Inquiring about rotary phase converters and VFDs.

[Tom Fitzpatrick]

I'm interested in learning more about rotary phase converters and VFDs. I just watched a recent You Tube video by Mr. Pete from his shop in the US and he is doing a 2 part series on 3 phase power, rotary phase converters and VFD's, plus one other conversion tool. In his video he talks about the noise that rotary phase converters make when running, which could be a deterrent to using one. I am currently restoring an old industrial compressor that came with a 7 hp motor operating at 3 phase power and in excess of 500 volts. As I don't have 3 phase power in my hobby shop I purchased a Baldor 7 hp single phase 30 amp motor that I will be plugging into my 50 amp welder plug when I require it to operate (once the restoration is complete). I also realize that 3 phase motors require considerably less amps to operate than single phase motors.
I have been searching for a vertical knee mill for several years and realize that most come with a 3 phase motor (although I realize they can be retrofitted with single phase motors such as those offered from H&W machinery in the US). I'm interested in hearing from members about their success/failures/what to watch for tips when using rotary phase converters or VFDs. Looking forward to hearing a few opinions. Thank you.

 

[Chipper5783]

Another phase conversion arrangement is called "Phase Perfect". They are pretty quite and the voltage regulation is absolutely spot of (still has the current being out of phase from the voltage - but that is a reality when just generating a 3rd leg). The PP units are expensive, but work very well (it is very rare to see used ones come up for sale - second hand rotary units come up frequently). I ran my shop on an RPC for ~35 years, it worked okay - it was noisy and was strictly speaking undersized for my needs (it didn't like starting motors that were on its' max rating). I got the PP because I had moved into the CNC world. There are higher end rotary units that are supposed to be suited for old 3 phase power CNC machines - but they ain't cheap either. Some rotary units are not too bad for noise - two other shops I've been in have rotary units and they are much quieter than the unit I had.

VFDs certainly work for the phase conversion. I have one 3 phase machine running on a single phase supplied VFD - it works great. However, in practical terms going the VFD route restricts the VFD to only that machine (you also have to run all your motor controls via the VFD).

Sort out an RPC (plus a transformer if need be) and get your shop up and running on 3 phase. If you are into the work shop games, getting your 3 phase system set up is pretty well a must. It seems that having a 3 phase system set up almost magically pulls machines into your shop (I got 4 of my machines at less than the value of scrap metal because I have space, power and an interest in old machines - one was free and that included delivery).

 

[ducdon]

I like the idea of the rotary as it leaves the possibility of adding future machines. As for the noise it could be mounted in a separate room or in a shelter box outside the shop wall? If it was done that way I would want an a light at the door so I wouldn't forget and leave it on.

 

[Susquatch]

I'll take the opposing view on this debate.

While the right rpc would enable an entire shop full of machines, most hobbiests don't have that. I only have three machines that might have required 3 phase. One of them (my lathe) is still single phase but I'll be converting it to 3 phase when the priority bumps up to the requisite level.

Perhaps I started out with a VFD on the other two machines (both mills) in order to solve the 3 phase problem. But it didn't end up that way. The VFD provides a Variable speed feature and other advantages that I would never walk away from in the future.

So ya, the RPC might be the way to go for a large shop with many machines. But if control and flexibility are factored in, I'd go VFD all day every day.

Lastly, you don't really need a dedicated VFD for each machine. If you keep the controls simple and generic you can put a plug connector on the different machines and use the same vfd for each.

 

[Hacker]

I'll take the opposing view on this debate.

While the right rpc would enable an entire shop full of machines, most hobbiests don't have that. I only have three machines that might have required 3 phase. One of them (my lathe) is still single phase but I'll be converting it to 3 phase when the priority bumps up to the requisite level.

Perhaps I started out with a VFD on the other two machines (both mills) in order to solve the 3 phase problem. But it didn't end up that way. The VFD provides a Variable speed feature and other advantages that I would never walk away from in the future.

So ya, the RPC might be the way to go for a large shop with many machines. But if control and flexibility are factored in, I'd go VFD all day every day.

Lastly, you don't really need a dedicated VFD for each machine. If you keep the controls simple and generic you can put a plug connector on the different machines and use the same vfd for each.

If you use your VFD for multiple motors do you tune it each time to match the motor?

 

[kevin.decelles]

I have ended up with both, VFDs and a RPC. The variable speed feature for milling / lathe is very nice, and I'm using it on a sand-mulling machine to avoid complex gear reductions.

