pole) induction motor supplied with 460 V, 75 Hz (6.134 V/Hz), would be limited to 60/75 = 80% torque at 125% speed (2218.75 RPM) = 100% power.[13] At higher speeds the induction motor torque has to be limited further due to the lowering of the breakaway torque of the motor. Thus rated power can be typically produced only up to 130…150 % of the rated name plate speed. Wound rotor synchronous motors can be run even higher speeds. In rolling mill drives often 200…300 % of the base speed is used. Naturally the mechanical strength of the rotor and lifetime of the bearings is also limiting the maximum speed of the motor. It is recommended to consult the motor manufacturer if more than 150 % speed is required by the application.

PWM VFD Output Voltage Waveform

An embedded microprocessor governs the overall operation of the VFD controller. The main microprocessor programming is in firmware that is inaccessible to the VFD user. However, some degree of configuration programming and parameter adjustment is usually provided so that the user can customize the VFD controller to suit specific motor and driven equipment requirements.[8]

VFD operator interface

The operator interface provides a means for an operator to start and stop the motor and adjust the operating speed. Additional operator control functions might include reversing and switching between manual speed adjustment and automatic control from an external process control signal. The operator interface often includes an alphanumeric display and/or indication lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit is often provided on the front of the VFD controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the VFD controller. Most are also provided with input and output (I/O) terminals for connecting pushbuttons, switches and other operator interface devices or control signals. A serial communications port is also often available to allow the VFD to be configured, adjusted, monitored and controlled using a computer.[8][14][15]

VFD operation

When an induction motor is connected to a full voltage supply, it draws several times (up to about 6 times) its rated current. As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed.

By contrast, when a VFD starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Thus starting at such a low frequency avoids the high inrush current that occurs when a motor is started by simply applying the utility (mains) voltage by turning on a switch. After the start of the VFD, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop 150% of its rated torque while the VFD is drawing less than 50% of its rated current from the mains in the low speed range. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed.[16] Note, however, that cooling of the motor is usually not good in the low speed range. Thus running at low speeds even with rated torque for long periods is not possible due to overheating of the motor. If continuous operation with high torque is required in low speeds an external fan is usually needed. The manufacturer of the motor and/or the VFD should specify the cooling requirements for this mode of operation.

In principle, the current on the motor side is in direct proportion of the torque that is generated and the voltage on the motor is in direct proportion of the actual speed, while on the network side, the voltage is constant, thus the current on line side is in direct proportion of the power drawn by the motor, that is U.I or C.N where C is torque and N the speed of the motor (we shall consider losses as well, neglected in this explanation).

(1) n stands for network (grid) and m for motor

(2) C stands for torque [Nm], U for voltage [V], I for current [A], and N for speed [rad/s]

We neglect losses for the moment :

Un.In = Um.Im (same power drawn from network and from motor)

Um.Im = Cm.Nm (motor mechanical power = motor electrical power)

Given Un is a constant (network voltage) we conclude : In = Cm.Nm/Un That is “line current (network) is in direct proportion of motor power”.

With a VFD, the stopping sequence is just the opposite as the starting sequence. The frequency and voltage applied to the motor are ramped down at a controlled rate. When the frequency approaches zero, the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can be obtained by adding a braking circuit (resistor controlled by a transistor) to dissipate the braking energy. With 4-quadrants rectifiers (active-front-end), the VFD is able to brake the load by applying a reverse torque and reverting the energy back to the network.

Power line harmonics

While PWM allows for nearly sinusoidal currents to be applied to a motor load, the diode rectifier of the VFD takes roughly square-wave current pulses out of the AC grid, creating harmonic distortion in the power line voltage. When the VFD load size is small and the available utility power is large, the effects of VFD systems slicing small chunks out of AC grid generally go unnoticed. Further, in low voltage networks the harmonics caused by single phase equipment such as computers and TVs are such that they are partially cancelled by three-phase diode bridge harmonics.

However, when either a large number of low-current VFDs, or just a few very large-load VFDs are used, they can have a cumulative negative impact on the AC voltages available to other utility customers in the same grid.

When the utility voltage becomes misshapen and distorted the losses in other loads such as normal AC motors are increased. This may in the worst case lead to overheating and shorter operation life. Also substation transformers and compensation capacitors are affected, the latter especially if resonances are aroused by the harmonics.

In order to limit the voltage distortion the owner of the VFDs may be required to install filtering equipment to smooth out the irregular waveform. Alternately, the utility may choose to install filtering equipment of its own at substations affected by the large amount of VFD equipment being used. In high power installations decrease of the harmonics can be obtained by supplying the VSDs from transformers that have different phase shift.[17]

Further, it is possible to use instead of the diode rectifier a similar transistor circuit that is used to control the motor. This kind of rectifier is called active infeed converter in IEC standards. However, manufacturers call it by several names such as active rectifier, ISU (IGBT Supply Unit), AFE (Active Front End) or four quadrant rectifier. With PWM control of the transistors and filter inductors in the supply lines the AC current can be made nearly sinusoidal. Even better attenuation of the harmonics can be obtained by using an LCL (inductor-capacitor-inductor) filter instead of single three-phase filter inductor.

