On 2008-09-30, David Lesher wrote:
"DoN. Nichols" writes:
A VFD is a big magic solid state box that delivers AC of from {say} 10Hz
to 100Hz. It USUALLY will be one able to deliver 3 phase power. [But
there may well be smaller single phase ones, so cabbage emperor...]
There are smaller ones designed to *run* only from single phase,
and power a 1/8 HP motor (I have one -- about the volume of a brick and
a lot lighter).
But not for driving normal single phase motors at variable
speeds.
Don, I was specifically trying to keep it simple and on-topic. The OP's
concern is getting a VFD {or other solution} for his case. I just didn't
want him buying some weird eBay VFD that WAS single phase...because
he assumed "VFD == 3 phase"
Have you *ever* seen a VFD -- on eBay or elsewhere -- which was
single phase output? I haven't, and I think that it because there is
not much benefit from variable frequency with single phase motors.
How it does that.... Well think of it as an audio oscillator and a
BIG amplifier for the speakers err motor.
Except that it is normally done by PWM (Pulse Width Modulation)
Again....TMI
Perhaps -- but I find people here like to know the "why" of an
answer, not just the "you can (or can't) do it" type of response.
[ ... ]
Air compressors can draw a *lot* of power, and running one from a VFD
(if you run it a lot) will probably cover the cost of the VFD in power
bill savings over two or three years. They are close to 100% efficient
because of the switching mode output, while a RPC is rather inefficient.
This runs a big hammer -- I suspect it's occasional use only. He had
3 phase very nearby, but the utility wanted lot$ to bring it in.
O.K. So to him, the benefit is not having a noisy rotary
converter generating heat between operations. He can leave the VFD on
full time if he wants, and get very little waste power.
A rotary converter is a semi-kludgey way of getting 3 phase from a single
....
Actually -- a *larger* idler motor than the load motor (1.5x or
2x) will probably provide all the torque you need -- especially if you
take time to balance the converter by adding capacitors between the
generated phase and the other two legs to produce approximately equal
current in each leg.
And how many ram types would do that?
You mean rcm? Quite a few have -- and I first learned about it
from the studies that Fitch (used to be a regular here) posted in a
series of articles as he studied the construction of a rotary converter
for his shop.
The balancing gives you lots quicker starting (more torque), and
the final capacitors across the input line to the rotary converter
corrects the power factor, so a 10A load won't draw 30A through the
breaker, causing lots of nuisance trips. It also keeps the wiring
cooler. Note that it does *not* save much money (just a little for the
extra heat generated in the wiring), because *residential* power meters
measure true Watts, not Volt-Amps which can be very different with an
uncorrected rotary converter.
I will admit I'd not considered the instant reverse aspect. That might be
reason for the rotary converter approach.
It is one reason. The ability to connect multiple machines to a
single converter, and not have to play tricks with the
forward/stop/reverse switching to pass the commands to the VFD (instead
of risking zapping the output transistors by putting a switch between
the VFD's output and the motor, as most machine tools are wired, since
they expect the three phase to be coming from the power company.
And *I* can't get commercial three phase (even though it runs
past about a quarter block from here (you've seen my place -- just up at
the top of the hill there is medium HV three phase passing by)) -- in
part because of town regulations -- which prohibit any hobby machine
tool with a 3HP or larger motor. Even if I could, the charge from the
power company is well beyond what I can afford -- and they charge
industrial rates for three phase -- which means that they base the
charge on the peak current draw during the month, and you pay as though
you have drawn that much full time through the month.
Enjoy,
DoN.
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