We’ve seen two instances recently of extensively damaged Toshiba VT130P2 800 horsepower VFD s. These particular variable frequency drives are often found in waste water treatment plants.
On the most recent call, we discovered that 12 cooling fans were damaged, resulting in excessive temperatures building up on the input rectifiers. The output IGBTs were fitted with thermistors, but the input hockey puck type diodes, did not have thermistors mounted on them.
This caused extensive heat damage to all of the input diodes. Additionally, the long term effects of over-heating damaged the bus caps(this is a very common failure with over-heating). In bus capacitors damaged from heat, the electrolytic dries up, and the excessive ripple on the DC Bus results in poor performance, and additional heating.
In this case, we were able to repair the inverter, but in a similar case recently, the damage was just too extensive and we replaced the unit with another vfd.
Cooling and other environmental issues can become a problem on any horsepower vfd, but is considerably 
exacerbated in high horsepower units. Annual preventive maintenance can allay many of these problems, and if downtime is an issue, then you should consider such a program.
EMA routinely works on both low voltage and medium voltage high horsepower applications and we’re accustomed to both preventing and repairing these problems. Click HERE for your nearest EMA facility, or EMAIL us to discuss these or any other drive and automation problems you may be having.