MOTOR EFFICIENCY CALCULATIONS IDENTIFY SOLUTIONS FOR DRAMATIC ENERGY AND COST SAVINGS

Phil Burge, Communications Manager for SKF, discusses why motor efficiency estimation is important to deliver a series of measurements and readings that can be used to the advantage of production or plant engineers in terms of cost control and legislative compliance.

Industrial motors have something of a bad reputation, which is hardly surprising when you consider the statistics. Indeed, they are responsible for consuming about 65% of all generated electrical energy, account for up to 75% of the total electricity costs in industrial plants and use 50% of the total electricity consumed by commercial buildings. In addition, it has been said that the annual cost of operating an electric motor can reach a staggering 25 times its original purchase cost.

Accordingly, in both North America and Europe there is increasingly stringent legislation, such as the US Energy Independence and Securities Act (EISA) which came into effect in December 2010, being introduced that regulates, among several other things, the energy efficiency of motors in order to protect the environment. This legislation dictates the mandatory minimum efficiency level of all types of motors that can be legally bought, sold and used throughout industry, and can cover both existing and future generations of motor technology.

This has far reaching implications that will be felt across many industries. With some 300 pages worth of goals and mandates, coupled with new laws designed to increase the efficiency of a wider range of motors than ever before, the EISA will affect all engineers that have the responsibility of specifying or purchasing electric motors, as well as those responsible for motor powered plant operations. The legislation even goes to the extent of dictating the physical size and design of motors, with the consequences of high taxes and hefty fines for companies that fail to maintain clean and efficient working practices.

Considering this it goes without saying that the ability to understand and calculate the efficiency of an electrical motor, which is essentially the ratio of mechanical output to electrical input, is vital in respect to energy conservation, cost savings, and reduced emissions. This is particularly true in today’s market, where prices continue to soar, while the value of currency declines, along with the growing concern for global warming.

In particular, engineers require this efficiency information for applications such as power conversion, conveyor belt motors and water pumps in order to save company money and take advantage of rebates made available by the government and local power companies. Therefore, having a method by which to obtain this efficiency calculation, as well as to identify key areas for improving efficiency in real life applications is essential to running a successful, safe and efficient manufacturing or process operation.

Options – not obstacles…..

There are many ways in which to calculate the efficiency of a motor, but some are cumbersome, while others are intrusive and completely impractical in real world scenarios. There are, however, currently four methods that are considered non instructive which can be used for efficiency estimation in the field. It is important to note the word estimate, as you have to expect some degree of inaccuracy when using in-service field tests over a no load uncoupled laboratory test on a dynamometer. These tests are the Nameplate method, Upper Bound with Resistance method, ORMEL 96 and the Instantaneous Current Method.

Of all four, the preceding method often proves to be the most accurate test and often the least intrusive, as all of the required data can be gathered easily and safely from the motor’s control cabinet (MCC). Furthermore, in a recent study carried out by Oregon (OSU) and Washington (WSU) State Universities, which looked at all four non- intrusive motor efficiency test methods, it was found that the Instantaneous Current Method does not try to estimate losses, but rather the output power. As this element is less sensitive to the efficiency of the predicted motor, this leaves a smaller margin for error on estimating lower end efficiencies ensuring maximum precision.

In essence, the Instantaneous Current Method requires a measurement of the motor’s input power and a calculated estimate of the motors output power, with current transformers and potential transformers being used to gather incoming rotating voltage and currents for all 3-phases. Calculations are then made based on these values for speed and torque, while air gap torque is calculated using the Park’s Vector, or 2-Axis, Theory. Friction, stray load and windage losses are estimated and then subtracted from the air gap torque calculation to get an estimate of the output torque, while speed is estimated through the gathered currents and voltages.

In order to simplify testing and help apply more efficient motor findings, a new generation of motor efficiency analysis instruments has been introduced that integrates a wide range of monitoring capabilities to deliver consistently high quality standards. For example, the EXP3000 from Baker Instruments, an SKF company, uses the proven Instantaneous Current Method to give users a comprehensive look at overall motor performance and integrity.

This type of innovative device calculates the operating efficiency then extracts a comparable efficiency from a motor database that contains over 22,000 different NEMA design motors from numerous motor manufacturers. The percentage difference in the losses between the tested motor and a comparable motor with the target efficiency is then evaluated with respect to the thresholds. Ultimately, it identifies motors that are performing under par and calculates the payback period if replaced by a new motor.

Once testing is complete, the results can then be saved and stored for each motor, allowing data to be recalled for trending capabilities and effective maintenance management, while reports can be generated and printed quickly, allowing operators visually to confirm and document the performance of each motor. These instruments also use a software and data transfer package that enables the creation of multiple databases that can be used to organise the gathered information to specifications set by the user, simplifying communication channels by handling data in a manner that is useful, complete and accessible.

As long as environmental and economic needs stay at the top of the agenda for modern businesses, a more reliable, safe and beneficial method of enabling efficiency will always be vital to reduce energy demands and the impact on global warming. As one of the biggest energy consumers in industry, motors must be scrutinised to ensure that they are running as efficiently and smoothly as possible. By determining the efficiency of a motor, users can then correct the issues that directly effect energy usage, such as cavitating pumps and improperly tuned drives, and start increasing the efficiency and service life of the motor while also reducing maintenance costs, downtime and production losses.

SKF (U.K.) Limited
Luton
Bedfordshire

Can be contacted on:

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E-mail: marketing.uk@skf.com
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