Applications of drives in the chemical industry BY PETER PIETERS & JARI RIIKONEN (ASDs) are often judged as much less reliable when compared with direct online (DOL) operation of motors This article discusses that most failures contributing to the (bad) image of ASDs are related to specification and engineering, rather than to the drive itself As a matter of fact, the usage rate of drives in the chemical industry is far below the usage rate of drives in other industries (3% versus >8%) Because failures with high-voltage (HV) frequency-controlled drives also contribute to the image of low-voltage (LV) drives, an overview is given on all types of failures and their possible prevention for LV as well as HV The rate of possible improvement by implementing proper techniques is given based on a split in failures caused by design versus the drive itself and expressed in mean time between failure (MTBF) figures Digital Object Identifier 10.1109/MIAS.2010.938401 © DIGITAL VISION 1077-2618/10/$26.00©2010 IEEE IEEE INDUSTRY APPLICATIONS MAGAZINE NOV j DEC 2010 WWW.IEEE.ORG/IAS A DJUSTABLE SPEED DRIVES 47 IEEE INDUSTRY APPLICATIONS MAGAZINE NOV j DEC 2010 WWW.IEEE.ORG/IAS 48 we all know typical failures associated ASDs and Energy with all of these drive types This Consumption THE USAGE RATE knowledge can still contribute to the The major concern for chemical oil and image of the whole product family gas (COG) industries is to have reliable OF DRIVES IN The conclusions of this article functioning installations In most cases might be used to decide on the use of a component failure can cause a loss of THE CHEMICAL LV frequency converters in applicaproduction exceeding the repair costs INDUSTRY IS FAR tions where the COG industry is still by a factor of more than a thousand reluctant to so This explains why the usage rate of BELOW THE ASDs in COG industries is not very Historical Failures high However, upcoming directives USAGE RATE OF and Solutions on energy policies will stimulate us to In this section, an overview is given on find solutions for reducing energy conDRIVES IN OTHER failures that have happened or could sumption In several publications, it happen Failures can be found, but not can be found that the large-scale appliINDUSTRIES (3% matching the reader’s expectations for cation of ASDs is expected to save VERSUS >8%) his or her situation (failures are not about 37 TWh by 2010 in the indusrecognized as such) The other way trial sector This is as much as the around, the reader might expect some energy consumption of all public railways in The Netherlands, Belgium, Luxemburg, Germany, failures not listed here The list is not a complete overview, and France and as much as 5% of the CO2 emission to be nor does the list give an indication of the number of failures reduced by the European Union in 2012 as agreed in the or the chance on occurrence Sometimes only one occurKyoto protocol A well-known example of reducing energy rence carries the image of the product for tens of years The consumption and thus reduce CO2 emissions is the use of a failures and inconveniences are listed in groups For each frequency-controlled pump instead of a fixed-speed pump identified failure possible preventive actions are indicated with a controlled valve, or even worse with an overflow or backflow This article discusses several failures on ASDs Design and Commissioning and demonstrates that these failures can be avoided by 1) Dimensioning: After ten years of use, very small proper engineering improvements in a production line cause an ASD to trip on overload It appears that the drive was Applicability designed for 1,200 r/min of the motor at a certain power The improvements resulted in a motor running at 1,320 r/min continuously Because of the HV Drives equipment characteristics, the load increased by ASDs can be divided into two groups One group consists more than 20% When investing in an ASD, the of special engineered drives for certain applications The electrical installation should not be designed for the drives in this group meet the specifications for the driven mechanical base case, but it should be designed for equipment and the supply of electricity In most cases, the limits of operation of an ASD These limits, these drives will have a primary connection to an HV suphowever, should not take into account changes in ply system via a three- or even five-winding transformer the mechanical design On a load curve of a pump, Because of the special attention to these systems and often using a certain impeller, the drive should be engithe lack of an alternative way to drive the equipment, this neered for the required power at synchronous motor group can be qualified as meeting the reliability