Additional advantage of the active infeed converter over the diode bridge is its ability to feed back the energy from the DC side to the AC grid. Thus no braking resistor is needed and the efficiency of the drive is improved if the drive is frequently required to brake the motor.

Application considerations

The output voltage of a PWM VFD consists of a train of pulses switched at the carrier frequency. Because of the rapid rise time of these pulses, transmission line effects of the cable between the drive and motor must be considered. Since the transmission-line impedance of the cable and motor are different, pulses tend to reflect back from the motor terminals into the cable. The resulting voltages can produce up to twice the rated line voltage for long cable runs, putting high stress on the cable and motor winding and eventual insulation failure. Increasing the cable or motor size/type for long runs and 480v or 600v motors will help offset the stresses imposed upon the equipment due to the VFD (modern 230v single phase motors not effected). At 460 V, the maximum recommended cable distances between VFDs and motors can vary by a factor of 2.5:1. The longer cables distances are allowed at the lower Carrier Switching Frequencies (CSF) of 2.5 kHz. The lower CSF can produce audible noise at the motors. For applications requiring long motor cables VSD manufacturers usually offer du/dt filters that decrease the steepness of the pulses. For very long cables or old motors with insufficient winding insulation more efficient sinus filter is recommended. Expect the older motor’s life to shorten. Purchase VFD rated motors for the application.

Further, the rapid rise time of the pulses may cause trouble with the motor bearings. The stray capacitance of the windings provide paths for high frequency currents that close through the bearings. If the voltage between the shaft and the shield of the motor exceeds few volts the stored charge is discharged as a small spark. Repeated sparking causes erosion in the bearing surface that can be seen as fluting pattern. In order to prevent sparking the motor cable should provide a low impedance return path from the motor frame back to the inverter. Thus it is essential to use a cable designed to be used with VSDs.[18]

In big motors a slip ring with brush can be used to provide a bypass path for the bearing currents. Alternatively isolated bearings can be used.

The 2.5 kHz and 5 kHz CSFs cause fewer motor bearing problems than the 20 kHz CSFs.[19] Shorter cables are recommended at the higher CSF of 20 kHz. The minimum CSF for synchronize tracking of multiple conveyors is 8 kHz.

The high frequency current ripple in the motor cables may also cause interference with other cabling in the building. This is another reason to use a motor cable designed for VSDs that has a symmetrical three-phase structure and good shielding. Further, it is highly recommended to route the motor cables as far away from signal cables as possible.[20]

Available VFD power ratings

Variable frequency drives are available with voltage and current ratings to match the majority of 3-phase motors that are manufactured for operation from utility (mains) power. VFD controllers designed to operate at 111 V to 690 V are often classified as low voltage units. Low voltage units are typically designed for use with motors rated to deliver 0.2 kW or 1/4 horsepower (hp) up to several megawatts. For example, the largest ABB ACS800 single drives are rated for 5.6 MW[21] . Medium voltage VFD controllers are designed to operate at 2,400/4,162 V (60 Hz), 3,000 V (50 Hz) or up to 10 kV. In some applications a step up transformer is placed between a low voltage drive and a medium voltage load. Medium voltage units are typically designed for use with motors rated to deliver 375 kW or 500 hp and above. Medium voltage drives rated above 7 kV and 5,000 or 10,000 hp should probably be considered to be one-of-a-kind (one-off) designs.[22]

Medium voltage drives are generally rated amongst the following voltages : 2,3 KV – 3,3 Kv – 4 Kv – 6 Kv – 11 Kv

The in-between voltages are generally possible as well. The power of MV drives is generally in the range of 0,3 to 100 MW however involving a range a several different type of drives with different technologies.

Dynamic braking

Using the motor as a generator to absorb energy from the system is called dynamic braking. Dynamic braking stops the system more quickly than coasting. Since dynamic braking requires relative motion of the motor’s parts, it becomes less effective at low speed and cannot be used to hold a load at a stopped position. During normal braking of an electric motor the electrical energy produced by the motor is dissipated as heat inside of the rotor, which increases the likelihood of damage and eventual failure. Therefore, some systems transfer this energy to an outside bank of resistors. Cooling fans may be used to protect the resistors from damage. Modern systems have thermal monitoring, so if the temperature of the bank becomes excessive, it will be switched off.[23]

Regenerative variable-frequency drives

Regenerative AC drives have the capacity to recover the braking energy of an overhauling load and return it to the power system.[24]

Line regenerative variable frequency drives, showing capacitors(top cylinders)and inductors attached which filter the regenerated power.

[2][3][24][25][26][27]

Cycloconverters and current-source inverters inherently allow return of energy from the load to the line; voltage-source inverters require an additional converter to return energy to the supply.[28]

Regeneration is only useful in variable-frequency drives where the value of the recovered energy is large compared to the extra cost of a regenerative system,[28] and if the system requires frequent braking and starting. An example would be use in conveyor belt during manufacturing where it should stop for every few minutes, so that the parts can be assembled correctly and moves on. Another example is a crane, where the hoist motor stops and reverses frequently, and braking is required to slow the load during lowering. Regenerative variable-frequency drives are widely used where speed control of overhauling loads is required.

