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Delta 3 Phase and VFD problems

Delta 3 Phase and VFD problems

Delta 3 Phase and VFD problems

(OP)
We have had problems with VFD’s and spindle drives with our 240V Delta three phase. The new Baldor drive required Y three phase only. Searching only provided thread237-106308
Our incoming power has a wild leg that is 210V to ground.
Some systems work, Fanuc, and Haas, but not Allen Bradley, or Benshaw.
Any Ideas?

RE: Delta 3 Phase and VFD problems

Floating delta sourced power will always be a problem for VFD's because, to properly protect the motor and motor leads from ground faults, the drive has to create a virtual ground if an actual ground isn't provided thru the source power.  As soon as it does this, power supply imbalance to ground and phase-to-ground leakage currents create conflicts which result in unacceptable nuisance faults, usually ground faults.

The solution is either to provide an isolation transformer with grounded wye secondary (preferred) or turn off all of the drive protections for ground and short-circuit faults and then hope the leakage currents aren't larger than the drive input fuses.  If they are larger than the fuses, your only remaining choice is the transformer.

Aren't floating delta supplies just wonderful!!  Just so you can keep running with a ground fault!  I will oppose those systems as long as I am able, if for no other reason than the hazards they present to operating and maintenance personnel.

RE: Delta 3 Phase and VFD problems

"We have had problems..."
What kind of problems?

"240V Delta three phase..."
What are your voltage measurements? This denotes only the "nominal" voltage.

"The new Baldor drive required Y three phase only..."
What series of Baldor drive? I see that the H2 Series recommends having a balanced voltage with respect to ground, but does not technically "require" it. If they did, they would sell very few 240V VFDs in the US, because 240V Wye does not even exist here. Did you actually contact Baldor and ask them? Sometimes they put this stuff into manuals just to cover the rare problems, not the general use. BTW, I have never seen this issue with Allen Bradley nor Benshaw either.

You may have some other pertinent problems with your incoming line, such as a severe voltage imbalance, but just being  a Delta system is not the sole cause. 240V "red-leg" Delta as it appears you have, is a low cost way of having 3 phase main power with a small amount of 120V for lighting and such by connecting to a center tap on one winding of the transformer. That is why you read 210V to ground on one leg, it is the one NOT common to that center tap. The down side with this arrangement is that those 1 phase 120V loads represent an immediate imbalance in current, which creates an imbalance in voltage. The original intent, that of a "small amount" of 1 phase loads tapped off, is often lost on the 2nd, 3rd or 4th owner/tenant and they keep adding on to the system without realizing the consequences. One day, your 3 phase voltage imbalance is so bad that your 3 phase equipment starts giving you trouble.

Bottom line, more investigation is warranted.

JRaef.com
Eng-Tips: Help for your job, not for your homework  Read FAQ731-376 pirate

RE: Delta 3 Phase and VFD problems

(OP)
Jraef,
We burnt up 2 Allen Bradley spindle drives and one Benshaw VFD This has been very expensive for a small company.
The incoming power is from our power company in rural western Washington.
The between leg voltage is 240V and we do use the other 2 legs for 120V and 240V single phase. There is a neutral provide for the 120V and 240V single phase. The power company has monitored the system as have we and there are no imbalances that should cause problems.

We did install an isolation transformer on the new Baldor drive, but the Fanuc and Haas do not have an isolation transformer do not seem affected. We may have had a similar problem with another VMC that the dealer and factory could not make work correctly, so they took the machine back.
Are there internal design differences that can make one system work and the other not?

RE: Delta 3 Phase and VFD problems

EdDanzer,

Consider what happens to the voltage of the DC bus as each rectifier diode (or thyristor - I guess diode for this application) conducts: when the device conducts, that pole of the DC bus is connected to the line voltage less a diode forward volt-drop. On a rectifier fed from a balanced Y system the DC bus voltage is roughly split in half relative to ground, and this value doesn't vary enormously as one device commutates to the next. With a grounded delta the bus voltage relative to ground jumps from near zero to full line voltage as the rectifiers commutate, causing a high dv/dt between the DC bus and ground.

I agree with DickDV's comments regarding the G/F detection and the possibility of the detection circuitry misbehaving as a result.

----------------------------------
  Sometimes I only open my mouth to swap feet...

