Electric motors are widely used in various sectors where mechanical energy is needed. It is an electromechanical device which converts electrical energy into rotary mechanical energy. This output is then further converted to provide the needed final use-‐energy. The two main components of motor are the stator (stationary element) and the rotor (rotating element).
Figure 1 illustrates the major parts of the electric motor. The different parts are described here and numbered according to the figure. It consists of a stator housing (1), ball-‐bearings (2) that support the rotor (9), bearing blocks (3) for positioning of the bearings and as a finish for the stator housing, fan (4) for motor cooling and valve casing (5) as protection against the rotating fan. On the side of the stator housing a box for electrical connections (6) is located. An iron core (7) which is situated inside stator housing is made of thin (0.3 to 0.5 mm) iron sheets. A magnetic field is generated by phase windings and the stator core. The speed is determined by the number of pairs of poles at which the magnetic field rotates. The speed at which the motor rotates at rated frequency is called the synchronous speed of the motor.
The rotor (9) is mounted on the motor shaft (10). The rotor is made of thin iron sheets like the stator. When the rotor rotates in the magnetic field, it cuts the magnetic flux which induces a current (Iw) in the rotor. Further, a force is generated from the interaction between an electric motor's magnetic field and winding currents according to the Faraday’s law of electromagnetic induction. This force is determined by the flux density (Φ), the induced current (Iw), the length (l) of the rotor and the angle (θ) between the force and the flux density: F = Φ x Iw x l x sinθ (http://www.motorsystems.org/motor-‐basics)
Electric motor systems account for about 60 to 70 percent of industrial electricity consumption depending on the industrial structure (UNIDO, 2011). There has been extensive usage of electric motors not only in the industry sectors but also in the commercial, residential, agricultural and transportation sectors. Pumping, compressed air and fan systems are the significant energy users where consumption of electricity is dominant as shown in Figure 2. Besides, material handling and processing also consume a lot of electricity, although they are heterogeneous and differ from each other (UNIDO, 2011).
Among the various sector that contribute to mitigate greenhouse gas (GHG) emissions, the role played by the industrial sector is also significant. Thus, reducing GHG emissions from the industrial sector would reduce the overall GHG emissions. Energy savings and emissions reductions can be achieved by 10-‐30% by reducing total energy use or by increasing the production rate per unit of energy used (Saidur et al., 2009). By contrast, to reduce GHG emissions, enhancing energy efficiency is a key role. Therefore, energy research organizations and governments should emphasize the importance of energy efficiency of motor in the industrial sector as a high priority.
There are number of benefits which Energy-‐efficient motors possess. Energy efficient motors have features with improved manufacturing techniques and superior materials, it usually have longer bearing lives, higher service factors as well as lower waste heat output, less vibration, all of which increase reliability.
Investments in improving energy efficiency of electric motor systems are often delayed or rejected due to barriers and market failures. Lacks of attention of the plant manager, higher initial cost for efficient motors, etc. are the major hindrances (UNIDO, 2011). Particularly in developing countries where access to capital is difficult to manage, very often energy efficient motors get less attention due to higher initial costs. In most cases, broken motors are rewound and reused, which do not help much to increase motor efficiency.
Policymakers have identified the opportunity of high energy efficient motor potentials. In line with this, some policies have already been introduced like minimum standards and motor labelling schemes in many countries. Besides, energy audit schemes as well as capacity development programs have also been focused in order to improve system efficiencies. However, progress has achieved to improve the efficiency of the motor system like use of variable speed drivers, rewinding the motor, power factor correction, etc. (UNIDO, 2011). Market has successfully transformed towards the higher efficiency motor market, while more new emerging technology with even higher efficiency are just about to enter the market.