for the speed If a drive is to run at over synchronous speed, application However, in this group, failures on drive systhe converter power should correspond to the maxitems contribute to the image of drives in general For this mum allowed speed for the motor following the reason, the failure causes and remedies of this group will load curve of the pump also be used in the following sections to demonstrate the 2) Hardware Limit Values: There is a difference beway reliability can be improved tween DOL operation and converter operation with respect to the response on thermal overload In LV Drives fact, the overload of converter-operated equipment LV drives have developed from engineered products about is limited to the maximum current a converter can two decades ago (similar to HV drives) to standardized supply, whereas a DOL motor can run up to its products A rapid change in techniques also caused a breakdown torque (Figure 1) A drive can be prolearning curve in which approximately every five years a grammed to decelerate, to keep the torque equal to new technology became available One can think of the the maximum current that can be provided The first types using thyristors, followed by transistors and response of an ASD on overloads should be investigate turn-offs, right up to the present-day use of insugated during the engineering This might result in lated-gate bipolar transistors Programming techniques a higher rated converter have also developed from analog potentiometer settings 3) Cabling (Length þ Type): If single-core cables are on printed circuit boards (PCBs) with operational ampliused, the high-frequency currents through the fiers and analog control via binary control to modern cable will create an electromagnetic field in the microprocessor-based solutions with built-in techniques earth shield proportional to f (f is the frequency) for torque vector control From the history in our minds, Time 3h 2h 1h 30 20 Max Current ASD 10 min Max Current DOL min 40 s 30 s 20 s t6x 30 s 20 s 10 s 10 s 5s 3s 2s 1s 5s 2s 1.05 I / Iθ Typical thermal overload protection curves for motors ASD, the expertise of the commissioning engineer is vital and can only be sustained if the activity is practiced regularly It is recommended to obtain this service from the manufacturer or an authorized representative Second, knowledge on the operational aspects of the equipment connected to the drive helps in tuning it to its application The commissioning engineer should be supported by a local technician During commissioning, the local technician gives information on the operational requirements of the equipment and learns about the drive, while the commissioning engineer gives information on parametering the drive and will learn from the operation of it It is recommended to test the commissioned drive on load 7) Project Cooperation: As far as possible, the owner, contractor, and subcontractor shift their responsibilities to the equipment vendor, which causes a suboptimal working system Each equipment vendor uses his own preferred vendor for frequency converters, thus leaving the owner with a wide variety of products and the problem of training maintenance personnel to understand all the different types of complex operating systems and programming tools Failures in such units can be a nightmare because of the relative long repair times On the other hand, if ASDs, as part of a system, need to be installed in a substation (i.e., nonexplosive atmosphere) while the rest of the unit is outside in the plant (i.e., explosive atmosphere or dusty), the equipment vendor will try to avoid using ASDs to keep control over his supply and to stick to the warranty he gives on the product The costs related to warranty, however, are often minor compared with production losses From an owner’s point IEEE INDUSTRY APPLICATIONS MAGAZINE NOV j DEC 2010 WWW.IEEE.ORG/IAS A fault in the outer cable insulation will cause a current to flow to earth in an uncontrolled way This will cause a thermal damage to the cable at the fault spot and in the ultimate failure of the primary insulation With ASDs, always use threecore cables If the capacity of the cable is not sufficient, two or more three-core cables should be mounted in parallel [1] 4) Redundancy Requirement: Because of the insufficient reliability (seals), a spare pump is often installed with an associated ASD A voltage drop will cause an ASD to generate a “not running” signal As a result, the spare drive will start up at the same moment Because the frequency converter is programmed to restart automatically on voltage dips, the first ASDs will start again after, say s This causes both pumps to run at the same time, which could lead to process failures