1. TOTAL COST OF OWNERSHIP
A Financial Approach to the Process Industry
TCO-Energy / October 1, 2006
A Complete System Approach to Energy Efficiency
Abstract:
Manufacturers of electric motor Variable Speed Drive (VSD) technologies advertise the energy
efficiency of their VSD’s as stand-alone devices. While these advertised efficiencies appear, on
the surface, to be impressive, the end-user needs to consider the energy efficiency of the entire
VSD system when selecting which technology to purchase and implement in their process.
Ancillary equipment as well as mechanical losses typically results in realized efficiencies
significantly lower than the published VSD numbers. A notable exception to this rule is found in
VSD’s based on permanent magnet technology.
System Energy Efficiency… The methods available to adjust the speed
What’s the truth? of a driven system are numerous. Pulley
Systems, Variable Frequency Drives, Eddy
It is an accepted fact that in variable load Current Drives, Fluid Drives, and Permanent
process applications, the use of speed Magnet Adjustable Speed Drives top the list
control on motor driven systems offers of options. But which one is the best
significant energy savings. A quick look at option?
the Affinity Laws that govern centrifugal
pump applications helps to explain and When making this decision it is important to
confirm this fact. In summary, the Affinity look at the Total Cost of Ownership
Laws are: associated with each of these options. Total
Cost of Ownership (TCO) analysis has to
1. Flow - Q1/Q2 = N1/N2 include all costs that go along with each
option. Initial purchase price typically is only
2. Head - P1/P2 = (N1/N2)2 10 to 25% of TCO. Drive system energy
efficiency, non-energy system operating
3. Horsepower - HP1/HP2 = (N1/N2)3 costs (such as long term maintenance
requirements), drive system life, and the
Where Q = Flow, N = Speed, P = Pressure, and cost of process downtime all need to play a
HP = Power. part in the purchase decision. Some of
these, such as initial price, are easy to get a
What the Affinity Laws tell us is that flow handle on. Drive system energy efficiency
changes are proportional to changes in however, tends to be a little more difficult to
pump speed but that the power required to determine.
drive the flow is proportional to the cube of
the pump speed. So, a 50% reduction in Most manufacturers of adjustable speed
flow requires a 50% reduction in pump systems focus the customer on the savings
speed. However, the power required to realized by taking advantage of the Affinity
produce this lower flow will only be 12.5% of Laws. While this is important, it is also
the original level! critical to determine the true efficiency of the
system. A system that provides its user with
It is this relationship between flow, speed, $100 of up front savings but that costs an
and power that makes adjustable speed additional $150 to run because of energy
control such an attractive option in the efficiency losses is not a good business
process world.

2. decision! This paper examines the actual While a pulley system may have better
system efficiencies for each of the above system energy efficiency than a
mentioned adjustable speed options. restrictive flow control device (such as a
valve or damper), a pulley system will
Pulley Systems: generally have the worst overall system
energy efficiency compared to all of the
Pulley systems are the oldest of the VSD other VSD technologies.
technologies having been in use even
before the introduction of motors to industry. Variable Frequency Drives:
There are several hundred suppliers in
North America and the technology has seen Variable Frequency Drives (VFD’s) were
very little innovation in the past few introduced to industry in the 1980’s. Major
decades. suppliers in North America include: Rockwell
Automation; ABB; General Electric (GE);
In a pulley system, speed of the driven load and Siemens. The VFD technology has
is adjusted by moving a drive belt to a seen significant amounts of innovation
different size pulley or by using an generally focused on getting more power out
adjustable pulley, or sheave. If the drive of a smaller package. The adoption rate of
pulley is kept at a constant diameter then a VFD’s by industry in the 1990’s was quite
doubling of the driven pulley diameter will high. This adoption rate has slowed
result in a halving of the speed. Unless a dramatically in recent years as more users
sheave is being used, this type of system become aware of the problems associated
requires that the process be stopped in with VFD’s. The biggest issues identified by
order to change the speed and provides a users are:
limited set of speed choices to the operator. • Reliability of drives
This is not an appropriate choice for speed • Need to replace or upgrade drives
control in a true process control loop. every 4 – 7 years
However, in a system requiring a one-time, • Harmful impacts to other equipment
permanent adjustment this seems, on the on same electrical service.
surface, to be a fairly viable option.
