Journal of Power and Energy Engineering, 2014, 2, 448-456
Published Online April 2014 in SciRes.
How to cite this paper: Dlamini, V., et al. (2014) A Motor Management Strategy for Optimising Energy Use and Reducing
Life Cycle Costs. Journal of Power and Energy Engineering, 2, 448-456.
A Motor Management Strategy for
Optimising Energy Use and Reducing Life
Cycle Costs
V. Dlamini, R. C. Bansal, R. Naidoo
Department of Electrical Electronic & Computer Engineering, University of Pretoria, Pretoria, South Africa
Received January 2014
With increasing energy costs and renewed focus on using energy in ways that support the env i-
ronment, a structured approach is required to ensure that energy is used efficiently. A compre-
hensive motor management strategy to reduce motor life cycle costs while increasing reliability is
presented. The application of energy management principles is combined with benefits that can
be obtained from using energy-efficiency motors. An economic model for determining the optimal
time a motor should be replaced with a higher efficiency motor is proposed. The strategy pre-
sented incorporates benefits that can be obtained from using in-situ motor efficiency estimation
and condition monitoring techniques as part of a motor management system.
Motor Efficiency; Motor Management; Maintenance; Vibration Signature Analysis; Energy
Efficiency; Motor Replace ment
1. Introduction
Electric motors are a key part of industry. They are used in a wide variety of equipment and processes. This in-
cludes fans, pumps, compressors, conveyor drives and machine tools. Motors are a leading power consumer due
to their widespread use in industry. Motors can account for more than two thirds of the electrical power con-
sumption in some countries. As the cost of electricity continues to increase, motors provide a great opportunity
to reduce energy consumption.
Energy conservation technologies can reduce the energy consumption by an estimated 11% to 18% [1]. The
reduced energy consumption results in a reduction in operating costs for businesses. This means less power has
to be generated which, in turn reduces the harmful greenhouse gases emitted into the atmosphere.
Motor management can be described as strategies that focus on reducing the total cost of ownership of motors
in a plant. The cost of ownership of motors includes the energy cost of running a motor, the cost of purchasing
motors, the cost of maintaining motors, and the business cost incurred as a result of motor-related process inter-
ruptions [2]. A comprehensive motor management strategy incorporates the benefits of the latest technology and
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the application of best practices to the repair of motors in a plant. The ultimate aim of a motor-management
strategy is to ensure a reliable plant at the lowest possible motor-related costs [3].
In this paper an overview of energy management is presented with a focus on electric motors. The benefits
that can be obtained from using energy efficient motors are discussed. The cost savings that can be obtained
through developing a structured approach to motor management are investigated. An economic model for de-
termining the optimal time a motor should be replaced with a high efficiency motor is presented. The application
of non-intrusive motor efficiency estimation and condition monitoring techniques is proposed as part of an inte-
grated real time motor management system.
2. Energy Management Overview
An energy management program must seek to minimize the adverse impact on the environment. This can be
achieved by understanding how the business uses energy and creating an awareness of energy saving. Efficient
maintenance structures must be put in place [4].
Motor energy management strategies focus on load management, efficiency management and power factor
correction [5]. A starting point for energy management is to perform an energy audit. This determines how
power is consumed by the plant. Once an audit has been conducted, energy-saving opportunities can be identi-
fied. Plans for implementing them can be put in place. The energy audit identifies the following areas for poten-
tial improvements [4].
The efficiency of the operations.
The efficiency of the billing systems.
Efficiency of the maintenance activity.
The efficiency of operations entails assessing the design and operation of the different processes in the plant
to determine if they use energy efficiently. A motor energy audit must focus on the motor sizes and determine
how well they are matched to the load requirements. Incorrect motor sizing has a negative impact on the effi-
ciency of the motor. A motor that is larger than required results in operation at a lower efficiency. This translates
to energy loss. Motors usually operate at their highest efficiency at between 75% to 80% of their rated load. The
efficiency and power factor both decline as the load reduces. Motors are often oversized to allow for higher fu-
ture loads or to make provision for short-term load requirements [3].
