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Phase converter VFD questions

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Thad in AR. View Drop Down
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    Posted: 01 Feb 2023 at 4:20am
I know nothing of Electricial devices.
The mill I bought is 3 phase and I do not have 3 phase. The mill also has two motors which I assume are both 3 phase.
I need to know what I’m up against to get this going?
There is a possibility of a rotary Phase converter for sale locally? I don’t even know what that is?
I hear of VFD’s
I’m happy with the speeds built in to this mill if need be.
I want the best system for longevity of these motors.
I also dont want to spend extra for something I don’t need.
There is a scrap yard locally that gets in 3 phase motors from Tysons all the time if that helps.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8070nc Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 5:12am
IIm an industrial matainence contractor. I deal with inverters on a daily basis. I dont know what size machineyou havebut ivr got a bridgeport clone thats 2 hp. Its been running on an automation direct single phase input 3 phasr output inverter for 15 years. I can control spindle speed without hahing to change belts. I have run a 1 inch end mill with it with no problems. To me an inverter is the easiest solution. If you machine is an Index with a table feed motor you will need 2 inverters. Id get a size larger inverter than what the motor size is for longevity. If you think I can help you ill talk to you on the phone to get you through the process. Donald Matthews
910 824 8856. Leave a message because i dont answer unfamilliar numbers because of scammers. Feel free to call
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8070nc Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 5:17am
The biggest advantage with an ivverter to me is you can get spindle speed and table feed speed exactly where you want it
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Thad in AR. Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 5:27am
Originally posted by 8070nc 8070nc wrote:

IIm an industrial matainence contractor. I deal with inverters on a daily basis. I dont know what size machineyou havebut ivr got a bridgeport clone thats 2 hp. Its been running on an automation direct single phase input 3 phasr output inverter for 15 years. I can control spindle speed without hahing to change belts. I have run a 1 inch end mill with it with no problems. To me an inverter is the easiest solution. If you machine is an Index with a table feed motor you will need 2 inverters. Id get a size larger inverter than what the motor size is for longevity. If you think I can help you ill talk to you on the phone to get you through the process. Donald Matthews
910 824 8856. Leave a message because i dont answer unfamilliar numbers because of scammers. Feel free to call

Yes it is an Index with two motors.
I haven’t received the machine yet. I’ll be contacting you soon.
Do I have to have a 3 phase motor to go with these inverters or just with a rotary phase converter?
Thank you
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8070nc Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 5:42am
Ive got an index and it has 3 phase motors. Unless they have been changed yours will be 3 phase. Yes 3phase motors are required for use with inverters
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Thad in AR. Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 5:44am
Originally posted by 8070nc 8070nc wrote:

Ive got an index and it has 3 phase motors. Unless they have been changed yours will be 3 phase. Yes 3phase motors are required for use with inverters

Would I have to buy 2 extra motors and how big?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Thad in AR. Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 5:45am
The spindle motor is 1.5 hp 230 volt 3 phase. I don’t know about the power feed motor
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8070nc Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 5:50am
No phase converter neede with an inverter. If you have 240 single phase you can run a 3 phase motor with the right inverter up to at least 5 hp. Maybe more than that im not sure untill i do some research. Another advantage of the inverter is better overload protection
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve(ill) Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 9:02am
Thad.... there are several ways to get 3 phase power from single phase 240v.... Your motor your trying to convert over has to run off 240v... not 480 V 3 phase.

Option 1 --STATIC CONVERTER-- is to use a few capacitors and wire up to get a  START and RUN  feeds to run the 3 phase motor.. Takes some type of start - momentary switch to KICK the motor over.

Second --ROTARY CONVETER--- way is to have an OLD 3 phase motor that you pull start or roll over with a 1/4 HP motor to get it going.. then use it as a GENERATOR to put out 3 phase for your motor..

