Question about Phase Converters

[7th Kahuna]

Last weekend I 'inadvertantly' purchased a 3 phase Unisaw. Three phase or not the deal was too good to pass up. I have no access to 3 phase power. No way to test the saw. This lead me to do some research into phase converters.

I should start out by saying I know just a little bit more than nothing about 3 phase, though I am quite comfortable with single phase.

Based upon my research it appears I would need to construct a static phase converter, and use that to power a 3ph motor, which in turn would act as a generator to provide power to any 3ph equipment.

I did a quick scan of Craigslist for larger 3ph motors. Naturally they are a bit expensive. This leads me to two questions.


1) Are there any special concerns in purchasing a used 3ph motor? Seems to me they should be simpler machines than single phase motors.

And perhaps more importantly,

2) Would it be reasonable to consider gutting the Unisaw to build the phase converter? Were I to sell it, my buyer won't be able to test it, so I can't expect to get a particularly good price. I am also missing the rails and fence. On the other hand, the unisaw (a 1978 model) would provide me with a 3hp, three phase motor and original Rockwell motor starter / safety package.

As I understand it, I can run up to a 3hp 3ph motor from a 3hp 3ph generator (phase converter). I can't imagine I would need to run anything much more than three horse power. Several of the online resources have suggested 5 to 7.5 hp. I understand there is some efficiency loss if you run the equipment directly from the static phase converter. Is this also a concern between the generator and the motor running the equipment?

I would appreciate hearing about anyone else's experience with phase converters or links that you have found particularly useful.

I realize there is a lot more to learn but I am trying to make an immediate decision about whether or not to sell the Unisaw.

Thanks

 

[Stuart in MN]

You should be able to run the saw directly from a static phase converter. If you'd like to buy one instead of build it yourself, Ronk is a decent brand: http://www.ronkelectrical.com/econo_phase_shifter.html I haven't used their products for a long time so I don't know what they go for these days - you'll have to call their 1-800 number to find a local distributor who can give you pricing.

edit: with a static phase converter you're only getting 2/3 of the power that you would with a true three phase input, plus they are not designed for continuous load or hard starting devices. However, for something like a table saw it should work okay.

I checked eBay and you can get them really cheap, like under $100, but chances are those are built offshore so who knows what kind of quality they have.

 

[gorilla]

I've built 4 rotary phase converters over the years two of witch have been running for 20+ years. The guide line I've used is that the idler motor needs to be twice the size of the motor you want to run. If you plan on using the saw think about a VFD not too expensive no moving parts etc.

 

[7th Kahuna]

Thanks for the input guys. It wasn't clear to me whether the static converter would be suitable for the saw. Gorilla, what are you running with the rotary converters?

I have gone down this road not so much because of an interest in running this particular saw, but more out of an interest in learning how to do it. Just another tool in the mental tool box. If components from the saw would support that effort, so much the better. If not, the saw will probably end up getting sold as I don't really have the space for it at the moment.

 

[rsanter]

You have a few options.

First is to replace the motor. Years back I bough a unisaw that was three phase. Same deal .... Too good of a deal to pass up. I found that the replacement motor from the grizzly saw went righ on the unisaw. I found an essentially new/used grizzly motor on eBay that cost me less than $200 shipped. Worked great

Another option is to just buy and use a static phase converter. The problem with this is that you will loose about 30% of the rated HP of that motor. If you can live with that then great. You should be able to get into a static converter for about $100

Next. You can go a rotary phase converter. All that is is a static converter that runs a spare three phase motor that will take the 30% losses. Then you connect the saw motor to the spare motor and the spare motor is actually generating the third leg of the three phase for you and you will then have over 90-95% of the rated Hp of the saws motor available.

Next you can use a VFD. You can buy the inexpensive TECO converters for $130-$160 for your unit. Get the bottom of the line unit that does not have the adjuster knob on the front as you are really not going to be adjusting the speed. With tuis you will also need to get a good enclosure to put the VFD into to meet the sawdust out of it. Go the largest enclosure you can justify so that it has some air space in it so the VfD does not get too hot.
You can mount the enclosure in the bottom cabinet but I think it will be best to to mount it to the back of the saw or under one of the wings.

