Electric drives

1. The electric drive is a system

The implementation of an electric motor drive in any application requires several types of components:

  • the component that will produce the useful effect: a pump, a fan, a compressor, drive wheels or rollers, etc.
  • a mechanical transmission (gears, pulley, chain, etc.)
  • an electric motor (asynchronous, synchronous or direct-current)
  • a control circuit (direct-start, star-delta, variable speed)

Given the widespread use of motorised systems in industry for a wide range of purposes (machinery, transport, ventilation and pumping, compression and refrigeration, etc.), particular attention should be paid to their energy efficiency.

2. Potential improvement measures

Implementation of a variable speed drive

When the demand in terms of speed varies, the installation of a variable speed drive is recommended in order to adjust the speed accurately to the requirements and thus save energy compared to permanently operating at full speed.

Relevant examples can be found in the fields of pumping or ventilation (reducing the flow rate has a big impact on energy consumption) and transporting materials (limiting full-speed operation).

Modern variable speed drives provide power factor correction (cos phi) and power distortion correction, which further improves the energy performance of the system and avoids disturbances on the power grid.

However, there are losses due to the conversion of electrical energy in a variable speed drive (2-4% losses), which means that the investment in a variable speed drive is not necessarily cost effective for constant-speed loads.

Soft starters mitigate current peaks on the electrical grid, but do not contribute to improving energy efficiency in operation.

In addition, it is sometimes possible to recover energy during braking (this requires a suitable variable speed drive and a simultaneous energy requirement).

Potential savings: from -4 to 50% in energy consumption 

High efficiency electric motors

The purchase cost of the motor alone is not the best criterion to take into consideration, as this cost accounts for only 5 to 20% of the total cost of an installation over its lifetime. On the contrary: energy consumption can reach a very significant portion of the costs over the life cycle (up to 90%) for high power engines with an intensive use profile. This must therefore be taken into account by buyers when purchasing this type of equipment.

These motors offer significantly higher efficiency than the previous ranges according to standardised energy efficiency classes: these classes define the efficiency of asynchronous motors from IE1 (standard efficiency: least efficient) to IE4 (new Super Premium class: most efficient).

The European “eco-design” regulation imposes the marketing of ever more efficient motors, profitable for users and protecting the environment through lower energy consumption:

  • since 16/06/2011, all new motors on the market must comply with efficiency level IE2 (High Efficiency), formerly EFF 1
  • since 01/01/2015, new motors from 7.5 to 375 kW must have an efficiency greater than or equal to efficiency level IE3 (Premium Efficiency), or achieve efficiency level IE2 and be equipped with a variable speed drive
  • since 01/01/2017, all new motors from 0.75 to 375 kW must have an efficiency greater than or equal to the efficiency level IE3, or reach efficiency level IE2 and be equipped with a variable speed drive

Potential savings: from 2 to 8% in energy consumption

However, the characteristics of IE2 and IE3 motors differentiate them from standard models: 

  • their cost is higher, due to the additional work needed in construction and the materials
  • internally, the magnetic and electrical circuits use more precious and more voluminous active materials (windings, magnetic sheets, etc.); the mechanical elements (bearings, ventilation) are more elaborate
  • externally, the mass and physical size can be greater
  • the rated speeds may be slightly higher due to the different electrical and magnetic characteristics (less slip compared with mains frequency)
  • their cooler running means a longer service life

Are these high-efficiency motors profitable in all cases?

The use of these motors, as a replacement for standard motors, is generally cost effective for low power (< 11 kW) and intensive use (>2,000 hours per year). However, a more precise calculation of profitability should be carried out before proceeding with the investment.

For more information, visit http://www.motorsystems.org/

Avoid oversizing

The load rate has a significant impact on efficiency. Ideally, a motor should always operate around its nominal point: no-load or part-load runs result in losses in terms of efficiency. For this reason, oversizing of electric motors should be avoided.

Potential savings: from 1 to 3% in energy consumption[1]

Schedule maintenance

The quality of repair work (rewinding) has a decisive influence on the performance of used motors.

The quality of the installation (shaft alignment, belt tension, etc.) and of the maintenance are decisive in order to guarantee the good long-term performance of the transmission.

Potential savings: from 0.5 to 2% in energy consumption [1]

Choose the right mechanical transmission

Inevitably, mechanical losses occur within the transmission components: gears, belts, chains, etc.

The choice of transmission mode must therefore include an energy efficiency criterion. Generally speaking, modern direct-drive solutions are the most efficient, followed by gears and chains.

Potential savings: from 2 to 10% in energy consumption