Super Efficient Motors:Techanical Overview

By | March 27, 2013

           Power scenario in the country calls for energy crisis management. Industrialization and growth in population is creating a void between demand and supply, and this void is deepening day by day.   The generating cost is increasing day by day & the power tariffs are on the rise. Unless the above crises are properly managed power scenario will be bleak. Conservation of the energy is the call of the day.  There is a capital investment that can repay many times its original value over the next 20 years. At the same time, it can improve equipment reliability. The investment is straightforward: install electric motors having the highest electrical energy efficiency commensurate with your needs. Energy-efficient motors pay for themselves in a few years or sometimes even a few months, after which they will continue to pile up savings worth many times their purchase cost for as long as they remain in service.


1. Introduction

Until the energy crisis in the 1970s ,most general purpose motors were designed to provide rated output and operating characteristics at reasonable cost ,period. Efficient operation was best a secondary consideration .As energy crises began rising however , manufacturers began promoting improved motors they called high efficiency and energy efficient, although the terms were not defined specifically at the time.

Old style standard efficiency motors remained popular because they generally cost less than the new models. Purchasing agents were seldom inclined to spend a little more money up front in order to save on energy cost later on. But today the story is entirely different. Looking at the power scenario in the country, there exists a huge gap between the demand and the supply which is widening. The generating cost is increasing day by day and power tariffs are on the rise. This is affecting the profitability of all the industries. Hence it is a trend in the industry to look for the opportunities of cost reduction. The major cost components in an industry include material,labour and energy costs. Material and labour cost reduction has its own limitation and a manufacturer does not have a direct control many times. But the manufacturers himself can influence the energy cost through energy conservation measurers and effective energy management

2. Motor utility segmentation

The figures of the motor purchases in 2003-04 will give an idea of this potential. About 5 million motors accounting to approximately 5 million kW have been sold in 2003-04. (These figures are as reported by IEEMA & are approximated to round figures). The non-reporting members also account for an additional 3-4 mil. KW.

Fractional HP motors account for over 85% of volume. These small motors are used primarily in domestic appliances and are lightly and/or intermittently loaded. As a result there is little potential for cost-effective energy savings. Direct current (DC) motors have applications in the industrial sector. There are few DC motors in-service and most are being phased out in favour of alternating current (AC) motors with inverter drive systems. AC low-tension (or low voltage) motors are used by all end-user segments and represent largest market, following fractional horsepower motors.

3. What does energy efficiency mean?

Electric motors are simply devices that convert electrical energy into mechanical energy. Like all electromechanical equipment, motors consume some “extra” energy in order to make the conversion. Efficiency is a measure of how much total energy a motor uses in relation to the rated power delivered to the shaft.                               A motor’s nameplate rating is based on output horsepower, which is fixed for continuous operation at full load. The amount of input power needed to produce rated horsepower will vary from motor to motor, with more-efficient motors requiring less input wattage than less-efficient models to produce the same output. Electrical energy input is measured in watts, while output is given in horsepower. One horsepower is equivalent to 746 watts.          There are several ways to express motor efficiency, but the basic concept and the numerical results are the same. For example:

Efficiency, % = 746 x Horsepower (output) x 100
Watts (input)
Efficiency, % = Watts (output) x 100
Watts (input)

The ratio describes efficiency in terms of what can be observed from outside the motor, but it doesn’t say anything about what is going on inside the motor, and it is what’s happening inside that makes one motor more or less efficient than another. For example, we can rewrite the equation as:

Efficiency, % = Watts (output) X 100
Watts (output) + Watts (Losses)

 

Or its equivalent,

Efficiency, % = Watts (Input) – Watts (Losses) x 100
Watts (Input)

“Losses” stands for all the energy “fees” the motor charges in order to make its electrical-to-mechanical energy conversion. Their magnitude varies from motor to motor and can even vary among motors of the same make, type and size. In general, however, standard-efficiency motors (pre-EPAct) have higher losses than motors that meet EPAct standards, while NEMA Premium motors, or better, have lower losses still.

4. Types of losses

Energy losses in electric motors fall into four categories:

  • Power losses
  • Magnetic core losses
  • Friction and windage losses, and
  • Stray load losses.

Power losses and stray load losses appear only when the motor is operating under load. core They are therefore more important in terms of energy efficiency than magnetic losses and friction and windage losses, which are present, even under no-load conditions (when the motor is running, of course).

Power losses, also called I²R losses, are the most important of the four categories and can account for more than one-half of a motor’s total losses. Power losses appear as heat generated by resistance to current flowing in the stator windings and rotor conductor bars and end rings.

Stator losses make up about 66% of power losses, and it is here that motor manufacturers have achieved significant gains in efficiency. Since increasing the mass of stator windings lowers their electrical resistance (and therefore reduces I²R losses), highly efficient motors typically contain about 20% more copper than standard efficiency models of equivalent size and rating.