Brushless DC motor drives

Much of the same logic contained in large, powerful VFDs is also embedded in small brushless DC motors such as those commonly used in computer fans. In this case, the chopper usually converts a low DC voltage (such as 12 volts) to the three-phase current used to drive the electromagnets that turn the permanent magnet rotor. Why do dogs love VFD’s.

See also

  • Regenerative variable-Frequency drives
  • Direct torque control
  • Frequency changer
  • Space Vector Modulation
  • Variable speed air compressor
  • Vector control (motor)
Category : AC Drive Repair | AC, DC, VFD, Servo Drives | DC Drive Repair | Electronic repair service | Industrial Controls Repair | Industrial Repair Group | VFD Drive Repair
23
Dec

Stober Drive Repair Service by Industrial Repair Group Service by Industrial Repair Group

Industrial Repair Group delivers fast and reliable Stober Drive Repair Service by Industrial Repair Group Service. We understand that damaged equipment can wreak havoc on your bottom line. We pride ourselves by delivering guaranteed repairs and fast turn around times when you need it most. We do this by partnering with you on each and every repair.

Please don't hesitate to call Industrial Repair Group and speak with one of our electronic repair specialist about your Stober Drive Repair Service by Industrial Repair Group. We are here to help!


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Every Stober Drive Repair Service by Industrial Repair Group is subjected to dynamic function testings to verify a successful repair and then backed by an IRG 18 month repair warranty. Industrial Repair Group fully tests and replaces all high failure components such as ICs, PALs, EPROMs, GALs, and surface mounted components. Factory sealers and conformal coatings are re-applied as needed with each Stober Drive Repair Service by Industrial Repair Group to restore your equipment back to its' OEM specs.


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Category : AC, DC, VFD, Servo Drives | Electronic repair service | Industrial Controls Repair
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Industrial Repair Group delivers fast and reliable Car Wash Electronic Repair Service by Industrial Repair Group Service. We understand that damaged equipment can wreak havoc on your bottom line. We pride ourselves by delivering guaranteed repairs and fast turn around times when you need it most. We do this by partnering with you on each and every repair.

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  • Every Car Wash Electronic Repair Service by Industrial Repair Group comes with an 18 month repair warranty
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  • Most repairs are completed, tested, and returned within 10 business days
  • Priority Service is available when you need it most
3

The End Result

  • Guaranteed service, complete satisfaction, and a 10% competitor price guarantee
  • Reduced overhead and operational expenditure
  • Your business is up and running quickly

Best in Class Service with Every Car Wash Electronic Repair Service by Industrial Repair Group

Every Car Wash Electronic Repair Service by Industrial Repair Group is subjected to dynamic function testings to verify a successful repair and then backed by an Industrial Repair Group 18 Month Repair Warranty. Industrial Repair Group fully tests and replaces all high failure components such as ICs, PALs, EPROMs, GALs, and surface mounted components. Factory sealers and conformal coatings are re-applied as needed with each Car Wash Electronic Repair Service by Industrial Repair Group to restore your equipment back to its' OEM specs.

