[Shop-talk] VFDs

[David C.]

I just purchased a used Delta 12/14" table saw.  It has a 5 HP, three 
phase Baldor motor on it.  I have been doing a lot of research over at 
the old woodworking machines site on how to power it.  I'm looking for a 
more or less plug and play solution, and would rather not start swapping 
motors at this point.  I have more or less reached the following 
conclusions:

- Most people feel that static converters are a Very Bad Thing, although 
a few folks report that they work fine;

- If you are running multiple 3 phase loads, or loads in the 5 to 7.5 HP 
and up range, a rotary phase converter is the way to go.

- Buying a rotary phase converter is expensive, and apparently you can 
build your own for a lot less, but it requires a lot of messing around 
on Ebay, not to mention a lot of assembly.

- VFDs are very highly regarded, but the "inexpensive" ones, that is, 
under about $400, top out at around 3 HP or 11 amps or so.  Also, 
anything over 3 HP generally wants a 3 phase input.  I'll be running it 
with a single phase, 220 input.

- Apparently VFDs are capable of running at 150% for a minute before 
tripping out.  My motor pulls 12.8 amps at FLA and the VFD I'm 
considering is rated at 11 amps.  Some Old Woodworking Machinery folks 
report running 5 HP machinery on a 3 HP VFD with no problems.

- Since I don't plan on ripping 12/4 maple all day long with a power 
feeder, I will probably never approach FLA on my saw, so a 3HP, 11 AMP 
will probably work fine for me.  Also, the existing saw has an Allen 
Bradley push button starter mounted on the front of the saw.  With a VFD 
I can get rid of the starter and use low voltage switching via the VFDs 
inputs to start and stop the saw.  If I go with a rotary or static 
converter, or a big single phase motor ($$$), then I will be messing 
around with sizing the overload heaters, getting the correct coil 
voltages, and so on.  In my experience with 3 phase motor starters, this 
can get expensive in a hurry, expecially for obsolete AB stuff.

Anyone got any experience with these thing or would otherwise like to 
wade in?

thanks
Dave C