TOTAL COST OF OWNERSHIP
A Financial Approach to the Process Industry
TCO-Maintenance & Operations / October 1, 2006
A Complete System Approach to Non-Energy Operating Costs
Much attention is paid to the energy efficiency of Variable Speed Drive (VSD) technologies.
However, energy efficiency is only the “tip of the iceberg” when considering the Total Cost of
Ownership of VSD alternatives. End users of VSD’s need to consider the costs associated with
maintaining, repairing, and operating the variable speed control technology they decide to use in
their plant. The differences in the costs of replacing parts, repairing vibration caused damage,
correcting misalignment, and replacing worn out or obsolete VSD components can be
considerably higher than the energy cost differences between the same technologies.
Total Cost of Ownership Analysis: The methods available to adjust the speed
of a driven system are numerous. Pulley
It is an accepted fact that in variable load Systems, Variable Frequency Drives, Eddy
process applications, the use of speed Current Drives, Fluid Drives, and Permanent
control on motor driven systems offers Magnet Adjustable Speed Drives top the list
significant energy savings. A quick look at of options. But which one is the best
the Affinity Laws that govern centrifugal option?
pump applications helps to explain and
confirm this fact. In summary, the Affinity When making this decision it is important to
Laws are: look at the Total Cost of Ownership
associated with each of these options. Total
1. Flow - Q1/Q2 = N1/N2 Cost of Ownership (TCO) analysis has to
include all costs that go along with each
2. Head - P1/P2 = (N1/N2)2 option. Initial purchase price typically is only
10 to 25% of TCO. Drive system energy
3. Horsepower - HP1/HP2 = (N1/N2) efficiency, non-energy system operating
costs (such as long term maintenance
Where Q = Flow, N = Speed, P = Pressure, and requirements), drive system life, and the
HP = Power.
cost of process downtime all need to play a
part in the purchase decision. Some of
What the Affinity Laws tell us is that flow
these, such as initial price, are easy to get a
changes are proportional to changes in
handle on. Non-Energy Operating Costs
pump speed but that the power required to
however, tend to be a little more difficult to
drive the flow is proportional to the cube of
quantify when comparing VSD options.
the pump speed. So, a 50% reduction in
flow requires a 50% reduction in pump
Most manufacturers of VSD’s pay very
speed. However, the power required to
careful attention to the reliability and
produce this lower flow will only be 12.5% of
maintainability of their VSD as a stand-alone
the original level!
piece of equipment. All too often, the
reliability and operation of the Process
It is this relationship between flow, speed,
System that the VSD is being used in is
and power that makes adjustable speed
neglected. The reason for this is simple.
control such an attractive option in the
Until recently, the VSD technologies
available to the industrial user were all very
similar in the impact they had on the The potential for seal damage and
complete system’s maintenance and subsequent leakage is very real. In fact, in a
operation. Breakthroughs in variable speed government publication that covers process
power transmission, specifically VSD’s that leaks and their impact on air quality, the
utilize permanent magnet eddy currents to EPA proposes that process operators plan
transmit torque across an air gap, have to replace 20% of the seals in their system
changed the VSD landscape significantly. annually as a standard leak prevention
This paper will examine the Non-Energy program. In an average oil refinery using
Operating Costs associated with traditional 3000 pumps this equates to 600 seal
VSD’s. Specific attention will be paid to the replacements annually… two seals every
differences and advantages of Permanent day! The maintenance demand created by
Magnet Adjustable Speed Drives. A this task is enormous and ties up talent that
summary and comparison of the actual could be used on process optimization
system Non-Energy Operating Costs for the instead.
different VSD options will also be provided.
Traditional VSD Technology:
Pulley systems (or Friction Drives) are the
For the purpose of this discussion, the term oldest of the VSD technologies having been
“Traditional VSD’s” will refer to: Pulley in use even before the introduction of motors
Systems, Variable Frequency Drives to industry. Pulley Systems are easy to set
(VFD’s), Eddy Current Drives, and Fluid up, require no specialized training, and offer
Drives. a small degree of overload protection.
