Need 460V three phase to run Monarch DC Lathe

I am looking for information regarding the proper selection of equipment to run A late model Monarch 10ee lathe with solid state controls. The lathe is set up for 460 three phase at 13 amps. I presently have a rotary phase converter that I have used for years to run my other equipment at 230 volts. I thought it would be a simple matter of just putting a 3 phase setup transformer on the lathe and bring the voltage up to 460. After hooking up the lathe in this fashion I had control issues with the lathe that seem power related as the lathe worked fine when hooked to 460 V from the utility company. I was informed by a phase converter manufacture that I would need to step up the voltage before it went into the converter and not after it comes out. He was also was worried that the Monarch lathe might regenerate power back into the system during breaking or speed changes which would require a double converter. Does this sound right? Any help would be appreciated.

Are you sure you used a THREE phase transformer after the converter and before the Monarch? Would need to be 12 kva minimum -- pretty expensive!!! Could you possibly have used a single phase transformer and just run the third leg straight from the converter to the lathe? If so, that explains your problem as the third leg would be only 240 volts.

If that is what you did, you need to rewire the converter motor to take 460 volts (if motor can do that) and do as you were advised about placing the transformer before the converter. That way the converter puts out 460 volt three phase on its third leg.

But WATCH OUT if you do that. Even if the motor can be rewired for 460 volts, that's no assurance the capacitors are rated for 460 volts, which they'd need to be. Also, the capacity of the capacitor banks would need to be reduced considerably. I can provide details if needed, but you whole converter would need re-engineering.

bnelson

The transformer that I used is a three phase unit rated at 25KVA. The thing weighs 230 pounds. So I do not think that it is the problem as long as it is ok to hook it up after the converter. I also put a post on the Monarch forum as well. I receved a suggestion that my converter is to small and I should check the wild leg under load for the proper voltage. I will try that to see what happens.

Well, I am wrong about the hookup. Just thought I'd check, as you don't often see such a big three phase transformer used this way. It should be OK to hook it up after the converter, going from 240 volts 3 phase delta to 460 volts delta.

I agree with checking the high leg under load. If it is below 400 volts that likely will give you control problems on the Monarch and could reflect an undersized converter.

Power is conserved across a transformer, so a converter of given hp rating at 240 volts should be able to power a load motor of exactly the same hp rating on the other side of a transformer at 480 volts. If you've got a clamp-on Amprobe, check all three leg currents of your Monarch under full load. If the voltages are all pretty close to 460 but one leg has essentially zero current compared to the other two legs (e.g., 13 amps, 14.5 amps, 1.3 amps), then your voltage balance is not good enough and your lathe is running essentilly single phase. A buck/boost transformer on the offending leg may remedy this.

bnelson

Just thought of another idea if the above bombs out. Try switching ALL THREE lines in sequence (just move each line clockwise to the next terminal, for all legs).

If it's touchy voltage and the controller is doing its thing on only two legs single phase, this could solve your problem.

Also, have you grounded things correctly? The way you have things hooked up, you have an NEC-defined separately derived system. (A usual phase converter is NOT a separately derived system). With no ground on the secondary 460 volt side, you do NOT have a high leg. To get it, you'd need to run a big ground wire from the main panel, to the transformer case, AND to a center tap on one of the secondary coils of the three phase transformer. You definitely don't need to do this, but be aware that if you've only run a ground from the panel bus (or the converter frame) to the transformer case, you have an UNGROUNDED SYSTEM. Even if your Monarch frame is grounded clear back to the panel ground bus, the three phase circuit feeding it is UNGROUNDED, and if one hot leg were to short to frame on your lathe, nothing would blow!

If you are indeed running your system ungrounded, I wonder if the Monarch control circuitry is meant to do that. Often in control circuits the ground is not just an overcurrent/safety thing, but it serves as a zero-reference point with respect to the phase conductors. If that point floats, it can make control networks behave very flaky.

bnelson

The Monarch manual is mute on the issue of a 480 volt "corner grounded" system, so I would assume such a system is acceptable to the 10EE. Particularly as such systems were quite common when the 10EEs were designed and used.

You would simulate a "corner grounded" system by grounding the H1 lead of the 240:480 transformer.

