I was checking a faulty Schneider VFD (ATV312H037N4) which is rated for a nominal output current of 1.5 A. Turns out the rectifier bridge of the inverter module (FP10R12YT3) blew up and after reading the datasheet I was surprised to find out that the IGBTs are rated for a nominal current of 10 A and the rectifier diodes are rated for 1600 V and 25 A. After checking some other drives from different brands I noticed the same thing.

I get that oversizing of components is normal for this kind of application but this seems a little extreme. Why would they do this? What's keeping this VFD from running a much larger motor that's closer to the ratings of the inverter module?

All the heavy lifting is done by the module so I don't see the reason to why the VFD is rated so low.

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    \$\begingroup\$ Current may have to be de-rated somewhat due to temperature rise. Voltage is probably over-spec'd due to transients which could arise from reflections when driving long lines and also maybe from other industrial loads (these things are operated in factories and such) or even maybe general resilience against nearby lightning strikes. Certain places (e.g., Florida) are infamous for destroying electrical stuff due to frequent lightning. Industrial inverters are expected to run trouble free for several years at least. \$\endgroup\$ – mkeith May 24 at 3:26

Those current ratings only consider conduction loss ratings. But there are also switching losses. Even then, sit down and calculate the temperature. It is never anywhere near the rating.

There are also startup currents and peak currents where the heat generated by \$I^2R\$ losses is produced faster than the heat can spread and equillibriate on the transistor substrate causing hot spots.

There are also all sorts of nasty voltage transients.

I was monitoring VFD currents the other day on a scope. For a 1HP, 230V 3-phase motor, startup currents on a single phase were repetitive 18-30A bursts lasting miliseconds while the motor was spooling up. The nameplate current was 2.68A.

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  • \$\begingroup\$ +1 Noting what run-current of motor was would help put the startup transient in presepctive. \$\endgroup\$ – Russell McMahon May 24 at 4:51
  • \$\begingroup\$ @RussellMcMahon Added. Did I also mention that the reason I was monitoring the VFD startup current was because the old VFD burst into flames when someone was trying to repeatedly start it up when it refused to spool up? That was fun. \$\endgroup\$ – DKNguyen May 24 at 5:14
  • \$\begingroup\$ Flames are fun, afterwards, often. I have a VFD that flamed here (input cct) which needs looking at after too long on the shelf. | Current playing is a 2 kW DC treadmill motor being used to drive a rotary vane vacuum pump. Seems to work. I MAY make a simple controller to give me slightly more flexibility over eg current limit but at present I am using the treadmill consoloe and controller. \$\endgroup\$ – Russell McMahon May 24 at 7:44

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