View Full Version : Phase converters

02-04-2008, 11:46 PM
Ok, I looked through the past threads and didn't run into any on phase converters. I just got a ca. 1991 Enco 10x54" 3hp mill, and need to get it running. Does anyone have any recommendations on rotary phase converter brands (I'm figuring on going with a 5hp size) and why they would suggest a particular unit? I don't want to build one...I'm not comfortable with electrics...I also don't do my own surgery either...:D I do have competent help in getting the shop area wired for standard 220v.

Montezuma, IA

02-05-2008, 12:08 AM
Modern way is to use a VFD . Most now do an excellent job of 1 phase to 3 phase conversion and give you lots more options..

A search for VFD will provide lots of info.
Very Simple to wire up for your 3 HP, a cheaper way to go ...

Will give you (with proper model) variable speed (rotary won't)

adjustable ramp up and down of start up/shut down speed speed (rotary won't)

easily switched with low voltage controls for FWD/REV (rotary won't)

All in a small 5 or 10 lb package.

Has overload protection to prevent smoking motor.

Late edit :)
No Bearings to fail either...

J Tiers
02-05-2008, 08:03 AM
VFD has options unavailable on an RPC.

But, an RPC is quite bulletproof, no "fragile semiconductors" etc. It can also be overloaded without problems if the overload is not prolonged.

When a VFD has an overload, it normally shuts down entirely, stopping your cut with the tool in the work, and looks at you like a disappointed puppy.

It can be utterly destroyed, beyond repair, by effectively unseeable and un-detectable conditions.

That is something to be considered when you have an item which is absolutely required in order to operate your shop at all. Especially when it is a "high tech" item which you have absolutely no way of repairing by yourself in any practical way.

It is bad to depend on what may as well be "magic".

That said, a VFD offers many alluring and seducing features, the main one of which is variable speed.

02-05-2008, 09:07 AM
You can also run multiple machines with a RPC,generally up to 3 times the rated value

02-05-2008, 09:27 AM
Rotaries are just the thing if you get a charge out of whipping up something functional using old parts you happen to have lying around already.

generally up to 3 times the rated value
Where does that come from? I don't believe I've ever heard of a limit.

Forrest Addy
02-05-2008, 10:00 AM
Looks like we're on out way to a nice controversy. Here's my input. It's been suggested that VFD's are less than perfectly reliable and contain fragile electronics. Nothing is perfectly reliable but VFD's are right up there. They are industrial electronics made to survive in a harsh industrial environment. Plant manager and industrial engineers are among the most knowledgably skeptical people on Earth and they run their process equipment from linked VFD's. I have seven now and have never had a failure in the 16 years sice I bought my first one. If you live in lightning country you might have trouble but otherwise a VFD will be about as reliable as they come.

You may not like electricity but you will appreciate the benefits a VFD will bring you once it's installed and the operating parameters are set up. VFD's and rotary phase converters have both good features and some limitations. A VFD's big feature is you can vary the connected motor's RPM over a wide range with good speed regulation at full torque. A rotary phase converter's big feature is you can riun more that one motor from it at the same time.

Whether you are comfortable about electrical work or not the merits of the two schemes have to be considered if you wish to make a good decision that you can live with. Here's my take on VFD's from something I wrote a few years ago. There's a lot of material here. You may wish to print it out and take it in sips.

Here's goes:

There’s a lot to learn when it comes to making shrewd decisions regarding VFD’s. Few home shop machinists have experience with heavy duty industrial electronics. The key to learning new things is to connect the basic concepts to a vocabulary. It’s no different than learning tennis or machine work than when learning enough about electricity to make a decision about VFD's and whether they will help you with powering your three phase motor. Be patient. The learning doesn't happen overnight. The understanding will come if you apply yourself.

The speed of an induction motor is controlled by the frequency of the AC power feeding it. When plugged in the wall induction motors run only at designed speed.

Variable frequency drives are a box of solid state electronics that converts three phase 60 Hz AC to variable frequency three phase. This allows you to operate a three phase (and three phase only) induction motor at any RPM you set. The motor runs at full torque from minimum practical RPM to 60 HZ and drops off in proportion above 60 Hz. They are very simple for ordinary home shop machnists to work with if you can follow instructions when setting them up.

