Reliability of adjustable speed drives

If a drive is to run at over synchronous speed, the converter power should correspond to the maxi-mum allowed speed for the motor following the load curve of the pump.. Because the frequ

Applications of drives

in the chemical industry

B Y P E T E R P I E T E R S

& J A R I R I I K O N E N

A DJUSTABLE SPEED DRIVES(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-volt-age (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

pre-vention 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

ASDs and Energy

Consumption

The major concern for chemical oil and

gas (COG) industries is to have reliable

functioning installations In most cases

a component failure can cause a loss of

production exceeding the repair costs

by a factor of more than a thousand

This explains why the usage rate of

ASDs in COG industries is not very

high However, upcoming directives

on energy policies will stimulate us to

find solutions for reducing energy

con-sumption In several publications, it

can be found that the large-scale

appli-cation of ASDs is expected to save

about 37 TWh by 2010 in the

indus-trial sector This is as much as the

energy consumption of all public

rail-ways in The Netherlands, Belgium, Luxemburg, Germany,

and France and as much as 5% of the CO2emission to be

reduced by the European Union in 2012 as agreed in the

Kyoto protocol A well-known example of reducing energy

consumption and thus reduce CO2emissions is the use of a

frequency-controlled pump instead of a fixed-speed pump

with a controlled valve, or even worse with an overflow or

backflow This article discusses several failures on ASDs

and demonstrates that these failures can be avoided by

proper engineering

Applicability

HV Drives

ASDs can be divided into two groups One group consists

of special engineered drives for certain applications The

drives in this group meet the specifications for the driven

equipment and the supply of electricity In most cases,

these drives will have a primary connection to an HV

sup-ply system via a three- or even five-winding transformer

Because of the special attention to these systems and often

the lack of an alternative way to drive the equipment, this

group can be qualified as meeting the reliability for the

application However, in this group, failures on drive

sys-tems contribute to the image of drives in general For this

reason, the failure causes and remedies of this group will

also be used in the following sections to demonstrate the

way reliability can be improved

LV Drives

LV drives have developed from engineered products about

two decades ago (similar to HV drives) to standardized

products A rapid change in techniques also caused a

learning curve in which approximately every five years a

new technology became available One can think of the

first types using thyristors, followed by transistors and

gate turn-offs, right up to the present-day use of

insu-lated-gate bipolar transistors Programming techniques

have also developed from analog potentiometer settings

on printed circuit boards (PCBs) with operational

ampli-fiers and analog control via binary control to modern

microprocessor-based solutions with built-in techniques

for torque vector control From the history in our minds,

we all know typical failures associated with all of these drive types This knowledge can still contribute to the image of the whole product family The conclusions of this article might be used to decide on the use of

LV frequency converters in applica-tions where the COG industry is still reluctant to do so

Historical Failures and Solutions

In this section, an overview is given on failures that have happened or could happen Failures can be found, but not matching the reader’s expectations for his or her situation (failures are not recognized as such) The other way around, the reader might expect some failures not listed here The list is not a complete overview, nor does the list give an indication of the number of failures

or the chance on occurrence Sometimes only one occur-rence carries the image of the product for tens of years The failures and inconveniences are listed in groups For each identified failure possible preventive actions are indicated

Design and Commissioning

1) Dimensioning: After ten years of use, very small improvements in a production line cause an ASD to trip on overload It appears that the drive was designed for 1,200 r/min of the motor at a certain power The improvements resulted in a motor run-ning at 1,320 r/min continuously Because of the equipment characteristics, the load increased by more than 20% When investing in an ASD, the electrical installation should not be designed for the mechanical base case, but it should be designed for the limits of operation of an ASD These limits, however, should not take into account changes in the mechanical design On a load curve of a pump, using a certain impeller, the drive should be engi-neered for the required power at synchronous motor speed If a drive is to run at over synchronous speed, the converter power should correspond to the maxi-mum allowed speed for the motor following the load curve of the pump

2) Hardware Limit Values: There is a difference be-tween DOL operation and converter operation with respect to the response on thermal overload In fact, the overload of converter-operated equipment

is limited to the maximum current a converter can supply, whereas a DOL motor can run up to its breakdown torque (Figure 1) A drive can be pro-grammed to decelerate, to keep the torque equal to the maximum current that can be provided The response of an ASD on overloads should be investi-gated during the engineering This might result in

a higher rated converter

3) Cabling (Length þ Type): If single-core cables are used, the high-frequency currents through the cable will create an electromagnetic field in the earth shield proportional to f2(f is the frequency)

THE USAGE RATE

OF DRIVES IN THE CHEMICAL INDUSTRY IS FAR BELOW THE USAGE RATE OF DRIVES IN OTHER INDUSTRIES (3%

VERSUS >8%).