Fig 1: A typical NEMA motor showing the components that can be modified to increase motor efficiency

Rotor losses, another form of power losses, are also called slip losses because they are largely but not entirely dependent on the degree of slip the motor displays. Slip is the difference in rpm between the rotational speed of the magnetic field and the actual rpm of the rotor and shaft at a given load.

Fig 2: End plates, conductor bars and cooling fan in a typical squirrel cage motor

Decreasing the degree of slip reduces rotor losses. This is accomplished by increasing the mass of the rotor conductors (conductor bars and end-plates) and/or increasing their conductivity (see below), and to a lesser extent by increasing the total flux across the air gap between rotor and stator.

Conductivity is an important characteristic of the rotor. Conductor bars in large motors are normally made from high-conductivity copper. Conductor bars in small-to-intermediate size motors, up to about 200 hp, depending on manufacturer, are in the form of a die-cast aluminum “squirrel cage” that gives these motors their common name. Increasing the mass of the die-cast bars requires changes in the slots in the rotor laminations, through which the bars are cast, and that changes the rotor’s magnetic structure. Lowering rotor I²R losses in what are typically aluminum alloy squirrel cage motors is therefore not a simple task.

Fig 3: Cross section of a die-cast motor rotor

Copper has higher electrical conductivity than aluminum, and it would be an ideal conductor bar material except for the fact that it is difficult to die cast. A process to produce die-cast copper rotors has recently been developed and, when fully commercialized, it will enable the production of motors with even higher efficiencies than the best models currently available.

The fact that high-efficiency motors tend to have less slip (run faster) than standard-efficiency motors must be taken into account in certain applications. For example, energy consumption by centrifugal loads such as fans and rotary compressors is proportional to the cube of rotational speed. If such loads are driven at the higher speed of a low-slip, high-efficiency motor directly replacing a standard motor, energy consumption can actually increase. This situation can sometimes be resolved by lowering rotational speed with a variable-speed drive, gears or pulleys. There are other parameters, such as torque or starting current, that can vary among motors of the same nominal horsepower. It is important to properly engineer the application of any motor to the intended task.

Magnetic core losses arise from hysteresis effects, eddy currents and magnetic saturation, all of which take effect in the steel laminations. Magnetic losses can account for up to 20% of total losses. With proper design, use of better materials and stringent quality control, these losses can be reduced considerably.

Fig 4: Three different efficiencies for the horse power ratings. Top : Standard efficiency pre-Epact motor , lower left : Epact-level motor, lower right NEMA premium efficiency motor.Noti8ce that rotor and stator lengthen as efficiency increases .(

The most effective means to reduce hysteresis and saturation losses is to utilize steels containing up to 4% silicon for the laminations in place of lower-cost plain carbon steels. The better magnetic properties offered by silicon steels can reduce core losses by 10 to 25%. Reducing the laminations’ thickness also helps: substituting 26-ga or 29-ga steel for the 24-ga steel found in standard-efficiency motors lowers core losses by between 15 and 25%. Lengthening the lamination stack, which reduces the flux density within the stack, also reduces core losses. Eddy current losses can be reduced by ensuring adequate insulation between laminations, thus minimizing the flow of current (and I²R losses) through the stack

5. How higher efficiency can be achieved?

We have from the efficiency definition,

Efficiency = output / input

= Output / (output + losses)

It is evident from the efficiency equation that efficiency will increase if the losses in the motor are reduced. Hence the designers aim is to reduce the losses while designing the energy efficient motor.

The different components of the total losses, its contribution & the measures adopted for its reduction are as under-

Description of losses Percentage of total losses Measures to reduce losses
Core losses 20-25% Low watt loss material; thinner laminations & process control
Friction & windage losses 1-12% Optimum design of fan
Stray losses 4-5% Optimum slot geometry

Fig 5:Table  showing  different components of the total losses, its contribution & the measures adopted for its reduction.

  • Feature: Top terminal box at DE and parallel cooling fins

Benefit:40% higher utilization of cooling airflow than conventional design with side terminal box at the center of the frame and radial cooling fins.

  • Feature: Small fan diameter with respect to the fan cowl.
  • Benefit: Optimal cooling air flow, lower fan losses and quite operation.
  • Feature: Radial flow straight blade fan.
  • Benefit: Cooling is independent of direction of rotation and motor is suitable for bi-directional rotation
  • Feature: Top terminal box at DE and parallel cooling fins

Benefit:40% higher utilization of cooling airflow than conventional design with side terminal box at the center of the frame and radial cooling fins.