There are several hundred suppliers in
All traditional VSD technologies offer North America and the technology has seen
significant advantages when compared to very little innovation in the past few
Control Valves and Dampers. Because of decades.
the Affinity Laws, the potential for energy
savings is substantial. Also, because VSD’s In a pulley system, speed of the driven load
control flow by controlling the driver speed is adjusted by moving a drive belt to a
and, therefore, the head of the system, the different size pulley or by using an
potential for cavitation and the damage that adjustable pulley, or sheave. If the drive
comes with it is greatly reduced. An pulley is kept at a constant diameter then a
advantage of Control Valve systems is that doubling of the driven pulley diameter will
they appear to be extremely simple and result in a halving of the speed. Unless a
easy to understand. The additional energy sheave is being used, this type of system
and non-energy costs of Control Valves, requires that the process be stopped in
however, can not be ignored. order to change the speed and provides a
limited set of speed choices to the operator.
Traditional VSD’s either control the load This is not an appropriate choice for speed
speed by changing the driver speed or by control in a true process control loop.
changing the amount of power being However, in a system requiring a one-time,
transferred to the load. In both cases, the semi-permanent adjustment this seems, on
driver is physically connected to the driven the surface, to be a fairly viable option.
load. Because of this physical connection,
any misalignment between the driver and Adjustable pulley drive systems, or sheaves,
the load will result in vibration leading to operate using the same principles and
shortened seal and bearing life in the function by moving the sides of an
system. Generally, realignment of the adjustable drive pulley towards or away from
system occurs at least once per year. Even each other. This changes the effective pitch
when this preventative maintenance occurs, diameter of the pulley, and therefore, the
vibration remains one of the biggest output speed. While capable of continuously
concerns for rotating equipment. In severe modulating speeds in response to a process
cases, the vibration can be enough to loop, these systems tend to be hard on belts
actually damage the driven equipment. and produce relatively high mechanical
In both of these systems, proper operation radial loads greatly reduce the life of
depends on friction between the belt and the bearings and will cause premature leakage
pulleys. Because of this necessary friction, in pump seals.
there are numerous opportunities for wear
which, in turn, leads to increased costs for Pulley Systems offer a simple and easy
system maintenance and repair. to understand approach to varying the
speed of a driven load. However, in
addition to the vibration-related
maintenance costs common to all
traditional VSD technologies, Pulley
Systems utilize belts that have maximum
life expectancies of 3 years. On average,
the use of Pulley Systems in variable
speed applications can result in
maintenance costs more than twice as
high as other traditional VSD options.
Figure 1: Pulley System
Variable Frequency Drives:
Belts typically are constructed in a manner
similar to an automobile tire. An elastic Variable Frequency Drives (VFD’s) were
compound is formed into a loop. introduced to industry in the 1980’s. Major
Strengthening belts made of fiberglass, suppliers in North America include: Rockwell
steel, or another cord material are Automation; ABB; General Electric (GE);
embedded into the loop and the cross and Siemens. VFD technology in general
section of the material is given a specific has seen significant amounts of innovation
shape. Typical life expectancy of a generally focused on getting more power out
properly installed and tensioned belt is of a smaller package. The adoption rate of
approximately 25,000 hours, or slightly less VFD’s by industry in the 1990’s was quite
than three years of continuous operation. high. This adoption rate has slowed
dramatically in recent years as more users
As is the case with automobile tires, these become aware of the problems associated
belts wear with use. Side wall friction and with VFD’s. The biggest issues identified by
belt/pulley slippage wears away belt material users are:
during normal operation. In the case of a
load seizure, these systems are designed to
• Reliability of drives
either slip or to break the belt. In either
• Need to replace or upgrade drives
case, the belt is typically rendered unusable
every 4 – 7 years
and must be replaced.
• Harmful impacts to other equipment
In addition to friction, the belts tend to on same electrical service.
deteriorate as a result of oxygen in the air,
heat, and lubricants in the system. Small Due to their apparent low initial cost, many
pulley diameters will also put extra strain on industrial users think of VFD’s as the de-
belts resulting in shorter life expectancies. It facto standard for process speed control.
has been shown that belt life can be cut in This technology has a fairly long history and
half simply due to a 35º F increase in the market price continues to decline.
ambient operating temperatures. These factors make the VFD appear, on the
surface, to be a good value. However,
To maintain the proper amount of friction VFD’s also carry with them several issues
between the belts and pulleys, tensioning of that negatively affect not only the efficiency
the belt is required. A belt that is too loose of the process they are used in, but also the
will rapidly wear out due to friction generated reliability of those processes.
