Energy saving opportunities - Motors
Motors represent 80% of electrical energy consumption in the industry segment
Motorised systems are one of the potential areas where energy savings can be made.
Many solutions exist to improve the energy efficiency of these motorized systems, as described below. You can also refer to the white paper "Energy efficiency of machines: the choice of motorization"
Choice/replacement of the motor
Those wishing to improve passive energy efficiency often consider replacing motors as a starting point, especially if the existing motors are old and require rewinding.
This trend is reinforced by the determination of major countries to stop low-efficiency motor sales in the near future. Based on the IEC60034-30 Standard’s definition of three efficiency classes (IE1, IE2,IE3), many countries have defined a plan to gradually force IE1 and IE2 motor sales to meet IE3 requirements.
In the EU, for example, motors of less than 375 kW have to be IE3-compliant by January 2015 (EC 640/2009).
There are two reasons for replacing an old motor:
- To benefit from the advantages offered by new high-performance motors (see Fig. K16)
Depending on their rated power, high-performance motors can improve operational efficiency by up to 10% compared to standard motors. By comparison, motors which have undergone rewinding see their efficiency reduced by 3% to 4% compared to the original motor.
- To avoid oversizing
- In the past, designers tended to install oversized motors in order to provide an adequate safety margin and eliminate the risk of failure, even in conditions which were highly unlikely to occur. Studies show that at least one-third of motors are clearly oversized and operate at below 50% of their nominal load.
- Oversized motors are more expensive.
- Oversized motors are sometimes less efficient than correctly sized motors: motors are at their most effective working point when operating between 30% and 100% of rated load and are built to sustain short periods at 120% of their rated load.
- Efficiency declines rapidly when loads are below 30%.
- The power factor drops drastically when the motor does not work at full load, which can lead to charges being levied for reactive power.
Knowing that energy costs account for over 97% of the lifecycle costs of a motor, investing in a more expensive but more efficient motor can quickly be very profitable.
However, before deciding whether to replace a motor, it is essential:
- to take the motor’s remaining life cycle into consideration.
- to remember that the expense of replacing a motor even if it is clearly oversized, may not be justified if its load is very small or if it is only used infrequently (e.g. less than 800 hours per year see Fig. K17).
- to ensure that the new motor’s critical performance characteristics (such as speed)are equivalent to those of the existing motor.
Operation of the motor
Savings can be made by:
- Replacing an oversized old motor with an appropriate high-efficiency motor
- Operating the motor cleverly
- Choosing an appropriate motor starter/controller
Other approaches are also possible to improve the energy efficiency of motors:
- Improving active energy efficiency by simply stopping motors when they no longer need to be running. This method may require improvements to be made in terms of automation, training or monitoring, and operator incentives may have to be offered. If an operator is not accountable for energy consumption, he/she may well forget to stop a motor at times when it is not required.
- Monitoring and correcting all the components in drive chains, starting with those on the larger motors, which may affect the overall efficiency. This may involve, for example, aligning shafts or couplings as required. An angular offset of 0.6 mm in a coupling can result in a power loss of as much as 8%.
Control of the motor
The method for starting/controlling a motor should always be based on a system-level analysis, considering several factors such as variable speed requirements, overall efficiency and cost, mechanical constraints, reliability, etc.
To ensure the best overall energy efficiency, the motor’s control system must be chosen carefully, depending on the motor’s application:
- For a constant speed application, motor starters provide cheap, low-energyconsumption solutions. Three kinds of starters can be used, depending on the system’s constraints:
- Direct on line starter (contactor)
- Star Delta starter: to limit the inrush current, provided that the load allows a starting torque of 1/3 of nominal torque
- Soft starter: when Star Delta starter is not suitable to perform a limited inrush current function and if soft braking is needed.
Example of constant speed applications: ventilation, water storage pumps, waste water treatment stirring units, conveyors, etc.
- When the application requires varying the speed, a Variable Speed Drive (VSD) provides a very efficient active solution as it adapts the speed of the motor to limit energy consumption.
- It competes favourably with conventional mechanical solutions (valves, dampers and throttles, etc.), used especially in pumps and fans, where their operating principle causes energy to be lost by blocking ducts while motors are operating at full speed.
- VSDs also offer improved control as well as reduced noise, transient effects and vibration. Further advantages can be obtained by using these VSDs in conjunction with control devices tailored to meet individual requirements.
- As VSDs are costly devices which generate additional energy losses and can be a source of electrical disturbances, their usage should be limited to applications that intrinsically require variable speed or fine control functions.
- Example of variable speed applications: hoisting, positioning in machine tools, closed-loop control, centrifugal pumping or ventilation (without throttle) or booster pumps, etc.
- To handle loads that change depending on application requirements, starters, VSDs, or a combination of both with an appropriate control strategy (see cascading pumps example Fig. K20) should be considered, in order to provide the most efficient and profitable overall solution.
- Example of applications: HVAC for buildings, goods transport, water supply systems, etc.
- The method for starting/controlling a motor should always be based on a systemlevel analysis, considering several factors such as variable speed requirements, overall efficiency and cost, mechanical constraints, reliability, etc.