The billing system must be analyzed to ensure that the economic tariff structure is optimal while considering
the plant’s operational requirements. It is also important to determine the contribution made by motors to the
overall energy consumption of a plant. This allows for the calculation of potential savings which can be realized
through motor management strategies. The maintenance activity within the plant has to be assessed to determine
the standard. Poor maintenance results in a reduction in efficiency.
A portable instrument for measuring and logging of motor load profiles and estimating efficiency is a valua-
ble tool to conduct an energy audit. It could further be used to determine the power factor of the motor.
Once the data from an audit has been collected, it must be analyzed and opportunities for energy savings
should be identified. Action plans must then be put in place. The following alternatives can be implemented:
a) The motor can be kept intact.
b) The motor can be replaced with a new standard motor.
c) The motor can be replaced immediately with a higher efficiency motor.
Control systems such as flux optimization or variable speed drives can be implemented to improve the effi-
ciency of the motor [5].
The action taken depends on the load or process requirements. After analyzing the plant processes, opportuni-
ties for optimizing the process efficiency through implementing a control strategy can be identified. A thorough
economic comparison of the available options is necessary to maximize savings.
3. Motor Replacement
In this section the development of a motor replacement strategy is discussed. Tools for economic analysis of the
potential benefits of replacing motors with energy efficient motors are presented.
3.1. Motor Replacement Strategy
The cost and environmental benefits of replacing a standard efficiency motor with an energy-efficient motor
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have been highlighted. A motor replacement strategy has to be developed to achieve the benefits. This ensures
that clear guidelines exist on how to ensure that motors are operated with the desired reliability and at optimal
life cycle costs. An installed motor can be replaced with an energy efficient motor under the following condi-
tions [6]:
a) When a motor has failed;
b) When a new motor is required for an application; and
c) When a motor currently in operation is to be replaced.
In each of the cases an economic evaluation of the available options has to be performed to quantify the bene-
fits. A repair/replacement strategy has to consider the following:
The impact on energy usage;
The cost of the capital to be spent;
The motor size;
The motor repair cost;
The motor operating and repair history;
The replacement motor cost; and
The availability of a replacement [7].
The energy usage and efficiency of the installed motor must be compared to an energy-efficient replacement
motor. An energy-efficient motor provides an opportunity to reduce the cost of energy for operating the plant.
There is an opportunity to assess if the motor is properly sized for the load. An oversized motor operates at a
lower efficiency level. This results in energy wastage. The repair cost can be used to make the repair/replace de-
cision. If the motor repair cost exceeds a certain percentage of the replacement cost, the motor must be replaced
with an energy-efficient motor. The motor operating and repair history is an important factor when making the
decision. A strategy for repairing/replacing a motor must consider the reliability of the motor and the probability
of future failures. If a motor has been repaired for a predetermined number of times, it should be scrapped. The
availability of an energy-efficient replacement motor needs to be investigated. If there is a long lead time for the
replacement motor, then the production losses incurred until replacement might be excessive. The lead time for
the motor repair has adverse consequences if there are no spares and it runs critical process equipment. The
above-mentioned factors need to be taken into account when doing an economic evaluation.
New installations present a good opportunity to introduce energy-efficient motors on a plant. The plant will
yield benefits of using energy-efficient motors. The motor strategy for a plant must specify that all new motor
installations use energy-efficient motors. A detailed analysis can be done to determine the feasibility of intro-
ducing an energy efficient motor as a replacement for a standard-efficiency motor that is still operational. A
proposal for how motor replacement decision should be made is presented after a discussion on how motor
mai ntenance influences such decisions.
3.2. Economic Analysis
In order to replace a standard efficiency motor with an energy-efficient motor, a capital investment is required.