By so grounding the transformer's secondary, you are creating a true "separately derived" system.

The all solid-state 10EE may be different, however.

Peterh,

What about an ungrounded system? Unless NickG's transformer is wired wye on the 480 volt secondary AND the Y point is bonded to the transformer case, I fear that he could have run a ground wire but have, literally, an ungrounded system electrically speaking (no grounded phase conductor; no solidly grounded system). A totally ungrounded transformer secondary still classifies as a separately derived system according to NEC.

Do you know if a Monarch can run in an industrial setting where they choose to run an ungrounded system? Some factories do that. My concern with the control network is that if it references a ground as a zero reference voltage and there is in fact no solid ground, there will be errors introduced. Even in the absence of a solid ground, everything is ultimately grounded via capacitative or inductive coupling to earth/frames; these latter are quite variable, or 'flaky', compared to a solid ground.

bnelson

I tried several things last night. I first checked my voltage out of the converter to ground and found that the wild leg to ground measures 201 volts before going into the transformer. When starting and stoping the lathe the voltage on that leg would jump around quite a bit while the other two legs would remain very steady. If I measured across the legs after the transformer with the lathe running I would get 450 volts on the line legs and 490 volts on the wild leg. I also noticed that the indicator light on the lathe control would dim slightly as I started and stoped the machine. Then I had the same idea as you to try different combinations on the legs to see if I could possibly get the lathe control off of the wild leg and working from the other two legs. I have ordered a schematic from Monarch but have not recieved it yet so I am not sure how it is exactly wired. Anyway after trying this I hit upon a combination in which the lathe would not trip the control anymore. It still would not go over 1900 RPM but I fiqured I might have damged the control with my previuos attemps at hooking up the lathe. However after cycling the lathe several times the lathe suddenly stopped working. The control stayed up but the lathe would not run. I think I have tracked this new problem down to three blown fuses inside the control. They are marked as FWH 40 amp semiconducter fuses. When I check them with an ohm meter they show as open. it is my quess at this point that I need to get my phase converter upgraded to unit that has better control of its voltage. I think that when I changed my line wires I just shifted my problem to a different part of the lathe. You are correct in the fact that lathe is grounded but is not hooked up to the transformer at a center tap point. I do not fully understand the issue with this or the difference between the transformer types. I would appreciate a further discussion on this issue.

I checked with Monarch and it appears that it is fairly straight forward to change the voltage of the lathe to 240 volts. Just change the wiring on three transformers and install bigger fuses. So I think I will go this route and then I will not need the set up transformer at all. However I am still uncomfortable about my present converter and the voltages it is producing. What should I be looking for to be sure that it will not cause further problems? Would I be better off to purchase a CNC grade converter if so what brand do you recomend?

A review ...

1) WiaD and Modular 10EEs have anode transformers which are strappable for 240 or 480, but these same 10EEs have filament transformers which are specific to 240 or 480. Plus, the main contactor is purpose-built and is single voltage (either 240 or 480).

2) Solid State 10EEs, not needing filament transformers, may indeed have all transformers which are strappable. I'd check the schematic to see if the machine is still running the main contactor off of line voltage, however.

3) Although some sites may run the transformer secondary ungrounded, this isn't a good practice. There are two choices (at least): a) ground any line, thereby creating a "corner grounded" situation, or b) install a grounding impedance.

The book "Electrical Grounding, 4th Edition", presently available on a close-out bases from HamiltonBooks.com for about $4, more than adequately discusses these options.

In fact, this particular book has the most cogent treatment of "corner grounded" systems that I have yet read (and I used to be an EE at this nation's largest municipal utility).

I agree with Peterh about ungrounded systems; I hate 'em. They may have some application in large factories with elborate ground fault alarms, where it may be important to keep an assembly line running if only one hot leg grounds out in a large motor. But they have no place in small shops. Corner grounding of a transformer secondary is entirely acceptaable and very easy; only drawback is really high fault currents can result in the event of a short to ground.

