# Rotary phase converter/Transformer troubleshooting



## 460 Delta (May 9, 2018)

Well back feeding a 208 xformer with 230-240 volt could be part of the problem. When unloaded the RPC is putting full voltage out and the xformer is being magnetically saturated, so it’ll be hot and drawing high current.

The control power needs to come off of the incoming single phase power and not the manufactured phase, it’s too unsteady.

I would consider putting the additional caps at the starters for the iron worker and shear so they switch in and out when the motor is started. It’s a bit of a guessing game to figure out what size you need, but a 20uf on the iron worker starter would be a place to start. It’s mostly trial and error putting them in like this, but with a little experience it goes pretty fast. I had a full size r-mix plant set up like this where caps switched in with the load and it kept the voltage in relatively good balance. Just make sure you secure the cap in the starter enclosure so it won’t fall over and short to ground.


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## wcord (Jan 23, 2011)

If i read your post correctly, the system was working until recently?
If that's the case, then I would check the capacitors to see if any of them have weakened. Burnt wires are just loose connections.
I would assume, that you are working on a Willet, so their service department is a wealth of information - call them


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## oldsparky52 (Feb 25, 2020)

wcord said:


> If i read your post correctly, the system was working until recently?
> If that's the case, then I would check the capacitors to see if any of them have weakened. Burnt wires are just loose connections.
> I would assume, that you are working on a Willet, so their service department is a wealth of information - call them


I would do a fall of potential test across the starter also. Wcord's advice on the caps is the first place I'd start. I believe if one of them goes bad, this is the kind of stuff you might see (but I'm not really educated on this stuff so don't take my word for it).


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## oldsparky52 (Feb 25, 2020)

Viggmundir said:


> Amp check(at phase converter) with the iron worker running was 95ishA on L1 and jumping 95-105A on L2. I did tighten breaker lugs, one side was hotter then the other, and tighten lugs thru out phase convertor control box. Also replaced burnt wiring to part of capacitor bank before any running tests. Customer says it running better now. Phase converter voltage output was* 235V - 260V - 270V with no load. With the transformer and iron worker running, voltage is 230V - 178V - 182V, so pretty low*, I never even tried testing to the 10Hp motor. Iron worker was* pulling 2.5A, 10A, 10.2A at the machine. Voltage was around 550V - 500V - 520V.* Had a brief call with original install guy (out of province), they always had low voltage at machine which is why they are putting the 240V 3 phase into the 208V tranny taps.


Seems the iron worker may have a problem on A phase? Not loading up A phase, or ... overloading B and C phases?


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## Viggmundir (Sep 13, 2019)

wcord said:


> If i read your post correctly, the system was working until recently?
> If that's the case, then I would check the capacitors to see if any of them have weakened. Burnt wires are just loose connections.
> I would assume, that you are working on a Willet, so their service department is a wealth of information - call them


It is a Willet... I never even looked at brand name when I was there, thankfully I have a nameplate picture. As far as the system 'working' until recently, I'm not actually sure that is the case. The impression I was getting is that the 600V stuff never worked quite right, but was only installed recently. The RPC itself was installed back in the 90's I think he said. I would love to check the caps, but I don't have a meter that can test capacitance, I'm looking at getting one that can, my meter is due for an upgrade anyways and this is a good excuse for it. And yes I realize the burnt wires are loose connections, I redid a couple of the crimp spades because they slide on and off very easy. It also didn't help that there was 4 wires and the O/L block bus stuck under 1 terminal screw of the contactor... I managed to pull 1 of those out without much of a tug at all.


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## Viggmundir (Sep 13, 2019)

460 Delta said:


> Well back feeding a 208 xformer with 230-240 volt could be part of the problem. When unloaded the RPC is putting full voltage out and the xformer is being magnetically saturated, so it’ll be hot and drawing high current.
> 
> The control power needs to come off of the incoming single phase power and not the manufactured phase, it’s too unsteady.
> 
> I would consider putting the additional caps at the starters for the iron worker and shear so they switch in and out when the motor is started. It’s a bit of a guessing game to figure out what size you need, but a 20uf on the iron worker starter would be a place to start. It’s mostly trial and error putting them in like this, but with a little experience it goes pretty fast. I had a full size r-mix plant set up like this where caps switched in with the load and it kept the voltage in relatively good balance. Just make sure you secure the cap in the starter enclosure so it won’t fall over and short to ground.


