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ebb
10-07-2016, 06:49 PM
Most all of my equipment is 3 phase. I have a radial arm (it is 3 phase) saw from years ago and set it up on something that was called a static converter. As best as I can tell it uses a capacitor to generate the third leg. Is this a VFD? If not does a VFD have enough advantages over my present set up to bother changing over?

alinwa
10-07-2016, 07:34 PM
I can't school you because I don't know anything, BUT I have successfully installed VFD's on two pieces of equipment. (Knowing nothing about a thing has never stopped me from working on it.....)

IMO they're different.

cuz I researched it a liddle bit before buying a VFD.

The VFD (Variable Frequency Drive) actually does "fake the third leg" by changing the frequency of one or both of the others to actually drive the third (armature? set of windings? rotor/stator/gator/howard/fnibbledegibbit thingie) and it allows the motor to run smoothly and allows one to adjust rpm's nearly infinitely. IMO it allows the motor to produce full power within the range of the ability of the VFD unit to handle the draw.

(I'm 'wayyy out of my depth here :) )

The static converter is just a means of using a capacitor (kindofa' fast-discharge battery) to START the motor, whereupon it continues to run, somewhat unevenly at 2/3 or 1/3 it's rated power output.

THERE!!!

If that doesn't get the goat of someone who DOES know what's going on, someone who'll correctly answer your question, then I've failed my quest....


hth

al

alinwa
10-07-2016, 07:39 PM
I will say this.....a 200.00 VFD and a 500.00 DRO installed onto an antique Bridgeport turns it into a mo'chine capable of great things! My old BP was originally 440V and I bought a VFD just to simplify the wiring when I moved it. The infinitely variable feature turned out to be AWESOME.....I'd never go back now to changing belts/pulleys every few minutes.

TRA
10-08-2016, 03:20 AM
They input single or 3 phase AC then Edison and Tesla do their magic AC/DC thing and 3ph AC comes out to power your device, which is tunable to achieve whatever motor characteristics and operation you desire.:rolleyes:

Ditch that static converter, all it does is on-off.

jim1K
10-08-2016, 08:54 AM
My old South Bend 10 is now powered by a 1hp. 1140 motor and a VFD.never change a belt again, life is simple........ jim

mks
10-08-2016, 10:51 AM
VFD's are sure nice on machines that need variable speed. But you only need one speed on your radial arm saw, and unless you are bogging down ripping 4-by lumber, you don't need the extra power. I have a static converter on my radial arm saw, and see no reason to change.

brickeyee
10-08-2016, 03:04 PM
Capacitors are used to dray the phase of the voltage around.
Ideally they would move it 90 degrees if in series with the motor winding.

They do not in actually.

The problem that occurs is large values of capacitors are often electrolytic and that means they cannot work 'both ways' on an AC voltage.

There ARE types of capacitors that are 'non-polarized' but creating a large value (measured in farads) gets very large physically especially at higher voltages.

Film style capacitors or two polarized capacitors in series (but this halves their value) do the job.

A VFD converts the incoming power to DC and can then control the voltage at the output with transistors (of various types).

It MAKES the 3-pahases with thet correct relationship.

There are all sorts of other schemes that can create usable 3-phase waveforms.

All the way to a full up motor-generator set.
You can use a large 1-phase motor to turn a 3-phase 'alternator.'

The alternator in your car often generates 3-phase power that is then rectified to rippling DC by a 3-phase bridge.
It actually takes 6 diodes.
The battery is what smooths it out the ripple.

It acts as a huge filter capacitor.
If any one of the six diodes fails the ripple can start to exceed what the battery can smooth.

TRA
10-08-2016, 10:03 PM
VFD's provide many advantages over low quality phase converters that result in loss of power due to voltage imbalance and magnify issues with the quality of your electrical service. They have become just as useful to most applications, whether fixed or variable speed. Not all electrical services are equal and the newer drives can clean up most of them.

brickeyee
10-16-2016, 11:41 AM
VFD's provide many advantages over low quality phase converters that result in loss of power due to voltage imbalance and magnify issues with the quality of your electrical service. They have become just as useful to most applications, whether fixed or variable speed. Not all electrical services are equal and the newer drives can clean up most of them.

