With over 60 percent of the electricity consumed by industry used by motor systems, there is a pressing need for their efficiency to be increased by reducing losses in the motors, or better still by matching load and energy use. This demands a number of considerations, including proper sizing of all components, improving the efficiency of the end-use devices such as pumps and fans, and reducing electrical and mechanical transmission losses.
The shocking truth is that some common systems operate at extremely low efficiencies; for example, compressed air systems often operate at efficiencies of around 10 percent - but with current technology the electricity use of industrial motor systems can be reduced by as much as 30 percent.
The key is to take a ‘systems approach’ and analyse both the energy supply and energy demand aspect of motor systems, as well as how both factors interact to optimise total system performance.
A motor management plan can support long-term motor system energy savings and ensure that failures are handled quickly and effectively, not only ensuring that an efficient motor is put in place after failure, but also that the most-efficient system is considered.
For example, repair and overhaul using energy efficient components can deliver cost savings once each motor returns to service without affecting capital budgets, but only if energy efficiency has been carefully calculated.
The Instantaneous Current Method is considered the more accurate and less intrusive of the options available when calculating motor efficiency, 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.
In essence, 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.
The EXP3000 Dynamic Motor Monitor 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. 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 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.
This instrument also uses 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.
A detailed look at the efficiency of the production system can pay dividends. In response to a sharp rise in local energy costs, Indonesian textile plant PT. Leuwijaya Utama implemented an urgent cost-saving programme, and discovered that 30 percent of the factory’s energy consumption was consumed by the twisting machines, which are critical in ensuring fabric quality.
So, as well as taking measures to correct electric motor energy losses and optimising frequency converters for the overall electricity supply, the energy consumption of the twisting machinery was examined in closer detail.
Inside the Leuwitex twisting machines, lines of high precision spindles are driven by two powerful motors. As these machines operate 24 hours per day, frictional losses (and ultimately energy losses) occur in the rotational motion dependant on the quality of the bearings fitted at the ends of each spindle.
With 176 twisting machines incorporating 256 spindles, this clearly presented an opportunity for a major energy saving. By installing SKF Energy Efficient (E2) deep groove ball bearings, frictional losses were reduced by 30 percent over a standard SKF bearing and achieved 10 percent total energy savings.
The statistics can be daunting when you consider the cost of inefficiency in motor systems, but they also underline the importance of working together as a coherent force with a consistent approach. A large amount of waste can be converted into a correspondingly large energy saving if attitudes to energy efficiency are transformed.
And, if engineering operations take a systems approach to calculate motor performance they can capitalise on significant savings and rest assured that they are truly maximising their energy efficiency, not just solving a part of the problem.
Phil Burge is with SKF in the UK