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The global importance of motor efficiency

01 July, 2018

Two thirds of the electrical energy used in industry is consumed by electric motors. Rob Wood, ABB’s sales manager for LV motors in the UK, explains how motor designers can reduce losses in their machines to boost efficiency and thus limit CO2 emissions.

The world is facing a serious energy challenge, with demand for electricity growing faster than for any other form of final energy. Pressures to reduce energy consumption and lower carbon dioxide emissions are coming from all sides.

One of the simplest ways to address this challenge is to seize the opportunities for energy reduction that come from using energy more efficiently.

More than 40% of generated electricity is consumed by industry, and two-thirds of that energy is used by electric motors.

By using more efficient motors it would be possible to achieve huge savings in both energy and carbon dioxide emissions. This has prompted governments around the world to introduce MEPS (Minimum Energy Performance Standards), setting mandatory minimum efficiency levels for electric motors.

So what is motor efficiency? Efficiency is simply the ratio of output power to input power and is represented as a percentage.

Let’s take a standard motor with a 75kW output, for example. Achieving this output requires a power input of 80.9kW. Using the above ratio we get an efficiency of 92.7%. This also means that the power losses amount to 5.9kW.

Where do these losses come from and how can they be minimised to increase the motor’s efficiency? Losses can be defined as follows:

  • Stator iron losses are present due to the energy required to magnetise the core material, and include losses due to creation of eddy currents that flow in the core. They can be reduced by using better and thinner electromagnetic steels and by lengthening the iron core.
  • Friction and windage losses are present due to the friction in motor bearings and seals, along with air resistance in cooling fans. They can be reduced by dimensioning and selecting bearings correctly, and by optimising fan and air flow designs. 
  • Stator winding losses or I2R losses are present due to the flow of currents through the resistance of stator windings. They can be reduced by optimising the stator slot design, with the windings tightly packed to ensure a good slot fill ratio, together with low-loss electrical steel laminations to minimise magnetic losses in the stator core.
  • Rotor losses The rotor also suffers I2R losses, caused by the rotor currents, as well as iron losses caused by the magnetic flux interacting with the rotor core. These can be reduced by increasing the size of the rotor bars and end-rings to produce a lower resistance.
  • Additional load losses or stray load losses are present due to the leakage flux caused by variations in the windings, mechanical imperfections in the air-gap, and irregularities in the air-gap flux density. They can be reduced by improving slot geometries and minimising deviation in the air-gap.

Unfortunately, losses are an inevitable part of running a motor and because they affect efficiency directly, motor manufacturers are constantly doing their best to reduce them.

By using higher quality materials, cutting-edge motor designs, and manufacturing in high-precision automated factories, losses can be minimised.
This means that with every successive generation of motor design, efficiency improves and contributes to solving part of the global energy challenge.

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