RE: Delta 3 Phase and VFD problems

I am not a VFD design expert, but it would seem if the VFD's rectifier is desinged to be fed by a balannced Y system, that is equal voltage to ground across diodes(scrs), but connected to a delta system with one center-tap would cause problems. Because the three phases to ground would read 120V,120V and 208V in a 240V/120V delta system. While the ground reference point of rectifier is looking for a balanced 120V or so input. See the attached diagram:

http://services.eng.uts.edu.au/~venkat/pe_html/ch05s1/ch05s1p1.htm

One of solutions would be to install a isolation tranforerm with Wye secondary as suggested before. I would suspect an ungrounded delta system would work too, but you need the ground the center tap for 120V.

RE: Delta 3 Phase and VFD problems

May be the diodes/scrs used in the VFDs that have no problems are rated for peak voltages of 240V nominal input and those they don't are rated for peak voltages of 120V nominal.

Does this make sense??

RE: Delta 3 Phase and VFD problems

(OP)
What I was trying to find out if there are different designs that allow one drive to work and the other fail.

Rbulsara,
The link shows one design that would not work with delta power. (thank you)

Is it possible to connect all the DC together and solve the imbalance?

RE: Delta 3 Phase and VFD problems

EdDanzer, the biggest difference might be the ratings of the diodes/SCRs.  Grounded wye would allow lower voltage ratings than would be necessary on other types of systems.

RE: Delta 3 Phase and VFD problems

eddanser:

I am afraid not. It is most likey what davidbeach and I are suspecting. You need to talk to the manufacurers to give you their reasoning.

RE: Delta 3 Phase and VFD problems

In fact all DC are tied together, it is the PIV (peak inverse voltage) that diode and SCRs will see.

RE: Delta 3 Phase and VFD problems

Huh, I for one am learning something new here. I agree that the only conceivable difference would be in the ratings of the power devices in the bridge. I know this is done by some manufacturers of Medium Voltage solid state equipment because it makes a fairly substantial difference in component cost and count, but for 230V VFDs? I had no idea that anyone was using Diodes or SCRs that were rated only for Y systems, it makes no sense other than to MAYBE save a few cents on the component costs. That would mean they are finding diodes rated for only 300PIV? Wow, now that is getting cheap!

Then again, to some bean counters those few cents may mean the world. The strange thing is that this just shifts the costs to the end user, in fact INCREASES cost substantially if they have Delta system, and as I mentioned, I have never heard of a 240V Y system in the US! 208/120 Y for certain, but not 240/138V Y. Ludicrous if you ask me.

Unless the issue is the potential voltage IMBALANCE that can occur in red leg delta system. Then again, how does one explain the fact that most 240V VFDs 3HP and under can operate on 1 phase input? You can't get any more of an imbalance that having one phase completely missing.

I do agree with DickDVs thoughts on the GF issue however. I have seen this a lot. We have quite a few 480V ungrounded Delta systems in the US, this has always been problematic with respect to GF tripping.

JRaef.com
Eng-Tips: Help for your job, not for your homework  Read FAQ731-376 pirate

RE: Delta 3 Phase and VFD problems

jraef:

GF tripping would not burn up drives...or would it? A higher than rated voltage would, I think. Trust me, I do not pretend to be an expert in VFD manufacturing.

As for bin counters...lets not get started...

RE: Delta 3 Phase and VFD problems

jraef, I do almost no 230V 3 ph work so I may have been making some useless assumptions on the type of  source.



Are you saying that 230V 3ph power in North America is almost always delta source, ie, floating with no neutral?

I'm learning some things in this post too.

RE: Delta 3 Phase and VFD problems

jraef:

Loss of a phase, for correctly sized devices in a rectifier, will not cause them to get damaged either. They may or may not work but will not burn up.

RE: Delta 3 Phase and VFD problems

Most common 230-240 V 3 phase is 4 wire in the US.  Much of this is supplied by an open, delta secondary transformer bank.  (In our area it is open wye/open delta supplied by transmission lines with 2 phase conductors and a neutral).  

There may be some corner grounded 230V delta and some ungrounded 230V delta somewhere, but not common like the 230V 3phase 4 wire center tapped delta.  Many power companies are replacing these as transmission lines are upgraded and all 3 phases are available for a wye transformer bank.   Our REA power company recently upgraded the line adding the 3rd phase, but they just moved all the residential and single phase on to the new phase.  Ok by me, I'm not ready for a shakeup, voltage wise.