Going to DC and then using various transistor styles to control the voltage at tens of kHz (or higher) works really well.

VFDs are vulnerable to excessive inductance in their leads both input and output.

Excessive inductance interacts with the filter networks in the VFD.

If it gets really bad it send them oscillating to destruction.

alinwa
10-16-2016, 12:48 PM
Going to DC and then using various transistor styles to control the voltage at tens of kHz (or higher) works really well.

VFDs are vulnerable to excessive inductance in their leads both input and output.

Excessive inductance interacts with the filter networks in the VFD.

If it gets really bad it send them oscillating to destruction.
Inductance from where? How do you know? How to prevent?

brickeyee
10-17-2016, 01:59 PM
Inductance from where? How do you know? How to prevent?

Inductance from excessive wire length.
Inductance if noise filtering t limit interference with other electronics in the area - like a DRO system.

We used to run lathe in SCIFs that require heavily filtered power to keep signal from 'leaking' out.

We actually had a VFD explode and burn.

Had a long talk with the manufacturer's design engineering staff.

Ended up making sure we added a bunch of high voltage film capacitors to swamp out the filter inductance.

No more exploding VFD.

You need to keep the wiring on them short and direct on both sides.
More is not better.
Keep the wires groups on both sides in close proximity to each other.
They do not need to be twisted (and probbaly should not be).

Just keep the input leads together and the output leads together.

TRA
10-18-2016, 12:29 AM
Inductance from excessive wire length.
Inductance if noise filtering t limit interference with other electronics in the area - like a DRO system.

We used to run lathe in SCIFs that require heavily filtered power to keep signal from 'leaking' out.

We actually had a VFD explode and burn.

Had a long talk with the manufacturer's design engineering staff.

Ended up making sure we added a bunch of high voltage film capacitors to swamp out the filter inductance.

No more exploding VFD.

You need to keep the wiring on them short and direct on both sides.
More is not better.
Keep the wires groups on both sides in close proximity to each other.
They do not need to be twisted (and probbaly should not be).

Just keep the input leads together and the output leads together.

Say what?:confused:

Why would a lathe be anywhere near a SCIF environment? Most of us live in the real world. BTW we don't coil our conductors, so inductance is not an issue. Voltage varies and voltage losses are a real issue outside the wall. IE, 208-210-220-230-240- A VFD can correct a lot of that and let a device operate at it's maximum efficiency and output level.

brickeyee
10-19-2016, 07:42 PM
"Why would a lathe be anywhere near a SCIF environment?"

How do you think you manufacture TS stuff?

Out in an open machine shop?

Rustystud
10-21-2016, 11:38 AM
Two schools of thought: buy one large enough to operate all your equipment, or buy one for each piece of your equipment. The best form of true three phase current without getting it directly from the power company. I think it is actually better. I converted my lathe from a 3 horsepower single phase motor to a 7 1/2 horse power 3 phase motor. Make sure you get a NEMA approved three phase motor, designed to be run with a VFD. Another piece of advise is if your lathe is metric get a metric motor.

For the best deals on VFDs and motors look up the Dealers Direct web site. They sell both new surplus VFDs and motors as well as used motors.

I bought a Westinghouse FM 215 in 2005, for my lathe. I think it was one of the best investments I have ever made in my shop. Be advised you may want an experienced electrician or engineer to wire it up. Mine has three wires going in and in the junction box there are 16 wires that have to be banked. Most VFDs will work on several different voltages.

Three phase will allow you to vary the frequency, have motor braking, and instant reverse. Due to the last you can rip the motor right off its bolts, and if your chuck is threaded send a spinning chuck loose in your shop. You can snap shear pins or break keyway keys or gear teeth. Wiring them correctly and using the features makes them much safer.
Nat Lambeth

ebb
10-23-2016, 03:20 PM
WOW the radial arm saw is not being used and I hooked up the lathe to the converter. I seemed to run OK and its 3 horse power motor was way over the top for anything I was doing. I disconnected the lathe when the mill arrived and put a twistlok on both machines and disconnect one or the other when using one. I must look into VFDs !