Why do capacitors in VFDs sometimes need to be reformed, and how does one accomplish that task?

An automatic capacitor reformer is an essential tool for any industrial electric motor asset management program. It allows a user to determine how much calculated capacitance of an electrolytic capacitor bank has remaining. The reason that this is important is that as an electrolytic capacitor ages, it loses its maximum capacitance and drops way below the original specs. As a result, it is important to know exactly how much capacitance remains in a capacitor to determine if it needs to be replaced to avoid failure when the power is needed or avoid replacing capacitors before it is absolutely necessary to do so.

This can be a problem when an AC drive has been in storage for more than two years. If you go for this period of time without charging them up, the protective metal-oxide layer degenerates, and if current is applied to a capacitor where this layer has degenerated, the current leakage could overheat the capacitor, boil the liquid in it and cause it to vent. This destroys the capacitor which will require it to be replaced and lead to unplanned downtime and cleanup.

The solution to the problem is to use an automatic capacitor reformer to “reform” the metal oxide insulating layer and restore it to its original characteristics. These capacitor reformers do this by stepping up the DC bus voltage gradually, typically in 100 VDC increments, so that metal oxide layer can reform gradually and prevent the capacitors from bulging or venting when the full three-phase power is applied to the drive and the drive works as expected when powered up.

Benefits of an Automatic Capacitor Reformer

• Saves money by avoiding replacement of aging capacitors before they reach the end of life, and getting rid of capacitors that have been in storage for too long.
• Prevents Failure and Damage—The ability to reform a capacitor allows them to be ready for operation when needed without the risk of venting or damage.
• Verifies the capacitance of stored drives quickly so they can be pressed into service when needed.
• Decreases maintenance time and cost.

Tekwell Services Provides Superior Electric Motor Preventative Maintenance Services

Tekwell uses the Bonitron M3628ACF automatic capacitor former rated for 1,000 VDC and capacitances up to 300,000 uF. This portable unit, powered by a standard 120 VAC outlet, can handle reforming of large AC drives (typically up to 1,000 HP, 600 VAC). Need to reform your capacitors in storage? Give us a call at 1-800-829-7454 or fill out the simple online form on our Contact page or for more information.