RE: Delta 3 Phase and VFD problems

rbulsara,
I didn't think GF would necessarily damage the drive, I was just agreeing that if can be a problem, i.e. nuisance tripping. EdDanzer never said the Baldor drives burned up, in fact he never said what his problems were, just that they didn't work. He did say that AB and Benshaw VFDs burned up though, and that bothers me. I completely agree that if properly sized, the semiconductor devices should be fine under these circumstances, but the point you and ScottyUK raised is that maybe some manufacturers are skimping on components and subjecting users to this kind of risk. I have  installed and commissioned a lot of 240V drives on Delta systems before, this has never been a problem. Admittedly though, I can say I have never done so with a Baldor, AB or Benshaw drive, so maybe there is something to this! Still conjecture for all of us at this point however.

DickDV, ccjersey's post answers your question quite well, that is my experience too. I know that overseas there is 380Y220 or 400Y230 or 415Y240, but here we have either 480Y277 or 208Y120. 240V is always delta in one way or another, the most common being the type with a spit winding and a grounded neutral to provide 120V. Here is a transformer connection book. Figures 11 and 15 are the most common connections for 240V 3 phase systems.

EdDanzer,
My experience having lived in Western Washington for 12 years is that there is a lot of the Open Delta version (figure 15) being used in legacy systems. It works fine, until, as I said, the user connects too much 120V load, then it gets severely unbalanced. It could very well be that the day your Utility came out to measure, you were not using as much as other times. It could also be as you suspect, some drives may not be capable of being hooked up without that grounded Y secondary transformer in front of them.

JRaef.com
Eng-Tips: Help for your job, not for your homework  Read FAQ731-376 pirate

RE: Delta 3 Phase and VFD problems

(OP)
The incoming service was installed in 1977. 2 of the phases will provide 120V to neutral, these two legs will provide 240V single phase. The wild leg is 210-215 VAC to ground or neutral. These 120V legs are fairly well balanced.
We monitored the power for about 5 days several years ago, and adjusted the 120V balance, and continue to monitor it when changing machinery mix or other loads.
Both the AB and Benshaw drives blew diodes and IGBT’s on the input or front side.
I should be getting a call from Benshaw with a more complete explanation later this week.
What I don’t understand is how converting the AC to DC should care about voltage to ground. This may change the DC buss voltage some?
Several years ago I designed and built 3ea. 100 amp DC power supply for some servo amplifiers and did not have voltage imbalance problems.

RE: Delta 3 Phase and VFD problems

Had it been designed around a 240V nominal voltage level, you are correct, it shouldn't really matter. Why it would "care" is if the PIV (Peak Inverse Voltage) rating, the semiconductor device's ability to block voltage, was selected based upon their reliance on there being a Y system, where the peak voltage that can be seen by the device is the Phase-to-Neutral voltage, not Phase-to-Phase. PIV ratings typically need to be 2.5 times the nominal line voltage. So in a 240 Ph-Ph voltage environment (delta), that is 240 x 2.5 = 600V, a common rating for diodes. If they designed it because they thought it would be a lower voltage to ground reference in a Y configuration, i.e. 120V or 138V, then the diodes they selected  may be unable to survive the peaks that could happen on that high leg. To be honest it doesn't make sense for them to do that, but it might explain why they insist on a grounded Y system. Again, just conjecture at this point. I have put in messages to some of my sources in VFD design to get their take on this, including someone I know at LS (formerly LG) in Korea, the people who make the Benshaw inverter.

An unbalanced voltage however is a problem in that the difference between phases can increase the ripple on the DC bus and if not able to be absorbed by the capacitors, can cause stress and possibly damage to the IGBTs. By the way, the balance between the 120V loads is somewhat irrelevant, the culprit would be the ratio of 1 phase to 3 phase loading on the transformers.

JRaef.com
Eng-Tips: Help for your job, not for your homework  Read FAQ731-376 pirate

RE: Delta 3 Phase and VFD problems

(OP)
One observation is both the AB and Benshaw units do not have very large capacitors.
How would you check the ration between phases?

RE: Delta 3 Phase and VFD problems

It seems to me that, with phase-to-phase voltage imbalance, in addition to causing ripple problems on the DC bus, you also are causing an imbalance in the loading of the various input rectifiers--the two on the highest voltage phase pair would be conducting longer and at higher current level in order to keep the DC bus pumped up to the highest peak value of the incoming phases.

This could, I suppose, cause unequal heating among the rectifiers and failure, especially if the heat sink was designed with little or no excess cooling capacity.