The last option....
If you can weld you can fab a mount that will replicate the adjusting mount things that the factory welds to the side of the original motor and make a motor mount that will allow you to mount a standard motor to that plate. The Steel City table saws that are made for rigid and craftsman are done this way. They are a close copy of the unisaw but they wanted to use a standard off the shelf motor so they fabbed a mounting plate. Take a look at those saws ( or perhaps a parts breakdown online) and you will see how they did that


No matter how you do it you are likly to spend a couple hundred bucks. Look around for a deal on one of those options and get that saw going

Bob

 

[alfredeneuman]

If you go with a VFD, then any switch between the VFD and the motor shouldn't be used. You should control it from the VFD.

Suddenly dropping the load from the VFD will cause damage to the output transistors and cause premature failure.

 

[7th Kahuna]

Bob, thank you for the detailed breakdown. I appreciate it. There is a lot of good information there. I had been thinking about the option of having a plate made to adapt a conventional motor. I was unaware that any other manufacturers used a similar style.

alfredeneuman, thanks, good to know.

 

[404]

Here is what I have done in the past...

The least complex and least expensive solution to getting this running is to add a motor run (not motor start) capacitor to the 3 phase motor. This capacitor is connected all the time. Let's call the 3 motor wires x y and z.

Wire 240 single phase to x and y. Attach the capacitor between x and z. If the motor turns the wrong way, change the capacitor wiring to be between y and z to reverse rotation.

The capacitor value can be pretty broad just to get this going. 10 microfarad per horsepower to start off. If the motor does not come up to speed quickly increase the capacitance. If the motor starts quickly but sounds harsh or buzzy, reduce the capacitance.

For best efficiency, a power factor meter like this:

http://www.ebay.com/itm/110-220-240...288?pt=LH_DefaultDomain_0&hash=item541dcb5750

allow tuning for best power factor (which would be 1) and minimum run amps. Test with both no load, and while cutting wood.. Make sure the motor comes up to full speed reasonably quickly.

The voltage rating of the capacitor should be much better than 220 volt. Here are some capacitor choices.

http://www.surpluscenter.com/Electr...=1&sort_by_options=price+asc&page_length=9999

370 volt or better would be okay , choose an assortment of capacitance values and try it out.

If you are smart enough not to zorch yourself with single phase there is a chance you will not zorch yourself with this..

Circuit breaker or overload protection is a separate topic.

 

[7th Kahuna]

If you are smart enough not to zorch yourself with single phase there is a chance you will not zorch yourself with this..

What you are describing there is a static phase converter. I appreciate the additional details and the links.

On another board I found a link to a book called "How to Run Three-Phase Motors on Single-Phase Power" by Lindsay Publications. I think I will order a copy.

 

[jask]

You really do not need to pay for this information, here are a couple links that will walk you through the process:

http://www.homemetalshopclub.org/projects/phconv/phconv.html

http://www.metalwebnews.com/howto/ph-conv/ph-conv.html

The first link uses a potential relay ( that can be found in scrap AC units ) to automatically stabilize the voltage of the false leg under load- this is the problem with pushbutton start designs or non rotary designs, the voltage collapsing under load is what causes stalling, overheating and damage.
I have used this for several machines and run a 3Ph tablesaw and bandsaw at home with this system and have checked it for both voltage and current under load ( ripping 2" red oak scraps ) and this design really works well...... as the load causes the voltage to start to collapse in the 3rd leg the potential relay engages the start caps to boost the voltage to the correst level, when it recovers the relay switches off to prevent overvoltage... all automatically and so seamlessly you will not even know it is happening without hooking up a meter.

 

[Norcal]

When I bought my Unisaw it had a 2 HP 3Ø motor, the original plan was to use a VFD for phase conversion, but due to a problem with the motor I elected to replace it w/ a 3 HP 1Ø motor which at the time was in the low $300 range including shipping, the VFD route for 3 HP & less is still cheaper then a new motor, even more so now since motors have increased in price since then.

http://www.electricmotorsite.com/mm...Y&Store_Code=EMS&Category_Code=sglwood_unisaw

 

[404]

What you are describing there is a static phase converter. I appreciate the additional details and the links.

On another board I found a link to a book called "How to Run Three-Phase Motors on Single-Phase Power" by Lindsay Publications. I think I will order a copy.

That exact publication was how I got started on this 25 years ago..

I bought and read the booklet, and I was like WTF... at the most basic level this is easy.


My description is a static phase converter, but it is also an electric motor featuring capacitor start and capacitor run. Same capacitor for both of course. Some single phase motors are made just this way with no centrifugal switch or potential relay. My 5HP 154T frame air compressor motor is like that.