THIRD--VFD--- way is the "NEW METHOD" that involves the VFD... Variable Frequency Drive.. Its a small shoe box with all the MAGIC inside to  start your motor, keep it running, convert 240v single phase to 3 phase 240v , and have the ability to change the FREQUENCY from 60 hz up or down to use as a SPEED CONTROL, without loss of horse power.  It can do WAY MORE than you will need... The one box could put out power to a "MILL" to run both of your 3 phase motors.....you can buy an el-cheap-o 2 HP converter on e-bay for under $100.

i made a static converter for my 3 HP lathe about 15 years ago.. if doing it today, i would buy a VFD and go that way.


Edited by steve(ill) - 01 Feb 2023 at 9:11am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve(ill) Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 9:13am
both your mill motors should be 3 phase, but might be different sizes.... Nobody would build a lathe powered by 3 phase and run a separate singe phase to it.

once you have 3 phase 240 v power, you can use that as 240v 3 phase or use 2 of the wires to get 240v single phase. ... you can open the elect box on the second motor and look at the wires and determine what was used.


Edited by steve(ill) - 01 Feb 2023 at 9:15am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Thad in AR. Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 10:28am
Originally posted by steve(ill) steve(ill) wrote:

Thad.... there are several ways to get 3 phase power from single phase 240v.... Your motor your trying to convert over has to run off 240v... not 480 V 3 phase.

Option 1 --STATIC CONVERTER-- is to use a few capacitors and wire up to get a  START and RUN  feeds to run the 3 phase motor.. Takes some type of start - momentary switch to KICK the motor over.

Second --ROTARY CONVETER--- way is to have an OLD 3 phase motor that you pull start or roll over with a 1/4 HP motor to get it going.. then use it as a GENERATOR to put out 3 phase for your motor..

THIRD--VFD--- way is the "NEW METHOD" that involves the VFD... Variable Frequency Drive.. Its a small shoe box with all the MAGIC inside to  start your motor, keep it running, convert 240v single phase to 3 phase 240v , and have the ability to change the FREQUENCY from 60 hz up or down to use as a SPEED CONTROL, without loss of horse power.  It can do WAY MORE than you will need... The one box could put out power to a "MILL" to run both of your 3 phase motors.....you can buy an el-cheap-o 2 HP converter on e-bay for under $100.

i made a static converter for my 3 HP lathe about 15 years ago.. if doing it today, i would buy a VFD and go that way.

I guess I thought one VFD per motor and especially if they vary in hp?
I hope I’m wrong.
I appreciate this info Steve.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote desertjoe Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 3:42pm
  Hey Steve,,I always thought that you would "lose" 1/3 of the HP that the 3 phase motor you were using,,,? I had located a nice 5HP  3 phase motor for my compressor and was told there would not be enough available HP for the compressor, by using the VFD,,,??
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jaybmiller Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 3:54pm
DJ, providing you've got enough electrons coming out of the wall... running a 5HP motor though a VFD will get 100% of the power it needs.
Now IF the VFD is rated for 2HP, you can't run a 5HP motor......
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve(ill) Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 4:09pm
Thad, i think you could run two motors with the same VFD .... The VFD has parameters so you can adjust the startup speed, amps , frequency... etc...... BUT.. if you set it up generically,  i dont see why one VFD would not run two motors, as long as all you want is "3 phase 240v" and not vary the frequency / speed .

I could be wrong, but i would think a 3 HP VFD would run a 1.5 + 1 HP motors.... 


Edited by steve(ill) - 01 Feb 2023 at 4:15pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve(ill) Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 4:11pm
A variable frequency drive (VFD) can be used to control multiple motors in some applications provided the right design considerations are made and appropriate protection is provided for each motor.

This offers several advantages such as lower cost, reduction in panel space and reduced control complexity.

