Product category:
Air compressors
News Release from: Energair Solutions | Subject: Retrofitting variable speed drives to compressors
Edited by the Manufacturingtalk Editorial
Team on 30 May 2003
Retrofitting variable speed drives to
compressors
A debate has arisen in the market place concerning the retrofitting of variable speed drives to existing non-VSD air compressors. John Sinker and Nigel Clark answer questions on the subject.
A debate has arisen in the market place concerning the retrofitting of variable speed drives to existing non-VSD air compressors The contention has been sparked by 'unofficial' claims from a manufacturer concerning the reliability and performance of a retrofit drives package compared to that of a new built-in VSD compressor
This article was originally published on Manufacturingtalk on 17 Jul 2001 at 8.00am (UK)
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In an attempt to set the record straight, John Sinker and Nigel Clark answer questions on the subject.
The two men were responsible for the design and introduction of one of the first integrated VSD air compressors while at CompAir UK;.
Subsequently, they have developed VSD technology to produce a dedicated retrofit system and went on to form EnergAir Solutions in 1999.
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Over 2,000 EnergAir compressed air energy management systems have been installed to date, including retrofit variable speed drives.
Q.
Does retrofitting an inverter to a single speed compressor provide the same energy savings as a new VSD unit? In a single compressor installation, yes, though we are yet to find a manufacturer with a monitoring system sufficiently advanced to provide an actual figure for the achieved savings, as ours does.
However, if the new VSD compressor is being connected to an air main fed by other compressors, it must also be controlled by an effective management system, or its contribution to energy savings could largely be wasted.
An effective management system, such as EnergAir's has to accurately tailor system pressure and maximise compressor efficiency across the whole site, it will also monitor many other maintenance-orientated elements to ensure any downtime is avoided.
Q.
Is there a difference between the performance of a retrofit VSD package and a new built-in VSD compressor? In terms of the results, no, a new VSD compressor might have a wider effective speed capacity range, but a compressor's most efficient operating speed range is 50 to 100% and even better, stopped.
These are the parameters our package works within, using a soft start function to ensure a smooth and efficient transition to operating speed.
The main difference only shows up in multiple compressor installations where a retrofit drive is designed to work with a management system that can incorporate any make or model of compressor and will effectively network all the compressors on any one site, regardless of size and age.
This ensures that air production on site becomes a carefully balanced and monitored whole, something that buying one new VSD compressor will not achieve.
Q.
Do the two alternatives compare in terms of accuracy in pressure delivery? Yes, a good quality VSD compressor will deliver and maintain system pressure as well as an EnergAir retrofit package, to within +/-0.1bar.
Q.
How about reliability? The reliability of any single speed compressor will be enhanced by a retrofit drive package and its life extended; controlled start ups, less off-line running and less full speed operation will reduce wear and tear considerably.
A new compressor does theoretically have a longer service life than a machine that has been in use for a while, but this is offset by the capital investment for a new machine, often ten times that of a retrofit package.
A new VSD compressor will be no more reliable than an existing compressor with a retrofit VSD package, in fact, if the new machine is a brand new model it stands a chance of suffering teething problems that would not be experienced with a tried and tested system.
EnergAir have even come across extreme examples where a retrofit system has prevented routine motor failures in some compressors.
Q.
How is the motor in a single speed compressor affected by VSD operation? A motor may run a few degrees Centigrade hotter when fed by a VSD but only at full load.
The aim of the compressor controlled by the VSD is to provide fine control over system pressure and so it spends far less time under full load operation than it would do otherwise (part of the reason why the system provides such large savings on energy usage).
On the other hand the reduction in shock loading during starting with an inverter actually protects the motor and extends its life.
At least one manufacturer EnergAir are aware of use a smaller air end on their VSD compressor compared to the equivalent fixed speed compressor, this cost cutting has resulted in higher motor and rotor speeds and the need for specialist 'high speed' bearings and grease.
The motor bearings and lubrication on all the compressors that we offer to upgrade (virtually all known brands to date) work well within their design parameters and so the issue does not arise.
Q.
Does running at lower speeds affect cooling? A good quality motor can in most cases run at full load down to around half speed without de-rating or overheating.
Further, the main cooling fan in an enclosed screw type air compressor draws a large amount of cooling air through the machine and provides significant additional cooling for the motor, hence the motor's own fan running at lower speeds is not often an issue.
In extreme cases where a compressor is running in an already high ambient temperature, a force vent fan unit can be retrofitted to the motor.
The EnergAir retrofit inverter is also equipped with motor temperature feedback to ensure any potential overheating, for whatever reason does not cause any damage or compromise safety.
Q.
How does a retrofit VSD package impact on EMC compliance? Fitting a VSD does raise the question of EMC compliance.