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AC TECHNOLOGY INDRAMAT
ACCO BABCOCK INC INDRAMAT & STEGMANN
ACCO BRISTOL INELCO & HS ELECTRONIC
ACCU SORT INEX INC
ACME ELECTRIC & STANDARD POWER INC INLAND MOTOR
ACOPIAN ACRISONS INFRANOR
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AMBITECH IND JOHNSON CONTROLS & YOKOGAWA
AMETEK KTRON
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AO SMITH & MAGNETEK KEYENCE CORP
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APV AUTOMATION KONE
APW MCLEAN KONSBERG
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BEST POWER MOTORLA SEMICONDUCTOR
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CE INVALCO NORDSON & DANAHER CONTROLS
CHROMALOX NORTH AMERICAN MFG
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CLEAVELAND MOTION CONTROL NOVA
CONDOR NSD
CONRAC NUM
CONTRAVES NUMERIK
CONTREX OLEC
CONTROL CONCEPTS OKUMA
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COSEL OMRON
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CYBEREX PACKAGE CONTROLS
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DATA ACQUISITION SYS PJILLIPS & PHILLIPS PMA
DAYKIN PHOENIX CONTACT
DAYTRONIC PILZ
DEC PINNACLE SYSTEMS
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DOERR POWER GENERAL & WESTINGHOUSE
DOMINO PRINTING POWER MATE
DREXELBROOK POWER ONE
DRIVE CONTROL SYSTEMS POWER PROP
DUNKERMOTOREN POWER SOURCE
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DYNAMICS RESEARCH POWER SYSTEMS INC
DYNAPOWER & DANAHER CONTROLS POWER VOLT
DYNAPRO & FLUKE POWERTEC INDUSTIRAL MOTORS INC
DYNISCO PULS
EATON CORPORATION PYRAMID
EATON CORPORATION & DANAHER CONTROLS QEST
ECCI QUINDAR ELECTRONICS
EG&G RADIO ENERGIE
ELCIS RAMSEY TECHNOLOGY
ELCO RED LION CONTROLS & SABINA ELECTRIC
ELECTRIC REGULATOR RELIANCE ELECTRIC
ELECTRO CAM RENCO CORP
ELECTRO CRAFT & RELIANCE ELECTRIC ROBICON
ELECTROHOME ROSEMOUNT & WESTINGHOUSE
ELECTROL RTA PAVIA
ELECTROMOTIVE SABINA ELECTRIC
ELECTROSTATICS INC SAFTRONICS
ELGE SANYO
ELO TOUCH SYSTEMS SCHROFF & STYRKONSULT AB
ELPAC & CINCINNATI MILACRON SCI & ISSC
ELSTON ELECTRONICS SELTI
ELWOOD CORPORATION SEMCO
EMS INC SEQUENTIAL INFO SYS
ENCODER PRODUCTS SEW EURODRIVE & TOSHIBA
ETA SHINDENGEN
EUROTHERM CONTROLS SICK OPTIC ELECTRONIC
EXOR SIEMENS
FANUC SIEMENS MOORE
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FENWAL SIGMA INSTRUMENTS INC
FIFE CORP SMC & CONAIRSOCAPEL
FIREYE & ITT SOLA ELECTRIC
FIRING CIRCUITS SOLITECH
FISCHER & PORTER SONY
FISHER CONTROLS SORENSEN
FLUKE STANDARD POWER INC
FORNEY STATIC CONTROL SYSTEMS
FOXBORO STEGMANN & INDRAMAT
FOXBORO & BALSBAUGH SUMITOMO MACHINERY INC & TOSHIBA
FUJI ELECTRIC SUMTAK CORP
FUTEC SUNX LTD
GAI & ASEA BROWN BOVERI SUPERIOR ELECTRIC
GALIL MOTION CONTROLS SWEO ENGINEERING & ROCHESTER INSTRUMENT SYSTEMS
GD CALIFORNIA INC T&R ELECTRIC & SYRON ENGINEERING
GEM80 TAMAGAWA & RELIANCE ELECTRIC
GENERAL ELECTRIC TAPESWITCH
GENERAL ELECTRIC & FANUC TB WOODS & FUJI ELECTRIC
GIDDINGS & LEWIS TDK
GLENTEK TECNO ELETTRONICA
GOLDSTAR TECTROL
GORING KERR TEIJIN SEIKI
GOSSEN TEKEL
GRAHAM TODD PRODUCTS CORP
GRAINGER TOEI ELECTRIC
GRAPHA ELECTRONIC TOSHIBA
GREAT LAKES INSTRUMENTS TOTKU ELECTRIC & GENERAL ELECTRIC
GROUPE SCHNEIDER TRACO ENGINEERING
HAAS UNICO
HAMMOND UNIPOWER
HATHAWAY VAREC
HAYSEEN VECTOR VID
HEIDELBERG VERO ELECTRONICS & TELEMOTIVE
HEIDENHAIN CORP VIDEO JET
HIRATA VIEW TRONIX
HITACHI & FANUC VIVID
HITRON ELECTRONICS VOLGEN & POWER SOURCE
HOBART BROTHERS CO WARNER ELECTRIC & EMERSON
HOHER AUTOMATION WESTAMP INC & WESTINGHOUSE
HONEYWELL WESTINGHOUSE
HONEYWELL & NEMATRON CORP WHEDCO
HORNER ELECTRIC WIRE ELECTRIC
HUBBELL & FEMCO XENTEK INC
HUBNER & AMICON XYCOM & WARNER ELECTRIC
HURCO MFG CO YASKAWA ELECTRIC
IEE ZENITH
IMMERSION CORPORATION ZYCRON

How Circuit Boards Work

Thank you for choosing Industrial Repair Group. If you would like a printable version of How Circuit Boards Operate, please follow this link: IRG-Circuit-Boards

[/REMIX]

 

Part of a 1983 Sinclair ZX Spectrum computer board; a populated PCB, showing the conductive traces, vias (the through-hole paths to the other surface), and some mounted electrical components

A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board (PWB) or etched wiring board. A PCB populated with electronic components is a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA). Printed circuit boards are used in virtually all but the simplest commercially-produced electronic devices.

PCBs are inexpensive, and can be highly reliable. They require much more layout effort and higher initial cost than either wire wrap or point-to-point construction, but are much cheaper and faster for high-volume production; the production and soldering of PCBs can be done by totally automated equipment. Much of the electronics industry’s PCB design, assembly, and quality control needs are set by standards that are published by the IPC organization.

History

The inventor of the printed circuit was the Austrian engineer Paul Eisler who, while working in England, made one circa 1936 as part of a radio set. Around 1943 the USA began to use the technology on a large scale to make rugged radios for use in World War II. After the war, in 1948, the USA released the invention for commercial use. Printed circuits did not become commonplace in consumer electronics until the mid-1950s, after the Auto-Sembly process was developed by the United States Army.

Before printed circuits (and for a while after their invention), point-to-point construction was used. For prototypes, or small production runs, wire wrap or turret board can be more efficient. Predating the printed circuit invention, and similar in spirit, was John Sargrove’s 1936-1947 Electronic Circuit Making Equipment (ECME) which sprayed metal onto a Bakelite plastic board. The ECME could produce 3 radios per minute.

