On 2008-07-15, BobH wrote:
Ignoramus19502 wrote:
So, everyone is saying that it is bad to place a switch between a
motor and a VFD (if it is switched when running, of course).

I am sure that it is true, but I am not sure why. Also, is there
something that can be done to mitigate a switch between VFD and
motor being switched when running.

The question is not entirely idle in nature and pertains to my lathe.

I think the inductive spike that occurs on opening the switch would be
really hard on the output FETS of the VFD.

The current surge of trying to start a motor at nominal speed would be
large. I am not sure it is always the case, but I have my VFD set up to
ramp up to 60 Hz over a couple of second period to reduce the startup
surge. I am not using the variable speed aspect of the VFD, just as a
source of 3 phase 60Hz because my lathe has a good variable speed drive.

As Joe mentioned, "plug" reversing would give really large current
surges and I think would put double the output voltage across the FETs
for a short time.

About the only thing I can think of that might mitigate the problem is a
large idler motor, but that would get rid of my two favorite aspects of
the VFD - quiet and low additional power drain. If you really want to
switch the outputs, I would probably just build a rotary phase
converter. You are 80% there with the idler motor.

Bob, thanks. I run the lathe from a phase converter now and it runs
great. So the reasons for putting a VFD on it are of minor nature,
such as making the lathe quieter, and having a working but cosmeticaly
flawed 5 HP VFD that is hard to sell (cracked front cover).

The issue with "switch between the drive and motor" is the two speed
forward/reverse lever on the lathe.

The lever lets me switch motor speeds on this reversible two speed
motor, REV2/REV1/OFF/FWD1/FWD2.

If I put the lathe on the VFD, everything would work well, I would
switch the VFD with the ON/OFF/BRAKE lever connected to its control
terminals.

It would work great, except:

This is all problem AS LONG AS I do not try to flip the
REV2/REV1/OFF/FWD1/FWD2 switch during running. If I ever do so, the
drive may be ruined as I would switch the motor live with VFD powered.

That is my issue. If I could, somehow, keep the drive safe when
accidentally flipping that speed selector lever during running, I
would be happy.

--
Due to extreme spam originating from Google Groups, and their inattention
to spammers, I and many others block all articles originating
from Google Groups. If you want your postings to be seen by
more readers you will need to find a different means of
posting on Usenet.
http://improve-usenet.org/