48

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

three-core cables If the capacity of the cable is not

suffi-cient, 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

pro-grammed to restart automatically on voltage dips,

the first ASDs will start again after, say 1 s This

causes both pumps to run at the same time,

which could lead to process failures such as

high-flow 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”

sec-tion, 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

possibil-ity of an auto reset on the thermal motor

protec-tion 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

ASD, the expertise of the commissioning engineer is vital and can only be sustained if the activity is prac-ticed regularly It is recommended to obtain this service from the manufacturer or an authorized repre-sentative 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 param-etering 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 responsibil-ities to the equipment vendor, which causes a subop-timal working system Each equipment vendor uses his own preferred vendor for frequency converters, thus leaving the owner with a wide variety of prod-ucts and the problem of training maintenance person-nel 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 com-pared with production losses From an owner’s point

Time

3 h

2 h

1 h

30 min

20 min

10 min

5 min

3 min

2 min

1 min

40 s

30 s

20 s

10 s

5 s

3 s

2 s

1 s

I I θ

t6x

30 s

20 s

10 s

5 s

2 s 1.05

Max Current ASD

Max Current DOL

1

Typical thermal overload protection curves for motors.

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

nMotors should be supplied from a dedicated

drawer in a motor control center (MCC) or a fixed

panel in a main switch board (MSB) made

avail-able by the contractor

nFrequency converter modules should not be

inte-grated in a process control panel or cabinet

The manufacturer should make available all required control loop calculations for speed con-trol 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 sys-tem The functionality of the operation of a steadi-ness system for preventing the consequences of a voltage dip is described in “Voltage Dip Versus

VOLTAGE DIP VERSUS ELECTRICAL FAILURE

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

nonfunc-tioning protection relay in a lower system The

selec-tivity 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

maxi-mum torque causes the motor current to increase

to a value necessary for accelerating, which is a

starting current of more than 4 times the nominal current Because a lot of motors will be reaccelerat-ing, the voltage will not return to the original level but to a level that could be less than 70% Acceler-ating 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 guaran-teed 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 3 s Following the volt-age dip, provided the voltvolt-age recovers to 70% within the mentioned delay times of 1.5 and 3 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 contac-tors (K11) energized during a voltage dip A mini-mum 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

S2

100 80 60 40 20 0

1.5 s

60 s

Time (s) 0.5 s

Typical voltage response to power dips.

~

~

M

0 V

i.e., 600 V 10-kV Board

690-V Board

400-V Board 2,000 V

80 V During 0.2 s

1,000-m Cable Connection

140 V

10-kV Board

Transformer Protection Working Within 0.2 s

S1

One-line diagram showing voltages for a fault situation.

50

information to the plant control system about the

status of the connected motor (running or

stand-ing) Tripping of ASDs on voltage dips occurred

when the status signal to the distributed control

tem (DCS)/programmable logic controller (PLC)

sys-tem 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 run-ning via relay K21, the ASD trips If this is a critical application, the whole plant could trip.

110 V 1 25

2

X2-2 –

2 8 Q1 7 95 F2

X4-9

X4-10 X4-5

X4-6

X4-11 X4-2

X2-1

X4-12 5 6

S21

S21

A2 A1

3

6

6

10

K21

K21

NOTE 3 S21

0

14

In

Start

22 13 21 NOTE 3 D

+

25 1 F11

K21

K11

K21 2 2 F12 25 A 1 L3

A1 A2 1

A 2 3 4

9 15 15 15

3N

X1 F1 160 A

Q1

F11–F12

F2 1.3.5 K11 2.4.6

X3 1 2 3

1 2 3

M

P = kW

In = A

n = omw/min

U 2,4,6

S21

P1 1 2

3 s

S3

A

Standard motor control circuit.