Adjustable pulley drive systems, or sheaves,
operate using the same principles and
function by moving the sides of an
adjustable drive pulley towards or away from
each other. This changes the effective pitch
diameter of the pulley, and therefore, the
output speed. While capable of continuously
modulating speeds in response to a process
loop, these systems tend to be hard on belts
and produce relatively high mechanical
losses. Unfortunately, the losses associated
with these types of systems can be fairly Figure 1: Low Horsepower VFD’s
substantial. In a pulley driven system there
are efficiency losses due to friction between Due to their apparent low initial cost, Many
the belt and the pulley, non-symmetrical industrial users think of VFD’s as the de-
loading of the motor bearings, and non- facto standard for process speed control.
symmetrical forces on the load bearings. This technology has a long history and the
Recent experience at a Wastewater market price continues to decline. These
Treatment plant in the U.S. demonstrated factors make the VFD appear, on the
energy efficiency losses of 16% - 25% due surface, to be a good value. However,
to belt slippage, friction, and bearing wear in VFD’s also carry with them several issues
a pulley system. that negatively affect not only the efficiency
of the process they are used in, but also the
reliability of those processes.

3. Manufacturers’ literature lists Harmonic
Filter inefficiencies that range from 1% to
3%.
Also, depending on the quality of power
available to the VFD, power-conditioning
equipment may be required. This
equipment is another source of lost
efficiency in the range of 1% - 2% and it is
critical that the process owner fully
understand what this impact will be.
VFD’s are essentially small, specialized
Figure 2: High Horsepower VFD’s computers and are subject to the same
environmental restrictions as a computer.
VFD’s control the speed of a motor by VFD’s should be installed in locations where
changing the input AC voltage into DC the maximum ambient temperature does not
voltage, and then “chopping” the DC voltage exceed 104° F, (40° C). If temperatures
into the desired frequency. Modern VFD’s higher than the VFD’s recommended levels
utilize microprocessors to control the are experienced, then de-rating of the VFD
frequency supplied to the motor in response will most likely occur. Because of this,
to the process loop’s feedback signal with cooling systems are often required for VFD’s
the most common type of VFD being the used on larger motors. Cooling systems,
pulse width modulating design. This whether water-cooling or air-conditioning,
process, however, has been directly tied to use energy to operate which is energy not
system harmonics, increased motor heating, being used to produce goods. (A rule of
damage to motor bearings (fluting), and thumb found in the literature is that
distortion of the voltage on the balance of approximately 1 kWh of cooling system
the grid. In addition, VFD’s often require energy is required to remove 3 kW of
ancillary equipment (air conditioning and generated heat.) Depending on the size of
transformers, for example) that also the VFD, the wasted energy required to
negatively impact the energy efficiency of operate the necessary cooling system can
the VFD installation. be as much as 10%.
Typically, VFD manufacturers will advertise Additionally, when sizing air conditioning for
efficiencies in the range of 96% - 99%. It is VFD’s installed in locations above 3000 ft
important to note, however, that these (1000 m) above sea level, it is important to
efficiencies are for the VFD alone and do not remember that cooling capacity must be de-
take into account losses associated with the rated as a result of the reduced capacity of
inefficiencies listed above. the air to convectively remove heat from the
system. A common rule of thumb is a 2%
As a variable frequency drive operates, it de-rating for every 1000 ft above the 3300 ft
introduces harmonic currents into the driven altitude level.
motor’s windings. These harmonics flow in
an opposite direction to the desired current In VFD installations that involve long wire
and effectively reduce the capacity of the lengths, it is often recommended that LC
windings to transfer usable power. Energy Filters be installed in the motor leads. (“L”
losses due to harmonics are typically seen stands for inductance and “C” stands
as heat losses. The effect of these for capacitance. Sized properly, this
harmonic currents can be up to an increase combination of inductance and capacitance
of 5 – 10% in motor heating and, therefore a will filter out harmful resonant frequencies.)
equivalent reduction in system efficiency It is a well established fact that VFD’s that
due to these energy losses. Harmonic Filters have long lead distances between the motor
can be used to minimize the impact of these and the drive unit generate reflected
currents on the system’s efficiency but these “standing waves” in the leads that can result
filters aren’t without an efficiency cost. in output pulsations of up to 3 times line

4. voltage. Since motor manufacturers So, anything that can be done to reduce the
typically use insulation rated for twice line Reactive Power component of electrical
voltage, the effect of these pulsations is that systems will result in increases in cost
the motor’s insulation begins to degrade efficiencies for that system’s user.
leading to shorts in the motor windings.