Providing the power cables are armoured, braided or run exclusively in trunking and an RFI filter is fitted there will not be a problem associated with conducted or radiated emissions.
The equipment will comply with the power drive standard EN61800-3 for the second environment or optionally for conducted and radiated emissions the generic standard EN50081-2.
Q.
Does that also apply to line harmonics? Regarding harmonics, there is little difference between manufacturing sites as to how much harmonic current they pull.
Factories with weak supplies tend to suffer more than those with stronger connections, however EnergAir would always advise on this issue to enable compliance with G5/4.
Where it is of particular concern, harmonics can be suppressed through active and passive filter networks, use of 12 pulse or active bridge networks Q, Could driving a single speed compressor at different speeds cause mechanical harmonic resonance? It might be possible, but EnergAir Drives have 4 skip frequencies, the position on the speed spectrum and width are both programmable.
It is quite normal on fan and pump systems to encounter such resonance, but not in practise on compressors.
An air end is designed to run reliably at all speeds, after all, the same air end will often be used on other models such as portable diesel engine driven compressors where the application and duty is highly variable and the speed of the diesel engine varies to satisfy demand.
Q.
Are there any other motor considerations with inverter operation? JS.
There is a theoretical issue with 'premature AC motor bearing failure due to Inverter PWM waveforms'.
The effect is said to be possible in AC motors of approximately 200kW and above, where the operation of an inverter unit has been reported to cause high frequency current pulses to pass through the motor bearings, causing pitting and premature bearing failure.
There are however, no firm statistics or evidence to support these reports.
A world leading motor manufacturer, US group Emerson, have researched this issue for a number of years, as the potential for this effect is greater Stateside due to higher supply voltages and longer cable runs.
They concluded there is little or no supporting evidence of this phenomenon if the drive is installed correctly.
The British VSD trade association GAMBICA are 'aware of this issue' but again have no supporting data or evidence.
The important point here is that the possible effects can be virtually ignored if the motor is correctly and effectively earthed.
In all our experience and that of our drives supplier, we are not aware of any problems with bearing life reduction due to Inverter PWM waveforms being found or reported in the field.
Q.
How does running at lower speeds affect internal lubrication in oil-free compressors? Oil-free compressors often use a pump mechanically driven from the main compressor drive shaft to deliver lubricating oil to the timing gears.
It could be that in a particular model there would not be an adequate oil supply to the gears at very low speeds, we have yet to come across one, but we would recommend the pump be replaced by a motor driven pump to resolve the issue.
It is a simple operation and something we would include in the quote.
Q.
Some suppliers would claim that it is better to stick to the OEM manufacturer for maintenance, how does this affect your customers? We do not maintain compressors, but our monitoring system offers 24hr real time information on everything from system pressure to dryer status, bearing condition, compressor efficiency, duty cycles and more.
This information is then automatically relayed to in-house maintenance departments, specialist service suppliers who operate 24hrs or both.
Messaging can be in the form of e-mails faxes, phone calls, SMS messaging and is completely scaleable in terms of urgency or importance.
Q.
What of the quality issues surrounding a retrofit VSD package? All our systems are installed and commissioned by us, they are guaranteed and supported 24hrs.
All EnergAir equipment is CE marked.
Component parts such as drives are sourced from market leading suppliers.
It is a guarantee of quality that few compressor manufacturers could or would be willing to match.
Q.
What do you think of some manufacturers claims that it is not possible to give accurate data on energy saving with a retrofit inverter? Rubbish, frankly.
EnergAir specialise in selling compressed air energy savings not compressors, I suspect this is why the claims have been made.
Manufacturers will state that in order for you to enjoy lower running costs you must buy another new (expensive) compressor, irrespective of the fact that your compressors may be capable of satisfying your production requirements.
I don't blame them for this; they are in the business of selling new compressors.
EnergAir, however would pre-audit your system to determine which compressor should be inverter retrofitted to minimise energy costs and maximum return on investment.
We will then guarantee those savings and report back with the results on an ongoing basis.
Q.
Do most compressor manufacturers insist their customers buy a new compressor when they express an interest in the benefits of VSD control? In a word, yes.
Compressor manufacturers have a vested interest in selling new machines and this undoubtedly affects their advice to customers.
Manufacturers do not seem to have a problem however, when fitting an external drive constitutes a 'special' or bespoke engineering project.
In this instance it represents a profitable exercise for them, but does legitimise the procedure somewhat and call in to question the validity of some documentation we have seen from manufacturers attempting to 'scare-off' customers from retrofit packages.
Q.
Which companies would recommend a retrofit system? The recommendation of a retrofit solution generally comes from a compressed air distributor who has been asked to suggest the most cost effective way of lowering the running costs of a non-VSD compressor house, or by referral from an EnergAir customer. Request a free brochure from Energair Solutions ...
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