51

Electrical Failure.” The status

signal of a DOL motor comes

from a contact of an auxiliary

relay still energized during the

voltage dip Engineering the

status signal of the motor, in

case of an ASD, it made sense

to derive this signal 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 motor signal for

the DCS system is indicating a

motor that is not running While

all other motors still indicate

run-ning, according to the information from the drawer

auxiliary system, the ASD fails If this is a critical

application, the failing ASD initiates a trip of the

whole plant There are two ways to avoid plant

trip-ping on short-voltage dips One possibility uses the

functionality of the drive to automatically restart after

a voltage dip In this case, the motor-run signal shall

be delayed for the same time as a power dip is

allowed This shall depend on the status of an

under-voltage detection signal This signal is available in the

MCC as a combination of energized K21 relay and

not energized K1 relay (see Figure S1) If this

situa-tion occurs, it shall be interlocked with the motor-run

signal The other possibility uses a minimum voltage

detection for each part of a power supply

independ-ently susceptible to voltage dips A well-developed

software program in the DCS can take care of

restart-ing the facility

2) Voltage Spikes: Before the mid-1990s, reinforced

insulation on motors was not common The

insula-tion of motors fed by an old-type ASD can fail

when frequency converters were not equipped with

output sinusoidal filters In new applications, motors

above 500 V should have special-designed insulation

systems for voltage source inverters It is

recom-mended in some cases to have a sinusoidal filter as

an option [3]

3) Earth Faults: Refer to the “Historical Failures and

Solutions: Design and Commissioning—Cabling

(Lengthþ Type)” section

4) ASD Bypass by DOL: As a solution to cope with

the reputation of less reliability from ASDs, an

engineering solution to create a DOL bypass is

chosen This gives operations the time to recontrol

the chemical process or to be able to shut down

the plant at restarting conditions At the moment,

a bypass switches on opposite phase of residual

voltage of the motor, and the torques to handle by

the coupling are very high After only one or a few

bypass switches, the coupling can break One must

be aware of this risk In most cases, after 3–5 s,

the residual voltage has dimmed It has to be

investigated that a switch overtime of at least 3 s

does not harm the continuity of the process If the

switch overtime needs to be shorter, the motor

foundation and shaft should be reinforced, and an

elastic type of coupling fitted to avoid hazardous over-torques on the equipment shaft

5) Electromagnetic Compatibility: A fre-quency converter is a type of high-frequency wave transmitter The disturbance can transmit either through the air or via the supply cabling back to the network Some incidents occurred on the control

of a smart MCC with built-in fre-quency converters Some of the PCBs for the communication of the DCS with the drawers in the MCC were affected They could not be ad-dressed anymore from the MCCs supervisory control and data acquisition system After experiencing some difficulties, there is only one solution Do not mix up control electronics with ASDs in the same metal enclosure If a smart MCC is being used, the ASD connected to this MCC should be outside the MCC Special atten-tion should be paid to cabling of the drive All high-frequency carrying power cables should be shielded and all shields mounted according to the installation requirements of the manufacturer If electronics (especially communication electronics) are mounted near frequency converters, these electron-ics should be metal enclosed [4]

6) Bearing Currents: The high-frequency voltage in a stator induces a voltage in the motor shaft If the voltage is high enough, the oil film in the bear-ings will break, and a high current discharge will occur The process is cyclic All the discharges result in a damaged bearing Typically, bearing problems start if the shaft voltage is higher than about 250 mV It is recommended to have iso-lated bearings on nondrive side of a frequency converter-fed motor [5]

Control

1) Corrupted Signals: The new generation of speed feedback equipment use double trains of pulses such as 1,024 per revolution of the shaft shifted by 90° They enable a precise control of the motor, allowing high dynamic processes such as steel mills

to be controlled precisely However, in the chemi-cal industry, the dynamics are generally very low Once a process runs at its optimum, the changes

in motor speeds are very small, and the used ramps are very gradual Frequency converters are devel-oped for both high dynamic and slow processes This means that for slow processes, a purchaser gets the high dynamics for free But the high dynamic signal requires the pulses to rapidly calcu-late firing angles for the electronics, whereas the low dynamic processes only require a firing angle calculated from a slowly changing analog value Any failure in the pulses or the phase shift between them causes the converter to shut down Since new generation of LV frequency converters use torque vector control, speed feedback devices

THE MAJOR CONCERN FOR COG INDUSTRIES

IS TO HAVE RELIABLE FUNCTIONING INSTALLATIONS.