The RPC came into play when I got my surface grinder, which came with 3 separate motors (spindle/coolant/hydraulics)..... the VFD doesn't work in that scenario (more for single motor solutions). The RPC handles this nicely -- but there is the noise factor that can be mitigated easy enough. I currently have the RPC portable (on a dolly) with a connector so I can use it on multiple machines. I have the same connectors on my VFD cords so I can choose VFD or RPC on any machine right now.

Since I operate solo -- never a problem as I only have one machine in play at a time. I have another couple of motors and will be building another RPC to learn more about the electronics. That one may be permanently mounted in a room and plumbed in.

 

[Hacker]

I use both a VFD and an RPC and the reason is I ended up with machines of different voltages and the cost of converting them to 240V was much higher then the cost of an RPC. I bought the VFD first and then added the RPC. I went the cheapest route at the time. In hind sight for my setup I should have gone with the RPC first. I use the gearbox to adjust speed as I will take a cut and stop the machine and look at the finish and chips then make the adjustment. I am not that good of a operator to do this on the fly.
If you use an RPC all you need is a transformer to step up the voltage from 240 to 440 or 600. I am not aware of any of the VFDs currently on the market that can do that.
As far as the noise the only time I ever really notice it is when I have shut down a machine and there is nothing else running in the shop. I have mine tucked under my work bench and it is enclosed on three sides and top and bottom to mask some of the noise. If your shop is in a basement or in an area where noise is going to be a concern then I would go with the VFD.

 

[Six O Two]

I installed an American Rotary 5hp phase converter in my shop. With two machines to power (for now), I figured that one RPC would save me some money vs 2 VFDs. It didn't quite end up that way, but I'm still happy with the result. Both my mill and lathe already had on-the-fly speed adjust, so that wasn't a draw for me towards a VFD. The noise is there, but in my garage shop, it's not really that big a deal.

Keep in mind that depending on how you set it up, there might be quite a bit of follow-on costs for getting everything wired. I had to hire an electrician, added in a dedicated 3ph panel, 2 disconnects and wiring + plugs and sockets, which added over $1k to the cost.

 

[Dabbler]

I, Like Kevin and Hacker have both RPC and VFSs. I wired my first RPC over 40 years ago.

@Hacker: you can move a VFD from motor to motor as long as they are the same current and voltage. It is too much bother to reprogram the VFD for motors of a different current rating. (you can also make a mistake, which isn't catastrophic, but will lead to extra motor heating and poor performance)

Rotary is perfect for multiple motors of different ratings, or for using transformers.

Never use a transformer to alter VFD output voltage.

 

[Susquatch]

If you use your VFD for multiple motors do you tune it each time to match the motor?

No. I haven't even tuned it for one motor let alone several. I have an open tech support question at TECO about it. But it's one of those "never let perfection be the enemy of the good" things. Autotuning certainly isn't needed on either of my mills. The VFD's work great with just some basic configuration. A normal person would call it done and move on to more important projects. I'm just one of those kind of guys who can't resist playing with things I should just leave alone. Why? Mostly just because I can.

 

[Tom Kitta]

Through I have used VFDs I have no machines wired for VFD use right now. I am pure RPC. I also have transformer. Most of my machines either are fully variable speed already or have rather generous number of available speeds or don't need any speed adjustment (like a surface grinder or TC&G) so little need for VFD. I add VFD to the CNC I am slowly building.

RPCs are cheap to make and no need to re-wire all machines with VFD. Once I went past 2 machines it was obvious that RPC is they way to go. You can use multiple motor machines without any issues, run bandsaw welders and transformers. If I needed to I would have made the voltage more balanced with better capacitor bank.

Sold state systems are even better then RPCs but much more pricey. They provide very balanced power (CNC grade, as good as power from transmission lines or maybe even better) and are very efficient.

My 15hp RPC cost me about $500 few years ago. Add to it cost of wiring the garage, which is mostly wire cost dependent on how many wires you need. Plus cost of outlets dependent on how many you need. Done few years back when prices were lower for cables it was another $500 for 8 outlets and about total of 100ft of cable. One should not really count cable and outlets as one would need some power anyways distributed for VFD use in the first place.

My RPC is next to the entry door and shop lights. Power use per hour is about 0.5kw/h idling. This adds to shop heat. Its less then all lights in the garage. Still would be nice if it was just say 0.1kw/h but overall cost per month is not much.

 

[Mcgyver]

or don't need any speed adjustment (like a surface grinder or TC&G) so little need for VFD.