If the application falls within these specific conditions – and each motor can operate at the same speed and can accommodate a VFD as a single point of failure – the next step is to review design considerations and to select the right components. The VFD must be sized properly, and each motor needs protection. The following guidelines are recommended:

  • Add FLA of all the motors and add 20% (to accommodate lower leakage inductance) to select the VFD size
  • Set correct current ratings
  • On some drives it is not recommended to size below 3 hp due to capacitive coupling (cable charging current)
  • Adding a common mode core and using XLPE cable will help to minimize cable charging current
  • Drives should be set in V/Hz motor control
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve(ill) Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 4:13pm
Joe, a STATIC or ROTARY phase converter does not give a 100% true third phase.. and there for they say you only get 60- 80% of the motor HP output.

A VFD is RATED for a given HP, so if it says 3HP ... i should put out 3 HP.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bigal121892 Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 5:24pm
In applications where a VFD is running more than one motor, the motor are all of the same size, and all turn at the same speed. If you try to use the VFD as a phase converter, running two different motors, and start/stopping with motor starters, the VFD will trip out, if not be damaged, as essentially you would be trying to start across the line. One other thing, when you signal phase power, to run a 3phase motor, the drive needs to be upsized by one size. In other words, if the motor is a 1Hp, the VFD will need to be a 2Hp 208 - 240 volt VFD. A 2Hp motor, would need a 3Hp VFD.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8070nc Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 5:36pm
This application on an index machine will require two (2) inverters
One for the spindle and one for table feed. If both were on one every time you changed spindle speed you would change table feed speed. It would be a nightmare
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DonBC Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 5:42pm
the one experience I had I found that it reduced the power output of the motor and I had to use a smaller drive pulley.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve(ill) Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 6:17pm
8070.... i would agree that two VFDS is a easier plan for most applications... In this application he is only looking for 3 phase 240v output..... He is not using the VFD for speed control ... the mill has its own setup for speed... It might work since the motors are same age, same design, and basically same size..

I agree, i would not try to use a VFD if i had a 1940  5 HP motor running 1750 rpm with a   1990 1/2 HP motor running 3600 rpm ..... that dont work.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaveKamp Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 6:23pm
Just to get my 'new to me' machines started up, I use a homemade rotary converter.  Once I know their state-of-function, I fit the spindles with VFDs, and find an alternate method of powering any other 3-phase-necessary equipment.

In the long run, you WILL want to run your SPINDLE motor on a VFD, and you can feed MANY VFDs with single-phase power.  SOME will require a 'derate' of the VFD's output, because it is only able to handle so-much-power coming in on just two of it's three phases, but as others noted, upsize the VFD by a bit, and you're fine.  SOME VFDs will NOT run on single phase input.  OTHERS will have 'phase loss detection' which can often be disabled in software programming.  YET OTHERS can be 'fooled' into operating on 3 phase by connecting the unused phase, to one of the operating phases by a small capacitor (the power detection circuit sees an out-of-phase sine wave and THINKS it's live, so it allows run).  And there are many that just plain old designed for, work on, and advertised as allowing single-phase in, with no sweat.

"Joe, a STATIC or ROTARY phase converter does not give a 100% true third phase.. and there for they say you only get 60- 80% of the motor HP output."

Not true.  Static converters will fit into the 'not 100%' category simply because the phasing of the 'generated leg' is very heavily reliant on tuning with a stable load, so it will be assymetrical under most points, but yet, still functional, and you'll get right to 100% IF you have the capacitor values dialed-in properly for THAT loadpoint. 

The ROTARY converter WILL produce a 'true' three phase, with those who claim otherwise are basing their argument on a neutral-to-generated-leg measurement, which in case of most any rotary design, is inappropriate, because the 'center' of the sinusoidal rotation is offset from neutral.  Measurements must be done strictly from leg-to-leg.

FWIW, the design that most guys use for building their own rotary converters, comes from Fitch Williams's white paper of several decades ago.  I don't know if Fitch is still kickin' around, but last I checked (half a decade ago), he was... and he was still answering questions now'n then, but his work still lives on, and many 'OEM' outfits use teh same architecture in what they sell.