That's what I thought about the control power, I didn't actually check which phase it was on when I was there. 20uf seems very low to me, I was thinking closer to about 10x that... But putting them at the motor starters would help with balancing per machine, if I can find enough space in the control cabinets, and find caps that are rated for the 600V rather then the 240V at the RPC.

With the transformer energized but no 600V motor loads on, the voltage at the transformer secondary was around 670V . Which is when I noticed they were tapped into the 208V with the incoming, and why I asked why the heck did you do this? But once the motor starts running, it drops off so bad that they were trying to compensate...


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## SWDweller (Dec 9, 2020)

Burned up wiring at the cap bank is a good place to start. I never liked replacing a single cap. I always did the whole set. This is from the days of 6 step VFD's. When the system stressed and took out the wiring, fuses in this case then for sure in my mind the caps are suspect to the point of replacement. 
Might be time for some interlocks to prevent the customer from being his own worst enemy. 
Lock out some loads so the phase converter is not overloaded, ever.


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## Viggmundir (Sep 13, 2019)

oldsparky52 said:


> Seems the iron worker may have a problem on A phase? Not loading up A phase, or ... overloading B and C phases?


Oh it sure seems like it has some kind of issue. I didn't have a ton of time when I was there, I had to run off for an inspection on another job, and had just stopped in to get a preliminary look at it. I was just trying to get some basic voltage/current measurements on all of it in the several minutes it would run before tripping the breaker.


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## 460 Delta (May 9, 2018)

Viggmundir said:


> That's what I thought about the control power, I didn't actually check which phase it was on when I was there. 20uf seems very low to me, I was thinking closer to about 10x that... But putting them at the motor starters would help with balancing per machine, if I can find enough space in the control cabinets, and find caps that are rated for the 600V rather then the 240V at the RPC.
> 
> With the transformer energized but no 600V motor loads on, the voltage at the transformer secondary was around 670V . Which is when I noticed they were tapped into the 208V with the incoming, and why I asked why the heck did you do this? But once the motor starts running, it drops off so bad that they were trying to compensate...


On the last plant I did, I used 23uf salvage caps out of MH lights. The 10 hp motors I used 1 or 2 depending on loading and “reasons”. The 15 hp on conveyors were closer to 50uf for some reason. This was all on a 240 volt system so the uf requirement should be less on a 600 volt system.


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## 460 Delta (May 9, 2018)

SWDweller said:


> Burned up wiring at the cap bank is a good place to start. I never liked replacing a single cap. I always did the whole set. This is from the days of 6 step VFD's. When the system stressed and took out the wiring, fuses in this case then for sure in my mind the caps are suspect to the point of replacement.
> Might be time for some interlocks to prevent the customer from being his own worst enemy.
> Lock out some loads so the phase converter is not overloaded, ever.


An interlock system looped through the main contactor so they can’t start any motors until the RPC is running is a good idea also.


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## Almost Retired (Sep 14, 2021)

There is a relationship of HP to mfd. I dont know the ratio. but more hp needs more mfd per phase.
Of that i am certain. I have seen commercially made rpc's that obviously follow that rule


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## CMP (Oct 30, 2019)

I think their killing the old girl, she worked fine for what she was sized for for many years, until the butchers showed up and started molesting her. You need to remember that for single phase input, you need to divide that input current number by the √3.
125A ÷ 1.73 = 72A 3Φ That is the most you could get out of that breaker if the rotary consumed zero power and your transformer was 100% efficient. The rotary does consume power in its function, and that saturated transformer as mentioned earlier is consuming a great deal of the output, driven way beyond it's voltage rating, as a kludge to try and get more power. Rated at 208V and fed with 270V no load. Its a heating element at this point.

They need a separate converter and the proper transformer for the 600V loads. An ironworker and shear are considered a heavy starting and cyclical load and need a sturdy rotary converter, especially with the transformer losses added in. 2¢

A sizing chart from one of the more popular RPC suppliers in the US for comparison.


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## SWDweller (Dec 9, 2020)

I just had a thought you could put the iron worker/other load on separate VFD's and let the VFD produce the third phase. I have seen this work before on motor loads.
I have been told to go square root of 3 larger on the primary side so you can get the third phase. Toshiba in Huston has a sheet every month for their scratch and dents. Full waranty just not full price.
This could keep your customer in business so that they do not kill their rotary converter. I had the occasion to work for a place where we got a rotary in to fix. Even the "brain surgeons" could not get the parts and labor down for what we bought a Toshiba VFD and an isolation transformer and installed. We needed the transformer to help the VFD against the 3 phase heating elements that were involved with that project.


