Drive train management
04 January 2012
Stephen Barker describes how a complete drive train management strategy can become a valuable part of a best practice approach to energy, cost and carbon emissions reduction
Motor driven systems represent the largest single consumer of electrical energy in the UK – accounting for around 22 per cent of all the electricity we generate – but the vast majority of such systems are designed, installed and operated inefficiently. In recent years, increasing regulatory focus has been attached to electric motors and driven machines, including mandatory minimum motor efficiency standards, which are now in place.
This is good news, as many users are simply not aware of the economics of running electric motors. Few users may be aware that a single 75kW motor may cost in excess of £1m to run during a typical 20 year lifecycle. The purchase price of the motor is less than one per cent of that lifecycle cost.
Taking some basic decisions on purchasing policy and implementing some elementary engineering design can substantially reduce costs. However, even better news is that dramatically larger savings opportunities will be released when organisations are able to take a ‘systems’ approach to motor driven systems.
The complete drive train
When considering motor drive systems, the impact of the ‘complete drive train’ must be included. This encompasses everything from the electrical supply through to the final mechanical output device and may well include the following:
• Form of motor control – direct on line, VSD, soft-start, etc.
• Selection of motor – efficiency class, rating, mechanical configuration, etc.
• Gearbox or coupling arrangement
• Driven machine
• ‘Mechanical’ design of the system
Typically, most systems are purchased for minimum capital cost and lifecycle costs are rarely included. And where users attempt to include life cycle costs, they are often incorrectly calculated. It should be remembered that the overall ‘system efficiency’, from the electrical supply through to the mechanical output, could be 30 per cent or less!
The starting point for the system design is the requirement for the driven machine, taking into consideration the complete profile of use. The more accurately this is defined, the better the system efficiency will be. Best-in-class organisations now use advanced simulation techniques to improve predictive calculations and designs. Consider the mechanical transmission:
The mechanical coupling technique and any gearbox arrangement will have a substantial impact on overall system efficiency. For example, a high efficiency helical bevel gearbox may be 30 per cent more efficient than a traditional worm gearbox. Armed with a robust mechanical specification, the motor can then be correctly specified – but do avoid over-sizing as this adversely impacts efficiency. Motor control is next, and there are a number of considerations to take into account:
Speed profile – if the motor is required to run continuously close to rated load and rated speed, consider direct online (DOL) starting, soft start or similar. If the speed varies, consider a variable speed drive (VSD) where that is financially viable, using readily available software tools to calculate the payback.
Load profile – even for fixed speed applications, if there is a significant load variation in the duty cycle, it may be necessary to consider a VSD.
Application specific – or many applications, the special features of VSDs are extremely useful; for example, using the regenerative braking feature for a batch centrifuge or the wide speed range on paper winding machines. Where wide speed ranges or precise control is demanded by the application, a VSD is the obvious choice.
For all motor control methods, the impact on the mains supply should be considered: what is the impact of DOL starting current; are there problems with power frequency harmonics; would a low harmonic drive offer greater system efficiency; are there compatibility issues between the motor and drive system?
But even if you have come this far and have correctly specified all the components in the system, this is still not the end of the story. Often, the overall system may be compromised by poor installation practice, incorrect commissioning techniques and lack of appropriate maintenance.
So, in summary, the two most common problems encountered in motor driven systems are incorrect system design, and poor installation and commissioning. Fortunately, comprehensive guidance is available from all equipment manufacturers, and for VSD driven systems, the GAMBICA guide to installation is a very good starting point. This can be downloaded free of charge from www.gambica.org.uk/publications.
Maintenance issues
Having established the importance of correct system design and execution, the next step would be to consider effective asset management programmes. Some organisations have developed basic motor management routines but the real opportunity is to manage all of the motor driven systems on a plant wide basis. Here is a typical action plan and checklist:
• Capture asset data for the complete ‘drive train’ – application, types, ratings, age, controls, duty, repair history and so on.
• Define action on failure; is it best to repair, rewind or replace.
• What about efficiency? Are motors correctly sized and are they high efficiency units?
• Review driven machine, gearbox, coupling, motor, electronic drive.
• Consider the financial viability and operational benefits of fitting VSDs (payback, reliability improvement, process benefits, etc)
• For key applications, consider more sophisticated ‘expert systems’
• Include as part of the overall energy management procedure for your plant or factory.
As likely as not, components will fail at the most inconvenient moments and if pre-defined strategies are not in place, the easiest, short-term solution will be applied and this could prove a costly, long-term mistake. Having a pre-defined action plan supported by access to high efficiency replacement parts is key to long term efficiency improvements.
For higher power ratings or for plant critical applications, users should consider innovative ‘expert systems, which blend enhanced (but low cost) condition monitoring techniques with the intelligent control algorithms embedded in an advanced SCADA or process control system. These systems can:
• Calculate dynamic system efficiency
• Offer predictive failure analysis
• Minimise downtime and improve equipment availability
• Reduce system lifecycle costs by up to 60 per cent
Energy, cost and carbon reduction
Whilst most organisations have implemented some form of energy management programme, despite the inexorable rise in energy prices, few have fully implemented a systematic approach to energy management.
The principles of a systematic approach to energy management can be found in the recently published international standard ISO 50001 or the earlier European norm EN 16001. Our experience is that organisations that are able to implement such an approach actually multiply their energy savings by a factor of four compared with the more usual ad hoc approach.
Extensive documentation, tools and support are available to support users across all elements of this process.
This article is based on a presentation given by Stephen Barker at the MDS 2011 conference. Stephen Barker is head of Energy Efficiency & Environmental Care, Siemens Industry Sector UK
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