The SG is one of the few machines I have put a VFD on (240 & 600 3P via RPC otherwise) and I can't believe I waited so long. I'm a convert; imo its the one machine that really needs a VFD. Why? 16 second soft start and stop means the wheel is never jarred and hence maintains its balance. Without it, because of the small amount of clearance between the wheel and spindle, you should dress each time you power up which can sometimes be impossible with a complicated set up.

 

[Tom Kitta]

The SG is one of the few machines I have put a VFD on (240 & 600 3P via RPC otherwise) and I can't believe I waited so long. I'm a convert; imo its the one machine that really needs a VFD. Why? 16 second soft start and stop means the wheel is never jarred and hence maintains its balance. Without it, because of the small amount of clearance between the wheel and spindle, you should dress each time you power up which can sometimes be impossible with a complicated set up.

I just googled it and the answer is maybe. You should dress the grinding wheel before grinding as it removes dirt that got into the wheel and to remove any dull abrasive crystals. I read a reply by a pro and they usually grind to two tenths and never balanced wheels.

 

[Mcgyver]

I just googled it and the answer is maybe. You should dress the grinding wheel before grinding as it removes dirt that got into the wheel and to remove any dull abrasive crystals. I read a reply by a pro and they usually grind to two tenths and never balanced wheels.

You googled it?

First point: People who don't balance the wheels don't balance them because the results are satisfactory using the unbalanced wheel. This is a result of the combination of the finish required and the rigidity of the machine and the job. Note a decent finish on a large heavy grinder is easier to obtain than on a light 6x12.

I wouldn't consider 2/10ths as exactly the pinnacle of grinding work. There are advantages to balancing a wheel however it has little to do with accuracy, it is about surface finish. Having said all that, I rarely balance as wheel as I don't find I need to to achieve the finish I want....but maybe be suspicious of the man says its never material, ever, anywhere.

The real point though is this: There is by design, clearance between a wheel and the spindle. Its not just to permit mounting and manufacturing tolerances, its a safety feature ensuring that as spindles warms and expand, it never puts an outward potentially bursting force on the wheel. So, when it is mounted, the wheel is always somewhat eccentric by this clearance amount. When you dress the wheel, you remove most of the imbalance created by the clearance - the OD is now perfectly concentric. This is ideal for balance and even contact. There remains the eccentricity of the bore, however imbalance is a function of mass and distance so this eccentricity has in comparison a tiny effect.

Without the soft start/stop, each time you start the grinder it can jar the wheel enough. i.e. the relative position of its bore position relative to the spindle. That's like remounting the wheel, it recreates the significant (to finish) ecentricity and imbalance on the outside of the wheel.

 

[Tom Kitta]

You googled it?

First point: People who don't balance the wheels don't balance them because the results are satisfactory using the unbalanced wheel. This is a result of the combination of the finish required and the rigidity of the machine and the job. Note a decent finish on a large heavy grinder is easier to obtain than on a light 6x12.

I wouldn't consider 2/10ths as exactly the pinnacle of grinding work. There are advantages to balancing a wheel however it has little to do with accuracy, it is about surface finish. Having said all that, I rarely balance as wheel as I don't find I need to to achieve the finish I want....but maybe be suspicious of the man says its never material, ever, anywhere.

The real point though is this: There is by design, clearance between a wheel and the spindle. Its not just to permit mounting and manufacturing tolerances, its a safety feature ensuring that as spindles warms and expand, it never puts an outward potentially bursting force on the wheel. So, when it is mounted, the wheel is always somewhat eccentric by this clearance amount. When you dress the wheel, you remove most of the imbalance created by the clearance - the OD is now perfectly concentric. This is ideal for balance and even contact. There remains the eccentricity of the bore, however imbalance is a function of mass and distance so this eccentricity has in comparison a tiny effect.

Without the soft start/stop, each time you start the grinder it can jar the wheel enough. i.e. the relative position of its bore position relative to the spindle. That's like remounting the wheel, it recreates the significant (to finish) ecentricity and imbalance on the outside of the wheel.

Maybe my SG is heavy & the wheels are grabbed well enough to prevent any movement, I never noticed stopping and starting moving the wheel or affecting the surface finish. I just re-ground hard jaws for a vise and the finish was silky smooth, like glass.

so far when measuring a part I got about +- tenth accuracy - but I think I can do better - I need to make sure wheel is freshly dressed.

