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Energy efficiency in hoisting motors

January 21, 2015

Development in crane hoisting motors has proceeded gradually over the past decades. A recent study shows possibilities to reduce carbon footprints by modifying material use and dimensions in motors, and by taking the actual purpose of the motors into closer consideration.

In the EU alone, motors account for 40% of all energy consumption. Energy efficiency in motors has become more relevant during the last ten years because of changes in energy legislations not only in the EU, but also in the USA and China.

A study on the energy efficiency of hoisting motors led by Anna-Kaisa Repo, Senior Research Engineer at Konecranes, concentrates on determining the life-span energy consumption of an intermittent-duty (S3) type hoisting motor. Repo presents her view of the big picture of improving the energy efficiency of motors in Europe.

“On the EU level the biggest benefits in improving energy efficiency lie in motors from 1 kW up to 50 kW because these types of cage-induction motors are plentiful in industrial locations.”  

Repo’s concentration on the matter of energy efficiency wasn’t prompted by the situation in the EU, but by her active involvement in developing environmental product declarations for Konecranes equipment. One of the key sections in these declarations is environmental impact, which includes carbon footprint. This provoked a logical starting point for the study.

The right motor for the right purpose

A key aspect in designing motors, which is also related to energy efficiency, has to do with taking the actual task and purpose of the machine into account. One of the starting points behind Repo’s study was to balance the energy consumption of a product’s manufacturing phase with its lifetime consumption. 

“If we talk about a typical continuously operated motor, such as an S1 motor in an air conditioning system for example, where the running time can be as much as 6,000 hours per year, the impact of manufacturing on total energy consumption is relatively low. But then, when we look at intermittent-duty motors in cranes, the impact can increase starkly. Manufacturing becomes more relevant because the operational energy consumption is low due to lower usage hours and dominating partial-load operation,” Repo explains.

Intermittent-duty S3 duty motors are the target for the study. These are motors that are active only for a while, and after that need a rest period. Workshop cranes are an example of equipment that use these types of motors. Two key variables for the study were materials used in rotors, and the stack length, which refers to the active parts of a motor.

The study shows that to make significant impacts on overall energy efficiency, eliminating leftover materials in manufacturing is important. Aluminum has the lowest manufacturing energy consumption, so it’s naturally the best choice for motor manufacturing in intermittent-duty motors. In this case increasing the stack length results in a slight reduction in consumption during operation.

When it comes to a typical grabber crane, which works around the clock carrying heavy loads of metal scraps in steel industry for example, the situation is different.

“It can be reasonable to change aluminum rotors to copper in grabber crane motors, because the reimbursement time can be higher. 

The status quo with these machines is satisfactory, but nevertheless steps could be made towards improvements in the area of energy efficiency,” Repo argues.

A system level perspective

Some heavy-duty industrial crane and port crane functions are already integrating energy efficiency to the whole process in the form of hoisting machinery that operate in generating mode while lowering loads. These types of motors supply energy back to the electrical network, making a significant impact on energy consumption particularly when handling heavy loads frequently.

Looking at the big picture, in the future, energy efficiency and CO2 footprint issues will be determined by the machines and what they are used for, rather than individual components. Repo highlights this as a key principle in sustainable design.

“Energy efficiency will be examined not by component, like we are currently looking at motors, inverters, or electronics. Instead it will be assessed from a system point of view. The focus will be on the system as an entity, and how it should be optimized. Konecranes wants to take a pioneering role in this progress.”

EXPERT SPOTLIGHT

Anna-Kaisa Repo, Senior Research Engineer, Konecranes

  • Anna-Kaisa Repo has been a member of the Konecranes Research team since 2009.
  • Her current tasks include leading research projects related to electric motors and drives, energy efficiency, and energy storage and participating in the work to standardize motors.