such as highflow or high-pressure trips This problem results from the interlocking of pumps being based on pulsed signals The best method would be to have no installed spares and to make the reliability of the drive acceptable To avoid a situation as described, the interlocking should be based on steady signals rather than pulsed signals As also described in the “Power—Black/Brown Outs” section, the better method is to keep the motor-run signal live during a short-time power outage No changeover will then take place during a power dip, and the automatic restart of the drive will prevent a shutdown of the system 5) Software-Based Limits and Functions: A motor can be overloaded close to its maximum torque From analysis reported in the “Contribution of Historical Failures to the Realistic MTBF” section, it can be seen that an ASD application is more sensitive to thermal overloads than a DOL application Very often, an ASD trip on thermal overload is judged as an ASD failure The fact that many DOL motors have a self-resetting thermal relay, whereas a converter more often needs a manual reset, has a contributing effect to this conclusion The possibility of an auto reset on the thermal motor protection of an ASD should be investigated In some applications, the required torque for eliminating process congestions should be evaluated, and the drive should be engineered correspondingly [2] 6) Training: The software in a frequency converter will give only the performance that is programmed into it In most cases, a certain profile can be chosen This makes it very easy to commission the drive Only a very few motor parameters have to be set, and the drive is ready to run However, sometimes, the application requires more than only a standard commissioning In the past, converters had to be trimmed for running with full torque at very low speed The torque vector control that is used today was not applicable at that time At first installation, the drive functioned well, often tested at uncoupled or unloaded equipment When the power at low speeds increased during the lifetime, starting the drive became difficult First, for commissioning an 49 of view, shifting the responsibility downward can be explained as “penny wise, pound foolish.” The owner’s specifications should incorporate requirements for standardization and control of motors Some examples of the requirements are shown n Motors should be supplied from a dedicated drawer in a motor control center (MCC) or a fixed panel in a main switch board (MSB) made available by the contractor n Frequency converter modules should not be integrated in a process control panel or cabinet n The manufacturer should make available all required control loop calculations for speed control either by 4–20 mA signal or by automation bus protocol Power 1) Black/Brown Outs: A typical voltage dip occurs when a failure occurs in the electrical supply system The functionality of the operation of a steadiness system for preventing the consequences of a voltage dip is described in “Voltage Dip Versus 50 A typical voltage dip occurs when a failure occurs in the electrical supply system The impedance of the circuit between power generation and fault spot will result in a momentary supply voltage dip to a distribution where frequency converters are in operation Figure S1 shows a simplified drawing for such a situation The voltage will not stay at that lower level Once the protection device has shut down the affected connection, the voltage will return to the original level Since in most HV systems, protection relays are used, the duration of the voltage dip will last until the time necessary for the relay to switch off For direct protection relays, this duration will be about 0.2 s, if a failure occurs in a system with higher voltages, such as regional or national grid, that could be 30 kV, 50 kV, 110 kV, etc The protection relay also acts as a selective backup for a nonfunctioning protection relay in a lower system The selectivity causes time delays up to 0.4 s before switching off the fault spot At the moment the voltage returns, most asynchronous motors will have dropped down their speed Applying voltage to a motor that is running with a speed below the maximum torque causes the motor current to increase to a value necessary for accelerating, which is a starting current of more than times the nominal current Because a lot of motors will be reaccelerating, the voltage will not return to the original level but to a level that could be less than 70% Accelerating at lower voltages causes the acceleration time to increase A typical diagram for response to voltage dips used within a petrochemical facility is shown in Figure S2 There is a limit to the number