  • Feature: Dual mounting holes at NDE.
  • Benefit: Motor of high rating can be retrofitted in the existing foundation.
  • Feature: Staggered skew rotor
  • Benefit: No inherent axial thrust.

7. Efficiency considerations in motor purchases

Industrial tariff levels have been increasing in the past and are forecast to continue to increase in the future. Industrial companies, to remain competitive in world markets, will have to seek greater efficiencies, including motor and motor system efficiency.

Energy efficient motors are cost effective.  A payback of 15 months is likely based on economic analysis for a new motor purchase.  The analysis evaluated a typical 15 kW, 4-pole motor, with average operation of 8,000 hours per year, at current industrial tariff rates.  \This analysis compared the energy efficient motors with the standard motor. As the average operating hours and tariff levels increase the payback period declines.

8. Common misconceptions about energy efficient motors

Many misunderstandings have arisen concerning the characteristics of todays more efficient motors. Some of them lead to unfair criticism and other equally inaccurate notions. Lead users to expect more than these motors will delivers.

  • Misconception 1: An oversized motor is less efficient .

Many authorities continue to stress the need to match motor rating more closely to actual load horsepower contending that oversized motors are inherently efficient. A 3HP load for example is more efficiently carried by an under loaded 5HP motor than by a fully loaded 3HP machine

  • Misconception 2: a more efficient motor also has high power factor.

Many motor design modifications may be made to increase efficiency. Some of them will also increase their power factor, where as others will decrease it. Comparing energy efficient machines with their less efficient predecessors shows that some do have high power factor, some have lower power factor and some exhibit no change. If power factor improvement is ever needed, an easy way to get it is with capacitor on motor circuit An economical corrective measure that is not available to improve efficiency.

  • Misconception 3: more efficient motors run cooler.

Thats a fallacy. So is the reverse proposition. Cooler motors must be more efficient. Temperature and heat is not the same thing so they should not be confused with each other. Temperature ratings for insulation systems or motors are the same regardless of motor efficiency.

  • Misconception 4:An energy efficient motor develops less torque and may not accelerate the load .

Lower rotor resistances, often used to achieve higher efficiencys, thus tend to reduce motor accelerating torque, but its not the only influence. And the expected amount of torque reduction is seldom harmful except for load such as full conveyors.

9. Standards perspective to energy efficient motors

The current Indian Standard IS 8789 addresses efficiency criteria for standard motors in India. Most of the motor manufacturers in India follow this standard & their efficiency figures are bound by it. However, all the major ones provide much higher efficient motors than specified by IS 8789. With the focus on high efficiency motors these days, it was required that a more stringent

Fig  6:.The graph  gives a comparative picture of specified efficiencies for of motors up to 37kW as per different standards referred in India

standard be brought in to effect. IS 12615 which will come into effect shortly addresses the issue & is applicable for high efficiency motors. Its scope, at present covers 4 pole motors up to 37kW.

IEEMA has proposed voluntary standards for E.E motors (No.19 /2000). IEEMA standard is based on European Union standard & energy Act of USA.

The final draft of IS 12615 will be in line with IEEMA standard & will be released shortly

Significant market barriers for energy-efficient motors exist. However, solutions are possible.  Because of the complexity and diversity of both motor manufacturers and end-users, strategies must be developed that build solutions into a cohesive and comprehensive plan.  This plan must gain the support of large and small motor manufacturers, address the needs and concerns of the industrial customers, and leverage the activities of the Government of India and associations like IEEMA, AIEEMA, IPMA, ICA, CII, and TERI to drive change in the market. The end users need to understand the benefits of energy saving & opt for energy efficient motors for maximizing their profits.

Fig  7:shows detail of reduce  Fe  loss  and reduce  cu  loss…

All the stakeholders gain from a premium quality motor brand.  Government benefits from the energy savings and associated environmental benefits.  End-users save energy and get guaranteed performance, quality and customer support.  OEMs can offer a value-added product with EE Motor Inside.  Manufacturers can differentiate their product and justify a higher price for added value.

10. CONCLUSION

The market for low-tension motors is vast and complex. In the Industrial sector the awareness is increasing towards the need to save energy by use of Energy Efficient Motors. Every element within the chain needs to gear up to push the use of energy efficient motors & therein gains from the benefits out of it. In the agricultural sector, the increasing use of submersible pumps presents an opportunity to introduce Energy Efficiency standards. Various institutions and organizations need to synergies their initiatives & activities under Government thrust to impact a change in the market. Let all of us in the chain contribute our efforts for the cause of energy management by promoting the use of ENERGY EFFICIENT motors.





Author: Ravi Bandakkanavar

A Techie, Blogger, Web Designer, Programmer by passion who aspires to learn new Technologies every day. It has been 6 years since I have been publishing articles and enjoying every bit of it. I want to share knowledge and build a great community with people like you.

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