Nick, try this as long as you're giving up on the big transformer anyway. Disconnect the big transformer, start your converter, run it unloaded, and measure the leg-to-ground voltage for all three legs. You ideally should have 120-120-208. Then measure the leg-to-leg voltage for all three legs; should ideally have 240-240-240. If possible connect a large 240 volt three phase load to the converter output and do all these measurements again. Post the data here. It should give a very good indication if your converter is either too small, or perhaps not fitted with the right capacitors. If you can also give us all three amps under load for the three hot legs, that would help even more. With this info it should be possible to tell if your converter is OK. Let us know either the nameplate amps or the hp of the load you use in this test.

bnelson

With respect to "corner grounded" systems, Square D, and perhaps others, sell a circuit breaker which is specifically rated for such service.

Very expensive, however, as the fault current required to be interrupted is indeed higher.

The most recent NEC (2000, IIRC), and perhaps one back (1996, IIRC) states, "Delta breakers are prohibited", but it is unclear whether this statement is specifically referring to "corner grounded" systems, or to another system. Plus, it is unclear whether this applies only to new installation, or to all systems.

Theoretically, all "old work" systems are grandfathered.

The previously cited book, "Electrical Grounding, 4th Edition", which is consistent with the 1996 NEC, discusses "corner grounded" systems with reasonable clarity.

bnelson,

I conducted the test you mentioned and listed the results below

Test One
Leg Current Voltage to Ground
1 0 120
2 0 119
3 0 228


Test Two (one HP load)
Leg Current Voltage to ground
1 2.9 120
2 .9 119
3 2.9 228
Across the legs Test Two
Leg 1-2 239 Volts
Leg 2-3 241 Volts
Leg 1-3 267 Volts


Test Three (Aproximate 5 HP Load)
Leg Current Voltage to ground
1 14.3 120
2 6.7 119
3 10.4 210
Across the legs Test Three
Leg 1-2 238 Volts
Leg 2-3 229 Volts
Leg 1-3 252 Volts

Any help would be appreciated.

Nick,

Thanks for such complete test results. Your converter is working perfectly, and I would assume it could probably handle a 7.5 - 10 hp load motor. What is the spindle rating of your 10EE?; hp rating of your converter?

I'll elaborate further later on about why your test results indicate your converter is fine and likely could power a > 5 hp load.

bnelson

bnelson,

My converter is rated at 7.5 HP. I am not sure of the true HP rating on the Monarch because of the DC drive motor. However the rating plate on the lathe shows 13amps at 460 volts. When I was running the lathe using 460 it seemed to be drawing about 1/2 of this. However the lathe seems to have some type of regenerative or breaking situation going on. When the lathe spindle direction is changed or the speed is quickly changed I can see my digital amp meter jump up and also hear the converter load down briefly. Because it happens so fast the amp meter does not have time to react fully and it is hard to really tell what the current draw is. So it is very possible that the lathe could have some 13 amp spikes.
Is it safe to assume that my problems are being caused by having the large 3 phase transformer installed after the converter and if I correctly change the lathe to 230 volts I should be ok. I did some trouble shooting on the lathe and found that it had blown three 40 amp semiconducter fuses which protect the SCR's inside the control cabinet. I replaced these and very briefly powered the lathe up under 460. It now seems to run ok but I am sure if I continued to run the speed up or down quickly or changed directions under power it would shut down again. Your help is very much appreciated.

Thanks

FWIW, the Ward Leonard 10EEs contain the following rotating machines:

1) ac motor, 220/440 volts, three-phase, 4.6 HP, 3450 rpm (60 Hz) [ * ] ,

2) dc generator, 230 volts, 2.5 kW, separately excited, direct coupled to (1)

3) exciter, 115 volts dc, belt coupled to (1) with a 1:1 ratio, and

4) dc motor, 230 volts armature (and series field), 115 volts (shunt) field, 3 HP.

All the dc machines (motors, generators, etcetera) in a 10EE are compound-wound.

[ * ] Machines sold in Buffalo and Philadelphia and immediately adjacent areas (say, Camden, NJ, e.g.) may be two-phase 220/440; machines sold in Canada may be 600 volts nominal; machines sold in Europe may be 380 or 415 volts, 50 Hz.

I just recieved the wiring diagram from Monarch today. It is titled 10" EE W/REGEN DRIVE. So I think that my assumption is correct that it has some type of regeneration going on at speed changes. It also shows that it is a 5HP machine.

Nick,

This weekend I’ll post how your above comprehensive test results show your converter is entirely OK.