of motors that can be allowed to reaccelerate after a voltage dip The resulting voltage dip caused by reaccelerating all motors at the same time results in some motors not reaccelerating at all This situation creates an unwanted impact on operation Motors should have a protection that prevents reaccelerating them when successful starting cannot be guaranteed Typically in the mentioned facility, motors with a power above 55 kW will trip after 1.5 s, and all smaller motors will trip after s Following the voltage dip, provided the voltage recovers to 70% within the mentioned delay times of 1.5 and s, the motors will be restarted The automatic restart is based on an uninterruptible power supply (UPS)supplied auxiliary circuit for MCCs as given in Figure S3 It will keep auxiliary relays (K21), controlling contactors (K11) energized during a voltage dip A minimum voltage protection relay in the auxiliary supply to all the drawers of the MCC (set at 70%) switches off the auxiliary supply at elapsed time delay The status of the auxiliary relay (K21) is used to give 10-kV Board Transformer i.e., 600 V 1,000-m Protection Working Cable Connection Within 0.2 s V 690-V Board 10-kV Board 2,000 V 100 Nominal Voltage (%) IEEE INDUSTRY APPLICATIONS MAGAZINE NOV j DEC 2010 WWW.IEEE.ORG/IAS VOLTAGE DIP VERSUS ELECTRICAL FAILURE 140 V 400-V Board 80 V During 0.2 s ~ ~ 80 60 40 20 0.5 s 1.5 s 60 s Time (s) M S1 One-line diagram showing voltages for a fault situation S2 Typical voltage response to power dips L2 3N s 110 V s – X2-2 1 25 A 25 F21 F1 A K21 Q1 96 160 D 95 X1 F11 A A1 F2 98 Q1 K11 A2 X4-9 F11–F12 F2 K21 2 NOTE 25 1.3.5 F12 A K11 2.4.6 X4-10 L3 X4-5 NO 15 15 15 21 S21 In Start 22 13 S21 X4-6 S21 14 NC X3 3 X4-12 K21 NOTE X4-11 NYCY × 2,5/2,5 A2 K21 U 2,4,6 S21 A1 X2-1 1,3,5 L1 L2 L3 + NO 6 10 NC P1 A NYCY × 2,5/2,5 s M P = kW In = A n = omw/min S3 Standard motor control circuit information to the plant control system about the status of the connected motor (running or standing) Tripping of ASDs on voltage dips occurred when the status signal to the distributed control system (DCS)/programmable logic controller (PLC) system was not derived from the K21 relay In former engineering, the status signal of the motor came from the drive software that indicates a controlled inverter Once the inverter pulses are blocked (which it does at the moment the supply voltage fails), the DCS/PLC system will notice a motor is not running While all non-ASD motors still indicate running via relay K21, the ASD trips If this is a critical application, the whole plant could trip IEEE INDUSTRY APPLICATIONS MAGAZINE NOV j DEC 2010 WWW.IEEE.ORG/IAS X4-2 51 IEEE INDUSTRY APPLICATIONS MAGAZINE NOV j DEC 2010 WWW.IEEE.ORG/IAS 52 elastic type of coupling fitted to Electrical Failure.” The status avoid hazardous over-torques on signal of a DOL motor comes THE MAJOR the equipment shaft from a contact of an auxiliary 5) Electromagnetic Compatibility: A frerelay still energized during the CONCERN FOR quency converter is a type of highvoltage dip Engineering the frequency wave transmitter The status signal of the motor, in COG INDUSTRIES disturbance can transmit either case of an ASD, it made sense through the air or via the supply to derive this signal from the IS TO HAVE cabling back to the network Some drive software that indicates a RELIABLE incidents occurred on the control controlled inverter Once the of a smart MCC with built-in freinverter pulses are blocked (which FUNCTIONING quency converters Some of the PCBs it does at the moment the supply for the communication of the DCS voltage fails), the motor signal for INSTALLATIONS with the drawers in the MCC were the DCS system is indicating a affected They could not be admotor that is not running While dressed anymore from the MCCs all other motors still indicate runsupervisory control and data acquisition system ning, according to the information from the drawer After experiencing some difficulties, there is only auxiliary system, the ASD fails If this is a critical one solution Do not mix up control electronics application, the failing ASD initiates a trip of the with ASDs in the same metal enclosure If a smart whole plant There are two ways to avoid plant tripMCC is being used, the ASD connected to this ping on short-voltage dips One possibility uses the MCC should be outside the MCC Special attenfunctionality of the drive to automatically restart after tion should be paid to cabling of the drive All a voltage dip In this case, the motor-run signal shall high-frequency carrying power cables should be be delayed for the same time as a power dip is shielded and all shields mounted according to the allowed This shall depend on the status of an