On 2013-08-22, wrote:
On Thursday, August 22, 2013 4:15:04 PM UTC-4, Spehro Pefhany wrote:



I'd guess the VFD- it's presumably ~10 years old, about time for the

caps to be dying.



A couple of thoughts. I have a vfd on my lathe. When I disconnect
the power the fan on the VFD continues to run until the caps are
discharged. Have you noticed this? And if you have is the time before
the fan stops shorter than it was.

I would check the DC voltage on the caps. Be very careful in doing
this. Make sure the power is completely disconnected for say five
minutes before opening the case. Then take a insulated wire with bare
ends and put the ends across the big capacitors. Fasten the wire to
some wood dowel or equivalent with electrical tape so you do not have to
have you hands close to the capacitor terminals.

Better to use a moderate value resistor (maybe about 100 ohms)
capable of handling 50 Watts or more ( it would take closer to 1200
Watts if you were leaving it connected while power was applied, but for
the time needed to discharge the capacitors, 100 Watts shodlu suffice.
The really high surge from shorting the terminals can shorten the life
of the caps if they are still fully functional.

Now connect your
multimeter across the caps and set to high DC volts before applying
power. Is the voltage about what you would expect? About 1.4 times the
input voltage to the VFD.

Shut the power off and discharge the caps as above before
disconnecting the multimeter.

Better yet is to get a friend who knows electronics and have him show
you how to check the voltage on the caps.

I would actually expect the same DC voltage even on quite old
caps, as long as they have *some* capacitance and the VFD is not driving
a load. Better to have it driving a load, and to measure the AC across
the caps, (which will show how well the capacitor is holding the voltage
up between charging cycles from the power line.) If both measure the
same, then both are probably good. Expect perhaps as much as 50 VAC on
one powered from 240 VAC if the load is close to a full load. If it is
less near a full load, they are in pretty good condition. If it is
significantly greater, the caps are letting the voltage get too low, and
that is triggering the voltage limit alarms. (Hmm ... that 50 VAC was
assuming a single phase power to the VFD -- with your full three phase,
if it gets much below 25 VAC consider replacing the caps. They ware
easy to spot -- bit fat long cylinders with two screws on one end.
(When replacing them, beware that each terminal is either positive or
negative -- the positive will have either a red dot near it, or a '+'
molded in the top. These must go exactly as the old ones were oriented,
and likely they will be arrange so the positive side of one will be near
the negative side of the other, so check both cap's orientation when you
take the board connecting them off. (And be sure to discharge them as
above just before touching the screws.) With my test equipment, and my
knowledge, I would perform the tests live, but if you are not
experienced, the above path with discharging is much safer. A simple
slip could result in a fat arc and blobs of molten metal spraying
around.

Good Luck,
DoN.

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