A Case for Energy-Efficient Motors

In most continuous running plants, the installed base of motors is already over a decade old, say ANIL NAIK and AJAY BHALERAO.

Electric motors are estimated to consume around 65 per cent of the electrical energy consumed by the industry. Moreover, energy costs over the typical life-cycle of a motor can be as high as twenty times the original capital cost of the motor. Energy-efficient motors thus offer an opportunity to significantly reduce energy costs and their collateral environmental effects.

Increasingly, there is a strong economic – and environmental – case for choosing high-efficiency motors over conventional ones. Instead of repairing or rewinding a failed motor, organisations may profitably consider replacing them with energy-efficient motors or motor driven systems; this can bring about significant reduction in energy consumption.

Over the past two decades, motor-efficiency measurement standards have undergone extensive changes. At the same time, technological advances and greater end-user awareness have resulted in higher availability and application of energy-efficient motors, particularly IE2, IE3 and now even IE4.

However in most continuous running plants, the installed base of motors is already over 10 years old, inefficient, oversized and operating on fluctuating load between 40-80 per cent without a Variable Frequency Drive. Rewound motors also cause a 10 to 15 per cent loss of power.

Motors in the Cement Industry
The cement industry uses a large number of electric motors, right from MW ratings in 11/6.6/3.3 kV to much smaller ratings in 415 V supply. Since energy costs form a significant component of the production costs in a cement plant, plant managers and plant designers are constantly looking for energy savings solutions. Since large power ratings already have high efficiencies, the emphasis is on smaller rating motors, where the improvement in efficiency is much higher. The increased efficiency can result in very short payback periods.

Some of the manufacturing stages (or areas) where these smaller rating motors are used in the cement plants are:

Almost all new cement plants now specify IE2 or IE3 motors. Existing plants can also greatly benefit from the replacement of existing old motors with new IE3 or even superior efficiency class motors.

The Indian government has mandated that with effect from October 2017, all induction motors manufactured and sold in the country must have minimum IE2 (high-efficiency) levels. This will ensure that no low-efficiency motor is sold in India. However, for those industries where motors run for significant amount of time selection of efficiencies even higher than IE2, IE3 (premium efficiency) or IE4 (super-premium efficiency) motors can make strong economic sense.

With power tariff rates increasing at a Compounded Annual Growth Rate (CAGR) of over 5 per cent, IE4 motors can have a payback period of less than a year. Considering a motor life of 15 years, the lifetime saving in energy costs for a typical 15 kW motor – for an incremental investment of Rs 50,000 – can be as high as Rs 11.7 lakh.

Performance of higher efficiency class motors
Operating speed and slip
In general, motors with higher efficiencies have a higher operating speed, i.e., a reduced slip compared to motors of lower efficiency. Usually the slip is reduced by some 20 to 30 per cent for the next higher-efficiency class for motors of the same rated output power.

Most of the difficulties observed in not obtaining the expected power savings during field trial, testing and at customers plants when they replace standard motors with high efficiency motors are due to the effect of increase in speed of high efficiency motors.

Operational Problems
Several customers have faced the problem of input power increasing when the customer has replaced existing standard motors with IE2/IE3/IE4 motors. The reason for the increase in power consumption is explained below.

Applications where load-torque is increasing with speed (pumps, fans and compressors)As a general rule, high-efficiency cage-induction motors, with more active material, have a lower slip (see table-1), i.e a higher speed of rotation, than motors of lower efficiency. On the average, higher efficiency class motors run 5 to 20 rpm faster than standard motors.

When the torque of the application is a function of the square of the speed (centrifugal loads), like in pumps, fans, compressors, etc., the increase in speed will lead to an increase in output power (torque) which could in some circumstances defeat the benefits from the improved energy efficiency.

Even a minor change in the motor’s full-load speed translates into a significant change in the magnitude of the load and energy consumption. The "fan" or "parabolic law" shows that the kilowatt loading on a motor varies as the third power (cube) of its rotational speed. In contrast, the quantity of air delivered varies linearly with speed.