underinstallation requirements of the manufacturer If voltage detection signal This signal is available in the electronics (especially communication electronics) MCC as a combination of energized K21 relay and are mounted near frequency converters, these electronnot energized K1 relay (see Figure S1) If this situaics should be metal enclosed [4] tion occurs, it shall be interlocked with the motor-run 6) Bearing Currents: The high-frequency voltage in a signal The other possibility uses a minimum voltage stator induces a voltage in the motor shaft If the detection for each part of a power supply independvoltage is high enough, the oil film in the bearently susceptible to voltage dips A well-developed ings will break, and a high current discharge will software program in the DCS can take care of restartoccur The process is cyclic All the discharges ing the facility result in a damaged bearing Typically, bearing 2) Voltage Spikes: Before the mid-1990s, reinforced problems start if the shaft voltage is higher than insulation on motors was not common The insulaabout 250 mV It is recommended to have isotion of motors fed by an old-type ASD can fail lated bearings on nondrive side of a frequency when frequency converters were not equipped with converter-fed motor [5] output sinusoidal filters In new applications, motors above 500 V should have special-designed insulation systems for voltage source inverters It is recom- Control mended in some cases to have a sinusoidal filter as 1) Corrupted Signals: The new generation of speed an option [3] feedback equipment use double trains of pulses 3) Earth Faults: Refer to the “Historical Failures and such as 1,024 per revolution of the shaft shifted by Solutions: Design and Commissioning—Cabling 90° They enable a precise control of the motor, (Length þ Type)” section allowing high dynamic processes such as steel mills 4) ASD Bypass by DOL: As a solution to cope with to be controlled precisely However, in the chemithe reputation of less reliability from ASDs, an cal industry, the dynamics are generally very low engineering solution to create a DOL bypass is Once a process runs at its optimum, the changes chosen This gives operations the time to recontrol in motor speeds are very small, and the used ramps the chemical process or to be able to shut down are very gradual Frequency converters are develthe plant at restarting conditions At the moment, oped for both high dynamic and slow processes a bypass switches on opposite phase of residual This means that for slow processes, a purchaser voltage of the motor, and the torques to handle by gets the high dynamics for free But the high the coupling are very high After only one or a few dynamic signal requires the pulses to rapidly calcubypass switches, the coupling can break One must late firing angles for the electronics, whereas the be aware of this risk In most cases, after 3–5 s, low dynamic processes only require a firing angle the residual voltage has dimmed It has to be calculated from a slowly changing analog value investigated that a switch overtime of at least s Any failure in the pulses or the phase shift does not harm the continuity of the process If the between them causes the converter to shut down switch overtime needs to be shorter, the motor Since new generation of LV frequency converters foundation and shaft should be reinforced, and an use torque vector control, speed feedback devices IEEE INDUSTRY APPLICATIONS MAGAZINE NOV j DEC 2010 WWW.IEEE.ORG/IAS the same Because there is no are not necessary anymore In coordination between electrinew drives, the problem could AFTER TEN YEARS cal and process control, both only occur at very low speeds implement the control philoso(less than 5% of control range) OF USE, VERY phy in their equipment Thus, 2) Alarm and Fault Indications: the time constants in the DCS One annoyance each engineer SMALL system can be different from the feels is that despite all the selftime constants the commissioning tests and diagnostics, a fault IMPROVEMENTS engineer for the drive uses Before cannot be found A transient starting a project, in which ASDs phenomenon with its root cause IN A form a part, a design philosophy in the motor, the cabling, or PRODUCTION for the control should be drafted, the speed feedback control loop and general application rules causes the drive to trip and LINE CAUSE AN should be established after resetting is ready to start again The information in the ASD TO TRIP ON ASD display is not sufficient to Mechanical locate the fault However, once 1) Under/Over Torque: Refer to the OVERLOAD discovered, the fault can look “Historical Failures and Solutions: very obvious and is often related to Design and Commissioning— installation