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## CMP (Oct 30, 2019)

In case your the do it yourself type, several years ago I helped another Canuck from Quebec build his own 240V to 600V rotary converter for his grinding machine. He's not a sparky but did an admirable job for a first time builder. If he is near your area, I'm sure he would be happy to consult with you on construction. The fab shop that needs it could help doing the mechanical portion and locating a used 600V idler motor.

Here is the thread that got the project started. 240 v to 575 v 3 phase converter project

He posted a few videos of his project on youtube, the early one has no narration but it shows the build and functionality.
Rotary Phase Converter to convert single phase 240v to 575v , 3 phase, 575v / 600v / 7.5 hp

The latest one from earlier this year has narration for French speakers and a views of his control panel. I still have a copy of the controls that I sketched out for him, but I'm sure he would be willing to share his final version if you were interested.
rotary phase converter 240 to 575 volt rpc roto phase 7.5 hp


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## Viggmundir (Sep 13, 2019)

CMP said:


> In case your the do it yourself type, several years ago I helped another Canuck from Quebec build his own 240V to 600V rotary converter for his grinding machine. He's not a sparky but did an admirable job for a first time builder. If he is near your area, I'm sure he would be happy to consult with you on construction. The fab shop that needs it could help doing the mechanical portion and locating a used 600V idler motor.
> 
> Here is the thread that got the project started. 240 v to 575 v 3 phase converter project
> 
> ...


Well that was an interesting read! I am not very close to Quebec though. I'm closer to Colorado and Kansas! And even though I am Canadian, I don't do the whole parler en francais thing very well...


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## Viggmundir (Sep 13, 2019)

Well thanks everyone for chipping in and commenting, lots of good info! I was in a bad cell area today, so didn't get to see them all earlier. I'm hoping to head out to this job tomorrow to see what I can do with it. I've got a bunch of various capacitors ordered so I can try different combinations out. I'm also using this as an excuse to upgrade my meter, I've got a Fluke 376FC showing up tomorrow morning  .
I had a quick chat with the customer earlier today, he said the transformer and 600V stuff install was recent, and has never worked properly.


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## Viggmundir (Sep 13, 2019)

SWDweller said:


> I just had a thought you could put the iron worker/other load on separate VFD's and let the VFD produce the third phase. I have seen this work before on motor loads.
> I have been told to go square root of 3 larger on the primary side so you can get the third phase. Toshiba in Huston has a sheet every month for their scratch and dents. Full waranty just not full price.
> This could keep your customer in business so that they do not kill their rotary converter. I had the occasion to work for a place where we got a rotary in to fix. Even the "brain surgeons" could not get the parts and labor down for what we bought a Toshiba VFD and an isolation transformer and installed. We needed the transformer to help the VFD against the 3 phase heating elements that were involved with that project.


From what I've seen, I think the limit for single phase supply to VFD convert to 3 phase is around 5Hp, and possibly only at 240V.


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## micromind (Aug 11, 2007)

Viggmundir said:


> From what I've seen, I think the limit for single phase supply to VFD convert to 3 phase is around 5Hp, and possibly only at 240V.


Usually you can get a VFD that is made for single phase input but usually not more than 5HP. Larger than that, it'll be a 3Ø model and you'll need to double the HP. For example, a 10HP motor would need a 20HP VFD. Most can be programmed to not go into a 'missing phase' alarm. 

Also, with the exception of 120 volt input models, no VFD can output more than the input voltage.


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## Viggmundir (Sep 13, 2019)

So an update for those who care: 

I spent most of yesterday installing a new 200A disconnect switch to feed the phase converter rather then coming thru the 200A panel that is loaded up with welders and lights and such. Cleaned out the splitter while I had it open, everything there is covered in years of welding/metal cutting dust. Had everything wired up and ready to go at the end of the day, just had to reroute the feed cable in the converter control cabinet because of some extra slack from pushing up the cable where it had pulled out of its connector, and one of the lines popped out of the main contactor. I couldn't get it to tighten it back down because apparently the lug screw was stripped 😫 ....