Before a capital investment is made, an economic analysis has to be performed to determine the return on in-
vestment. The return on investment is used to determine the economical feasibility of purchasing a new motor.
A challenge in implementing a motor replacement or repair strategy is that the financial benefits of the invest-
ment may only be realised a few years later. The justification for the capital investment has to be made at the
time the motor is replaced. Methods for performing the required economic analysis are presented in this section.
When comparing different economic investment options, it is necessary to convert them to a common base.
Numerous techniques can be used to enable such a comparison. The most widely used methods for enabling
economic comparison are the payback, net present value, internal rate of return, project balance and annual
equivalent methods. Although all of the tools mentioned can be used, the preferred methods are the net present
value and payback methods. These two methods and their application to motor comparison are investigated in
detail [4].
To evaluate as to whether replacing a motor with an energy-efficient motor is feasible, information on the
process and motors is required. The electricity tariff structure, annual motor load profiles and motor efficiency
curves are required to determine the annual power consumption of the motors to be compared. In a plant where
there is an established energy-management structure, motor load profiles for each motor may exist. Where such
information is not readily available, a power-logging instrument can be used to determine the motor load profile.
V. Dlamini et al.
The efficiency estimation technique presented in [8] can be used to determine the motor efficiency curve at
each point of the motor load cycle. The efficiency curve of the energy-efficiency replacement motor can be re-
quest ed from the motor manufacturer. The effective interest, energy cost inflation rate, cost of motor replace-
ment and its expected operating life are additional information required to determine the net present value of the
inves t me nt.
The annual savings that will be realised by replacing a motor with an energy-efficient motor are given by (1),
savingout rcr
= −
is the motor rated power in kW,
is the percentage of full load divided by 100,
is the annual
motor operating hours,
is the average energy cost per kWh,
is the efficiency of the motor currently in-
stalled and
is the efficiency of the energy-efficient replacement motor. In (1) it is assumed that the motor
will operate at the same load when it is in service. In order to be able to compare the total savings that will be
achieved, the net present value of the savings have to be calculated over the motor’s expected operating life.
Once the annual savings have been determined, the net present value of the savings that will accrue over the
motor’s operating life can be determined. The net present value is a method that is used to bring the savings that
will be realised over an extended period to their present equivalent. It takes the time value of money [4] into ac-
count. This calculation must include the cost of the capital required to purchase the new motor and a projection
of the expected inflation rate for the cost of energy over the operating life of the motor. To determine the present
value of the savings, it is necessary to calculate the effective interest rate using (2).
100 1
= −
is the effective interest rate,
is the expected annual energy cost inflation rate and
is the re-
quired internal rate of return on investments. The inflation rate and internal rate of return are assumed to be con-
stant over the calculation period. After the effective interest rate has been calculated, the present value of the
savings to be obtained can be determined from (3).
( )
( )
saving savingn
is the annual savings,
is the effective interest rate and
is the expected operating life of the
new motor. The present value of the savings obtained can then be compared to the expense that will be incurred
in purchasing the new motor. If the cost of the new motor is less than the net present value of the savings that
will be achieved, a business case can be presented.
In providing economic justification for motor replacement, the payback period can be used as an alternative
method for making the decision. The payback period for a motor replacement study is the time it will take for
the benefits of replacing the current motor to exceed the capital invested in purchasing the new motor. The pay-
back period can be calculated using (4), [9].
( )
ln 1
motor saving
iC A
is the payback period in years,
is the annual savings,
is the effective interest rate and
is the cost of the replacement motor. The cost of the new motor must include labour and downtime for
the installation and uninstalling. When using the payback period for deciding the feasibility of replacing a motor,
a project with a shorter payback period is most feasible.