failures or bad quality Hardware Limit Values” section components somewhere outside the converter cabinet 2) Vibrations: The first generation of frequency conBased on experience, most failures indicated by the verters needed a motor tacho for speed feedback drive have their origin in cabling and motor If unexEither the accuracy or the control at very low speeds plained faults occur, this is the first place to look Today, could not be handled by the analog electronics of many measuring techniques are available to identify these types of converters These tachos most often whether a problem is in the motor and cabling or in were direct current (dc) generators with a linear speed the converter itself If the failure appears not to be in to voltage characteristic The construction to the motor or cabling, the modular design of frequency conmotor was kind of artificial Vibrations from the enviverters today make it easy to replace certain comporonment, the driven equipment, or the motor itself nents or even a complete drive Parametering a new could easily affect the tacho causing the voltage outdrive often is done by entering the original hand-held put of the tacho to generate a ripple This ripple display and synchronize the data with the drive data results in continued torque pulses of the converter to 3) Conditions for Start/Stop: In some cases, the use of the motor Depending on the application, these pulses maintenance safety switches close to the motor are could damage the coupling or gearbox However, mandatory As the name says, these switches are high dynamic feedback is not required (see also the installed for safety reasons The maintenance safety “Control—Corrupted Signals” section) The tachosigswitch disconnects the power from the MCC or nal shall have a filter for ripple frequencies ASD to the motor The distance between the con3) Max/Min Values: From the age of dc variable speed tacts in the switch and the guaranteed position of motors, many protections have been inherited To the lever with a one-way lockable construction allow protect the motor from a broken coupling, a maximechanics to proof their own safety by adding a permum speed limit device is used Since protections sonal lock to a multilock For normal DOL motors, are evaluated in a safety integrity procedure, it is hard these switches can also be used to stop a motor It is to abandon these devices, which give the ASD the as simple as remote stopping or stopping through the image of being expensive The same is with necessary process safety system It has no further consequences positive temperature coefficient (PTC) relays for for the system With an ASD, it is different If the motors in hazardous (explosive) areas and minimum cable from the ASD to the motor is interrupted by a power relays for protection against dry running of a switch, the pulsed voltage to the motor will be interpump An effort can be expected from the drive manrupted The absence of an induction ufacturers In the design of new drives, causes the converter to trip on loss of there will be facilities for safety-related field The trip of the converter will protections meeting the International Switching Off Sequence be often first noticed when operaElectrotechnical Commission (IEC) tions want to start the motor again, 61508 and IEC 61511 standards which fails of course A manual reset of the converter is necessary If safety Maintenance maintenance switches are necessary, 1) Frequency: Electronics in general OFF it is possible to use a switch with a not need maintenance The more preopening contact before the main often cabinets are opened for inspeccontacts are opened (see Figure 2) tion, the higher the chance that 4) Limits: Sometimes in the control components are inadvertently touched design, the requirements for process Typical maintenance Preventive maintenance is limited to control and electrical control are switch for ASD exchanging fans and capacitors and 53 MTBF (Units) IEEE INDUSTRY APPLICATIONS MAGAZINE NOV j DEC 2010 WWW.IEEE.ORG/IAS 54 months were registered as bus sometimes switch transition resisfailure, but no indication that it tances No other inspections are THERE IS A could be the supply voltage In necessary, so the best is to keep 99% of the cases, a frequency the cabinet closed DIFFERENCE converter is maintenance free Do 2) Critical Components: Some parts not try to measure PCBs, because of a converter have to be exBETWEEN DOL they will neither give you any changed during their lifetime, OPERATION AND information on the condition of a in particular fans and capacitors drive nor help to avoid trips As has happened, one discovers CONVERTER However, it makes sense to check a fan has to be exchanged, but the old-fashioned