This is explained in the following example:
Existing standard motor speed = 1440 rpm. New higher-efficiency class motor considered runs at = 1460 rpm.

As per the centrifugal fan or pump affinity rules,

(Where kW2 and kW1 are pump motor loads at RPM2 and RPM1).

A relatively minor 20 rpm increase in a motor’s rotational speed, from 1,440 to 1,460 rpm, results in a 1.39 per cent increase in the load placed upon the motor by the rotating equipment; at the same time, with little increase in delivery, boosting energy consumption by 4.22 per cent, exceeding any efficiency advantages expected from purchase of a higher efficiency class motor. Predicted energy savings will not materialize -in fact, energy consumption will substantially increase.

Therefore, in applications when a motor of lower efficiency is retrofitted by a motor of increased efficiency, the input power may not reduce as much as anticipated when comparing the efficiencies of the two motors.

In some cases the input power of the energy-efficient motor may actually increase compared to the motor of lower efficiency. The user should be aware of the sensitivity of load and energy requirements to rated motor speed while replacing a standard motor with a higher efficiency class motor in a centrifugal pump or fan application.

One method is to use a VFD to reduce the speed to the original value, but this introduces the additional losses of the VFD, which may defeat the purpose of using an energy-efficient motor, unless the customer is already using a VFD for energy savings on the pump.

If a belt and pulley system is being used, one can reduce the pulley diameter and bring down the speed of the pump. If the pump is directly coupled to the motor, the only other alternative is to trim the impellor.

If the motor operates at a higher, an appropriate retrofit arrangement to trim the pump, impellers must be adopted to capture the full energy-conservation benefits. As a thumb rule, one could reduce the diameter (trim) of the impeller inversely to the increase in the speed, e.g., if the speed increases by 3 per cent, then reduce the diameter of the impellor by 3 per cent. This is valid for a trim of maximum 5 per cent. Instructions on how to calculate the amount of trimming are available on many Internet sites. A simple search will give a lot of information. The pump manufacturer can be contacted for guidelines.

Due to the increase in rated speed of high-efficiency motors, it is possible that the input power does not come down as expected in pumps and fans after replacing the motor. This is because the pump is delivering more output. Using the affinity laws, one can estimate the power savings by reducing the input power in cube ratio of the speed increase and confirm that there are savings. However, to actually save that power, it is necessary to trim the impeller to get the true savings, while continuing to get the same (existing) output from the pump. This is clearly mentioned in the CIGRE report.

It should be also noted that in spite of the increased speed of high efficiency motors sometimes we have measured energy savings even in pumps and fans. This can be attributed to the possibility that the efficiency of the existing motor with the customer is actually very low. As per the affinity laws of pumps the power input will always increase in cube of the speed increase.

Starting performance
Energy efficient cage-induction motors are typically built with more active material, i.e., longer core length and/or higher core diameter in order to achieve higher efficiencies. For these reasons, the starting performance of energy-efficient motors differs somewhat from motors with a lower efficiency. On an average, the locked-rotor motor rotor current increases by 10 to 15 per cent for motors from one efficiency class compared to motors of the next higher efficiency class with the same output power. Individually, this difference depends on the design principle of the motors, and should be checked with the manufacturer when replacing motors in an existing installation.

Power Factor
Some customers give more importance to the total kVAR consumption rather that the kW consumption. In other words, they would prefer a motor with high PF rather than high efficiency, as long as the total kVA comes down.

References:

• IEC 60034-30-1
• CIGRE Draft Report: "GUIDE ON USE OF PREMIUM EFFICIENCY IE3 MOTORS & DETERMINING BENEFITS OF GREEN HOUSE GAS EMISSION REDUCTION.

About the authorsAnil Naik is Chairman, IEEMA’s Rotating Machines Division and Ajay Bhalerao is former Managing Committee Member Electrical Research and Development Association, and Former Member, Standards Committee of BIS – ETD 15 & 22. Both are associated with Bharat Bijlee.