Description of the technology
Efficient electric motors achieve greater efficiency by reducing the losses which account for only 3-‐6% of the energy that flows through the motor. As shown in Table 1, there are five categories of losses that occurr in a motor, including stator power losses, rotor power losses, magnetic core losses, friction and windage losses, and stray load losses (Emadi & Andreas, 2005). Among them, stator power losses consume the highest percentage (37% of total energy loss) share of energy loss that a motor accounts. Besides, stray load losses which are 16% of total energy loss can be reduced by redesigning stator winding, but each design change may increase losses in other areas. Moreover, rotor power losses, magnetic core losses and friction and windage losses can be minimized by using higher quality materials and optimizing the design for larger magnetic fields and greater electricity flow (Kreith & Goswami, 2007).
The most common practice in industry is to rewind burnt-‐out motors which exceed 50% of the total number of motors in some industries. It is a technique which can maintain motor efficiency at previous levels. But careful measures should be taken care off to rewind the motors, because in most cases it also results in efficiency losses. The effect of rewinding can reduce the motor efficiency such as winding material, winding and slot design insulation performance, and operating temperature. For example, when the windings get heated, this can damage the insulation between lamination, which further raises the eddy current losses. However, proper measures such as using wires of greater cross section and slot size permitting etc. would result in a reduction of stator losses and thereby increasing efficiency. However, original design and structure of the motor should remain the same during the rewind, unless there are specific load-‐related reasons for redesign (BEE, 2005).
Power factor correction by installing capacitors
Capacitors are often used to improve the power factor which is connected in parallel (shunted) with the motor. The capacitor itself will not be responsible to improve the power factor of the motor, but of the starter terminals where power is generated or distributed. The benefits of power factor correction include reduced I2R losses in cables upstream of the capacitor (and hence reduced energy charges), reduced kVA demand (and hence reduced utility demand charges), reduced voltage drop in the cables (leading to improved voltage regulation), and an increase in the overall efficiency of the plant electrical system (BEE, 2005).
Variable speed drives
Electric motors have traditional control methods using mainly two states: stop and operate at maximum speed. Motors are sized to provide the maximum power output required in most motor installations. In order to provide the maximum designed load, the rotational speed is kept constant at its optimum value and to match with the load the power input to the motor also remains constant at the maximum value. However, in order to have significant energy savings, rotational speed of the motor should be decreased when load decreases. Nevertheless, the majority of motors are operated only at 100% speed for short periods of time which often results systems operating inefficiently and significant energy losses during the operation time. To match the speed of the motor with the related load, VSD technique is a very popular choice. The speed of a motor or generator can be controlled and adjusted to any desired speed by using VSD. In addition, VSD can also keep an electric motor speed at a constant level where the load is variable (Saidur et al., 2009). Figure 3 illustrates the different components of conventional pumping system and energy efficient pumping systems.
This section illustrates how energy efficient motors have been successful in making contributions to significant energy savings with short payback period. Table 2 explains some of the case studies in different countries where efficient motor technology has been effectively implemented in companies and this give an idea of the energy savings that can be realized.
For more than a decade, many countries have started implementing labelling and minimum energy performance standard (MEPS) schemes with an aim to phase out the least efficient motor classes by setting minimum standards for the efficiency. The labelling helps to provide the necessary information which allows for easy comparisons of motor efficiency among producers and hence contributes to transforming the motor market towards high efficiency motors.
Boteler et al. (2009) reported that, both labelling and MEPS started in Brazil, China, USA, Europe, Mexico, Australia and Taiwan, resulting in several different national standards. But, due to variation in motor efficiency classes in different countries, it is difficult to make comparison and it turns out to be a considerable trade barrier. Therefore, the International Electrotechnical Commission (IEC) developed a test standards and labels as well as international efficiency classification for electric motors. The classification introduced by IEC had different efficiency levels with the label IE1 for the least efficient motors and IE4 for the highest efficiency motors. The defined efficiency classes are presented in Figure 4 for 50 Hz motors. The IE4 class has not yet been defined, but is expected to demand a further 15 percent reduction of losses in comparison to IE3. According to Figure 4, it is seen that, the expected savings and differences in efficiency are particularly high for smaller motors. The gap closes with increasing motor size which only have about 2 percent difference from IE1 to IE3 for 375 kW motors.