I am more worried about the magnetic chuck distorting things followed by dirty grinding wheel. Also don't get the part to change temperature too much.

Maybe I try to measure today or tomorrow whatever the wheel actually moves even a tiny bit when starting and stopping on my SG. I did hear about it before.

 

[Dabbler]

I was taught by Bert - he dressed the wheel regularly. He was able to grind a stack of inserts, 40 tall and hold to 2 tenths over the whole stack. So hos individual error was +/- about 1/13 of that on each piece individually.

If you don't dress you downside is that you create more heat in teh part and can glaze the work. Your pro was using advanced techniques to minimize the downside. It's faster but much arder to grind without regularly dressing.

Your easy way is to keep your wheel sharp by dressing it.

 

[Tom Kitta]

I was taught by Bert - he dressed the wheel regularly. He was able to grind a stack of inserts, 40 tall and hold to 2 tenths over the whole stack. So hos individual error was +/- about 1/13 of that on each piece individually.

If you don't dress you downside is that you create more heat in teh part and can glaze the work. Your pro was using advanced techniques to minimize the downside. It's faster but much arder to grind without regularly dressing.

Your easy way is to keep your wheel sharp by dressing it.

Me == easy way
Once I perfected easy way maybe I do more advanced stuff. I am thinking of adding DRO to SG - good idea?

 

[Dabbler]

Bert says they are useless. However Robin Renzetti uses one and he's the most accomplished grinder hand I know of. FWIW...

 

[Mcgyver]

Maybe I try to measure today or tomorrow whatever the wheel actually moves even a tiny bit when starting and stopping on my SG. I did hear about it before.

right, and that creates imbalance and uneven contact. It will still work, but will affect finish. There is a limit to how tight you can clamp the wheel, but afaik without a soft start its pretty much SOP to dress the wheel when you start. The difference in finish is notable. Also, its a good idea to let a grinder run; turn it on, let it warm up, dress and let it stay running. It keeps the spindle bearings the same temp and doesn't disturb the wheel. However some complex set ups make leaving it running about impossible to do so a soft start/stop is a godsend.

As for mag chuck distortion, it can be a real issue depending on part geometry. There are several ways to deal with it. Before that, you have to make sure the chuck is seated on the table fully. scrape if necessary, then grind the top of the chuck - with coolant. I've even scraped one surface of a part so when its used on the mag chuck, it can't/won't distort. Scraping is just as accurate as grinding and can be more so with spindly parts because there are no forces distorting thing when you scrape. Get one side flat before grinding spindly parts is a good thing to have in the bag of grinding tricks.

On part temp change, imo you have to run coolant when grinding, I mean, you don't have to, its your shop lol, and it'll still work grinding dry....but if you want the best professional results, accuracy and finish it is necessary. It keeps parts the same temp (size) and the wheels clean. The propensity of temp change will depend on fixturing, part geometry and how much there to be ground. It will be less of an issue with a light cut on a flat part on the mag chuck (a lot of thermal mass) for example, but no matter the scenario grinding with coolant is better than without.

I put a Mitutoyo DRO on mine. Had an extra box, just needed a scale (was I in for shock there!) Its a Norton that came out of a high school so was hardly used but it suffered the conversion to metric. I disliked that, work in both but mostly imperial, and also felt even if willing to do the conversion, the downfeed system & dial didn't give enough resolution. I've grown to really like have it, probably more valuable than having one on a mill in that there was nothing I couldn't do a mill without one (but its an awesome convenience), whereas on the grinder I could never get the same accuracy I can with the 10ths DRO. That might just be a function of that particular grinder and its feed, but I wouldn't want to give it up.

 

[Hacker]

I, Like Kevin and Hacker have both RPC and VFSs. I wired my first RPC over 40 years ago.

@Hacker: you can move a VFD from motor to motor as long as they are the same current and voltage. It is too much bother to reprogram the VFD for motors of a different current rating. (you can also make a mistake, which isn't catastrophic, but will lead to extra motor heating and poor performance)

Rotary is perfect for multiple motors of different ratings, or for using transformers.

Never use a transformer to alter VFD output voltage.

You can do it I just feel it is a PIA and adds to the potential of an error and as you mentioned causeds heating which leads to shortening the life of motors and VFDs.

This fellow has used a step up VFD

This is the unit on AliExpress: https://www.aliexpress.com/item/10000404081162.html?spm=a2g0s.9042311.0.0.40694c4dl8JPjJ

It looks like they are doing it, I just haven't come across one that would be compatible with our voltages.