“Inverter Duty” motors address this issue by The VFD energy efficiency losses due to
using higher voltage insulation in their ancillary equipment such as filters,
construction. While this is an effective power conditioning and cooling
solution, the cost of these motors can be equipment, and transformers can be
significantly higher than regular duty motors. significant. Therefore, despite advertised
The option of using LC Filters, therefore, is efficiencies by manufacturers of 96 to
widely used in industrial applications using 99% for the VFD, true VFD system energy
VFD’s. While these filters are effective at efficiency could be below 75%!
minimizing the voltage surges being seen by
the motor, they do reduce the system’s Eddy Current Drives:
efficiency. Typical inefficiencies for LC
Filters are in the range of 3%. Eddy Current Drives were first introduced in
the 1930’s for use on railcars. Their use
Industrial customers who have medium and quickly spread throughout industry such that
high voltage applications in their facility also the leading supplier, Eaton, was selling over
face another efficiency drain when they $100 million per year of these drives during
utilize transformers to drop the line voltage the 1970’s. Since that time, there has been
down to a level that can be handled by very little product innovation in this category
standard VFD’s. Medium and high voltage of VSD’s and relative market share has
VFD’s are much more costly than standard fallen. Two of the largest suppliers in North
VFD’s. A quick study of transformers listed America are TECO/Westinghouse and
in various manufacturers’ literature shows Dynamatic.
inefficiencies ranging from 1% - 4%.
In addition to the direct losses of efficiency Eddy Current Drives use an electromagnetic
caused by the ancillary equipment typically coupling between the motor and the load
required by VFD installations, another, often that it is driving. The motor runs at its rated
disregarded, concern is the effect on the speed at all times while the speed of the
user’s power factor. load is adjusted. There are two basic
configurations of Eddy Current Drives
currently on the market – foot-mounted
Apparent Power (Figure 3) and shaft-mounted (Figure 4).
(KVA) Reactive
Power
(KVAR)
O
Actual Power (KW)
Power Actual Power
= = Cosine O
Factor Apparent Power
Figure 3: Foot Mounted Eddy Current Drive
In the equations above, Actual Power is the
electrical power actually used to do work in
a system and Reactive Power is the
electrical power that is absorbed/wasted as
a result of any non-work producing loads
within the same system. Apparent Power is
the trigonometric addition of Actual and
Reactive Power. Unfortunately, the power
company charges customers for Apparent
Power. Figure 4: Shaft Mounted Eddy Current Drive

5. The basic principle of operation of an Eddy
Current Drive is that an armature, typically a
steel drum that may have another
conductive material as a lining, is attached
to the drive motor. This armature assembly
turns at the nameplate speed of the motor at
all times. Attached to the load shaft is a
multi-pole electromagnet. A variable DC
current is supplied to the electromagnet. As
the current to the electromagnet is
increased, the magnetic field increases. As
the magnetic field moves relative to the
armature, eddy currents are created,
magnetically coupling the two elements and
transferring torque from the motor to the
load. Alignment between the armature and
the electromagnet is maintained with
supporting bearings within the drive unit
(Figure 5). Figure 6: Comparison of VSD Efficiencies
Despite the data showing that the stand-
alone Eddy Current Drive has
approximately 6% worse energy
efficiency than a stand-alone VFD, true
Eddy Current Drive system energy
efficiency will often be better than found
with a VFD system due to VFD ancillary
equipment inefficiencies.
Figure 5: Eddy Current Drive Schematic Fluid Drives:
According to published data, Eddy Current The principle of using fluid to transmit power
Drives will provide increased efficiency when was introduced in 1905 by the Vulcan
compared to restrictive flow control. Again, Engineering Company and was originally
this is because Eddy Current Drives follow applied to driving a low-speed ship’s
the principles of the Affinity Laws. However, propeller with a high-speed steam turbine.
because the Eddy Current Drive technology The advantage of this new Fluid Drive was
uses electrical power to energize the that it allowed for speed reduction without
electromagnets, there is a loss of efficiency the use of complicated and temperamental
when compared to stand-alone VFD’s. gear systems. In 1930 the first variable
Several industry publications present this speed Fluid Drive was installed in England
graph showing an energy efficiency with the first U.S. installation following
difference between VFD’s and Eddy Current shortly in 1932. At the time of introduction,
Drives of about 6% (Figure 6). Fluid Drives offered the benefits of smooth
and continuous control of speed using a
Generally, Eddy Current Drives require constant speed motor. Compared to the
water cooling above 200 HP. This water options available at the time (gears and
cooling requirement is a direct reduction in pulley systems), the Fluid Drive was an
total system energy efficiency. Also, shaft attractive technology.