United States was the first country to introduce ambitious MEPS for electric motors. MEPS were passed into law as early as 1992, but it took five years to adapt to the standards and redesign their motors. This is the so called Energy Policy Act (EPAct) 92 standard is comparable to the international IE2. Figure 5 provides a detailed picture on implementation dates of the different standards by countries. The labels IE1 and IE2 have already been applied in Australia, New Zealand, Brazil, Mexico, and China. Besides, by 2015, the Implementation of IE3 class will be predominant in USA, Canada and EU countries. In South East Asia, Thailand and the Philippines are playing the leading role towards the development of national standards for energy conservation (Yanti & Mahlia, 2009). In Brazil, the first regulation of the energy efficiency act for electric motors was introduced in 2002 which established two sets of minimum efficiency performance standards (MEPS): one is the ‘standard’ (mandatory) and other is the ‘high-‐efficiency’ (voluntary) motors. Later, an updated regulation was launched in 2005 (Edict 553/2005) which strongly recommends the use of previous high-‐efficiency MEPS as mandatory for all motors in the Brazilian market (Garcia et al., 2007).
A look at the historic motor market data reveals that market transformation towards more efficient motors has taken place in the past. Figure 6 illustrates market share of motors in EU and USA. In Europe, the labelling has significant contribution to reduce the market share of the least efficient (Eff3) motors which dropped from about 68 percent in 1998 to 16 percent in 2001, and only 2 percent in 2007 (Boteler, 2009). However, labelling could not significantly improve the diffusion of high efficient IE2 (former Eff1) motor in EU due its high upfront cost. Moreover, the efficiency class IE1 (Eff2) has high percentage of market share (about 80 percent) in comparison to only 12 percent of IE2 (Eff1) in the EU. In the USA, NEMA premium motors (equal IE3 motors), have increased steadily since 2001 and reached close to 30 percent in 2006. In Canada, motors with IE3 or higher even accounted for 39 percent of the market in 2007. In Korea, IE1 motors had a market share of 10 percent in 2005, while 90 percent of the motors were less efficient.
Barriers to further development
There is still a need to educate the end users about the benefits of using energy efficient motors within the industrial sector. It is still a widely held belief that by purchasing a motor rated higher than the application demands, the motor will last longer and be more reliable (Meckrow & Jack, 2008).
Regardless of its advantages, some technical problems of efficient motors have created a lot of concern among motor users. The technical barriers which a motor generally have is the generation of harmonics into the mains supply, electromagnetic interference (EMI) susceptible equipment, earlier failure of old motors due to faster voltage rates of rise in the Pulse-‐Width Modulation (PWM) synthesized waveforms presented in most VSD designs, unreliable earlier versions of VSDs, etc. (Almeida et al., 2003). Moreover, high radiation noise may be generated by high PWM frequencies which may cause significant damage to the motor by producing bearing currents and insulation voltage stress.
There are several reasons which may led the energy efficient motors be rejected for economic reasons: insufficient running hours to give an acceptable payback, high ratings of equipment leading to higher initial costs that will adversely affect the payback, equipment with limited lifetime, earlier bad experiences of energy saving products or applications that have not delivered the expected benefits, etc. (Almeida et al., 2003). Furthermore, in developing countries the most efficient equipment are generally not produced locally and has to be imported at high prices. A case in China shows that imported VSD had 90% market share in year 2000, mainly due to poor quality/feature of local products (Nadel et al., 2002).
Within the industrial sector, the majority of motor and drive purchases are made by the original equipment manufacturer (OEM) and not by the end user. The OEM is concerned predominantly with selling cost, rather than lifetime cost and therefore has little motivation to improve efficiency (Meckrow & Jack, 2008).
