ENERGY EFFICIENT MOTORS
ABSTRACT
In the present day world, energy conservation by a small means plays a significant role in making huge profits for any industry. The present paper explains the need of improving energy efficiency and particularly in motors. In industries, motor driven systems represent about 60% of all the electrical energy used. Energy efficient motors are the best solution to solve energy crisis. NEMA (National Electrical Manufacturers Association) set standards for energy efficient motors and named them as premium efficiency motors.
1.0 INTRODUCTION: Over half of all electrical energy consumed is used by electric motors as shown in figure1.
. Improving the efficiency of electric motors and the equipment they drive can save energy and Energy Efficient Motors are the best solution to solve energy crisis NEMA (National Electrical Manufacturers Association) set standards for energy efficient motors and named them as premium efficiency motors. As efficiency depends on losses, reducing them itself improves efficiency. Improved design specifications help to improve the efficiency. Comparison between the energy efficient motors and standard motors indicate the annual energy saved and thereby cost savings.
reduce operating costs. . Energy efficiency should be a major consideration when we purchase a motor, as well as the more common considerations.
"What’s the price and how soon can we get it?” There are three factors to be considered: 1Energy-efficient motors only provide savings when they're running.
2. Maximum savings and the fastest returns on investment.
Any motor selection has to be properly
engineered for its intended application .
1.1Definition
An "energy efficient" motor is a motor that gives you the same output strength by consuming lesser amounts of power. For energy-efficient motor’s performance must be equal or exceed the nominal full-load efficiency values.
The efficiency of a motor is the ratio of the mechanical power output to the
electrical power input. This may be expressed as:
In 1989, the National Electrical Manufacturers Association (NEMA) developed a standard definition for energy-efficient motors. The definition, designed to help users identify and compare electric motor efficiencies on an equal basis.
A sample database listing is shown in Table 1.1. The database contains the manufacturers’ name, motor model, full-load RPM, service factor, frame size, and list price.
Table1.1
Table1.1howing the comparision of standard motors and energy efficient motors of two different types viz. ODP, TEFC.
Note that the nominal full-load motor efficiencies vary from 86.5 to 93.2 percent. Prices also vary. In many cases, motors with identical list prices exhibit very different efficiency ratings.
2.0 FACTORS AFFECTING EFFICIENCY AND LOSS DISTRIBUTION
Types of Losses :Various Types of losses that incur in a standard motor and a high efficient motor are indicated in the figure2.1. Figure2.1
Figure2.1showing comparison of losses for a standard motor and an energy efficient motor.
Stator and Rotor I2R losses: Stator and Rotor I²R losses appear when heat is generated by resistance from current flowing in the stator windings, rotor conductor bars. These are major losses, typically account for 55% to 60% of total losses.
Core losses: It has hysterisis and eddy current effects. These are independent of load and account for 20 to 25% of total losses. Efficiency is improved by the use of thinner gauge, lower loss core steel reduces eddy current loss. Motor operation closer to synchronous speed will reduce rotor I²R losses. Efficiency can be improved by using more copper and large conductors, lowers resistance and losses due to current flow. Efficiency is improved by the use of longer core steel to design, reduces core losses due to lower operating flux densities Table2.2
Table2.2 shows various types of motor losses for a 1800 rpm motor for different ratings.
Friction and Windage losses: These losses are independent of load. They account for 8 to 12% of total losses. Efficiency is improved by the use of optimized design, strict quality control procedures minimizes stray load losses.
Stray load losses: These vary according to square of the load current, caused by leakage flux induced by load currents in laminations. They account for 4 to 5% of total losses. Efficiency is improved by the use of low loss fan design, reduces losses due to air movement.
3.0Modifications of the design to improve efficiency: Higher efficiencies are achieved by redesign of motors as follows:
1.Low loss of thinner laminations reduce the Iron losses even at partial loads.
2.Thicker conductors and more copper contents reduce copper losses due to lower resistance.
3. Longer core length, uniform air gap between stator and rotor reduce stray losses.
4. Special design of fan reduce windage losses.
5. An energy efficient motor results in lower energy costs when compared to a rewound standard-efficiency motor.
3.1Cost effectiveness: Energy-efficient motor depends on factors like motor price, efficiency rating, annual hours of use, energy rates, cost of installation and downtime, and the availability of utility rebates or other incentives.
Figure3.1.1
Figure3.1.1 shows the drooping nature of efficiency of a standard at partial loads.
Figure3.1.2;Figure3.1.2 depicts the initial cost and the life time costs of a motor. Hence one should be careful in choosing motor which will bring down the life time cost.
3.2 Energy Efficient Motors:(EEMs)
EEMs operate with efficiencies that are typically 4 to 6 % higher than the standard motor. The power factor is same or may be higher compared to standard motors. EEMs withstand wider voltage, frequency fluctuations and higher ambient temperatures without breakdown. In the event of field repairs it retains rated capacity when repairs are carried out by less skilled workers and in inexact conditions
Figure3.2.1
Figure3.2.1 shows the average life of the motors increase with the increase in size of the motors of different ratings.
Energy efficient motors require less maintenance and have longer life because of lower motor operating temperatures. The energy efficient designs of multi-speed motors are not available. They are not suited for highly intermittent duty and special torque applications as hoists and cranes, traction drives, punch presses, machine tools and centrifuges.
Figure3..2.2. shows the annual energy savings of the motor with its increase in size.
3.3 Case Study: Higher Efficiency Motors Mean Big Savings and Better Reliability
Table3.3 shows the typical examples of high efficient motors and standard motors of different ratings, with their efficiency at full load ,efficiency at 75% load ,additional cost incurred for a premium motor and annual energy savings.
|
Example One |
Example Two |
||
EPAct Standard |
NEMA Premium |
EPAct Standard |
NEMA |
|
Full Load Efficiency |
93.1 |
94.5 |
92.4 |
93.6 |
Efficiency (at 75% load) |
93.6 |
95.1 |
93.1 |
94.1 |
Meets EPAct? |
Yes |
yes |
Yes |
yes |
Incremental Cost |
NA |
$176 |
NA |
$96 |
Energy Savings at 75% Load (6000 hrs/year) |
NA |
2,829 kWh/y |
NA |
958 kWh/y |
4. Conclusion
Higher efficiency motors (HEMs or EEMs) are preferred than that of standard efficiency motors, because increased efficiency saves significant amount of energy consumed and therefore saves energy bill, costs incurred over maintenance of the standard motor . However EEMs may not always lead to lower power consumption.Sometimes there would be an increase in energy consumption rather than a decrease due to the smaller slip in selecting a suitable motor. So proper selection of high efficiency motors must be made according to the type of application and energy required. One should be careful in choosing proper rating, always avoiding over-sized motors.
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