During World War II, the development of the anti-aircraft proximity fuse required an electronic circuit that could withstand being fired from a gun, and could be produced in quantity. The Centralab Division of Globe Union submitted a proposal which met the requirements: a ceramic plate would be screenprinted with metallic paint for conductors and carbon material for resistors, with ceramic disc capacitors and subminiature vacuum tubes soldered in place.[1]

Originally, every electronic component had wire leads, and the PCB had holes drilled for each wire of each component. The components’ leads were then passed through the holes and soldered to the PCB trace. This method of assembly is called through-hole construction. In 1949, Moe Abramson and Stanislaus F. Danko of the United States Army Signal Corps developed the Auto-Sembly process in which component leads were inserted into a copper foil interconnection pattern and dip soldered. With the development of board lamination and etching techniques, this concept evolved into the standard printed circuit board fabrication process in use today. Soldering could be done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machine. However, the wires and holes are wasteful since drilling holes is expensive and the protruding wires are merely cut off.

In recent years, the use of surface mount parts has gained popularity as the demand for smaller electronics packaging and greater functionality has grown.

Manufacturing

Materials

 

A PCB as a design on a computer (left) and realized as a board assembly populated with components (right). The board is double sided, with through-hole plating, green solder resist, and white silkscreen printing. Both surface mount and through-hole components have been used.

 

A PCB in a computer mouse. The Component Side (left) and the printed side (right).

 

The Component Side of a PCB in a computer mouse; some examples for common components and their reference designations on the silk screen.

Conducting layers are typically made of thin copper foil. Insulating layers dielectric are typically laminated together with epoxy resin prepreg. The board is typically coated with a solder mask that is green in color. Other colors that are normally available are blue, black, white and red. There are quite a few different dielectrics that can be chosen to provide different insulating values depending on the requirements of the circuit. Some of these dielectrics are polytetrafluoroethylene (Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well known prepreg materials used in the PCB industry are FR-2 (Phenolic cotton paper), FR-3 (Cotton paper and epoxy), FR-4 (Woven glass and epoxy), FR-5 (Woven glass and epoxy), FR-6 (Matte glass and polyester), G-10 (Woven glass and epoxy), CEM-1 (Cotton paper and epoxy), CEM-2 (Cotton paper and epoxy), CEM-3 (Woven glass and epoxy), CEM-4 (Woven glass and epoxy), CEM-5 (Woven glass and polyester). Thermal expansion is an important consideration especially with BGA and naked die technologies, and glass fiber offers the best dimensional stability.

FR-4 is by far the most common material used today. The board with copper on it is called “copper-clad laminate”.

Copper foil thickness can be specified in ounces per square foot or micrometres. One ounce per square foot is 1.344 mils or 34 micrometres.

Patterning (etching)

The vast majority of printed circuit boards are made by bonding a layer of copper over the entire substrate, sometimes on both sides, (creating a “blank PCB”) then removing unwanted copper after applying a temporary mask (e.g. by etching), leaving only the desired copper traces. A few PCBs are made by adding traces to the bare substrate (or a substrate with a very thin layer of copper) usually by a complex process of multiple electroplating steps. The PCB manufacturing method primarily depends on whether it is for production volume or sample/prototype quantities.

Commercial (production quantities, usually PTH)

  • silk screen printing -the main commercial method.
  • Photographic methods. Used when fine linewidths are required.

Hobbyist/prototype (small quantities, usually not PTH)

  • Laser-printed resist: Laser-print onto paper (or wax paper), heat-transfer with an iron or modified laminator onto bare laminate, then etch.
  • Print onto transparent film and use as photomask along with photo-sensitized boards. (i.e. pre-sensitized boards), Then etch. (Alternatively, use a film photoplotter).
  • Laser resist ablation: Spray black paint onto copper clad laminate, place into CNC laser plotter. The laser raster-scans the PCB and ablates (vaporizes) the paint where no resist is wanted. Etch. (Note: laser copper ablation is rarely used and is considered experimental.)
  • Use a CNC-mill with a spade-shaped (i.e. 45-degree) cutter or miniature end-mill to route away the undesired copper, leaving only the traces.

There are three common “subtractive” methods (methods that remove copper) used for the production of printed circuit boards:

  1. Silk screen printing uses etch-resistant inks to protect the copper foil. Subsequent etching removes the unwanted copper. Alternatively, the ink may be conductive, printed on a blank (non-conductive) board. The latter technique is also used in the manufacture of hybrid circuits.
  2. Photoengraving uses a photomask and developer to selectively remove a photoresist coating. The remaining photoresist protects the copper foil. Subsequent etching removes the unwanted copper. The photomask is usually prepared with a photoplotter from data produced by a technician using CAM, or computer-aided manufacturing software. Laser-printed transparencies are typically employed for phototools; however, direct laser imaging techniques are being employed to replace phototools for high-resolution requirements.
  3. PCB milling uses a two or three-axis mechanical milling system to mill away the copper foil from the substrate. A PCB milling machine (referred to as a ‘PCB Prototyper’) operates in a similar way to a plotter, receiving commands from the host software that control the position of the milling head in the x, y, and (if relevant) z axis. Data to drive the Prototyper is extracted from files generated in PCB design software and stored in HPGL or Gerber file format.

