In article ,
Richard Ferguson wrote:
I have been hearing about VFDs and other types of solid state controls.
The Phase Inverter seems not to offer variable speed, but lots of
features. I presume similar units are available with variable speed.
However, it would seem to me that you would need a specific variable
speed drive for each tool, that you would not want to walk to the corner
to adjust the speed. That could get pricy.
To me the most practical (cheapest) thing would be to have one convertor
for the whole shop, asuming that you might eventually have multiple
devices with 3-phase motors.
Yep -- and if you have a single machine which can seriously
benefit from the variable speed, put a VFD on that one item. That can
be powered from a rotary converter, or from a single-phase line, with
the possibility that you will want a slightly higher rating (depending
on the manufacturer and model) to run it from single-phase.
The way I understand it, the alternatives go something like this.
1. Static rotory converter, simple, only gives 2/3 power, probably costs
around $100 for under 5 hp. Probably could be shared by several machines.
Static and rotary don't belong in the same breath. A "static
converter" is (usually) a set of starting capacitor and a
voltage-sensitive relay which uses the capacitor to only start the motor
spinning in the proper direction, and then switches it out. (There are
some versions which have tapped inductors as part of the system which is
a bit more flexible, but a *lot* more expensive. The static converter's
drawbacks a
a) Only gives you 2/3 of the motor's nameplate horsepower
as you have already noted.
b) The voltage-sensitive relay and the sizing of the
capacitors is right for only a narrow range of
horsepower, and may also have problems starting a motor
under load. If you have multiple tools with differing
horsepowers, you may not be able to use a single static
converter for all of them.
One advantage is that is is probably the least expensive bought
purely at *new* prices.
Another is that it can be used as part of a rotary converter
built from an old motor -- used to start the motor.
2. Rotary converter - full power, could be cheap if you use an old 3
phase motor to make your own, or more expensive if you buy one new.
Could be noisy. Often shared by multiple machines.
There are techniques for starting them other than the one
mentioned under "static converter", including:
a) "self-starting": where the tuning capacitors are a
compromise value which gets the motor spinning without
any switching at all.
b) "Pony motor": A smaller motor is connected with belt
to the converter motor, to get it started spinning, and
then the belt is thrown by raising the pony motor to
slack the belt.
c) A pull rope, used to start the motor spinning, just
before switching on the power.
Note that a rotary converter is better made self starting, either
from the inclusion of the parts of a static converter, or from
self-starting capacitor balance. One reason for this is that if
you walk out of the shop leaving something running (even just
the rotary converter), and power drops for long enough for the
converter to spin down, when it returns, the motor will sit
there humming and drawing excess current. If the circuit
breakers are properly sized, it will pop the breaker.
Otherwise, it might fry the motor without a self-starting mode.
Note also that a rotary converter can be made better (for a
given size) by "tuning" (the addition of run capacitors -- not
the starting capacitors) to optimize the balance of voltages and
currents in the generated phase to match the other two.
An additional step beyond tuning is a power-factor correction
capacitor across the line power leads coming into the converter,
best done *after* the rest of the tuning. This minimizes the
out-of-phase current which flows into the idler motor, which may
not affect your electricity costs with home power, though it can
if you are being charged commercial rates. However, it *can*
trip circuit breakers if you are close to the rating, which can
be a nuisance.
3. Phase inverter - solid state, I think that you said $700. I think
that you said that they could be shared, but you lose some of the
sophisticated features in that case.
Hmm ... not sure about this -- I think that it is the same as a
VFD, and the prices need not be that high -- unless you are into
the serious horsepower range. That is probably a brand-new
price for the current model -- which is more than you need.
4. Variable Frequency Drive - the top of the line solution, provides
electronic variable speed control. I would assume the top price. I
would guess that with some machines, you might want to run part of the
machine with the VFD, while running other parts with straight 3-phase
power, like the controls. It would seem unlikely that one would share
the VFD, but maybe some people do.
I have shared one VFD -- but it is an oversize one for my shop.
It is a 30A one (7.5 HP running from single phase). However, I
now have individual VFDs on a per-machine basis.
One way to avoid running to the corner for each change is to
remote the controls. Most VFDs have terminals which offer you
the ability to stop it, start it in forward or reverse, and
adjust the speed with a potentiometer. These wires should be
run in a shielded cable -- especially if the reach is more than
just a few feet. Just make the cable long enough to reach to
the worst-case location machine.
Note that you *don't* want to switch the motor leads at the
machine when running from a VFD, as the spikes created can zap
the output transistors. Instead, you command the VFD to start,
stop, and reverse -- ideally from controls mounted on the
machine.
If there are electronics on the machine (e.g. a CNC machine, or
one equipped with a DRO, you do *not* want to power those parts
through the VFD, as they may not handle the variable frequency
gracefully -- let alone the variable voltage which comes with
the slower speeds.
What is the price range here?
I'll let others specify the prices of the other systems, and I
will deal with the kind of prices which *can* be found for VFDs -- if
you are lucky, and don't insist on new. That 30A VFD which I mentioned
above cost me $100.00 -- at a hamfest perhaps five years ago.
The other VFDs that I have cost around $300.00 each (some
adjustment, as one was 3HP and the other 2HP rated running from single
phase). These were when a local individual, looking for a VFD to power
his own machines, got to talking with Mitsubishi USA at the right time.
It turns out that the had just come out with a new line, slightly
smaller footprint, and a few more features (beyond what is needed for a
HSM type -- more for industrial controls). He was able to buy a large
number of them for a good bulk price, and was selling them locally for
equally good prices.
Then, someone at Mitsubishi got the idea of selling the VFDs on
eBay -- usually bringing in more money, though still well below the
price of the current models.
There are at least two web locations which sell VFDs -- both
used, and new, for good prices. The one which I remember is
http://www.dealerselectric.com. I'm sure that someone else will point
to the other source.
A VFD from one of these sources (even eBay) can cost you less
than a commercially-built rotary converter, though unless you luck into
one at a hamfest as I did, it will be hard to beat the price of a rotary
built from used parts.
Which of these solutions can be shared
between multiple devices? Am I wrong about what I said above? I am
trying to make sure that I understand.
Most of the above *can* be shared -- with some compromises with
some. For running multiple machines, a good oversized rotary converter
is probably the best bet. A "static converter" is the least likely to
be a good choice for this, especially if you have a wide range of
horsepower ratings for your machines. A VFD can work, with the remoted
control box, as long as you don't need to run more than one machine at a
time, but it can be a nuisance as you move from machine to machine.
Now to see what others have posted.
Good Luck,
DoN.
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