In determining the annual savings it should be noted that Equation (1) is only applicable to motors that oper-
ate under constant load. This is because the efficiency of both motors under consideration will vary with differ-
ent loading points. The annual savings do not take into account the demand charge for electricity. In cases where
the motor is considered to make an appreciable contribution to the maximum demand, the annual energy savings
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calculation has to be modified to take this into account [10]. The power saved in kW can be calculated using (5).
saving outcr
= −
The annual savings that will be achieved by reducing the maximum demand are determined using (6).
savingssaving c
D PD=××
is the power saving in kW and
is the demand charge. The total annual saving,
given by (7). The total annual saving can be substituted for the annual savings in the expression for calculating
the net present value and the payback period:
saving saving savings
T AD= +
If the motor load profile is not constant the equations presented need to be applied to each relatively constant
portion of the motor load cycle that is relatively constant.
An example of such a load profile is shown in Figure 1.
The annual savings are determined as follows:
1. The calculation would be done for each of the three loading points, A, B and C. The annual operating hours
would be determined by multiplying the daily hours at each loading point by the number of days the motor
operates per year.
2. The output power for each of the loading points would be determined.
3. The annual saving would be calculated using the efficiency at each loading point.
4. The total annual saving would be determined using (8) as the sum of the savings for each loading point:
savingout r
= −
In developing a motor replacement policy the tools that have been presented can be applied. It is recom-
mended that minimum economic requirements be determined and adopted for a company. This guides decision
making on motor replacement based on either the net present value or the payback period. A decision should not
only be based on motor replacement cost. It must factor in the expected future repair costs of the older motor in-
stalled in the plant. A comprehensive economic analysis must also explore the benefits obtained from power
factor correction and the impact of available rebate programmes for energy-efficient motors. This will form an
integral part of a motor-management strategy.
Figure 1. Motor load daily load profile.
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4. Motor Maintenance
The importance of a good maintenance strategy in an organization is often understated. Table 1 shows the con-
tribution made by maintenance-related to the total operating costs [11]. This varies across different industry
sectors. The maintenance cost is made up of the following components.
a) The direct cost of repairs
b) The cost of any pro-active work (labor, materials, contractor, etc)
c) The cost of lost revenue and reputation due to downtime
d) The cost of any penalties that are incurred as a result of damaged products or operating systems
It is can be seen from Table 1 that the cost of maintenance can be too significant to ignore in any plant. A
good motor maintenance program extends the life of motors and improves system availability. This will result in
a reduction in maintenance costs and downtime-related losses [3]. A motor-maintenance strategy must have
guidelines for motor storage, installation, operation and repair.
Motors that are kept in storage for have a higher probability of failure if they are not stored according to best
practices. The way in which a motor is installed has a significant impact on its reliability and operating life.
When a motor is installed it is essential to ensure that is has the proper foundation and alignment procedures in
place to minimize additional stresses to the motor. This can exceed the design limits [3]. Motors should be oper-
ated according to t he manufacturers’ guidelines to ensure that they achieve the design life.
A reliability-centred maintenance (RCM) approach has been found to be effective for motors [12]. RCM is a
proactive maintenance strategy that has processes for anticipating which failure modes will occur [11]. Once the
failure modes have been determined the consequences of each failure mode are analyzed. Plans are developed to
eliminate or minimize the consequences of each of the failure modes.
The repair of motors presents an opportunity to implement policies that will result in long-term savings. The
repair of motors must be governed by guidelines to ensure repairs are only carried out when it is financially
feasible. Managing motor repairs starts with establishing repair specifications and identifying suitable suppliers
that can provide high-quality repairs [7]. Agreements must be put in place with supplies to ensure that the repair
of motors is done according to industry best practices. The motor repair decision flowchart in Figure 2 illu-
strates how decisions can be made when a motor fails in order to minimize the life cycle costs. Keeping a de-
tailed motor repair history is important. Analyzing the trends can allow for estimating the life of the motor based
on its reliability and age. This can prevent the repair of a motor that is near the end of its life and allows for the
introduction of an energy-efficient motor in its place. The proposed decision-making process focuses on using
all available data to make decisions that will realize cost savings over the operating life of the motor and plant. It
guides a motor manager to take a holistic approach that takes into account energy efficiency and long-term ben-
5. Motor Management Strategy
The discussions of the preceding sections lead to the development of the proposed motor-management strategy
to ensure that motors in a plant have the lowest possible operating cost and very high reliability. A total mo-
tor-management approach with real time efficiency, load factor, power factor and vibration-based condition
monitoring is proposed. The collected date will be stored on a database for use in the RCM based approach.