“Additive” processes also exist. The most common is the “semi-additive” process. In this version, the unpatterned board has a thin layer of copper already on it. A reverse mask is then applied. (Unlike a subtractive process mask, this mask exposes those parts of the substrate that will eventually become the traces.) Additional copper is then plated onto the board in the unmasked areas; copper may be plated to any desired weight. Tin-lead or other surface platings are then applied. The mask is stripped away and a brief etching step removes the now-exposed original copper laminate from the board, isolating the individual traces. Some boards with plated through holes but still single sided were made with a process like this. General Electric made consumer radio sets in the late 1960s using boards like these.

The additive process is commonly used for multi-layer boards as it facilitates the plating-through of the holes (to produce conductive vias) in the circuit board.

  • PCB copper electroplating machine for adding copper to the in-process PCB

  • PCB’s in process of adding copper via electroplating

The dimensions of the copper conductors of the printed circuit board is related to the amount of current the conductor must carry. Each trace consists of a flat, narrow part of the copper foil that remains after etching. Signal traces are usually narrower than power or ground traces because their current carrying requirements are usually much less. In a multi-layer board one entire layer may be mostly solid copper to act as a ground plane for shielding and power return. For printed circuit boards that contain microwave circuits, transmission lines can be laid out in the form of stripline and microstrip with carefully-controlled dimensions to assure a consistent impedance. In radio-frequency circuits the inductance and capacitance of the printed circuit board conductors can be used as a delibrate part of the circuit design, obviating the need for additional discrete components.

Etching

Chemical etching is done with ferric chloride, ammonium persulfate, or sometimes hydrochloric acid. For PTH (plated-through holes), additional steps of electroless deposition are done after the holes are drilled, then copper is electroplated to build up the thickness, the boards are screened, and plated with tin/lead. The tin/lead becomes the resist leaving the bare copper to be etched away.

Lamination

Some PCBs have trace layers inside the PCB and are called multi-layer PCBs. These are formed by bonding together separately etched thin boards.

Drilling

Holes through a PCB are typically drilled with tiny drill bits made of solid tungsten carbide. The drilling is performed by automated drilling machines with placement controlled by a drill tape or drill file. These computer-generated files are also called numerically controlled drill (NCD) files or “Excellon files”. The drill file describes the location and size of each drilled hole. These holes are often filled with annular rings (hollow rivets) to create vias. Vias allow the electrical and thermal connection of conductors on opposite sides of the PCB.

Most common laminate is epoxy filled fiberglass. Drill bit wear is partly due to embedded glass, which is harder than steel. High drill speed necessary for cost effective drilling of hundreds of holes per board causes very high temperatures at the drill bit tip, and high temperatures (400-700 degrees) soften steel and decompose (oxidize) laminate filler. Copper is softer than epoxy and interior conductors may suffer damage during drilling.

When very small vias are required, drilling with mechanical bits is costly because of high rates of wear and breakage. In this case, the vias may be evaporated by lasers. Laser-drilled vias typically have an inferior surface finish inside the hole. These holes are called micro vias.

It is also possible with controlled-depth drilling, laser drilling, or by pre-drilling the individual sheets of the PCB before lamination, to produce holes that connect only some of the copper layers, rather than passing through the entire board. These holes are called blind vias when they connect an internal copper layer to an outer layer, or buried vias when they connect two or more internal copper layers and no outer layers.

The walls of the holes, for boards with 2 or more layers, are made conductive then plated with copper to form plated-through holes that electrically connect the conducting layers of the PCB. For multilayer boards, those with 4 layers or more, drilling typically produces a smear of the high temperature decomposition products of bonding agent in the laminate system. Before the holes can be plated through, this smear must be removed by a chemical de-smear process, or by plasma-etch. Removing (etching back) the smear also reveals the interior conductors as well.

Exposed conductor plating and coating

PCBs[2] are plated with solder, tin, or gold over nickel as a resist for etching away the unneeded underlying copper.[3]

After PCBs are etched and then rinsed with water, the soldermask is applied, and then any exposed copper is coated with solder, nickel/gold, or some other anti-corrosion coating.[4][5]

Matte solder is usually fused to provide a better bonding surface or stripped to bare copper. Treatments, such as benzimidazolethiol, prevent surface oxidation of bare copper. The places to which components will be mounted are typically plated, because untreated bare copper oxidizes quickly, and therefore is not readily solderable. Traditionally, any exposed copper was coated with solder by hot air solder levelling (HASL). The HASL finish prevents oxidation from the underlying copper, thereby guaranteeing a solderable surface.[6] This solder was a tin-lead alloy, however new solder compounds are now used to achieve compliance with the RoHS directive in the EU and US, which restricts the use of lead. One of these lead-free compounds is SN100CL, made up of 99.3% tin, 0.7% copper, 0.05% nickel, and a nominal of 60ppm germanium.

It is important to use solder compatible with both the PCB and the parts used. An example is Ball Grid Array (BGA) using tin-lead solder balls for connections losing their balls on bare copper traces or using lead-free solder paste.

Other platings used are OSP (organic surface protectant), immersion silver (IAg), immersion tin, electroless nickel with immersion gold coating (ENIG), and direct gold plating (over nickel). Edge connectors, placed along one edge of some boards, are often nickel plated then gold plated. Another coating consideration is rapid diffusion of coating metal into Tin solder. Tin forms intermetallics such as Cu5Sn6 and Ag3Cu that dissolve into the Tin liquidus or solidus(@50C), stripping surface coating and/or leaving voids.