heat. In fact, one of the primary causes of
belt failure is improper tensioning. Properly VFDs control the speed of the motor by
tensioned systems lead to longer belt life but changing the sine wave AC supply voltage
also place high radial loads on the motor to DC voltage, which is then converted into a
and load bearings and/or seals. These pulse width modulated (PWM) AC voltage of
variable frequency. The frequency of these
for leaks, checking exterior paint condition
and touching up when necessary, checking
the radiator surfaces for foreign materials,
confirmation of fan and cooling system
Figure 2: Small Variable Frequency Drives
Figure 4: Example of Bearing Fluting
Air conditioning systems also require annual
maintenance schedules. When considering
the maintenance impact of an air
conditioning system, purchasers of VFD’s
need to consider the following activities as a
base minimum: checking of refrigerant level
and quality, measurement of condenser coil
airflow, cleaning and inspection of the
system’s motor, compressor, ducting, and
Figure 3: Large Variable Frequency Drive System
VFD’s often appear to be the most
economical solution to adjustable speed
pulses ranges from 1 kHz up to 20 kHz. As applications. However, the general
these pulses are sent to the motor, a maintenance costs associated with
capacitive charge is created between the system vibration combined with the
motor’s stator and rotor, which induces a additional costs of maintaining the
voltage on the rotor shaft. As this induced system’s ancillary equipment typically
voltage builds, it can discharge to ground results in annual non-energy operating
through the motor’s bearings. This expenses that are, at a minimum, equal
phenomenon repeats on a continuous basis. to 20% of the system’s initial total
This repeating cycle of voltage build-up and installed costs.
discharge causes pitting of the motor’s
bearings and bearing race in a manner Eddy Current Drives:
similar to the process of Electric Discharge
Machining (EDM). This pitting can become Eddy Current Drives were first introduced in
quite severe over time – creating a pattern the 1930’s for use on railcars. Their use
of grooves in the bearing race. This pattern quickly spread throughout industry such that
of wear is called “fluting”. the leading supplier, Eaton, was selling over
$100 million per year of these drives during
When considering a VFD solution for an the 1970’s. Since that time, there has been
adjustable speed application, many users very little product innovation in this category
neglect to consider the ancillary equipment of VSD’s and relative market share has
often needed when comparing maintenance fallen. Two of the largest remaining
and operating costs. Air conditioning suppliers in North America of Eddy Current
systems and transformers both require Drive technology are TECO/Westinghouse
regularly scheduled maintenance. At a and Dynamatic.
minimum, preventative maintenance for a
transformer includes: power factor testing, Eddy Current Drives use an electromagnetic
oil level checks, oil sampling and testing, coupling between the motor and the driven
max and min temperature checks, checking load. The motor runs at its rated speed at
all times while the speed of the load is these drives every two years which may
adjusted. There are two basic types of Eddy require a full dismantling and rebuild of the
Current Drives currently on the market – Eddy Current Drive system.
foot-mounted (Figure 5) and shaft-mounted
Figure 7: Eddy Current Drive Schematic
In addition to the above maintenance,
proper Eddy Current Drive operation is
Figure 5: Foot Mounted Eddy Current Drive
dependent on the dissipation of heat within
the drive’s components. Annual inspections
of the cooling air passages within the drive
are critical, especially in dusty environments.
End users who are considering the
installation of Shaft Mounted Eddy Current
Drives also need to include the maintenance
and operating costs associated with the use
of belts and pulleys. As mentioned earlier,
Figure 6: Shaft Mounted Eddy Current Drive the non-energy operating costs associated
with these systems can be substantial when
The basic principle of operation of an Eddy compared to other VSD system options.
Current Drive is that an armature, typically a
steel drum that may have another A recent case study published by a North
conductive material as a lining, is attached American municipality determined that
to the drive motor. This armature assembly there are significant costs associated
turns at the nameplate speed of the motor at with maintaining and operating Eddy
all times. Attached to the load shaft is a Current Drives in their process! On
multi-pole electromagnet. A variable DC average, the non-energy operating costs
current is supplied to the electromagnet. As associated with the use of Eddy Current
the current to the electromagnet is Drives can be as high as 18% of the
increased, the magnetic field increases. As system’s initial total installed cost.
the magnetic field moves relative to the
armature, eddy currents are created, Fluid Drives:
magnetically coupling the two elements and
transferring torque from the motor to the The principle of using fluid to transmit power
load. Alignment between the armature and was introduced in 1905 by the Vulcan
the electromagnet is maintained with Engineering Company and was originally
supporting bearings within the drive unit applied to driving a low-speed ship’s
(Figure 7). propeller with a high-speed steam turbine.