RE: Delta 3 Phase and VFD problems

Certainly when something is designed on the margins, it can only survive in the best of environments.

EdDanzer, what size VFDs are we talking about here, i.e. 3HP and under, or larger than 3HP? Also Series number of the drives? The reason I ask is that both companies sell a wide range of drive offerings and the Series may have very different topologies between them. I heard back from my freind at LS in Korea, and he says that their drives do NOT stipulate that the incoming line is Y or Delta, they don't really care, which means they are using devices rated as high as necessary in case it is a delta supply. He also pointed out that LS is not the only supplier of VFDs to Benshaw, so the ones you tried may have been different.

JRaef.com
Eng-Tips: Help for your job, not for your homework  Read FAQ731-376 pirate

RE: Delta 3 Phase and VFD problems

(OP)
This drive is for a 40hp machine tool spindle. The motor has been rewound as it megged marginal before the Benshaw drive was installed but after the AB units burnt up.

RE: Delta 3 Phase and VFD problems

Hello jraef;
I found your comments on single phase loading on open delta systems interesting. I grew up in western Canada, and the open delta systems that I remember were installed for large single phase loads and small 3phase loads. The most common was a single large 3 phase pump in a rural or agricultural area. It was common to see a 50KVA or 75 KVA transformer paired with a &.5 KVA, 10 KVA or a 15 KVA to supply the 3 phase.
I still see legacy full delta systems, but for 40 years or so, wye connections have been used here for larger systems.
Different strokes eh?
respectfully

RE: Delta 3 Phase and VFD problems

At a plant where ai worked in Akron, Ohio we had a number of variable frequency drives that operated off of 240 volts corner grounded, which sa even more voltage imbalance to ground than 4-wire delta. These never had problem because the 30 amp branch circuits buffered the commutation notching. These ran 3 horsepower motors.

When operating a drive off an asymetrically grounded 3-phase system such as corner grounded or 4-wire delta, the extra ripple that shows up on the direct current buses really only affects how much power line frequency current flows through the motor winding capacitance to ground and the capacitance of the wiring between the drive and the motor. This current can also couple to the rotor and then damage the bearings. Usually, you need to worry more about zero sequence current at the carrier frequency of the drive because this goes right through winding and wiring capacitance. You may need an inductance between the drive and the motor to decouple the capacitance from the drive even if the power source is solidly grounded wye. Also, XHHW-2 and EPR (RHW-2) wire insulation has less capacitance to ground than THWN and in fact a VFD circuit that is running more than 10 feet using THWN wire will ot work when wet. Water increases the capacitance of allegedly waterproof insulation because the water acts as a big capacitor plate that is in intimate contact with the surface of the insulation. The thinner THWN insulation really acts up when wet - essentially less waterproof than XHHW.

What you also need is an inductance between the branch circuit breaker and the drive, a drive isolating transformer which also acts as an inductance, or a unversal harmonic filter or a long enough branch circuit that the resistance alleviates commutation notching. As each input diode switches off the incoming diode creates a momentary short circuit because the outgoing diode does not trun off instantaeously. If you do not have an inductance, resistance, or unversal harmonic filter in each branch circuit this momentary short circuit chews up the diodes. It is also know now that when 2 3-phase rectifiers both operate from the same bus with no decoupling inductance or resistance the voltage uncertainty from 21 rectifier's commutation cn cause the other rectifier to fail to commutate, resulting in a blown drive.

You can get a unversal harmonic filter from Mirus International that provides both the input decoupling inductance and correct the harmonic current distortion to that of an 18 pulse rectifier which helps a lot on weak power systems. They also make a version of this filter that also does single phase to 3 phase conversion so as to operate 3-phase input drives off of single phase power.

Mike Cole

RE: Delta 3 Phase and VFD problems

jraef,

132Y230 volts is a standard drive isolation transformer secondary here is the U.S. Depending on how you connect the primary taps you can get 138Y240 or 127Y220 out of it. A standard 12-lead standby generator can also be wired to produce this voltage. You cannot get utility supplied 138Y240 but you can get it as a separately derived system where the customer supplies the transformer or generator.