If your Monarch is rated 5 hp, it should draw only about 5 amps under full load at 460 volts:

amps = rated hp * 746 / (460 volts * 1.732) = 5 * 746/460*1.732 = 4.68 amps; at 85% efficiency, that translates to 5.5 amps.

What you are doing with the transformer should not be a problem; I really don’t think we can declare it to be the culprit responsible for blowing your fuses.

The regeneration issue is the likely culprit. And here is where I’m confused. I thought there were two kinds of braking, dynamic resistive (in which the load motor slows by acting as a generator and bleeding the excess kinetic energy amps to a big wirewound external resistor or heating coil); and regenerative (where the excess amps are back fed into the supply power system as usable watts for a different load).

How can a Ward Leonard drive do regenerative braking as I just defined it? How does it sync up going from DC back to AC? Do the tubes act as gate thyristors during braking? Does it somehow act as a synchronous generator? Peterh, here’s where we need a REAL engineer. The reason for this question is that, if a Ward Leonard truly does do regenerative braking, then the first “target” to get the excess power will be the idler motor, which will be severely overdriven and result in the currents going sky high during the braking operation. Just a guess, but that may be the root of your control problem.

If I’m right, you need to at least double the hp rating of your converter so it is not so severely overdriven during slowing/reversing of your 10EE, or get yourself a Phase Perfect solid state inverter which can handle the regenerative load with no idler, and pump it back directly to the single phase feed line (give it back to the lower company).

bnelson

"Regenerative" braking is indeed where the power to be dissipated is returned to the source.

This is practical in electrified railways where there is a large bus from which power for all locomotives is obtained, and to which power from any locomotive (or set of locomotives) may be returned.

However, these railways use ac motors, usually at least four per locomotive, not a single dc motor, as does the Monarch 10EE.

There is a converter topology which is bi-directional, where the commutating devices, usually thyristers now, but initially mercury vapor converter tubes not unlike thyratrons, can indeed receive or send power.

My former employer, this nation's largest municipal utility, used this concept and system to receive and send more than 1,400 megawatts of power between Oregon and Southern California. This was the first such system in the United States, and it has been in operation for more than thirty years, first at +/- 400 kV (800,000 volts line-to-line) and currently at +/- 500 kV (1,000,000 volts line-to-line). The Pacific Ocean is utilized as the ground return.

Since the 10EE actually uses its brake for such a short time, essentially from the instant the operator commands the machine to stop (or reverse) the spindle motor, until the instant the spindle motor has been stopped (or has been reversed), I wonder what the advantages, if any, of true regenerative braking are, over the "dynamic" braking used in most 10EEs.

The Ward Leonard 10EEs use dynamic braking, as do the WiaD and Modular 10EEs (and most probably the first Solid State 10EEs), and this covers most 10EEs which have been produced to date.

If someone would e-mail or snail-mail me a schematic of the 10EE's regenerative brake and motor control system, I would certainly be willing to analyze that design and advise the group of any implications with respect to rotary converters.

Since the ac motor in a Ward Leonard 10EE is a conventional induction motor and not a "wound rotor" motor, this motor cannot be turned into a generator and thereby used to return power to the ac source.

Besides, such a wound rotor motor would need a separate exciter, and the Ward Leonard 10EE's exciter is driven by the ac motor itself, which, under this theory, would no longer be acting as a motor, but, instead, would be acting as a generator.

Peterh,

Thanks for your very informative comments. I lived in Oregon back then and am aware of that HVDC transmission link. High tech IS art!!! As beautiful as Palestrina's music, or Mona Lisa.

But getting back to NickG, I think the most important question that needs to be answered for his 10EE is this: assuming he does rewire for 240 VAC input, and pulls his big transformer out altogether, is he still going to have serious problems running off his 7.5 hp converter?

I guess we really need to know if his 10EE TRULY does regenerative braking. I don't know at this point. If it does, and even if he removes the transformer, is his converter (which is running perfectly by data he posted above) too small to support the regenerative braking feature?

If someone can post convincing info here that Nick's machine really does regenerative braking, I would feel comfortable in recommending to Nick that he have a converter rated at least for 12 hp, and better yet, for 15 hp, to solve his severe control problems.

bnelson