Good luck and don't zorch yourself.

 

[Norcal]

If you use a static converter, all it does is start the motor, then after starting the motor is single phasing, not a good thing.

Replacing the motor, (not always possible since with some machines without the motor there is no machine), rotary converter, or VFD, are the best choices, a static converter should be avoided, IMHO.

 

[7th Kahuna]

If you use a static converter, all it does is start the motor, then after starting the motor is single phasing, not a good thing.

Replacing the motor, (not always possible since with some machines without the motor there is no machine), rotary converter, or VFD, are the best choices, a static converter should be avoided, IMHO.

As noted, I am entirely new to this, but in my research, I have seen static phase converters that use a bank of run capacitors to effectively create the third leg or phase and a single additional capacitor as a start capacitor on that same winding for additional starting torque. The start capacitor is temporary using either a manual switch or a relay to pull it in and out of the loop. Is this not different than what you are describing? I understand that in this arrangement there is a 20 percent or greater loss of efficiency but the third leg is still being powered.

 

[7th Kahuna]

That exact publication was how I got started on this 25 years ago..

Good to know.

Thanks

 

[7th Kahuna]

The first link uses a potential relay ( that can be found in scrap AC units ) to automatically stabilize the voltage of the false leg under load ...

Thanks for the links. I will check them out. Just handed an old AC unit over to the recyclers . . .

 

[Milton Shaw]

New single phase motors are available on Ebay, just looked, for $416. That would give you full power and full speed starts. After you spend $$$$ on converting it you still don't have full power. And if you got that good deal on the saw then you should still come out ahead by getting the right motor.

 

[Norcal]

3 HP 1Ø motor can be had for a lot less the $416 as mentioned in post # 17, but if the 3Ø motor is retained and a VFD or RPC is used there is no loss in power.

 

[404]

If you use a static converter, all it does is start the motor, then after starting the motor is single phasing, not a good thing.

Replacing the motor, (not always possible since with some machines without the motor there is no machine), rotary converter, or VFD, are the best choices, a static converter should be avoided, IMHO.

The version of the circuit having a potential relay to disconnect the start cap does just as you say. This is an inferior circuit and I am agog that anyone uses it in this exact form today. A version that also had a run cap connected to the third leg all the time could be a good circuit.

The correct circuit has a motor run capacitor always connected to the motor. Run caps are rated for this service. Start caps are only good for a few seconds of being connected before they boil and vent.

 

[offroadsteve]

I recently completed a very similar conversion on a similar age 3-phase unisaw, mine was only 2 HP instead of 3.

I built up my saw to keep rather than try to resell, so I went with a TECO VFD from Factorymation.com and could not be any happier with the setup. I pulled all of the original controls out and wired up the start/stop switch to the VFD and all the controls are now low-voltage (24v). Edit: The exact model I have is a TECO FM50-203-C, best price I found was $196 from Factorymation.com. I would recommend them as a vendor, was very happy with the customer service.

One of the advantages of the VFD is you can control the ramp-up / ramp-down speed, so it doesn't have a large inrush current, and it actively brakes on shutdown rather than just letting it coast to a stop, I look at it as a safety feature. The drive also acts as a overload for the motor, you can set the max current draw to the motor and the drive shuts down if the motor gets overloaded. I also added a second emergency-stop button on the rear of the saw, just in case.

Once you consider all of the requirements, I think you can get into an inexpensive VFD for about the same money as a static converter. Having now used my saw for a few months, even if you plan on multiple tools over time, I think a VFD on each tool would be the way to go.

 

[MTW]

7th Kahuna, I realize there is a lot more to learn but I am trying to make an immediate decision about whether or not to sell the Unisaw.

Only you can make the decision on whether to keep or sell the saw.

As to running it on single phase, there are many ways to make the conversion if it's worth the learning curve and investments to you. If you want to play with other 3Φ equipment, a converter for your shop might be worth the investment. If it's just to see if the saw runs, take it somewhere that has 3Φ for a test.

Means of conversion from 1Φ to 3Φ are many and they all have a place, including: Static, Rotary, Autotransformer, VFD, Phase Perfect electronic. These methods can all be applied to a single machine or a group of machines. It really depends on use and budgets.

A good place to educate yourself on the subject is here:
Phase Converters Designs & Plans

MTW