The VFD goes between the power and the motor. On the box are some control push buttons including the "start" and "stop" button. There is also a means to increase and decrease the speed of the motor - usually a knob like the volume control of an older radio. The initial connection amounts to hooking up five wires and a ground. Inside the box are terminals for connecting it to a more convenient arrangement of external push-button controls (the original control buttons furnished with you machine if you like) but you can also run it from the push buttons on the box.

A manual should come with your VFD should you make the plunge. Do not buy any VFD without the factory manual. If you're electrically challenged you won't decipher the manual overnight. You'll have to study it, taking in bit at a time like you'd eat an elephant.

As for cost, $200 (used price for a pre-owned 3 HP single phase rated unit) doesn't seem expensive to me for such a talented box. It will power (separately by means of a receptacle on the VFD and a plug for each three phase motor) other three phase motors should you mount them on your drill press, lathe, mill, etc. That smooth, quiet, three phase power and variable speed adds a lot of functionality to a machine shop, and the gadget has built in economizer circuitry that will lower you power bill a tad.

Be sure to get all the manuals that go with them. There's up to a hundred settings (parameters) that customize it to suit your motor and application including maximum current, acceleration and a deceleration, stall prevention, and lots of other features. You can run a 1/4 HP motor from a 10 HP drive if you wish but that's stretching it.

There are many drive brands: Yaskawa, Hitachi, Mitsubishi, Baldor, GE, Teco to mention only a few. Don't rush off and buy one over the counter from a retail dealer. First determine your needs, then check around your local big city mechanical drives or electric motor suppliers. Ask for take-offs or trade-ins. You may be able to get last year's drive for 1/2 price. And there is eBay. Be sure to get a 230 Volt drive preferably one suited for single input although a 3 phase only drive will work on single phase if appropriately de-rated. If you screw up and get a 460 volt VFD there's no solution but a step up transformer.

Some drives will step up 115 volt to 230 volts internally up to 1 HP. This is a drive targeted at consumers with a bad home shop habit and no way to get 230 volts in their shops. They are pricier than comparable 230 or 460 volt drives. If you’re looking at 115 Volt VFD’s and wondering how they will work on a 5 HP motor maybe it would be a good time to run in a 230 volt service to your shop.

Used later model VFD's from the internet auction site may be quite acceptable. Beware of older technology units - not because they function poorly but because they can be noisy and emit an aduible squeal that even I can hear. eBay usually has two pages of VFD's. Select "Business and Industry as a category and key in "VFD", "inverter drive", or "AC drive" as search objects.

Here are a couple of on-line sources. Once you get comfortable with the concepts of VFD technology and feel like shopping, phone them and dicker. Check out http://www.dealerselectric.com/ . ." Another one is at http://vfds.com/vfdprice.htm#120 Volt Models. These people (and many others) sell 120 volt input drives. They have a step-up circuit inside that lets them run 230 volt motors. These are more expensive. http://www.driveswarehouse.com

I have seven VFD's powering my machine tools from my 20HP metalworking hydraulic planer to my smallest drill press. I'll never go back to single phase power for major shop equipment except air compressors.

The de-rating question of VFD's for the home shop user may require more than a simple 50% answer. It's a matter of how much: The math works out to a three phase rated VFD running on single phase adequately serves a motor up to 2/3 it's rated capacity. If the VFD is designed for single phase power then you can use it to full load capacity.

More advanced consideration for de-rating VFD’s include questions like. How much single phase 120 Hz ripple in the DC buss can the VFD can handle without the logic or the power section being adversely affected under full load at @ 60 Hz? How much extra current and filter capacity is there in the rectifier section?

Continued on next post

Forrest Addy
02-05-2008, 10:03 AM
Continued from previous post:

As the motor speed for a full torque load is reduced, the demand from the DC buss decreases accordingly until about 41 Hz is reached. At that point full load motor current can be theoretically be drawn from the VFD's load terminals. The rectifier and DC buss still thinks it's working into a de-rated load because of the output transistor's PWM duty cycle at the lower voltage/Hz.