Got a replacement contactor sent out today, but it came with a 120V coil, no 240V coils in stock, so I rewired part of the controls this afternoon to account for that, including installing a DPDT 8 pin cube relay to isolate the 120V coil from the 240V coil of the time delay relay. Went to fire it up and it didn't work.... Cube relay would light up, but main contactor wouldn't pull in. After troubleshooting, I figured out that the movable contact in the cube relay had somehow come off its pivot point, and so while it would energize, the contact wouldn't actually switch... knocked it back into place and now it works! 

Finished out the day doing voltage/current readings with different combinations of capacitors. I also changed the taps on the transformer. Its an autotransformer, but I changed A and B (the original single phase legs) back to the 240V taps, and left C (generated leg) on the 208V tap. Starting voltage readings on the 600V line were 692V-561V-542V. When I left they were 597V-544V-570V. Added one more 50uF cap to the converter, and 2 series sets of 2 50uF at a 600V JB, one set A-C, one set B-C.


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## micromind (Aug 11, 2007)

Ever notice how 1 thing goes wrong than about a dozen more follow........lol. 

Good work though, eventually you got it running!


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## CMP (Oct 30, 2019)

When you go to try the iron worker and shear, make sure that the 600V generated leg does not power the control circuit in either machine. 

In addition if you need or want to try additional capacitance, since you already balanced the RPC, add the capacitors on the load side of each of the machine motor starters. In this way they will only be brought in when needed and not upset the RPC balance when they are not required. 460 Delta had previously mentioned this method. You don't want the voltage boosted too high, for when its only running the crane load.

Show us some pictures of what your working with and a RPC tag if there is one.


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## Viggmundir (Sep 13, 2019)

Update:

Well I'm stumped and confused now...

I've been trying to balance out the transformer, but have been doing it with no load on it. Should I be doing it under load?
I know a transformer will have a no load saturation current, but it seems excessive to me. The rated current for the xformer @ 240V is 36.1A, but its pulling 41.9A, 42.6A, 47.8A with no other load. It is an autotransformer in a Wye. I find its generating too much heat, I'm worried about cooking it. Room temp is around 20 Celsius, but after 5min of the xformer running, its surface temp has hit over 150 Celsius... I just don't see that being sustainable. The xformer is a 115 rise, 180 class though... Voltages with the xformer running were 202.1V, 209V, 236.3V. Are the higher currents due to the low input voltage? Would pushing that voltage higher bring the current down?I also found I have current on my bond conductor, around 30A or so at the disconnect before the RPC. Only there when the xformer is running. The original install they installed a 3 wire cable to the xformer and bonded the X0/H0 to the frame... I don't believe that an auto transformer qualifies as a "separately derived source", and so it shouldn't actually have the X0 bonded to the frame correct? But instead they should have run a 4 wire cable to feed it? So I figured the current over the bond wire should be over the neutral wire instead, so I wired up a length of #6 bare copper temporarily to check. Put tape anywhere it was touching metal, ran it back to the main RPC disconnect neutral block. Bond wire current dropped to around 2OA, but when I checked the #6, it had *136A !!!* flowing on it. Didn't take long for it to start getting hot. I clamped it with both a TRMS meter and a non-TRMS meter, they were within an amp of each other, so I don't think its harmonics, but where is that much current coming from?? The xformer is bolted to the overhead crane support leg, so maybe that much was going to ground thru the building structure before I unbonded the neutral?

I did also start and run the 10Hp motor, it started pretty easy, and current at the transformer didn't even go up very much, 52A, 42.8A, 46.7A.

Thoughts and advice?


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## Viggmundir (Sep 13, 2019)

Pics of the RPC, inside and out, and install location, a little tight!


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## micromind (Aug 11, 2007)

Since the voltage is 202, 209 and 236, is the supply 2 legs of a 120/208 Y? If so, is it possible to get the 3rd phase pulled in?

Also, are all 3 taps at the same position? If they're not, there will be circulating currents because all 3 windings are on the same core.

An autotransformer is not a separately derived system because there's no isolation between the primary and secondary. 

I would not bond or ground the center of the Y in the transformer.

I'd use the 240 taps, it'll cut back on saturating current though the 600 will be a bit low. 

RPCs need to be tested under load, the voltage of the manufactured leg will vary quite a bit with load. It should be high with no load and come down with load.