The advantage of this new Fluid Drive was
While Eddy Current Drives use magnetically that it allowed for speed reduction without
induced forces to transfer torque, they still the use of complicated and temperamental
depend on a series of bearing surfaces to gear systems. In 1930 the first variable
keep their rotating components aligned. In speed Fluid Drive was installed in England
fact, Eddy Current Drives typically have up with the first U.S. installation following
to four more bearings than the motor they shortly in 1932. At the time of introduction,
are attached to. As the number of bearing Fluid Drives offered the benefits of smooth
surfaces increases, so to does the likelihood and continuous control of speed using a
of bearing failure. Also, end users need to constant speed motor. Compared to the
replace brushes and dress the slip rings in options available at the time (gears and
pulley systems), the Fluid Drive was an Fluid Drives have been used in industrial
attractive technology. applications for over 100 years. Smooth
load speed control has been the
While the technology has continued to be advertised benefit of this technology.
available to this time, the operating However, when long-term non-energy
principles and designs of Fluid Drives have operating costs are considered the
remained rather stagnant. The largest Fluid choice of a Fluid Drive for adjustable
Drive suppliers today, (Falk and Voith), have speed applications quickly becomes a
introduced small changes to their drives, but questionable decision. The age of this
overall the technology is the same as it was technology’s installed base combined
in the 1930’s. with the industry’s maintenance
experience produce annual non-energy
When considering the installation of a Fluid operating costs in excess of 15% of
Drive, a process owner needs to consider initial installed costs.
the extensive maintenance activities
required of this VSD technology. Proper Permanent Magnet Adjustable
operation of industrial Fluid Drives is Speed Drives
dependent on the volume and pressure of oil
flow through the system. Heat dissipation, The introduction of Permanent Magnet
transmitted power, and component Adjustable Speed Drives (PMASD’s) by
lubrication are all dependent on proper oil MagnaDrive Corporation, a U.S. company
flow. Excessive system vibration (such as based in Bellevue, Washington, has given
that caused by shaft misalignment) can the process industry another option to
result in damage to the bearings, shafts and consider when reviewing adjustable speed
sealing components of a fluid drive. When applications. The company’s technology
vibration is excessive, damage to the drive’s has one of the fastest adoption rates for a
internal rotating components is also a very new introduction in its targeted industries
real possibility. with over 5,000 installations currently
A major industrial fluid drive maintenance
shop in the U.S. lists the following PMASD‘s consist of two primary
maintenance trends and concerns components. The first component, a set of
associated with fluid drives: copper conductor plates, is connected to the
• Ball or Roller Bearing Wear motor shaft; the second component of the
• Internal oil pump impeller wear permanent magnet drive is a rigid assembly
• Internal oil pump reversing valves of permanent, rare earth magnets which is
• Shaft sealing surfaces connected to the load. During operation,
• Scoop tube assembly and seals relative motion between the parts creates an
• Gaskets and o-ring condition interwoven, eddy current field that transmits
• Oil quality/contamination torque across the air gap. To adjust the
• Vibration speed of the driven load, the amount of
• Oil cooing system torque transmitted from the motor to its load
• Wear of the input shaft thrust bearing is controlled by changing the distance
• Fatigue cracking of input rotor vanes between the conductor plates and the
• Fatigue cracking of lower bearing support magnet assembly. During operation and
throughout the entire speed range, no
In addition, it is suggested in industrial physical connection exists between the
maintenance bulletins that the stresses motor and the load (Figure 8). This
experienced in fluid drive system subjected “disconnected connection” has
to repeated start/stop cycles can increase demonstrated vibration reductions up to
the loads experienced by the internal rotors. 85%.
These increased loads can cause damage
that shows up as either increased vibration
or unstable load speed.
across all industries. In comparison to
available baseline energy information,
the use of a permanent magnet drive can
result in significant non-energy
operational savings with annual non-
energy operating costs as low as 3% of
initial installed costs.