So it's full rated current to the load terminals up to about 41 Hz and a linear decrease thereafter to about 70 percent of full load current. This will limit the VFD's input diodes to no more then the three phase full load current. As for ripple current that's a function of the installed filter capacity and has to be figured (or scoped) for your particular VFD.

A three phase rated VFD destined to feed a 1.15 service factor motor run to full name plate ratings from single phase power needs to be DOUBLE rated, that is a 5 HP motor requires a 10 nominal HP drive. The de-rating protects the input diodes which may be rated for three phase amps.

Most manual machine tool drives seldom require full motor HP. A three phase rated VFD will be acceptable if 1 1/2 times over rated, that is a 5 HP motor will work well from a 7 1/2 HP drive. There will be a corner between about 45 hz @ full torque and 60 Hz @ 3/4 rated torque where the VFD limits motor performance. Most modern feature-rich drives allows the drive to be programmed at constant HP in this band of operation. There will be no harm to the motor or drive. The motor will merely act as if overloaded.

Most of these concerns are moot because few - very few - people in home shops take full HP cuts for more than a minute or so.

I just bought a 15 HP Allen Bradley 1336F VFD to run my lathe to a full 10 HP (the old stone age 7 1/2 - 10 HP Magnetek wasn't handling the lower RPM's very well even on low torque). I was thinking I'd do that piggy-back power supply trick on it.

There's never a simple answer to any question regarding AC.

By the way, connect nothing to the third line terminal of the VFD. No ground, not to the single phase line. Resist temptation and leave it unused and innocent ot wire. Someone mentioned connecting the neutral to something.

Connect the neutral to nothing but the return leg of any 115 circuits forming part of the electrical system. Do NOT connect the branch circuit neutral to ground at any point in the electrical circuit except to the neutral/ground rail at the service entrance in compliance with local code and NEC. Connect the ground to the chassis of the VFD at the ground terminal provided.

Also be sure to continue the ground conductor through to the motor and other electrical loads following the VFD's installation book guidelines to avoid ground loops.

Concerning questions about running motors from VFD's at low RPMS

Often home shop machinists wish to take advantage of VFD technology for their single phase machine tools but a conversion from single phase to VFD control takes some thinking. For one thing a VFD will not run a single phase motor. Period. For another you cannot replace a step pulley or multi-speed geared spindle transmission and vary the spindle RPM’s with the VFD alone. Any attempt to do so results in frustration as the spindle speed is reduced at the VFD’s speed control knob. It’s a matter of mechanical advantage.

Some complain that “VFDs lose torque at low RPM’s.” That’s false. Torque is twisting effort. Power is a function of torque times RPM. An induction motor is a constant torque device. As you reduce the motor RPM with the VFD the motor's torque remains constant but the RPM's and consequently power drops proportionate to the VFD's setpoint frequency. A 1 HP motor designed to run at 60 Hz develops 1/2 HP at 30 Hz, 1/4 hp at 15 Hz, and so on. No motor torque is lost as the RPM's are reduced only the delivered power.

What is mistakenly called "torque loss" is actually loss of tangential force. When it's necessary to run larger cutters at lower RPM the torque requirement goes up. If you attempt to obtain lower RPM by dialing down the VFD because the moment at the cutting edges is larger you also reduce the available power at the cutter. If a full HP cut is desired the spindle speed has to be mechanically - not electronically - reduced to suit the cutter RPM.

For that reason you cannot dispense with a machine tool's multi-step mechanical reductions if you wish to take advantage of the motor's full HP. Use the VFD to fill in between the steps for max HP and for low power operations use the VFD. If you wish to be lazy and avoid shifting belts or gears it's OK to use the VFD alone but only for lighter cuts in proportion to motor RPM.

Thrift store or garage sale? Don't buy it if pre-1995. Get a return agreement and have a local motor guy check it out. These drives are nearly bullet proof but their NEMA 1 enclosures are built like a bird cage. If the internal electronics are permeated by conductive dust (like from an abrasive shop saw) the logic and switching circuits will suffer. Older VFD's are 6 step and squeal when they run and are bereft of important features. The newer PWM drives are quiet and more resistant to faults.

