On 9 Jul 2003 16:58:57 -0700, (David Heidary) wrote:
What is a vfd??

Variable Frequency Drive. It is basically a rectifier driven inverter. It rectifies
the input AC voltage and uses that to run a power oscillator (in the case of a
3 ph inverter, 3 power oscillators, synchronized and phase shifted to synthesize
3 ph). The outputs of the oscillators drive the load. By varying the frequency
of the oscillators, with a control provided on the VFD, you vary the speed of
the driven 3 ph motor.

Because the VFD first rectifies the line voltage, ie turns it to DC, it doesn't
matter what frequency or phase the input voltage has. The output voltage
will be the frequency, and phases, the inverter control sets it to be.

For relatively small VFDs, the input can be single phase while the output
is made to be 3 phase. For example, I use a VFD to drive my 2 hp mill.
That VFD runs just fine off of 1 ph 230 volts, producing 3 ph 230 volts
for the mill motor. The VFD cost me $200 new from Dealer's Electric.
It is a simple, easy, solution to getting 3 ph power in a shop with only
1 ph power. Plus you get variable speed control of the mill.

For larger VFDs, the input rectifiers and filters are designed for 3 phase
input power only. Using a large VFD on single phase can overstress the
active rectifiers and filters. So you have to *derate* larger VFDs if you
intend to power them with 1 phase. Typically, they shouldn't be asked
to provide more than 2/3 of their nameplate power. In other words, to
power a 5 hp 3 ph motor with aVFD running from single phase power,
you should use a 7.5 hp rated VFD.

Note that since VFDs above about 3 hp are not designed to be run from
1 ph power, you may need to modify them to work that way, as well as
also derating them. Usually this means defeating a phase loss detector
in the VFD which shuts it down when a missing input phase is detected.

Defeating this circuit can be simple, difficult, or essentially impossible,
depending on the design of the particular VFD. You're somewhat on
your own here because you're using the VFD in a way for which it
wasn't designed.

Note also that high power VFDs, greater than about 3 hp, start getting
expensive. If you just need to make 3 ph from 1 ph power, a home made
rotary converter will be much less expensive in the higher power ranges.

I'm currently facing that dilemma. My new lathe is 7.5 hp 3 ph. That
means I'll need a 11.25 hp VFD to run it on single phase. Since that's
not a standard size, I have to go up to a 15 hp VFD. That's $929 from
Dealer's Electric. Then I have to hope I can fool its phase loss detector
into letting it run off of 1 ph power.

But I can build a rotary for nearly nothing, using a used 15 hp 3 ph motor,
some surplus capacitors, and a couple of contactors. That will limit me to
using only the speeds provided by the headstock gearing, but that's not
really a problem with this lathe.

Perhaps I shouldn't make a rotary converter sound like such a casual
no brainer. For a 15 hp rotary, we're talking about 50 amps of running
current, and a starting surge well in excess of 200 amps. Starting it can
tax a 200 amp service panel severely if you don't take steps to soft
start it.

One good way to do that is to spin it up first with a *pony* motor.
That's simply a small 1 ph motor belted to the shaft of the big 3 ph
rotary converter motor. Spin it up with the pony, then hit the big
motor with line voltage. Power can then be disconnected from the
pony. That reduces starting surge down to a more comfortable
range (ie something that can be handled by a 60 A motor rated
circuit breaker).

It can still be cheap if home made from scrounged parts, but it is
starting to become a bit more Rube Goldberg. OTOH, a large
rotary like that can be used to power every 3 ph piece of equipment
in your shop (perhaps not all at once, depending on the total current
draw). That can be considerably cheaper than buying a VFD for
each piece of equipment.

Gary