Re: [CAD_CAM_EDM_DRO] Dead VFD

Egroupscdh (E-mail) 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!).
>
>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?
>
>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?
>
>

Yes, you are thinking along the right track. But, WHY did the power FET (or
IGBT) fail? Are you absolutely sure the driver circuit is OK? Even if the
driver did not cause the failure, when the transistors blow, they can fry
the driver after the fact. It would be a big mistake to put $50+ of
transistors
into the unit with much kluging of heat sinks, etc, and then have it blow
again.

>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
>
>He is using it in 'derated' mode, feeding it with single phase 230 volts and
>driving a 7.5, 208 3-phase volt motor.
>
>Is it possible that he blew it because he's using 230 V to drive a 208 V
>motor? He hadn't told me until yesterday about the 230 .vs. 208 issue.
>Could he be trying to drive a motor wired in 'Y' configuration? (I'm over
>my head on 'Y' .vs. Delta other than knowing that they are different) Is
>this doable?
>
>

Not likely. If you check the specs on these units, they have a pretty wide
tolerance of input voltage. The real problem with running on single phase
is the ripple current in the filter caps, and the peak current in the
rectifier.
that is why they have to be derated. The computer inside corrects for the
higher line voltage by reducing the on-time of the transistors.
The Wye-Delta business is entirely moot, as long as the motor is
correctly wired for 208/230 3-phase. Changing from Wye to Delta or
vice-versa makes a very large change in the motor's operating voltage,
a factor of 1.732 (or the square root of 3). Almost all 2xx V motors can
be operated on 208 or 230 V. The run current is less on 230 that 208
(induction motors are far from being resistive loads).

First, is the unit fan-cooled? If not, did it run hot before it blew?
What was he doing when it blew? Did he just shut the lathe off?
I'm thinking that this application may need a braking resistor. As
the machine is stopping, energy flows back from the motor to the
VFD, where it piles up in the filter caps, raising the voltage. Most
drives will shut down with an overvoltage error message when this
happens. Has he seen that before the fault occurred?

Does he have any switches, relays or whatever between the VFD's
output and the motor? Most VFD makes strongly state that putting
relays between the motor and VFD is very bad, as the sparking that
will occur when the contacts open can blow the transistors.

Jon