Figure 8: Operation of Permanent Magnet ASD
1. AC Drive Worldwide Outlook. ARC
Advisory Group Market Study.
The advantage of PMASD’s is found in the
fact that they operate without any physical 2. California Energy Commission. Energy
connection between the driver and the load. Solutions For California Industry: Ways
To Improve Operations And Profitability.
By completely isolating these components 2001.
from each other, PMASD’s eliminate the
3. Coyote Electronics, Inc. Fort Worth, TX.
alignment-related issues experienced by all
traditional VSD technologies. The result is 4. Dabbs, Tom, The True Cost of
Maintenance, Pumps & Systems
increased seal and bearing life in the overall Magazine, April 2004, pump-zone.com
system and a reduction in the cost of
operating and maintaining that system over 5. DSI – Dynamatic, Inc., Sturtevant, WI.
its life. 6. Emerson Electric, St. Louis, MO.
7. Frost & Sullivan. European Variable
Also, because most PMASD’s require no Speed Drives Market Report. 2005.
ancillary equipment, the cost of maintaining 8. Frost & Sullivan. North American
and operating these additional components Variable Speed Drives Market Report.
is completely avoided. Larger horsepower 2005.
PMASD’s may require water cooling to 9. Hydraulic Institute, Pump Life Cycle
prevent excessive heat build-up in the Costs, 2005.
conductor rotors. In these applications, the 10. Iowa Energy Center website.
minimal cost of maintaining a heat www.energy.iastate.edu/news/pr/pr-
exchanger skid does need to be considered. acmaintenance.html
11. Jones, Garr. Pumping Station Design,
However, the costs associated with these Butterworth-Heinemann; 3rd ed. Woburn,
systems are historically low, comparing very MA, 2005.
favorably to compressed refrigerant cooling 12. MagnaDrive Corp., Bellevue, WA.
systems. 13. Maintenance Resource.com website.
PMASAD construction is, from both a 14. PowerFlow Engineering, Inc. Westland,
mechanical and electrical design point of MI. PowerFlow Engineering Maintenance
view, identical to the construction of the Bulletins. www.power-
electrical motors they are typically attached flowengineer.com/bulletins.html.
to. PMASD manufacturer’s maintenance and 15. Progress Energy. Electrical Maintenance
operations manuals suggest that the only Workshop: Transformer Maintenance &
required maintenance is to lubricate any Testing. 2003. http://www.progress-
bearing surfaces on the same schedule as is tr/presentations/ElectricalTestingWorksh
used for the motor bearings. One opk.pdf
manufacturer’s maintenance recommenda- 16. Reliance Electric. Maintenance and
tion for motor lubrication in standard duty Troubleshooting of Electric Motors
applications is to check and re-lubricate website. www.reliance.com.
every 1 to 7 years depending on the size of 17. Stock Equipment Company. Chagrin
the motor. Falls, OH.
The potential value of permanent magnet 18. US Department of Energy. Industrial
technology is clear. In site evaluations, Electric Motor Systems Market Study.
the permanent magnet coupling concept 19. Voith Turbo GmbH & Co. KG.
for speed control has been proven Crailsheim, Germany
Total System Non-Energy Operating Cost Comparison of Various Technologies
Annual Cost of
Technology Typical Failure Maintenance and Repair as
Modes a Percentage of Installed
• System penetration leaks
• Mechanical linkage hysteresis &
Restrictive Flow deadband
Control • Cavitation / Flashing damage
(Valves or Dampers) • Misalignment between driver and
load causing premature seal &
• Radial loads on bearings & pump
seals causing premature wear
• Belt wear
• Environmental degradation of belt
Pulley Systems 30%
• Improper tensioning causing loss of
• Misalignment between driver and
load causing premature seal &
• Electronic component failure
• Bearing fluting
• Failure of ancillary equipment
• Overheating of circuitry
• Motor winding damage due to
insufficient cooling air flow
• Misalignment between driver and
load causing premature seal &
• Mechanical weakening of
Eddy Current Drives components due to rapidly changing 18%
• Complex and aging controls
• Replacing brushes
• Misalignment between driver and
load causing premature seal &
• Fluid leakage leading to loss of
Fluid Drives 15%
• Inadequate cooling of torque transfer
• Demagnetization caused by
operation outside of specified range.
• Failure of actuator