With the availability of the relevant data, the RCM process should provide a comprehensive preventative main-
tenance program that will ensure that motors operate at high reliability.
Table 1. Contribution of maintenance to operating costs.
Industry Contribution (%)
Min ing 20 - 50
Primary metals 15 - 25
Electric utilities 15 - 25
Manufacturing 5 - 15
Processing 3 - 15
Fabrication and assembly 3 - 5
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Figure 2. Motor repair decision flowchart.
An accurate method of estimating the efficiency of in-service motors is needed in order to determine the per-
formance of installed motors without disrupting the motor driven process. In the proposed system the motor ef-
ficiency is estimated using a non-intrusive implementation of the compensated slip method. The motor speed is
accurately estimated using motor vibration signature analysis [8]. The estimated efficiency can then be used to
calculate the annual savings that will be realized by replacing an installed motor with an energy-efficient motor
using (1). If the load in not constant than potential savings it can be obtained with (8). This can be incorporated
into a motor management support system as described below.
The vibration signal may also used for condition monitoring since motor vibration signature analysis has been
widely studied and applied in this field. Various types of motor faults can be detected from the vibration signa-
ture of a motor. These include faults such as winding faults, unbalanced stator and rotor parameters, broken ro-
tor bars, eccentricity and bearing faults. The faults are detected as harmonics using vibration signature analysis
[13]. The condition monitoring data should be used as input data in the RCM system. It provides the data re-
quired to successfully implement the replacement decision model in Figure 2.
A motor management support system that will make implementing the motor-management strategy easier and
more efficient should be used. In Figure 3 the proposed plant wide motor management system is shown. This
system consists of local field instruments at each motor for measuring its voltage, current and vibration. The
collected data are sent via a wireless network to a central processor. At the central processor the non-intrusive
speed and efficiency estimation technique is implemented [8].
Energy usage and reliability indicators for each motor can also be calculated at the central server.
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Figure 3. Motor management support system.
The proposed system should include the vibration based condition-monitoring tools with the fault detection
algorithms implemented on the centralised processor. The processed data are sent to a Supervisory Control and
Data Acquisition (SCADA) system so that historic data can be accessed for use in motor replacement and ener-
gy-manage me nt studies. Alarms can be sent to the SCADA system to inform control room operators or main-
tenance personnel of developing motor faults so that corrective action can be planned and scheduled. This will
result in cost saving because unplanned downtime will be reduced.
6. Conclusions
An overview of motor energy management has been presented. Tools for performing an economic comparison
of motors were discussed. The tools can be used to determine the annual savings that can be realized by using an
energy-effic iency motor, as well as the payback period on the initial capital investment. The non-intrusive com-
pensated slip method can be used to estimate the efficiency of an installed motor. This enables the above-men -
tioned calculations to be made.
The motor speed estimation technique based on vibration signature analysis can be used for detecting numer-
ous fault conditions. A method for deciding how to handle motor failures has been proposed. This method takes
into account the economic aspects of repairing the motor, whilst considering the reliability history of the motor.
It provides a guideline for how energy efficient motors can be introduced taking into account the life cycle costs
including the cost of energy.
A comprehensive motor-management strategy is proposed. This motor management strategy centres on find-
ing the correct balance for energy efficiency, motor reliability and maintenance costs. This is achieved by using
a motor-management system built around a network of field sensors to provide real-time data on the condition
and performance of motors in a plant. This allows for proactive decisions to be made to minimize breakdowns
and optimize energy usage.
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