Now, the easy way to build a SIMPLE rotary (and one lots of guys use), is just a three-phase motor, a hand-operated 3-phase switch, and a piece of plywood.  Bolt the motor to the plywood, connect any TWO phases of the motor to one side of the knife switch, connect the other two to your mains.  As others noted, make certain your motor is wired to match your mains (ie. 240v single phase mains, wire the motor for 230v 3ph).  Throw the switch (the motor will growl) Now shut it off, spin the shaft with your foot, or a piece of string, then throw the switch, the motor will spin up.  At this point, there will be three-phase voltage at that motor... one at each of your 240v mains, and the third on the 'unused' wire.  Connect those three to your mill, leave the mill power switch off... repeat the Cro-Magnon starting procedure, and once the idler motor is spinning, throw the machine's power switch, and it's on.

If your mill's spindle motor is 2hp, use a 4hp idler motor.  A junked-up shaft is fine, just so long as the windings still hold electrons and the bearings aren't too wiped out, it'll do.

I ran my first Bridgeport, and my Monarch 10EE off a homemade rotary converter using a 7.5hp 3600rpm pump motor that I got for free.  After I had it all working, I added some capacitors (using Fitch's diagram) to help balance out voltages, and I added a capacitor-start system to kick it over automatically, and I wired in a couple of control relays and a timer to make sure it started safely, without the load accidentally engaged, and if something went wrong, it'd shut down and lock itself out for safety.  Handy for testing machines, and I still use a rotary for this today.

My Monarch 10EE has a 480v Allen-Bradley 1336 VFD and a 480-240v single phase transformer in it's belly, powering a 7.5hp Allis-Chalmers motor driving a 3.5:1 Gates Polychain belt to the spindle.  My Bridgeport J-mill uses it's original 'pancake' motor and spindle V-belts (basically permanently set to the center sheaves) fed by a 240v Allen-Bradley 1305 type AA12A inverter.  My Johannsen radial drill also runs an AA12A, powering a 3hp 1800rpm motor into a 3:1 toothed belt, to it's Bridgeport J-head spindle.  On all these, the upper frequency limit is around 210hz, so my motor speeds are up in the 6k range.  NONE of them have cooling fans on the motor shafts, instead, I've fitted constant-flow fans over the motors, so when they're turning slow, they have excellent cooling airflow, and when they're turning fast, they're not howling air-raid sirens in my workshop.

My Whiteman power trowel runs an Allen-Bradley 1336S, on 480v power.  I set a 480-240v transformer by a utility panel, feed 240v in at the secondary, take 480v out of the primary, through a 600v 3-wire drop-cord, hanging above a work area, and there's a 3hp electric motor driving the Whiteman's gearbox.  I can float concrete with ordinary blades at silly-slow speeds, with full torque, and when it's set up and ready to burnish, the blades can be tilted up, motor raced, to wail it down shiny.

I have a tiny Omron VFD on my HSS tool grinder... and I have an inexpensive analog (Minarik, I think) on the carbide tool grinder.

The table feed motors, you will NOT want to have on the VFD WITH a spindle motor.  Chances are, even if they're connected to 3-phase, that they're DC motors driving substantial gear reductions and have a variable speed circuit integrated into the drive electronics... it is also equally likely that they're single-phase fed off the machine's main power contactor.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve(ill) Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 6:23pm
the other question is COST... smaller 1 and 2 HP VFDS are CHEAP..... when you get to 5 HP the cost is much higher... Might be CHEAPER to use 2 of the 2 HP units..
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Thad in AR. Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 7:57pm
My cousin has a rotary phase converter that uses an electric motor. He hits the switch and the motor starts and runs on its own.
He bought everything put together in a panel for $138.00 and then got a cheap 5 hp motor to use.
I saw some Dps VFD for 2 Hp motors for just over $100.
Slick little deal if that’s all you have to have.