52

are not necessary anymore In

new drives, the problem could

only occur at very low speeds

(less than 5% of control range)

2) Alarm and Fault Indications:

One annoyance each engineer

feels is that despite all the

self-tests and diagnostics, a fault

cannot be found A transient

phenomenon with its root cause

in the motor, the cabling, or

the speed feedback control loop

causes the drive to trip and

after resetting is ready to start

again The information in the

ASD display is not sufficient to

locate the fault However, once

discovered, the fault can look

very obvious and is often related to

installation failures or bad quality

components somewhere outside the converter cabinet

Based on experience, most failures indicated by the

drive have their origin in cabling and motor If

unex-plained faults occur, this is the first place to look Today,

many measuring techniques are available to identify

whether a problem is in the motor and cabling or in

the converter itself If the failure appears not to be in

motor or cabling, the modular design of frequency

con-verters today make it easy to replace certain

compo-nents or even a complete drive Parametering a new

drive often is done by entering the original hand-held

display and synchronize the data with the drive data

3) Conditions for Start/Stop: In some cases, the use of

maintenance safety switches close to the motor are

mandatory As the name says, these switches are

installed for safety reasons The maintenance safety

switch disconnects the power from the MCC or

ASD to the motor The distance between the

con-tacts in the switch and the guaranteed position of

the lever with a one-way lockable construction allow

mechanics to proof their own safety by adding a

per-sonal lock to a multilock For normal DOL motors,

these switches can also be used to stop a motor It is

as simple as remote stopping or stopping through the

process safety system It has no further consequences

for the system With an ASD, it is different If the

cable from the ASD to the motor is interrupted by a

switch, the pulsed voltage to the motor will be

inter-rupted The absence of an induction

causes the converter to trip on loss of

field The trip of the converter will

be often first noticed when

opera-tions want to start the motor again,

which fails of course A manual reset

of the converter is necessary If safety

maintenance switches are necessary,

it is possible to use a switch with a

preopening contact before the main

contacts are opened (see Figure 2)

4) Limits: Sometimes in the control

design, the requirements for process

control and electrical control are

the same Because there is no coordination between electri-cal and process control, both implement the control philoso-phy in their equipment Thus, the time constants in the DCS system can be different from the time constants the commissioning engineer for the drive uses Before starting a project, in which ASDs form a part, a design philosophy for the control should be drafted, and general application rules should be established

Mechanical

1) Under/Over Torque: Refer to the

“Historical Failures and Solutions: Design and Commissioning— Hardware Limit Values” section

2) Vibrations: The first generation of frequency con-verters needed a motor tacho for speed feedback Either the accuracy or the control at very low speeds could not be handled by the analog electronics of these types of converters These tachos most often were direct current (dc) generators with a linear speed

to voltage characteristic The construction to the motor was kind of artificial Vibrations from the envi-ronment, the driven equipment, or the motor itself could easily affect the tacho causing the voltage out-put of the tacho to generate a ripple This ripple results in continued torque pulses of the converter to the motor Depending on the application, these pulses could damage the coupling or gearbox However, high dynamic feedback is not required (see also the

“Control—Corrupted Signals” section) The tachosig-nal shall have a filter for ripple frequencies

3) Max/Min Values: From the age of dc variable speed motors, many protections have been inherited To protect the motor from a broken coupling, a maxi-mum speed limit device is used Since protections are evaluated in a safety integrity procedure, it is hard

to abandon these devices, which give the ASD the image of being expensive The same is with necessary positive temperature coefficient (PTC) relays for motors in hazardous (explosive) areas and minimum power relays for protection against dry running of a pump An effort can be expected from the drive

man-ufacturers In the design of new drives, there will be facilities for safety-related protections meeting the International Electrotechnical Commission (IEC)

61508 and IEC 61511 standards

Maintenance

1) Frequency: Electronics in general do not need maintenance The more often cabinets are opened for inspec-tion, the higher the chance that components are inadvertently touched

Preventive maintenance is limited to exchanging fans and capacitors and

Switching Off Sequence

1

2 OFF

2

Typical maintenance switch for ASD.