components in it is only accessible from behind OPERATION WITH a drive, such as switches, door Since the cabinet is standing interlocks, and water leakage detecagainst a wall between other RESPECT TO THE tors (transition resistances) cabinets, the entire cabinet has 5) Training: After an overhaul of a to be dismounted to disconnect RESPONSE ON motor supplied by an ASD, the the fan and replace it If possiTHERMAL service lifetime reduced to less ble, select frequency converters than the turnaround interval The without any moving parts If fans OVERLOAD bearings were damaged by pit corare necessary, the (dis)mounting rosion This occurs only if currents should be easy without affecting flow through a bearing During other parts in the drive When the repair, it appeared that the mounted bearings engineering a drive system, attention should be paid were not insulated, while the original bearings had to the position of the cabinet in relation to the an oxidized insulation layer The people in the repair required maintenance To avoid shut downs, fans and shop did not notice that insulated bearings had to be capacitors should be exchanged routinely used Personnel in a motor repair shop should be 3) Settings of the Values: To speed up commissioning, trained to maintain frequency-controlled motors Spethe manufacturer provides standard programs for cial attention shall be taken to the typical motor data pumps and fans If not all data for the application in combination with an ASD These are, in essence, have been checked, then unaware faults are introthe reinforced insulation, isolated bearings, PTC eleduced, which only appear if a certain area of the ments (resistor) in the windings, shielding in cable application is employed One can be unaware of box, and sometimes special precautions to avoid high these faults for years Always try to get as much temperatures at low speed information as possible on the application (pump characteristics, process characteristic, process behavior, etc.) Call in a commissioning engineer from the Contribution of Historical Failures manufacturer to the parameter setting job, because to the Realistic MTBF For a project study in 2005 [6], the question rose whether this person is trained to mark variances 4) Components to Be Checked: One of the risky mainte- to use ASDs for compressors or to use steam turbines The nance activities is to measure actual values in a MTBF for both types of equipment was critical in the deciconverter cabinet In one experience, during these sion An investigation was made on failures on existing measuring activities, a short circuit occurred between drives of the same kind (HV) This resulted in: a 24-V connection and a microprocessor bus signal n Number of running hours: 337,250 Five PCBs had to be replaced, and once in a while, n Number of trips: 20 the drive tripped for unclear reasons during eight n MTBF: 16,863 h years months after the incident happened It appeared For this case study, the failure data on existing #107 LV that also a supply PCB generating 5-V dc had been ASD applications in operation in a petrochemical facility affected, but in such a way that the output voltage [6] have been investigated: was 3.5 V, just above the threshold for failure n Number of running hours: about 2,000,000 mode The failures that occurred during the eight n Number of high-priority notifications: 67 n MTBF: 28,500 h 3.2 years The same investigation has been made on 2,801 DOL Product Comparison motors in the same facility: n Number of running hours: about 50,000,000 n Number of high-priority notifications: 157 n MTBF: 320,000 h 36 years This difference between ASDs and DOL motors confirms the image ASDs have When considering failure causes, the MTBF will go up 1985 1992 1995 2003 noticeably, but still the total reliability of an ASD would not Product in the Market (Year) yet reach the reliability of a DOL motor Reliability of frequency converters has risen considerably when comparing ASD improvement rate reliability of present products to previous generation products TABLE ASD AND DOL MOTOR FAILURE INVESTIGATION Reported Failures During Last 4.1 Years Total Number Installed (SAP) Total High-Priority Notifications High-Priority Notifications Per Year (%) MTBF (Years) Average Service Factor ¼ 0.5 LV-motors DOL 2,801 157 1.4 37 LV-motors freq controlled 107 67 15.3 Motorþprocess related 2,908 219 1.8 27 Freq converter related 107 1.1 44 2,908 18 0.2 331 Motor related Conclusions The reliability of ASDs has improved very much The resulting MTBF from realistic estimation (which is 44 years) shows an MTBF for standard LV drives that will be acceptable for most applications in the chemical industry where programs such as “Improve Equipment