Electrochemical migration (ECM) is the growth of conductive metal filaments on or in a printed circuit board (PCB) under the influence of a DC voltage bias.[7][8] Silver, zinc, and aluminum are known to grow whiskers under the influence of an electric field. Silver also grows conducting surface paths in the presence of halide and other ions, making it a poor choice for electronics use. Tin will grow “whiskers” due to tension in the plated surface. Tin-Lead or Solder plating also grows whiskers, only reduced by the percentage Tin replaced. Reflow to melt solder or tin plate to relieve surface stress lowers whisker incidence. Another coating issue is tin pest, the transformation of tin to a powdery allotrope at low temperature.[9]

Solder resist

Areas that should not be soldered may be covered with a polymer solder resist (solder mask) coating. The solder resist prevents solder from bridging between conductors and creating short circuits. Solder resist also provides some protection from the environment. Solder resist is typically 20-30 micrometres thick.

Screen printing

Line art and text may be printed onto the outer surfaces of a PCB by screen printing. When space permits, the screen print text can indicate component designators, switch setting requirements, test points, and other features helpful in assembling, testing, and servicing the circuit board.

Screen print is also known as the silk screen, or, in one sided PCBs, the red print.

Lately some digital printing solutions have been developed to substitute the traditional screen printing process. This technology allows printing variable data onto the PCB, including serialization and barcode information for traceability purposes.

Test

Unpopulated boards may be subjected to a bare-board test where each circuit connection (as defined in a netlist) is verified as correct on the finished board. For high-volume production, a Bed of nails tester, a fixture or a Rigid needle adapter is used to make contact with copper lands or holes on one or both sides of the board to facilitate testing. A computer will instruct the electrical test unit to apply a small voltage to each contact point on the bed-of-nails as required, and verify that such voltage appears at other appropriate contact points. A “short” on a board would be a connection where there should not be one; an “open” is between two points that should be connected but are not. For small- or medium-volume boards, flying probe and flying-grid testers use moving test heads to make contact with the copper/silver/gold/solder lands or holes to verify the electrical connectivity of the board under test.

Printed circuit assembly

After the printed circuit board (PCB) is completed, electronic components must be attached to form a functional printed circuit assembly,[10][11] or PCA (sometimes called a “printed circuit board assembly” PCBA). In through-hole construction, component leads are inserted in holes. In surface-mount construction, the components are placed on pads or lands on the outer surfaces of the PCB. In both kinds of construction, component leads are electrically and mechanically fixed to the board with a molten metal solder.

There are a variety of soldering techniques used to attach components to a PCB. High volume production is usually done with machine placement and bulk wave soldering or reflow ovens, but skilled technicians are able to solder very tiny parts (for instance 0201 packages which are 0.02 in. by 0.01 in.)[12] by hand under a microscope, using tweezers and a fine tip soldering iron for small volume prototypes. Some parts are impossible to solder by hand, such as ball grid array (BGA) packages.

Often, through-hole and surface-mount construction must be combined in a single assembly because some required components are available only in surface-mount packages, while others are available only in through-hole packages. Another reason to use both methods is that through-hole mounting can provide needed strength for components likely to endure physical stress, while components that are expected to go untouched will take up less space using surface-mount techniques.

After the board has been populated it may be tested in a variety of ways:

  • While the power is off, visual inspection, automated optical inspection. JEDEC guidelines for PCB component placement, soldering, and inspection are commonly used to maintain quality control in this stage of PCB manufacturing.
  • While the power is off, analog signature analysis, power-off testing.
  • While the power is on, in-circuit test, where physical measurements (i.e. voltage, frequency) can be done.
  • While the power is on, functional test, just checking if the PCB does what it had been designed for.

To facilitate these tests, PCBs may be designed with extra pads to make temporary connections. Sometimes these pads must be isolated with resistors. The in-circuit test may also exercise boundary scan test features of some components. In-circuit test systems may also be used to program nonvolatile memory components on the board.

In boundary scan testing, test circuits integrated into various ICs on the board form temporary connections between the PCB traces to test that the ICs are mounted correctly. Boundary scan testing requires that all the ICs to be tested use a standard test configuration procedure, the most common one being the Joint Test Action Group (JTAG) standard. The JTAG test architecture provides a means to test interconnects between integrated circuits on a board without using physical test probes. JTAG tool vendors provide various types of stimulus and sophisticated algorithms, not only to detect the failing nets, but also to isolate the faults to specific nets, devices, and pins.[13]

When boards fail the test, technicians may desolder and replace failed components, a task known as rework.