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## Viggmundir (Sep 13, 2019)

No, it is from single phase 240V. the 236V is the original, with the 202/209 being either original leg to the generated leg. I have all taps at 240V on the xformer, but it still pulls more then full amps, even with no load after the transformer. 
I didn't think it should be bonded either, but if I unbond it, i then have 136A on what I used as a neutral wire back...

With the xformer off, those 202/209 voltages are more like 265/280V.

At this point, I'm wondering if the auto transformer is the wrong thing to use here, and it should be a delta/delta instead.


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## oldsparky52 (Feb 25, 2020)

I'm a little slow these days, maybe I should know, but what terminals are you connecting your circuits to?

In the transformer that is.

Are you bringing a neutral to the XO or floating it?


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## Viggmundir (Sep 13, 2019)

oldsparky52 said:


> I'm a little slow these days, maybe I should know, but what terminals are you connecting your circuits to?
> 
> In the transformer that is.
> 
> Are you bringing a neutral to the XO or floating it?


Load is connected from H1,H2,H3. Load to X4,X5,X6. X0/H0 (same wire) is currently bonded to the xformer frame. If I unbond it and connect a neutral to X0, it has 137A on the neutral.


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## micromind (Aug 11, 2007)

A ∆ transformer connection tends to balance voltage, A Y does not.

Also, i would not connect HO/XO to anything. It's a 3Ø 3 wire autotransformer, the service bonding jumper should be the only connection to ground.


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## Viggmundir (Sep 13, 2019)

micromind said:


> A ∆ transformer connection tends to balance voltage, A Y does not.
> 
> Also, i would not connect HO/XO to anything. It's a 3Ø 3 wire autotransformer, the service bonding jumper should be the only connection to ground.


Leaving it totally floating I did not try, I'll have to give that a whirl tomorrow.


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## 460 Delta (May 9, 2018)

I think a Delta-Delta stepup is the better setup in this case.

The voltage should be running higher on the manufactured leg by 20 volts or so no load. Maybe take the wood block spacer out in the cap bank and install another cap.

If the tag is correct on the RPC, it’s too small by a lot. The RPC is a 7 HP and the shear is a 15 HP. That’s a big overload, if it was lightly loaded, maybe, but with a big flywheel to turn you’re lucky it worked at all.

A bigger RPC is needed, or another to parallel with the present one to effectively make a larger unit.


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## Viggmundir (Sep 13, 2019)

460 Delta said:


> I think a Delta-Delta stepup is the better setup in this case.
> 
> The voltage should be running higher on the manufactured leg by 20 volts or so no load. Maybe take the wood block spacer out in the cap bank and install another cap.
> 
> ...


Sorry the label in the pic is hard to read. 
The RPC is 15Hp. Max of 20Hp load, with single biggest motor of 7.5Hp. The shear is a 10Hp, but in theory it isn't going to be running at full capacity. I started the shear briefly on Friday to see what effect it would have running, and even starting it didn't bog the RPC down at all. 

Removing the wood block is a thought, but more caps on the RPC directly makes it run rough unloaded (It has 7x50uF caps). Any more and the unloaded voltage goes up around 290-300V. I do also have (1x50uF + 2x45uF) on the transformer side of the transformer disconnect. Any more that I add will be there I think.

It's also running unbalanced on the single phase amps. With all the caps in there its around 55A leg A, and 96A leg B. I was hoping I could bring those closer together. All the caps are B to the generated leg. Maybe a few A-B or A- generated would help? But that also messes with phase angles then.


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## Viggmundir (Sep 13, 2019)

The shear in question. Also the starter, I had one arc chute off to look at the contacts. Built in the 50's apparently, and I would believe the starter is that vintage (But I could be wrong!). Split core coil on the left to pull it in, and overload at the bottom. All controls are 600V.


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## CMP (Oct 30, 2019)

The problem your having is the selection of an autotransformer to be powered from a RPC. This was likely done because it is much less expensive than a full isolation step up 240 Delta to 600 Wye which would be the correct choice in this application.

To help you understand why this is a problem, you must first realize that a RPC output is a center tapped delta. Meaning that its output is not symmetrical to ground or neutral, it has a wild leg on its output with respect to neutral or ground. Normally a center tapped delta phase would have one leg (B phase) around 208V to neutral. With a RPC this voltage will be quite a bit higher due to the capacitance added to boost the generated leg voltage, higher than normal so that when this weaker leg becomes loaded it will not drop too far to adequately power the connected load.