Edited by Thad in AR. - 01 Feb 2023 at 7:59pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve(ill) Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 8:01pm
The VFDs are small and CHEAP... Rotary works good, but takes up more space and its a separate motor to startup .... and you have to get it rotating "somehow" before you power it up ... or have an electric "start" circuit built in..  I have done that also... but more complicated than just a VFD ready to go.

Thad, take a look at the wiring to the second motor and determine if its running off 3 phase or a single leg of 240v .... also is the motor size "similar" to the main motor or much smaller HP.


Edited by steve(ill) - 01 Feb 2023 at 8:06pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve(ill) Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 8:08pm
Long term, a VFD can be programmed to have SPEED CONTROL on the motors... if you ever want to get fancy..... the rotary converter is just a standard 240v 3 phase output..
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Thad in AR. Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2023 at 8:16pm
Originally posted by steve(ill) steve(ill) wrote:

The VFDs are small and CHEAP... Rotary works good, but takes up more space and its a separate motor to startup .... and you have to get it rotating "somehow" before you power it up ... or have an electric "start" circuit built in..  I have done that also... but more complicated than just a VFD ready to go.

Thad, take a look at the wiring to the second motor and determine if its running off 3 phase or a single leg of 240v .... also is the motor size "similar" to the main motor or much smaller HP.

Will do. I just got an email saying it just left the warehouse and will be here Monday.
I’ll let you know as soon as I get it in the shop.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote DaveKamp Quote  Post ReplyReply Direct Link To This Post Posted: 04 Feb 2023 at 9:07pm
Originally posted by steve(ill) steve(ill) wrote:

Long term, a VFD can be programmed to have SPEED CONTROL on the motors... if you ever want to get fancy..... the rotary converter is just a standard 240v 3 phase output..


This is #1 of 2 VERY BIG reasons to opt for a VFD on a SPINDLE...
(because a spindle is either a workholding, or toolholding device, where cut SURFACE SPEED is very important)

When using a VFD on a LATHE, it means that you can adjust the speed to maintain SFPM while going from a large diameter cut, to a smaller diameter cut, or vise versal... as diameter of the cut path has a very remarkable affect on SFPM (Circumference = Pi x D right?).  On a mill, the cutting radius won't change as RAPIDLY as a lathe (because the lathe crossfeed and compound move while working), but if you're running say... a Criterion-type boring head, and you've adjusted a boring bar to cut a 4" diameter, and you're in backgear, with the lowest ratio belt combination, and your boring bar is STILL going too fast for the material and feedrate, you can dial the VFD back to whatever RPM fits the boring bar's SFPM to the material feedrate and cutter type.  In essence, the VFD's ability to give an infinite variability, solves lots of cutting problems, just by turning a knob.

The #2 reason:  DYNAMIC BRAKING.  With exception of the power trowel, my machines all have VERY PROMPT dynamic braking curves programmed in... because STOP is the most important performance function when something bad is either about to go wrong, or is in the process of going wrong.

Running a VFD on 2 motors of two different types, at the same time, though... not wise... especially a table feed motor.  Fortunately, it is rarely an issue, As it usually isn't necessary to do so.
Ten Amendments, Ten Commandments, and one Golden Rule solve most every problem. Citrus hand-cleaner with Pumice does the rest.
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Thad in AR. View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Thad in AR. Quote  Post ReplyReply Direct Link To This Post Posted: 05 Feb 2023 at 8:00am
I went down the wrong path this morning but found DPS digital phase shifters.
They are strictly a phase converter with no VFD.
3”x4” little box. Very simple to hook up. No speed control. $89 for my 1.5 Ho spindle motor.
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steve(ill) View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote steve(ill) Quote  Post ReplyReply Direct Link To This Post Posted: 05 Feb 2023 at 8:53am
In your application they should work fine... The GOOD News is they are very simple, have no programming, just IN- OUT capability..... The BAD News is they are very simple, have no programming, and just IN -OUT capability...   Wink   Clap   





Edited by steve(ill) - 05 Feb 2023 at 8:57am
Like them all, but love the "B"s.
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