Electric motors are part of our way of life: in the home, in industry and in commercial buildings.  According to Schneider Electric, motors consume 30% of the energy of a building, and 60% of the energy of industries and infrastructures. In business, 85% of electric motors control pumps, fans and compressors and by reducing motor speed, these applications have the potential to save up to 70% of their running costs.

Really?

If variable speed drives (VSDs) are so good at saving energy, why is every motor not fitted with one? Most modern systems are optimised for energy saving and do now include VSDs. However, electric motors can operate effectively for ten or twenty years, so there are many older ones without speed control, hidden away in basements or on roofs that are ignored or not known about.

Electric motors are designed to operate continuously at full speed, and without VSDs it is normal to use valves or dampers to control the delivery of fluids through the system. Installing a VSD to run a motor at a slower speed allows for the removal of the valves and dampers. Reducing a motor’s speed also reduces the power consumed by the motor, but there is more to it than that as the energy reduction is not linear.

CUBE RULE

As motor speed falls, the reduction in energy required to drive the motor is determined by the ‘cube rule’.  Simply put, reducing the motor speed by 10% reduces the power used by the motor to 90% x 90% x 90% =72.9% of that required at full speed, giving an energy saving of 27.1%.  By the same rule, if you reduce the motor speed to 60% then 60% x 60% x 60% = 21.6% or a reduction in energy of 78.4%.

Not every motor is suitable for variable speed control, but most are. Even a modest speed reduction will give a pay-back time of less than two years and will continue to save money for the life of the motor. Other benefits can include lower noise levels, longer filter life and improved control.

Open the Schneider Electric brochure here

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