Spare Equipment Needs Maintenance Too

Failure to maintain spare equipment can cost you money and downtime, as well as being a safety issue.

Scott SullivanBlog post by Scott Sullivan, electrical engineer at Electronic Drives and Controls who specializes in on-site field service of AC drives.

Nobody likes downtime. In industry, you lose money every minute that product isn’t going through a machine. In HVAC applications, downtime usually leads to complaints from sweaty people in the summer or freezing people in the winter. In extreme cases, having one machine down can cause other machines to run improperly, resulting in damage and more downtime. For simple applications, a bypass circuit can be implemented to prevent downtime but this comes at the cost of only being able to run at 100% speed. This forgoes the energy savings usually associated with drives and is incompatible with processes that require variable speed. When downtime isn’t an option and a drive is required, the most common solution is to have a spare AC drive on site for a quick installation. However, failure to maintain this spare equipment can lead to downtime anyway- and added costs.

Similar to how you can’t put a battery that’s been sitting in your garage for years right into your car, AC drives can’t just be wired in if they’ve been sitting on a shelf for years. AC drives contain capacitors, which have a very small amount of insulating oxide separating the conductive material. This material degrades over time but is chemically replenished within the capacitor as long as voltage is applied. If no voltage is applied, for example if the drive is in storage, the material still degrades. If you were to try to apply full line voltage to a degraded capacitor, it will most likely fail in a horrendous fashion due to this material shorting out.

So what do you do if you have a drive that has been sitting around for a while? Using the previous example of a car battery, you would hook the battery up to a trickle charger and let it sit for a day. AC drives require a very similar procedure where you hook up a lower voltage circuit to provide a small current to the capacitors. This allows them to chemically rebuild the insulating oxide layer without the danger of shorting the capacitors. For safety, a series resistor or a fuse is commonly used in case too much current is generated. This process is known as “reaging” or “reforming” the capacitors. Consult your user’s manual for the drive for specifics on how this is done. Typical times are 1 hour of charging per year of inactivity and should be performed every 1-2 years of inactivity. My recommendation is to write on the drive, in big black letters, the last date the drive was in service. That way you know if a charging is required and how long to charge if it is needed. If you are unsure of how long it has been since a drive was in use, call a service center. Most drive service companies will either let you send them the drive for capacitor testing or reform the caps on site at your location.

I’m reminded of a service call I had not too long ago. A customer’s drive had failed and it was going to be a couple weeks until I could get spare parts to repair it. The customer had a spare drive from a similar unit that was decommissioned and he asked if I could just install that drive in the meantime. The drive was compatible, but I could see from the manufacturer’s label that the drive was close to a decade old. I asked the customer when it was last in service and he said, rather vaguely, “not too long ago.” I explained the risks and he asked me to install it anyway. I installed the drive and shortly after powering it on there was a loud bang. One of the drive’s internal capacitors had exploded and damaged most of the drive’s other internal components. The drive was beyond economical repair, so the customer had to wait a few days until I could get another drive to install. It was at this point I learned the customer’s definition of “not too long ago” was about 6 years, whereas the manufacturer’s maximum idle time for the drive was 2 years.

Had the customer in my example been aware of this or kept some record of the last time the drive was in use, I would never have installed it. Capacitors exploding is dangerous to both equipment and nearby personnel. Always keep your spare equipment maintained and have the maintenance performed by qualified personnel. This customer ended up having to buy a new drive to replace the spare, money that he would not have had to spend if the drive was kept in working order or the capacitors were reformed.