One answer will be when they fail, but at that stage the pressure is on. Do you repair the one that has failed, assuming we know why is has failed; can you get the parts, and how long will it take? If you decide to replace it, will you choose the product best suited to your application, or leave the decision to the contractor and maybe go for the quick fix?

According to ABB, between ten and fifteen years is a reasonable operating life for an electronic variable speed drive (VSD). After that time, spares are either expensive or impossible to get hold of. Buyers of VSDs can also claim Enhanced Capital Allowances in their tax return on the purchase, enabling them to claim the full cost against corporation tax in the year of purchase.

New drives offer more information that can be fed into a central computer or building management system (BMS), for instance motor current, motor torque, drive fault status and other information critical to the function of the drive. Equipped with Ethernet, they can be interrogated remotely. The latest generations of drive provide improved energy savings, are smaller, and their more recent technology, will reduce mains harmonic pollution.

Most drives now come with built-in harmonics filters to reduce supply network pollution and meet the UK harmonic standard, G5/4. If the total sum of harmonics on the site is getting high, replacing an old drive with a more modern equivalent could make a difference.

Replacing the drive

By the time the drive is 10-15 years old, its power devices will have switched millions of pulses, modulating the motor current to the desired voltage and frequency. As the entire load of the motor passes through here, these components are subjected to considerable stress. Any problems with the load can put additional stress on the devices. Capacitors can begin to dry out and cooling fan bearings may need to be changed before they fail. Soldered joints may dry out and become brittle, causing loose connections. It is time to consider replacing the drive.
The new drive is simply fitted in place of the old one. The terminals may look different but will still be passing the same current and are therefore, in most cases, fine to use. The exception is if the drive is supplied via busbars, which may need modifying. However, if the drive is fitted in a cubicle, the whole cubicle, with busbars, is normally replaced.

Once the drive is installed, conduct annual checks to ensure that the connections are tight, and that any insulating dust is removed. Heat expansion inside the drive can cause screws to work loose, while dust build-up could cause the drive to overheat. With the new drive in place, your application has another 15-20 years ahead of it.

VSD recycling
When defunct drives are removed, they should be removed by a registered waste carrier, as it is illegal for anyone else to do so. ABB has an agreement with a recycling company, RID UK, which is a registered waste carrier and which will also ensure the drives are recycled properly.

Still not sure, then see what others do. To help, ABB has published a free guide to drive replacement, which can be requested from info@gb.abb.com

Go to 999 Inverters for ABB drives at discounted prices