Re: Dead VFD

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Egroupscdh \(E-mail\)"
<egroupscdh@W...> wrote:

> A friend of mine has a Motortronics series KP1-210 inverter
> on a lathe conversion (in his home shop) and the inverter
> has gone 'toes up' indicating a 'Transistor Fault'.
>
> First question: Does anyone have access to a schematic of
> this unit? He has contacted the manufacturer but they said
> that they don't have schematics for out-of-production units
> (hard to believe they threw out EVERY copy!).

I'm sure they have them, but they're not about to give them
out. They have nothing to gain - they make more money selling
replacement drives or repair services. Also, in today's lawyer
driven world, they don't dare help you do something that might
get you hurt. (It doesn't matter how skilled you are, they
have to _assume_ that you are an idiot to protect themselves.)

> Upon preliminary examination it looks like one of the sections
> of the 'output module' (potted module bolted to the large heat
> sink and directly connected to the motor terminals) is shorted.
> I have not studied the module yet but isn't this likely to be
> something like two FETs connecting the motor lead to either
> power rail?

More or less... It is actually 6 devices, two per output phase.
Most likely they are IGBTs (insulated gate bipolar transistors),
not FETs. The module might also contain the input rectifier
diodes - or they might be in a separate module.

> My thinking is that I could replace the bad section with a
> suitable power FET module to get him up and running again
> but I wonder if I should replace all three sections so that
> they are matched?
>
> The unit is rated at (front panel label):
>
> Input: 200-240 Volts, 3-phase
> Output: 200-240 Volts, 3-phase, 13.9 KVA, 35 Amps

You do _not_ want to cobble something together. The IGBTs in
that module are switching 35 amps at 320 or so volts in about
100 nano-seconds. That kind of switching generates voltage
spikes that are strongly dependent on stray inductance. The
drive needs to be carefully designed to minimize inductance
and absorb the spikes.

I design drives like this (actually bigger ones, 200HP and up)
for a living. I know all about the pitfalls and traps, and
have the tools and experience to avoid them. Even with all
that, I would not try to substitute power components in a
drive - everything interacts and works together. Even changing
from one vendor to another's supposedly "identical" module
requires extensive testing and sometimes circuit changes to
make it work.

The only fix I can recommend would be to replace the entire
module with an identical one (if you can find one). The
manufacturer and part number of the module should be printed
somewhere on it.

> Is it possible that he blew it because he's using 230 V to
> drive a 208 V motor?

Not likely. Most likely causes of failure:

1) switches or relays in the output leads
2) shorts or other wiring problems in the output leads or motor
3) high DC bus caused by trying to stop a large inertia load
4) misc...

the drive _should_ be able to protect itself against 3 and
probably 2. Number 1 is a drive killer - never open the
motor leads when the drive is running.

> If I can replace the output module, can anyone suggest a
> rating, model, supplier?

Get manufacturer and part number from the module - if you can't
find that module, then the drive is probably scrap. And as
Jon pointed out, whatever killed the module might have also
damaged other components, in fact a failure elsewhere in the
drive may have caused the module to fail.

Sorry to be the bearer of bad news.

John Kasunich