Re: [CAD_CAM_EDM_DRO] Re: Dead VFD

Egroupscdh (E-mail) wrote:

>My friend thinks that the VFD failed when he did a stop and then a restart
>before the spindle had come to a stop. But I don't see how this would have
>caused a problem because, on my VFD at least, all this causes is a ramp down
>'till you hit start again and then a ramp back up.
>
>

Most modern VFDs will have no problem with this. It really doesn't seem
terribly
likely that it would cause a failure. Of course, if the motor was still
spinning at
a high speed, and the inverter started out with a low frequency and
voltage, it
would end up returning energy back to the DC bus. But, that is the same as
slowing the load down, a normal operation.

>He suspects that he needs to add a braking resister ...
>
>

Yes, that is a good idea. I have put a braking resistor on both my mill and
lathe VFD's. Especially on the lathe, due to the large inertia of the
chuck,
it is really needed. It stops my lathe spindle in less than one second.

You don't need to buy several hundred $ braking resistors. Scrounge a
10 to 40 Ohm 100 W wirewound resistor and connect it to the braking
terminals. If the current through the resistor at 400 V would be roughly
equal to twice the normal running amps of the motor, it should work
fine. So, for a 1 Hp 230 V motor, running amps would be about 3.3 A.
A 40 Ohm resistor would draw 10 A at 400 V (the DC bus voltage
of an inverter running on 240 V single phase power.) The formula
is I (current in Amps) = E (voltage in volts) / R (reistance in Ohms).

Jon