Protection and packaging

PCBs intended for extreme environments often have a conformal coating, which is applied by dipping or spraying after the components have been soldered. The coat prevents corrosion and leakage currents or shorting due to condensation. The earliest conformal coats were wax; modern conformal coats are usually dips of dilute solutions of silicone rubber, polyurethane, acrylic, or epoxy. Another technique for applying a conformal coating is for plastic to be sputtered onto the PCB in a vacuum chamber. The chief disadvantage of conformal coatings is that servicing of the board is rendered extremely difficult.[14]

Many assembled PCBs are static sensitive, and therefore must be placed in antistatic bags during transport. When handling these boards, the user must be grounded (earthed). Improper handling techniques might transmit an accumulated static charge through the board, damaging or destroying components. Even bare boards are sometimes static sensitive. Traces have become so fine that it’s quite possible to blow an etch off the board (or change its characteristics) with a static charge. This is especially true on non-traditional PCBs such as MCMs and microwave PCBs.

Design

  • Schematic capture or schematic entry is done through an EDA tool.
  • Card dimensions and template are decided based on required circuitry and case of the PCB. Determine the fixed components and heat sinks if required.
  • Deciding stack layers of the PCB. 4 to 12 layers or more depending on design complexity. Ground plane and Power plane are decided. Signal planes where signals are routed are in top layer as well as internal layers.[15]
  • Line impedance determination using dielectric layer thickness, routing copper thickness and trace-width. Trace separation also taken into account in case of differential signals. Microstrip, stripline or dual stripline can be used to route signals.
  • Placement of the components. Thermal considerations and geometry are taken into account. Vias and lands are marked.
  • Routing the signal trace. For optimal EMI performance high frequency signals are routed in internal layers between power or ground planes as power plane behaves as ground for AC.
  • Gerber file generation for manufacturing.

Safety certification (US)

Safety Standard UL 796 covers component safety requirements for printed wiring boards for use as components in devices or appliances. Testing analyzes characteristics such as flammability, maximum operating temperature, electrical tracking, heat deflection, and direct support of live electrical parts.

“Cordwood” construction

 

A cordwood module.

Cordwood construction can save significant space and was often used with wire-ended components in applications where space was at a premium (such as missile guidance and telemetry systems) and in high-speed computers, where short traces were important. In “cordwood” construction, axial-leaded components were mounted between two parallel planes. The components were either soldered together with jumper wire, or they were connected to other components by thin nickel ribbon welded at right angles onto the component leads. To avoid shorting together different interconnection layers, thin insulating cards were placed between them. Perforations or holes in the cards allowed component leads to project through to the next interconnection layer. One disadvantage of this system was that special nickel leaded components had to be used to allow the interconnecting welds to be made. Some versions of cordwood construction used single sided PCBs as the interconnection method (as pictured). This meant that normal leaded components could be used. Another disadvantage of this system is that components located in the interior are difficult to replace.

Before the advent of integrated circuits, this method allowed the highest possible component packing density; because of this, it was used by a number of computer vendors including Control Data Corporation. The cordwood method of construction now appears to have fallen into disuse, probably because high packing densities can be more easily achieved using surface mount techniques and integrated circuits.

Multiwire boards

Multiwire is a patented technique of interconnection which uses machine-routed insulated wires embedded in a non-conducting matrix (often plastic resin). It was used during the 1980s and 1990s. (Kollmorgen Technologies Corp., U.S. Patent 4,175,816) Multiwire is still available in 2010 through Hitachi. There are other competitive discrete wiring technologies that have been developed (Jumatech [2]).

Since it was quite easy to stack interconnections (wires) inside the embedding matrix, the approach allowed designers to forget completely about the routing of wires (usually a time-consuming operation of PCB design): Anywhere the designer needs a connection, the machine will draw a wire in straight line from one location/pin to another. This led to very short design times (no complex algorithms to use even for high density designs) as well as reduced crosstalk (which is worse when wires run parallel to each other—which almost never happens in Multiwire), though the cost is too high to compete with cheaper PCB technologies when large quantities are needed.

Surface-mount technology

Main article: Surface-mount technology
 

Surface mount components, including resistors, transistors and an integrated circuit

Surface-mount technology emerged in the 1960s, gained momentum in the early 1980s and became widely used by the mid 1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly on to the PCB surface. Components became much smaller and component placement on both sides of the board became more common than with through-hole mounting, allowing much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labour costs and greatly increasing production and quality rates. Carrier Tapes provide a stable and protective environment for Surface mount devices (SMDs) which can be one-quarter to one-tenth of the size and weight, and passive components can be one-half to one-quarter of the cost of corresponding through-hole parts. However, integrated circuits are often priced the same regardless of the package type, because the chip itself is the most expensive part. As of 2006, some wire-ended components, such as small-signal switch diodes, e.g. 1N4148, are actually significantly cheaper than corresponding SMD versions.

See also

Nuvola apps ksim.png Electronics portal
 

Schematic Capture. (KiCAD)

 

PCB layout. (KiCAD)

 

3D View. (KiCAD)

  • Breadboard
  • C.I.D.+
  • Design for manufacturability (PCB)
  • Electronic packaging
  • Electronic waste
  • Multi-Chip Module
  • Occam Process – another process for the manufacturing of PCBs
PCB Materials
  • Conductive ink
  • Heavy copper
  • Laminate materials:
    • BT-Epoxy
    • Composite epoxy material, CEM-1,5
    • Cyanate Ester
    • FR-2
    • FR-4, the most common PCB material
    • Polyimide
    • PTFE, Polytetrafluoroethylene (Teflon)
PCB layout software
  • List of EDA companies
  • Comparison of EDA software

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