The aoutotransformer is expecting all lines to be symmetrical voltage to neutral and ground. If you look closely at the tag, it only lists wye supplied system voltages. It doesn't explicitly state not to uses a center tapped delta for supply, but it is implied by listing approved supply voltage systems, which does not include the center tapped delta. The 240/139V it shows is a symmetrical wye system and that is not what your feeding it.









In this drawing Peter Hass graphically shows the difference between the two system types, overlayed over top of each other.








If you look at the two neutral points of both system types, you will notice that there is 69V difference between the two. One just shows the single phase supply, but that is the system powering the RPC, adding in the generated wild leg would make the drawing more confusing and cluttered, and isn't necessary to show the 69V difference between the systems.

In effect, the two transformers are fighting each other over the status of the neutral potential. The utility transformer is much larger, so it always wins. The autotransformer is the looser and it will die trying to bring the neutral potential over to it's center. This is why you have huge currents on the center point common, whether you bond it to neutral or common, they both go back to the utility transformer common. 

The only way you can make it function, is to leave the common center point floating, not connected to anything, so there is no fight to control the neutral potential. In doing this it leaves you without a solidly grounded system to carry fault currents when a short circuit happens in the downstream equipment. Voltage readings to ground will be nonstandard. Any fault currents will try to get back through the much higher impedance's of the transformer winding's and cause heating of the winding's and upset the voltage output to the connected equipment. It's not a good thing to do for a permanent fix, but should prove out the theory presented of what is going on. Ungrounded 600V is not a good thing for the average user, and would require a ground fault detection system, and a trained person on staff to know how to deal with the problem (NEC Rule).


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## Viggmundir (Sep 13, 2019)

CMP said:


> The problem your having is the selection of an autotransformer to be powered from a RPC. This was likely done because it is much less expensive than a full isolation step up 240 Delta to 600 Wye which would be the correct choice in this application.
> 
> To help you understand why this is a problem, you must first realize that a RPC output is a center tapped delta. Meaning that its output is not symmetrical to ground or neutral, it has a wild leg on its output with respect to neutral or ground. Normally a center tapped delta phase would have one leg (B phase) around 208V to neutral. With a RPC this voltage will be quite a bit higher due to the capacitance added to boost the generated leg voltage, higher than normal so that when this weaker leg becomes loaded it will not drop too far to adequately power the connected load.
> 
> ...


Aha! That is so very helpful, I will try floating the neutral tomorrow, that diagram is a good explanation of the theory. And also a good explanation of the neutral current, makes sense.

Thank you!


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## CMP (Oct 30, 2019)

Also be aware as others have mentioned that the RPC should be sized at twice the load motor.
When you add in the transformer losses it becomes even more of an issue. If you let them continue to run the shear on the undersized RPC you might be back there repairing it more times than anybody likes. Refer back to the PDF chart I posted earlier for recommended sizes.


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## 460 Delta (May 9, 2018)

Something to consider is swapping out the motors on that shear (nice looking old Cincinnati Shaper) and iron worker to a 240 volt motors. A proper xformer to make 575 for it and the ironworker isn’t going to be cheap.

Another thing to consider is changing out that old 3 Star CH starter for something newer and using 120 volt control voltage. A newer unit will have some extra space in it if you decide to put a cap in to switch in with the motor to help voltage balance. Line voltage control is fine if it’s local and doesn’t leave the starter enclosure. With the hand and foot safety switches on that shear on 575 now, and floating too if you float the xformer, it should be an easy sell for a brand used (or new) NEMA size 2 starter.

If you leave the xformer floating, the controls will be also. The capacitive voltage on a 575 system could be 300+, enough for a serious bite if and when that old wiring gets frayed.


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## Almost Retired (Sep 14, 2021)

like 460 said:

So they bought an RPC (with all the problems that brings), they bought an autotransformer

Did no one ever suggest some single phase motors ????????

do some shopping and price comparisons (plus labor for connections) then consider reliability and ease of repair

Unless I missed something, this _should_ be a no brainer


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## micromind (Aug 11, 2007)

Going with single phase motors is a good idea but a modern motor won't fit exactly. 

Standard motor framed are as follows.......before 1952, there was no standard. From 1952 to 1964, they were U frames. From 1964 to present, T frames. 