AFTER TEN YEARS

OF USE, VERY SMALL IMPROVEMENTS

IN A PRODUCTION LINE CAUSE AN ASD TO TRIP ON OVERLOAD.

53

sometimes switch transition

resis-tances No other inspections are

necessary, so the best is to keep

the cabinet closed

2) Critical Components: Some parts

of a converter have to be

ex-changed during their lifetime,

in particular fans and capacitors

As has happened, one discovers

a fan has to be exchanged, but

it is only accessible from behind

Since the cabinet is standing

against a wall between other

cabinets, the entire cabinet has

to be dismounted to disconnect

the fan and replace it If

possi-ble, select frequency converters

without any moving parts If fans

are necessary, the (dis)mounting

should be easy without affecting

other parts in the drive When

engineering a drive system, attention should be paid

to the position of the cabinet in relation to the

required maintenance To avoid shut downs, fans and

capacitors should be exchanged routinely

3) Settings of the Values: To speed up commissioning,

the manufacturer provides standard programs for

pumps and fans If not all data for the application

have been checked, then unaware faults are

intro-duced, which only appear if a certain area of the

application is employed One can be unaware of

these faults for years Always try to get as much

information as possible on the application (pump

characteristics, process characteristic, process

behav-ior, etc.) Call in a commissioning engineer from the

manufacturer to do the parameter setting job, because

this person is trained to mark variances

4) Components to Be Checked: One of the risky

mainte-nance activities is to measure actual values in a

converter cabinet In one experience, during these

measuring activities, a short circuit occurred between

a 24-V connection and a microprocessor bus signal

Five PCBs had to be replaced, and once in a while,

the drive tripped for unclear reasons during eight

months after the incident happened It appeared

that also a supply PCB generating 5-V dc had been

affected, but in such a way that the output voltage

was 3.5 V, just above the threshold for failure

mode The failures that occurred during the eight

months were registered as bus failure, but no indication that it could be the supply voltage In 99% of the cases, a frequency converter is maintenance free Do not try to measure PCBs, because they will neither give you any information on the condition of a drive nor help to avoid trips However, it makes sense to check the old-fashioned components in

a drive, such as switches, door interlocks, and water leakage detec-tors (transition resistances)

5) Training: After an overhaul of a motor supplied by an ASD, the service lifetime reduced to less than the turnaround interval The bearings were damaged by pit cor-rosion This occurs only if currents flow through a bearing During the repair, it appeared that the mounted bearings were not insulated, while the original bearings had

an oxidized insulation layer The people in the repair shop did not notice that insulated bearings had to be used Personnel in a motor repair shop should be trained to maintain frequency-controlled motors Spe-cial attention shall be taken to the typical motor data

in combination with an ASD These are, in essence, the reinforced insulation, isolated bearings, PTC ele-ments (resistor) in the windings, shielding in cable box, and sometimes special precautions to avoid high temperatures at low speed

Contribution of Historical Failures

to the Realistic MTBF For a project study in 2005 [6], the question rose whether

to use ASDs for compressors or to use steam turbines The MTBF for both types of equipment was critical in the deci-sion An investigation was made on failures on existing drives of the same kind (HV) This resulted in:

n Number of running hours: 337,250

n Number of trips: 20

n MTBF: 16,863 h 2 years

For this case study, the failure data on existing #107 LV ASD applications in operation in a petrochemical facility [6] have been investigated:

n Number of running hours: about 2,000,000

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 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 con-firms the image ASDs have

When considering failure causes, the MTBF will go up noticeably, but still the total reliability of an ASD would not yet reach the reliability of a DOL motor Reliability of frequency converters has risen considerably when comparing reliability of present products to previous generation products

THERE IS A DIFFERENCE BETWEEN DOL OPERATION AND CONVERTER OPERATION WITH RESPECT TO THE RESPONSE ON THERMAL OVERLOAD.

Product Comparison

Product in the Market (Year)

3

ASD improvement rate.

54

The bar chart in Figure 3 shows an improvement of

bet-ter 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: 5

n MTBF: 67,450 h¼ 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

con-cerned 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

noti-fications 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

Elim-inating 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)

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

Re-liability,” “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 electron-ics 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 ad-vantage 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, “Common-mode 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 surveil-lance 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@europe.com) is with SABIC-Europe 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

TABLE 1 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 freq.

controlled

Motorþprocess

related

Freq converter

related

55