As for the economies of VFD Vs a phase converter, there are several. A spinning phase converter running idle draws idle current depending on about a dozen variables. Actual charges are strongly dependent on duty cycle and power factor. As long as the phase converter is running you pay for the KW/hrs the converter ticks up on the meter whether you are drawing power from it or not.

As for conversion efficiency, I'd guess that a machine tool drive motor run from a VFD consumes about 1/3 the power over a year that a comparable motor fed from an across the line starter. This is a controvercial statement to some but I point out that most home shop machine tools are run at a fraction of full load A VFD's economiser circuitry exploits that mode thus reducing metered power demand.

All modern VFD’s have "economizer circuitry". If the motor is run at part load, the VFD drops the line voltage to some level where the motor runs at reduced voltage closer to full load amps and its greatest conversion efficiency. My lathe motor is 10 HP and is rated for 230 Volts 3 phase @ 34 Amps. When I'm coasting along at 30% motor RPM taking a light cut, the motor draws about 23 Amps from the VFD at reduced voltage but the VFD draws only about 4 Amps from the line. No this is not smoke and mirrors but simply how real-world VFD's work with real world three phase motors. It's like a variable step-down transformer.

Most industrial motors are over-rated in their application and therefore run inefficiently. About 1/4 of all electrical energy is consumed by motors 50 HP or less. We in the US wouldn’t have to build new power generating plants for years if every three phase motor was furnished with a VFD.

The subject of an older motor's behavior with VFD's includes many urban legend style stories based more on self-accreting hearsay than actual performance. Not to trivialize the point but motor heating and insulation failure are concerns frequently overstated by popular belief particularly on 230 Volt systems.

Every motor has a duty cycle rating which is the percentage of time it can be safely run to full ratings. Most induction motors have duty cycle ratings of 100% or more meaning there is no limitation to operation except ambient the air temperature can be no greater than the safe insulation class temperature rating minus the motor's rated temperature rise. Since motors have thermal mass it takes time for the motor's copper temperature to increase to dangerous levels even at overload current. Makes sense; otherwise a motor would burn out at start-up.

Most induction motors running power tools have a relatively low duty cycle, even during stock reduction roughing cuts. The time between cuts counts as idle time permitting the motor a breather when it can cool somewhat.

You can run only one motor at a time from a VFD and you really should run the motor from the control panel not switch the motor after the drive. You may desire to purchase one VFD and run several items of equipment from it by plugging them in separately as you need them. Just remember that you may wish to adjust parameters every time you change motor for the built-in motor protection features to work properly.

Inverter rated induction motors have extra insulation strength to resist transient over-voltages induced in the windings by the switching frequency. New motors have stator iron properties favoring high frequencies therefore high voltage transients and insulation breakdown can be more of a problem when run from VFD's. Older motors have older iron which incidentally better blots up high frequency switching transients so it causes less of a voltage spike problem than would seem. Thus older tech motors mitigate effects newer motors have to be designed for.

It's long been the rule that induction motor insulation (and any other electrical appliance or device for that matter) has to with stand double the connectable voltage plus 500 Volts. So long as the motor is dry, VFD switching transients will cause no trouble for the home shop machinist running hid equipment from 230 Volts.

Don't be dismayed; I hope this thread doesn't overwhelm you. There's a lot of info posted here in response you your questions and that's good. A new generation of lurkers is always coming along who wish to know stuff that's been posted before. Your question stimulated a recurring topic back the the surface.

02-05-2008, 12:12 PM
I certainly have no problem with VFD's, but I'd have to say if a guy is afraid of wiring up an old motor and some capacitors, jumping in and re-wiring his reversing switch to control the VFD may scare him away as well. VFD's need to control the power to the motor by having their control inputs switched. Just connecting a three phase device to the output of the VFD and switching power input to the motor can damage a VFD.

I figure you are going to have to do some wiring whether you buy or build an RPC (which is the term you may want to search on)or buy a VFD. There's plenty of info on building an RPC out there and I would be happy to share what I did if you opt to go that route. Purchased RPC's are pretty expensive and if you are sure you will only run one three-phase device, the VFD likely makes financial sense. There is or was a forum dedicated to this and other electrical topics over on the PM site so you can look there too.

Electricity is not, however, brain surgery.


02-05-2008, 06:50 PM
Thanks, gents, for the information. May take a while to digest, but your help is appreciated.

