Design by Objective: Efficiency

April 1, 2004
Sometimes misused components drag systems down; other times compromised setups limit output. With the need for improved efficiency, the editors of Motion System Design asked industry experts how efficiency can be increased

What is considered efficient today?
Mike//Reliance Electric:
Efficiency is technically defined as output over input, or how much work is being accomplished for the power, energy, and money consumed. From a system standpoint, efficiency can also be defined in terms of equipment life or operation.

Production efficiency is not only related to power consumption, but also the ratio of time and overhead costs to the final results measured in product or profitability.

Andy//Rockford: The leading edge of efficiency for ballscrews is considered to be 95% efficient. This efficiency allows the designer to utilize a smaller drive motor than required for the standard lead screw.

In what sorts of applications is efficiency of greatest importance?
Mike//Reliance Electric:
Applications where energy consumption is a large part of production costs need greater efficiency from machinery and processes; those that require constant starting and stopping or continuous system changes need greater efficiency especially. Ramping speed up or down to start and stop uses more energy, more quickly than systems capable of regulating speed to slow down or ramp up without operating at full, constant speed.

Motor/drive packages are one solution. They regulate the amount of energy needed to accomplish each task in the process so that only the amount of energy required is used. This is more efficient than a system consistently running at constant speed.

What is the biggest limiting factor in a motion system when trying to maximize efficiency?
Mike//Reliance Electric:
Reduced demands for energy and the ability to reclaim energy are two practices that are beneficial to energy conservation. They reduce energy consumption demand placed on the utility and result in greater efficiency for the system because heat and other waste is not being put back into the environment.

Andy//Rockford: One of the factors limiting utilizing a screw assembly efficiently is the tendency for vertical applications to backdrive under load. This back-driving can be overcome by including a brake on the screw or on the motor.

What should engineers know about component interactions and how they affect efficiency?
Mike//Reliance Electric:
Consider each part of the system when thinking of ways to increase efficiency. Why? Because every component contributes to the efficiency (or inefficiency) of a system. Some mechanical components are often overlooked as potential barriers to efficiency because they are small. Another point: Components designed to improve efficiency can be used to replace inefficient components. However, the substitution of one component for another may be beneficial in reducing inefficiencies, but could also result in the loss of needed forces or actions.

Andy//Rockford: The designer must consider total system efficiency to determine required motor drive size. The theoretical efficiency of the components can be reduced if the application has inherent misalignments or inadequate lubrication.

What helps engineers achieve maximize efficiency in their designs?
Mike//Reliance Electric:
To maximize efficiency the limitations of the entire system must be defined; a systematic method is helpful. Begin at the energy source and continue through the system until the product leaves your control. By defining the entire system, each mechanical and electrical component can be identified as either a help or a hindrance to efficiency. Focus can then be placed on manageable areas that can be improved.

Actually, we’ve accomplished something we consider rather impressive in this area: a developed and tested 1,000-hp high-temperature superconducting motor. Its benefit is smaller size, which reduces friction and losses in armature materials for more efficient performance. Lower cooling costs associated with the higher acceptable operating temperatures of the motor also provide benefits in efficiency. High-temperature superconducting motors utilize variable speed drives for further efficiency in air and fluid handling applications. The estimated benefit is a 50% reduction in inefficiency and size compared to similarly rated conventional motors.

Andy//Rockford: To increase efficiency of ball screw assemblies, sometimes it’s necessary to alternate ball sizes within the ball nut assembly. This eliminates friction between balls by changing the rotational direction between successive balls.

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