If the existing motor is a U frame, there are adapter rails available that will bolt on to the existing holes and a T frame motor can be directly mounted without any modification. 

If the existing motor is not a U frame, you'll need to drill and maybe tap new mounting holes. 

Also, look at the RPM of the existing motor, a lot of older machinery had motors with slower speeds.


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## Almost Retired (Sep 14, 2021)

micromind said:


> Going with single phase motors is a good idea but a modern motor won't fit exactly.
> 
> Standard motor framed are as follows.......before 1952, there was no standard. From 1952 to 1964, they were U frames. From 1964 to present, T frames.
> 
> ...


still cheaper and easier for future maintenance


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## 460 Delta (May 9, 2018)

When I replied to switch to 240 volt motors, I meant 240 3 phase, not single phase.


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## Almost Retired (Sep 14, 2021)

460 Delta said:


> When I replied to switch to 240 volt motors, I meant 240 3 phase, not single phase.


That would eliminate the txr but not the rpc


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## 460 Delta (May 9, 2018)

Almost Retired said:


> That would eliminate the txr but not the rpc


In motors over 5 hp I would rather deal with a RPC than single phase motors with start caps and centrifugal switches. Once a RPC is dialed in it’s pretty much trouble free. Single phase motors are a hassle to keep going, and the long start time from flywheel inertia will play havoc with the start caps.


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## Almost Retired (Sep 14, 2021)

460 Delta said:


> In motors over 5 hp I would rather deal with a RPC than single phase motors with start caps and centrifugal switches. Once a RPC is dialed in it’s pretty much trouble free. Single phase motors are a hassle to keep going, and the long start time from flywheel inertia will play havoc with the start caps.


now i know


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## Viggmundir (Sep 13, 2019)

@CMP you get to be my hero this week    

Floating the neutral on the auto transformer made a massive difference. It went from 40-50A per phase, down to around 5A. I ended up removing about 130uF from the RPC, and moved 140uF from B-C in the transformer disconnect to A-C. With the shear running, voltages on the 240V side were: 241.4V, 241V, 239.7V. 600V side was 593V, 595.3V, 595V. Seems good enough to me. Amps at the shear were 5.2A, 5.2A, 4.8A. Iron worker was 4.0A, 4.1A, 4.4A. Xformer current with the shear running was 15.5A, 14.7A, 15.2A. 240V single phase was 16.2A and 16.1A (compared to 65A and 105A on Friday!) Line to ground voltages were 313V, 313V, 414V.

The shear actually had another control box on it I hadn't seen. The main start stop station is 600V to the starter, but all the foot pedals and air/hydraulic solenoids are 120V. Starting current on the shear was 126A to the transformer. The RPC is rated for a max start current of 250A, so no issue there. At one point today we fired up the iron worker while the shear was running at the same time, all good. My client was able to get some of his cutting done that has been piling up.

I asked why they went with the auto transformer and not an isolation type. Recommended by the supplier he got the xformer from, but had also sent pics of it to the RPC company and they didn't have an issue with it either.🤷‍♂️

Obviously a 3 phase service would be a better option here, but POCO gave my client a price of around $70k just to bring it from the existing lines to the building... not going to happen...

The RPC was installed in the 90's, and been working pretty well since then. Now that I have it all dialed in, I don't think its going to work it too hard.


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## oldsparky52 (Feb 25, 2020)

Great work! Thank you so much for sharing this with us.


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## oldsparky52 (Feb 25, 2020)

CMP said:


> Ungrounded 600V is not a good thing for the average user, and would require a ground fault detection system, and a trained person on staff to know how to deal with the problem (NEC Rule).


Humm, is there a ground fault detector on this setup?


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## micromind (Aug 11, 2007)

oldsparky52 said:


> Humm, is there a ground fault detector on this setup?


There's no need for a ground fault detector, the 600 volt system is solidly grounded by the single phase system and the autotransformer.


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## oldsparky52 (Feb 25, 2020)

micromind said:


> There's no need for a ground fault detector, the 600 volt system is solidly grounded by the single phase system and the autotransformer.


May be why the autotransformer was selected originally.


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## Viggmundir (Sep 13, 2019)

oldsparky52 said:


> May be why the autotransformer was selected originally.


Oh I doubt that. Grounding an isolation transformer would have been fairly easy. Its on the backside of the wall that the service disconnect is on. 12' of #6 would have done it.


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