Energy saving opportunities - Motors
Motorised systems are one of the potential areas where energy savings can be made.
Those wishing to improve passive energy efficiency often consider replacing motors as a starting point. There are two reasons for this:
- To benefit from the advantages offered by new high-performance motors (see. Fig. K8)
Fig. K8: Definition of energy efficiency classes for LV motors established by the European Commission and the European Committee of Manufacturers of Electrical Machines and Power Electronics (CEMEP)
- To rectify oversizing
Motors operating for long periods are obvious candidates for replacement by high-performance motors, particularly if these existing motors are old and require rewinding.
Depending on the power they generate, high-performance motors can improve operational efficiency by up to 10% compared to standard motors. Where motors have undergone rewinding, efficiency is reduced by 3% to 4% compared to the original motor.
By contrast, replacement with high-performance motors will not prove to be cost effective if the existing standard-efficiency motor – particularly if it has not undergone rewinding – experiences low or moderate levels of use (e.g. less than 3,000 hours per year). It is also important to ensure that the new motor’s critical performance characteristics (such as speed) are equivalent to those of the existing motor.
- As well as being inefficient, oversized motors are more expensive to buy than correctly sized motors. Motors are at their most effective when operating at between 60% and 100% of their nominal load. Efficiency reduces rapidly at loads below 50%. In the past, designers tended to develop 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 a third of motors are clearly oversized and operate at below 50% of their nominal load. The average load for a motor is around 60%.
Larger motors also tend to have lower power factors, which can lead to charges being levied for reactive power. When deciding whether to replace a motor, it is essential to take these factors, as well as the motor’s remaining life cycle, into consideration. It is also important to remember that the expense of replacing an admittedly oversized motor may not be justified if its load is very small or it is only used infrequently.
All things considered, every parameter needs to be taken into account before making a decision on replacing a motor.
Other approaches are also possible, as far as motors are concerned:
- 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 within the drive chains, starting with those on the larger motors capable of affecting 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%.
- Paying special attention to pumps and fans, because:
- 63% of the energy used by motors is for fluid propulsion in components such as pumps and fans.
- Flow control often uses valves, dampers and throttles, all of which cause energy to be lost by blocking ducts whilst motors are
operating at full speed.
- Effective project planning can often recoup investments in less than ten months.