Experience of many energy saving initiatives around the world, Nadel et al. (2011) found that, the most effective way to transform the market towards improved energy efficiency is a combination of technical information and financial incentives.
Almeida et al. (2003) noted that, in order to transform the motor market, there should have promotional/educational materials and schemes which will address their needs to implement successful energy saving projects. Some of these materials will also become helpful including, attendance at exhibitions, technical information on energy saving options, product selectors such as the motor data base EURODEEM1 and accreditation / labeling of products. Seminars, dissemination of information both through software programs (discs, internet) and printed documents, energy saving helplines, etc., also seem to have an effect in the motor market transformation.
To stimulate the market for energy saving products, financial incentives are playing very crucial measures. Such incentives can achieved by leasing, bidding, payment by savings rebates, penalties and loans. A successful example of penalties is the green taxes in Denmark, in which the collected funds are used to pay for the investments in energy-‐ efficient equipment, including EEMs and VSDs (Almeida et al., 2003).
The major benefit which energy-‐efficient motors have is the improved energy efficiency. The new technology has the potential to increase the productivity of the industry sector by providing the same output with less consumption of electricity. It has socio-‐economic benefits in terms of increased energy security and environmental benefits, i.e. lower GHG emissions, and lower environmental impact of electricity generation. Additionally, in case of financial return, energy efficient motors have significant cost-‐savings for the consumer over the life-‐cycle of the appliance and short payback period.
Efficient electric motor has the potential to reduce emissions associated with the energy savings by using VSD. The main Purpose of VSD is to control motor’s speed as per the load requirements. Introducing the VSD helps optimize the efficiency of the entire system (Mecrow & Jack, 2008). Table 3 shows the estimation of annual emission reductions by introducing VSD for only 91 industries in Malaysia.
Efficient energy motors are slightly costlier than standard motors. Motor-‐energy costs typically account for over 95% of the motor’s life-‐cycle cost, with over 1000 hours of operation per year; efficient electric motors are more cost effective over the system life. (Waide & Brunner, 2011). Hamer et al., (1996) have proposed a simple approach based on the purchase price of the motor and the present value of the losses to calculate the life cycle cost of electric motors. Life cycle cost of a standard and efficient motor with 25 hp is $ 1,500 and $ 1,900, respectively.
Several factors can affect the cost effectiveness of efficient electric motors, such as, motor price, efficiency rating, and annual hours of use, energy rates, costs of installation and downtime, payback criteria, and the availability of utility rebates. The additional cost that is incurred in efficient electric motors is repaid by energy savings. A single point efficient gain for continuously operating 50 hp motor with energy cost of $0.04/kWh and 75% load factor saves 4,079 kWh, or $163 annually. Thus, an energy-‐ efficient motor that offers four points of efficiency gain can cost up to $1,304 more than a standard model and still meet a 2-‐year simple payback criterion (DOE, undated).
Energy savings and payback period
According to a study done by Saidur et al. (2009), a walkthrough audit in 91 industries in Malaysia shows that by using energy-‐efficient motors for 50%, 75% and 100% motor loading, total energy saved was 1765, 2703 and 3605 MWh respectively. Similarly, associated cost savings for the estimated amount of energy savings are US$115,936, US$173,019 and US$230,693. Table 4 depicts the details of energy savings and pay back years for motors of power ranging from 1 hp to 50 hp. It has also been found that the payback period for using energy-‐efficient motors ranges from 0.53 to 5.05 years for different percentages of motor loading. These payback periods indicate the introduction/implementation of energy-‐efficient motors would be cost effective, as their payback periods are less than one-‐third of the motor life (if average motor life 20 years is considered) in some cases.
From Table 5, it is evident that a significant energy savings can be achieved for different percentages of speed reductions. More energy can be saved for higher speed reductions. Along with energy savings, a substantial amount in expense can be saved and associated emission reductions can be achieved using VSD for industrial motors.
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