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How to maintain a VFD


Courtesy of VFD Manufacturers

Many of the VFDs used by the water and wastewater industry are approaching 20 years old. With careful maintenance throughout this period, the drives will still be giving good service. But there comes a point in any drive's life cycle where the decision has to be taken: do you replace, upgrade or retrofit? But which is best and when is each relevant? This article explains the choices and the decisions that need to be made about each.

So your VFD is 20 years old. No doubt it has given you very good service throughout that time and in fact could continue to do so for many years to come. So why and when should you consider replace, upgrade or retrofit?

If you have been maintaining your VFD well over the past 20 years then it is highly likely you have in place the prerequisite for all VFD owners, a preventive maintenance programme.

Such a programme ensures that any components likely to degrade by their very nature, such as the capacitors used within VFDs, are replaced at regular intervals. And here lies our first definition – replacement. It is widely accepted that component parts of an VFD (such as the capacitors) or a motor (such as the bearings) will need replacement during their lifetime and this is the role of an effective preventive maintenance programme.

However, a replacement programme can extend beyond the component level. At some stage, the cost of maintaining the old installation, such as hydraulic coupling or throttle control, is out-weighed by the potential energy savings offered by an VFD. In some cases it is more cost-effective to replace the old drive with a new one that, not only is more efficient but uses, among other things, the very latest motor control method, communications and user interfaces. The warranty that comes with the new VFD will also play a part in this equation, as it ensures low service costs for the first years.

Frequently, VFDs and AC induction motors are installed to replace slip-ring motors, direct-on-line (DOL) starting or soft-starters. There is not much of a price difference between a small drive and soft-start anymore. Replacing with an VFD means that the benefits of energy savings and improved process control come as part of the package. Old soft-starters can suffer from reliability problems, particularly if they are damp. If an unreliable soft starter is replaced with an VFD, the improvement in energy efficiency can quickly pay for the replacement. It is also possible to change out old PLCs and let the logic in the VFDs do the number-crunching.

Applications with 12-pulse drives, used to help mitigate harmonics, can also benefit from being updated with more current VFD technology. 12-pulse drives use transformers that have losses of about 3 percent. These VFDs can be replaced with new active rectifier drives that will dispense with the need for transformers – and their associated losses – altogether. This will also make the site less prone to disturbances, as a faulty transformer can take weeks to replace, due to its sheer size and weight. A faulty drive, by contrast, can be repaired by a service engineer carrying a case of spares.

Whenever there is a need to replace an entire VFD - from ABB or any other drive manufacturer – or to replace another technology such as mechanical valves or dampers, ABB offers the customer the world's widest portfolio of VFDs.

Many water and wastewater companies use a professional installation service, such as that provided by authorized channel partners, to assist with the replacement. The authorized partners can undertake a pre-installation survey and advise about any modifications required for a successful installation, for instance if the ventilation needs improving or if the existing cabling is inadequate. They can also look at existing applications and advise about potential energy savings.

The next choice to be considered is whether the VFD is actually giving you what you want. During 20 years, technology moves on and today's VFDs offer a whole raft of new functions that were only dreamed about in the early years. In many cases this does not mean you have to throw away your existing drive. Depending on its age, you could decide to upgrade the technology to add functionality, either by downloading the latest software or applying a hardware update.

There are many reasons why this may be a good course of action. It could be that faster production levels are needed; or more process intelligence is required including exact energy saving levels measured in kWh, CO2 emissions or in monetary terms. In fact an upgrade can have a major impact on the actual process by bringing new levels of efficiency and end-product quality.

So an existing VFD can often be upgraded to the latest software or hardware to improve the performance of the application. Drive upgrades are designed for improving the performance and extending both the functionality and the lifetime of the drive to provide end-users with the best possible return on their investments.

Many ABB drives, for example, only need a simple upgrade of firmware before they can benefit from the new software such as ABB's pump control program, available with the company's new VFDs. This control program incorporates all the functions generally required by pump users, for instance multi-pump control, pump priority, flow measurement and a pump cleaning cycle, also known as anti-jam or de-ragging.

When making changes to the plant, it is a good idea to look at the overall operation of the site. Software like the pump control program can make alterations both simple and affordable. It may for instance be better to have two pumps running at low speed than one running flat out.

That then leaves retrofit. It may be that no process changes are required but that the existing drive system, in its entirety, is coming to the end of its life cycle. ABB calls this the classic phase and it is here when the product support and access to spares is being tapered off.

A retrofit will endeavour to salvage as much of the existing drive system infrastructure as possible, while introducing a far more compact, robust and efficient drive. It also brings the possibility to have some new features such as an improved user interface or new functionality such as fault loggers.

Today, VFD manufacturers like ABB offer a well defined life cycle management plan which ensures that any VFD is maintained to the highest standards, thereby ensuring that the risk of failure is minimized. The life cycle management plan looks after all likely areas of drive failure and makes sure there is a plan in place to combat any risk.

For example, inadequate cooling is a common source of failure. If VFDs are not maintained according to the manufacturer's maintenance schedules - such as regular replacement of air filters and annual checking and cleaning of heat sink fins - then insufficient air flows through the drive, causing failure.

However, there comes a point in any product's life cycle when it has to be phased out as newer models with the latest technology just make the support of existing technology counter-productive.

It is at this point that taking out the old drive and fitting a new drive becomes the most economical option. Some manufacturers are developing services in anticipation of their legacy products being retrofitted.

In a retrofit project, existing cabling, motors and cabinets often remain in use, while the drive or drive system is replaced either completely or partly. Retrofitting is typically carried out during planned production shutdowns, without causing extra production downtime.

For example, if a plant is using an early ABB VFD such as the SAMI STAR, then a retrofit could not be easier. ABB has developed a specific retrofit service that comprises engineering expertise and a drives retrofit kit. ABB engineers assess the current drive installation and recommend the appropriately dimensioned retrofit kit.

The retrofit kit replaces the existing internal components of the SAMI STAR drive. By reusing as much of the existing ancillary electrical and mechanical components such as cabinets and cabling, the retrofit kit typically costs less than replacing the complete drive and cabinet.

The benefits of such a retrofit kit include lower capital expenses, reduced engineering costs, lower labor costs, no production losses and quick project execution.

Don't forget the motor
The decision to replace, upgrade or retrofit a drive will be usually the result of either: stricter production or process requirements calling for existing drive replacement; a complete failure of the drive, making it irreparable; or the original manufacturer unable to support the product with spares.

Whichever project type is being undertaken, getting payback on drive installations is much easier now than it was when the sites were built, as energy prices have gone up while the prices of VFDs have fallen to about a quarter of what they used to be 20 years ago.

When choose a VFD, the motor should also be considered. The typical life of a motor is 15 to 20 years, so if the drive is due for renewal, so is the motor. New motors are more efficient than older ones and are better working with VFDs.

In many countries, financial incentives are available to support the installation of VFDs where these are used for energy-saving purposes. Across Europe, governments are introducing tax incentives that enable companies to offset the entire cost against tax in the year of purchase. This has made drives more affordable and is a driver for change in the water and wastewater industry.

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