With increasingly stringent legislation dictating minimum energy efficiency of motors, now is the time for manufacturers to calculate productivity gains against these new performance levels, suggests SKF communication manager Phil Burge
The ability to understand and calculate the efficiency of an electrical motor is vital when it comes to driving energy conservation, cost savings and reduced emissions. This information is particularly important to the engineers who are responsible for managing energy intensive applications, where energy prices continue to rise by alarming increments, and growing concerns over global warming are putting consumer pressure on industry, leading to ever more legislation to cut CO2 emissions.
Having a method by which to obtain this efficiency calculation, as well as to identify key areas for improving productivity is essential to running a successful, safe and effective operation. In 2010, industry accounted for 31 per cent of the UK electricity consumption, with two thirds of this consumed by electric motors – that is a total of 20 per cent of the UK’s electricity used by one source.
Just as importantly, industrial motor energy estimations often hold the key to meeting the increasingly stringent environmental legislation that has come into play across the world.
The EU has issued mandatory legislation for minimum motor efficiencies to level IE2 for most general purpose motors under its minimum energy performance standard. The legislation laid out in Commission Regulation EC 640/2009 came into effect in June 2011 and further revisions are expected to impose even higher efficiencies for all types of motors used in industry.
In North America, corresponding legislation came into effect in December 2010 setting the mandatory minimum efficiency at National Electrical Manufacturers’ Association (NEMA) premium level for all types of motors used throughout industry.
Finding the right method
A number of methods can be used to calculate the efficiency of a motor, but some are cumbersome, intrusive and simply impractical for use in real world scenarios. However, one in particular, the Instantaneous Current Method is considered more accurate and less intrusive, and looks at output power rather than trying to estimate losses. As this element is less sensitive to the efficiency of the predicted motor, the margin for error for estimating lower end efficiencies is smaller and ensures maximum precision.
The Instantaneous Current Method requires a measurement of the motor input power and a calculated estimate of the motor output power, with current and potential transformers being used to gather incoming rotating voltage and currents for all three phases.
Calculations are then made based on these values for speed and torque, while air gap torque is worked out 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.
To simplify testing and help apply more efficient motor findings, a new generation of motor efficiency analysis instruments has been introduced that integrates a range of monitoring capabilities to deliver consistently high quality standards. For example, the EXP3000 from SKF uses the proven Instantaneous Current Method to give users a comprehensive look at overall motor performance and integrity.
This instrument 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. 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 trend analysis and effective maintenance management, while reports can be generated and printed quickly, allowing operators to visually confirm and document the performance of each motor. Through motor efficiency analysis, a quick and simple assessment can help address deficiencies in excessive energy consumption, boosting overall productivity and service life of motors, while leading to reduced costs and enhanced profitability.