mounted Eddy Current Drives typically are
used in combination with belts and pulleys. While the technology has continued to be
Belts and pulleys will reduce overall system available to this time, the operating
efficiency. However, the inefficiencies due principles and designs of Fluid Drives have
to ancillary equipment with Eddy Current remained rather stagnant. The largest Fluid
Drives are generally much less than the Drive suppliers today, (Falk and Voith), have
inefficiencies associated with VFD’s. introduced small changes to their drives, but

6. overall the technology is the same as it was found in magnetic drives, in a Hydrokinetic
in the 1930’s. Drive it results in a temperature rise within
the drive’s oil. As the oil temperature
The category of Fluid Drives consists of increases, its viscosity and ability to transmit
Hydroviscous (HV) Drives and Hydrokinetic torque decreases. Because of this, oil
(HK) Drives. Fluid Drives use a hydraulic cooling heat exchangers are typically
fluid to transmit torque between an impeller required in a Hydrokinetic drive. The pump
on the motor side of the drive and a rotor on used to circulate the oil through the heat
the adjustable-speed load shaft. exchanger is driven through a power take-off
Hydroviscous Drives transmit motor torque from the motor shaft.
though a thin film of oil that is captured
between discs attached to the motor shaft Fluid Drives are advertised as one of the
and corresponding discs that are attached to most energy efficient VSD options
the load shaft. The amount of torque that is available to industrial users. However,
transferred between the two sets of discs is despite manufacturers’ claims, Fluid
determined by varying the amount of Drives (both HK and HV) are actually less
pressure being used to press the two sets of efficient than Eddy Current Drives due to
discs together. A separate hydraulic the losses described above.
cylinder is used to provide this pressure.
Permanent Magnet Adjustable
Hydroviscous Drives offer the benefit of Speed Drives
being mechanically “locked” together at
100% speed. This theoretically provides for The introduction of Permanent Magnet
complete transfer of torque from the motor Adjustable Speed Drives (PMASD’s) by
to the load. Of course, any misalignments MagnaDrive Corporation, a U.S. company
between the motor and load shafts will result based in Bellevue, Washington, has given
in a constant flexing of the drive the process industry another option to
components. Energy spent in this way is consider when reviewing adjustable speed
energy that is not being used to drive the applications. The company’s technology
process and is a loss of efficiency. has one of the fastest adoption rates for a
new introduction in its targeted industries
Hydrokinetic Drives operate in a manner with over 5,000 installations currently
similar to the automatic transmission in an operating.
automobile. While there is no physical
connection between the impeller and the The benefits of MagnaDrive’s technology
rotor, there are supporting bearings within are evidenced by the company’s close
the drive to maintain the proper clearances relationship with the US government. Early
and alignments. During operation of a funding for MagnaDrive was supplied by a
Hydrokinetic Drive, there is a small amount grant from the US Department of Energy
of slip between the impeller and the rotor. (DOE). Also, DOE testing demonstrated
Additionally, as the hydraulic fluid moves that MagnaDrive’s products reduce energy
within the drive housing, there will be usage by up to 70%. The DOE operates
frictional losses between the oil and the several MagnaDrive units in mission critical
walls of the housing. Industry data shows applications at nuclear facilities. The US
that these losses can be as much as 5% - Navy and US Air Force also utilize
7% when compared to Eddy Current Drives. MagnaDrive technology. The Navy operates
One feature of a Hydrokinetic Drive that is well over 1,000 units across ten different
found in manufactures’ literature is its ability ship classes in its fleet.
to accept some degree of misalignment
between the motor and load shafts. It is PMASD‘s consist of two primary
important to note that, in order to allow for components. The first component, a set of
this misalignment, there are clearances copper conductor plates, is connected to the
between the impeller and the rotor which motor shaft; the second component of the
may cause inefficiencies within the drive. permanent magnet drive is a rigid assembly
While the small amount of slip in a of permanent, rare earth magnets which is
Hydrokinetic Drive is consistent with what is connected to the load. During operation,

7. relative motion between the parts creates an speeds from 90% - 100% of rated motor
interwoven, eddy current field that transmits speed PMASD’s are about 5% more efficient
torque across the air gap. To adjust the than VFD’s. From 80% - 90% of rated motor
speed of the driven load, the amount of speed the energy efficiency of the two
torque transmitted from the motor to its load technologies are the same. Below 80% of
is controlled by changing the distance rated motor speed, VFD’s are approximately
between the conductor plates and the 5% more efficient than PMASD’s. Because
magnet assembly. During operation and more energy is used at the higher load
throughout the entire speed range, no speeds, the average efficiency of a PMASD
physical connection exists between the is approximately 3% - 4% higher than an
motor and the load (Figure 7). This equivalent stand-alone VFD. When total
“disconnected connection” has system energy efficiency is looked at, the
demonstrated vibration reductions up to PMASD is clearly the better alternative.
