Control.com - Nerds in Control

VFD found offline and unable to power up.
AC TECH SM 4100 VFD (10HP). On initial checks the line reactor as well as other electrical equipment in enclosure are in good working order. Power is not passing through VFD.

One capacitor out of two capacitors is blown. The very same programming that was used on many other VFDs I installed over the past two (2) years was set.

What could be the causes (high voltage, external interferences, bad electrolytes, etc.)??
There is a straight slit along the blown capacitor. The top of the capacitor seems to have bulged out as well.

Reverse polarity is about the most dramatic way to blow up electrolytic caps. If the unit had recently put into service, it's possible the dielectric film had deformed from storage, some manufacturers have a reforming procedure to be followed after long storage.

Excessive ripple current and leakage are ways blow up these caps in service. An open diode can greatly increase the ripple. They do have a finite life and while most dry out peacefully and simply lose capacitance, in a high current application they can dissipate excessive power due to the increased ESR and get hot enough to pop. That slit may well be a weak spot provided to vent excessive pressure without violently blowing your high voltage section full of aluminum foil, paper and highly conductive liquids. These capacitors tend to get conditioned to the voltage they are used at and a high line condition may push them over the edge. Most of the time they will reform for the higher voltage but sometimes the leakage gets excessive and the power gets out of hand before this can happen. Or they can simply break down and short but usually the fuses will open before they get too hot. They are really a lot more complex that most people realize. If the unit has been in service for a while I would operate on the high ripple/increased ESR or high voltage assumptions in the absence of a detailed failure analysis. And check the rectifier and for abnormal input power both high line and high harmonic content which can raise peak currents.

Regards

cww
Who has unraveled many miles of foil looking for evidence in failure analysis.

MS, please provide some additional detail such as the capacitor nameplate rating and their location in the drive circuitry! That is, are the three capacitors at:

a) AC-input to the drive;
b) on the DC-link;
c) at AC-output of the drive;
d) t the motor terminal-box?

Regards,
Phil corso (cepsicon@aol.com)