Reliability,” “Equipment Justification and Minimization,” and “Risk-Based Engineering” are common The highest effect in improving the MTBF lays in optimizing the engineering and failure response with respect to thermal overload trips There is still one more remark to make on reliability The problem with frequency converters, in situations where production losses and repair costs are very high, such as fans for a furnace, is the inability to monitor the electronics and smoothly shut down the plant before trip occurs Mechanical equipment, such as a gearbox with all available vibration monitoring techniques, should still have an advantage in these cases Acknowledgments The authors acknowledge Chris Lee of SABIC-United Kingdom for his valuable challenge of bringing thoughts to a legible paper The authors also acknowledge Klaus Kangas of ABB Drives, Finland, for his contribution to the data on drives development References [1] M Jin, Z Lei, M Weiming, Z Zhihua, and P Qijun, “Commonmode current inductively coupled emission of AC PWM drives,” in Proc Asia-Pacific Symp Electromagnetic Compatibility 2008 (APEMC’08), pp 650–653 [2] M Griggs and G D Hartzo, “Overload for ASD applications—How much is required?” in Proc Petroleum and Chemical Industry Tech Conf 2004 (PCIC’04), Alpharetta, GA, pp 309–318 [3] Z Peroutka, “Requirements for insulation system of motors fed by modern voltage source converters,” in Proc Power Electronics Specialists Conf (PESC’04), 2004, pp 4383–4389 [4] J Rajamaki, A Kasanen, and M Axelsson, “An EMC market surveillance project for frequency converters in Finland,” in Proc IEEE Int Symp Electromagnetic Compatibility (EMC), 2001, pp 94–99 [5] D Busse, J Erdman, R J Kerkman, D Schlegel, and G Skibinski, “Bearing currents and their relationship to PWM drives,” in Proc Industrial Electronics Conf (IECON), 1995, pp 698–705 [6] SABIC Europe, Chemelot site, 2005 [7] ABB Automation, private communication Peter Pieters (peter.pieters@SABIC-europe.com) is with SABICEurope in Geleen, The Netherlands Jari Riikonen is with ABB Drives in Helsinki, Finland Pieters and Riikonen are Members of the IEEE This article first appeared as “Improving ASD Reliability: Adjustable Speed Drives Failure Causes and Solutions” at the 2008 Petroleum and Chemical Industry Conference IEEE INDUSTRY APPLICATIONS MAGAZINE NOV j DEC 2010 WWW.IEEE.ORG/IAS The bar chart in Figure shows an improvement of better than 500% in the last 20 years [7] Learning from the historical failures and not repeating the same failures again in the new products is a matter-of-course action However, this alone may not be sufficient when reliability has a very high priority in the whole system The failure modes discussed earlier—the failures in the project study in 2005 [6]—could be avoided by proper engineering and improved by converter design subtraction The results were promising: n Number of trips: n MTBF: 67,450 h ¼ 7.5 years The failure data for the mentioned 107 ASDs applications and 2,801 DOL applications in a petrochemical facility have been analyzed in detail (see Table 1, before 2007) Only the high-priority notifications have been studied, because the failures causing such notifications result in production loss It appeared that not each high-priority notification had a root cause in the ASD In fact, only five notifications really concerned a drive failure, and 62 other failures had their cause outside the ASD The same exercise has been made for LV DOL motors From the 157 reported notifications, only 18 appeared to concern a motor failure A large number of the used drives were built before 1995 Further study pointed out that most of the high-priority notifications for both motors and drives originated from reset actions of the thermal overload trip The figures confirm the observations in the “Historical Failures and Solutions: Design and Commissioning—Hardware Limit Values” section Eliminating these failures would have the highest contribution to the image of ASDs in chemical industry, resulting in an MTBF for drives of 44 years This figure is close to the implicit accepted MTBF of DOL motors (37 years) 55 ... 1995 2003 noticeably, but still the total reliability of an ASD would not Product in the Market (Year) yet reach the reliability of a DOL motor Reliability of frequency converters has risen considerably... Riikonen is with ABB Drives in Helsinki, Finland Pieters and Riikonen are Members of the IEEE This article first appeared as “Improving ASD Reliability: Adjustable Speed Drives Failure Causes... to have reliable OF DRIVES IN The conclusions of this article functioning installations In most cases might be used to decide on the use of a component failure can cause a loss of THE CHEMICAL