Montezuma, IA

J Tiers
02-05-2008, 08:20 PM
Forrest, I do NOT hate VFDs.

I use them at work, I have DESIGNED items essentially equal to a VFD, etc, etc.

I agree they are pretty durable, although we have gotten some DOA units in shipments, and have had them fail also.

Yes, failures are usually due to some form of "abuse". SO WHAT? The RPC would have laughed at that "abuse".

My point is that a VFD is exactly the ticket for many situations, and they work well. They offer many features that are not otherwise available.

However, while an overload probably will not DAMAGE a VFD, it will also usually shut it off, possibly stopping you in mid-cut.
A VFD can be destroyed by any of several relatively well-known problems, and when damaged, normally must be replaced.

You normally need one VFD for each machine, although it IS "possible" to power a whole shop, it is more-or-less of a hassle to do so.

The thermally limited nature of an RPC means that it is somewhat "matched" to the overload characteristics of the driven motor.

The RPC will deal with an overload fairly transparently, usually powering through without "slamming on the brakes".

To get equal performance from a VFD, it must be radically over-specified, to 2 or 3 times motor rating.

The VFD may well be superior for the application, and the best choice.

For people who do not need the features, or who want a single power source for a shop load of machines, an RPC may be a better, cheaper, choice.

I object to the notion that a VFD is the first, obvious, and ONLY choice.

That said, I am probably going to put a VFD on a large drill press soon. I already have an RPC for other machines, but would like the VFD features.

Forrest Addy
02-05-2008, 09:16 PM
Jerry, I agree with everthing you say except for the perceived reliability. I agree that PRC's are electrically more robust than a VFD just as an ax is more robust than a chainsaw. My experience suggested you stressed the failure rate for VFD's. In my readng of your post you apparently gave then a bum rap - but maybe that was me.

I don't think I favored VFD's over RPC's except on the length of technical treatment in the above two posts. I have about two full posts on RPC's too but I didn't feel I should take up the space. My post was lengthy because the choosing of a VFD over a phase converter is technical and needs discussion to cover all the points. Maybe I over-did the explanation thus unbalancing an issue which should be decided on technical merits.

I don't "hate" RPC's. I really don't understand your use of such an emotive word to characterize a technical treatment of a single subject. An RPC is a well tested means of running three phase equipment from single phase. I've made many in my time and used one to power my shop until 1994 and never had a bit of trouble except for a reduction in delivered power from the load motor.

If someone has a need for three phase to run a shop having several three phase motors from single phase power and no need of variable speed I will of course reccommend an RPC or an electronic phase converter like a Phase Perfect.

However the original poster has a single Enco turret mill and needed to get it running. The conventional wisdom these days is to suggest a VFD for this application as it offers many advantages unattainable from an RPC. This is not always the case but it works well for the original poster's declared situation.

The decision of RPC vs VFD is a technical one that should be based on the merits of both compared with the requirements of the end user.

02-05-2008, 09:49 PM
If you go with a VFD, plan on a few late nights of reading (and maybe a week to understand). You'll need the instruction manual to set it up. And to turn the dang thing on! :rolleyes:

J Tiers
02-05-2008, 10:28 PM
I suspect that a week is a bit over-conservative.

Depending on the interface, and the degree of "techie-savvy" of the user, a couple hours is probably the most "required". But that does not count "getting your mind wrapped around" the whole VFD concept and approach.

If you want to use a VFD in a control system with PID voltage feedback etc (as I have), you can count on some extra study. But that is hardly the position of the shop user wanting power to a machine.

Stressed the failure possibility? Maybe. I wasn't intending to claim they are unduly failure prone.

There IS however, a degree of failure-proneness that is not shared by an RPC, and that has to be part of the consideration.

if you are in a major metropolitan area, with good shipping, and multiple suppliers, that may not be an issue. If you are located 2 hours from the nearest (small) town, you may put up with less capability in order to have more robustness and reliability.

02-06-2008, 12:07 AM
I have a couple of VFD's sitting on the shelf. In the somewhat distant past, I recall having read that the caps can go bad from disuse.
Is there anything I should do to ensure they will still be functional when I need them?