85%.
In comparison to stand-alone Eddy Current
Drives, stand-alone PMASD’s have higher
energy efficiency because they utilize
permanent magnets while the Eddy Current
drive must energize its electromagnets from
an external power source. This advantage
for PMASD’s can only grow when looking at
total system energy efficiency due to Eddy
Current drive system losses in pulleys and
water-cooling.
Figure 7: Operation of Permanent Magnet ASD
Generally, Fluid Drives have much lower
PMASD’s can be used on any centrifugal total system energy efficiencies than
load application with the current technology PMASD’s. The advantage of PMASD’s
able to handle up to 4,000 HP loads. becomes even more obvious when non-
Because the speed of the load is changed energy operating lifecycle costs are
while allowing the motor to continuously considered.
operate at its nameplate speed, motor
The potential value of permanent magnet
heating and the resulting inefficiencies are
technology is clear. In site evaluations,
not an issue. Also, because the permanent
the permanent magnet coupling concept
magnet drives are mechanical devices, they
for speed control has been proven
do not introduce harmonics into the user’s
across all industries. In comparison to
power grid. Filters, transformers, and air
available baseline energy information,
conditioning systems are not necessary.
the use of a permanent magnet drive can
result in significant energy savings.
The eddy currents being produced in a
Additionally, the disconnected nature of
PMASD are created, as noted above, by
these drives also provides the industrial
relative motion, called “slip”, between the
user with significant savings in
conductor rotors and the magnet rotors. In a
maintenance and operational costs.
typical PMASD, this slip results in an
approximate speed loss of 1% - 2% at full
References:
speed. The only ancillary equipment
1. AC Drive Worldwide Outlook. ARC Market Study.
inefficiencies are due to an actuator (used to 2. Coyote Electronics, Inc. Fort Worth, TX.
vary the size of the air gap), and the need 3. DSI – Dynamatic, Inc., Sturtevant, WI.
for water cooling above 600 HP. 4. Emerson Electric, St. Louis, MO.
5. Frost & Sullivan. N Amer & Euro VSD 2005 Report.
6. Hydraulic Institute, Variable Speed Pumping, 2005.
Studies conducted by the DOE show that 7. Jones, Garr. Pumping Station Design, Butterworth-
PMASD’s save 60% - 70% on energy costs rd
Heinemann, 3 ed., Woburn, MA, 2005.
versus restrictive flow control methods. 8. Kuphaldt, Tony (2003). URL:
www.allaboutcircuits.com.
Also, looking only at stand-alone efficiencies 9. MagnaDrive Corp., Bellevue, WA.
of the VFD (ie: not taking into account 10. US DOE. Industrial Electric Motor Market Study.
ancillary equipment losses) at operating 11. Voith Turbo GmbH & Co., Crailsheim, Germany.

8. Total System Energy Efficiency Comparison of Various Technologies
Technology Stand-Alone Total System Description of
Energy Efficiency Energy Efficiency Potential Losses
Restrictive Flow Highest energy cost – does
30% - 40% 30% - 40%
Control not follow Affinity Laws
Energy savings due to
Affinity Laws. Energy losses
Pulley Systems 85% - 90% 75% - 85%
from belt slip, friction, and
bearing wear
Energy savings due to
Affinity Laws. Losses from
VFD’s 96% - 99% 75% - 90%
ancillary equipment and
vibration from misalignment
Energy savings due to
Affinity Laws. Losses due to
Eddy Current Drives 90% - 93% 80% - 90% electromagnet energizing,
pulleys, and water cooling
above 200 HP
Energy savings due to
Affinity Laws. Losses due to
Fluid Drives 83% - 87% 80% - 85% fluid viscosity changes, oil
circulation pump, and
vibration
Energy savings due to
Affinity Laws. Losses due to
PMASD’s 97% - 99% 94% - 97%
actuator and water cooling
above 600 HP