Actually, UNGROUNDED 3-wire 3-phase has more than its fair share of motor and power electronics damage due to silent or invisible lighting during rainstorms. I have experience with both 277Y480 solidly grounded and 480 volts ungrounded and there is no comparison in the rate of motor damage. I also worked at a small factory complex that had 6 480 volt systems, 1 solidly grounded, 3 definitely ungrounded, 1 where the grounding method was unknown, and 1 padmount transformer that was no longer connected to a service. All of the 277Y480 volt solidly gtounded systems had NO surge arrestors and had zero lighting damage to motors, drives, and soft starts.

I have also figured out that there is a 200 volt to 800 volt gap between the 30 minute voltage withstand rating of a motor and the clamping level of surge arrestors that are UL listed for ungrounded and resistance grounded systems. This means that the surge arrestor WILL NOT protect the motors from a 30 minute Saint Elmo's Fire incident unless bleeder resistors are connected phase to ground. The recommendation of an old Audel electrical book was to connect 1 megOhm 5 Watt resistors from each phase to ground. What you can get are 240,000 Ohm 3 Watt metal film resistors that have a 750 volt peak rating. You would connect these in series strings as follows:

3 resistors 240 volts ungrounded
4 resistors 480 volts ungrounded
5 resistors 600 volts ungrounded

The peak voltage rating of these series strings should coordinate with most surge arrestors.

By the way, when a motor controller is in the off state, the motor circuit downstream of the controller is UNGROUNDED! This means that if you are using PVC conduit, unshielded tray cable or UF cable or service entrance cable or whatever, the motor could suffer static electricity damage during rainstorms if bleeder resistors are not connected to the output of the motor controller and a surge arrestor installed at the motor terminals. This explains how a spare pump motor in a sewage plant can blow up the first of second time that it is needed! Also, I would not depend on the antikick diodes in a variable frequency drives to pass output static charge buildup back to a grounded power source. Theoretically, the drive does pass statice electricity back to a grounded source when energized but would you really want the antikick diodes to be doing that?

Mike Cole

RE: Delta 3 Phase and VFD problems

I forgot to mention that the 30 minute voltage withstand rating of a 480 volt motor is from Baldor's compact disk catalog. The 30 minute rating is 1,000 volts RMS alternating current. The 1/2 cycle withstand is 3,000 volts RMS and the 1 second withstand is 2,500 volts RMS. This corresponds to 1,400 volts DC for 30 minutes provided that the DC response of the insulation is reasonable. What I mean by reasonable is that the DC rating of a plastic film capacitor is usually very different ( nonlinear ) from the AC rating because the capacitor manufacturer ususally provides a conservative rating of the capacitor that is not alinear function.

For an MCG model 160MXT-480D arrestor the clamping level at the surge protector is 2,000 volts and at the end of the 2 foot connecting cable is 2,050 volts. This means that static electricity can cook the motor insulation unless there are bleeder resistors. For that matter, static electricity can cook the surge protector unless bleeder resistors are installed.

Mike Cole

RE: Delta 3 Phase and VFD problems

(OP)
To update everyone, Benshaw will be warranting the drive and have stated it should not be used with delta three phase.
It may be prudent to check any drive for delta compatability.

RE: Delta 3 Phase and VFD problems

EdDanzer,
That is very interesting. As a result of this thread, I did an informal scan of 230V drive manuals I have laying around. Of them all, ONLY the Baldor manual makes reference to recommending use on grounded Y systems. Nobody else mentions it at all. Those small Benshaw drives are actually made by LS Industrial (used to be LG, the big Korean electronics mfr.), and of the 3 other brands that drive is available under, not one of them mentions this issue in their manuals. I also had a long relationship with Teco, and posed that question to one of my inside engineering contacts, they didn't actually seem to understand the question! I had to explain it in detail, then they came back only with the old "It hasn't come up as an issue before" response.

Maybe they don't know? I wonder if the concept of a 230V pure delta system is so foreign to them that they never considered it in their design. If the devices were selected so as to not matter, then it has never been an issue, i.e. the Teco response.

I did find this in an ABB manual however:

Quote:

Procedure
1. On IT (ungrounded) systems and corner grounded TN systems, disconnect the internal EMC filter by removing the screw at EMC. For 3-phase U-type drives (with type code ACS150-03U-), the screw at EMC is already removed at the
factory and replaced by a plastic screw.
WARNING! If a drive whose EMC filter is not disconnected is installed on an IT system [an ungrounded power system or a high resistance-grounded (over 30 ohms) power system], the system will be connected to earth potential through the EMC filter capacitors of the drive. This may cause danger or damage the drive. If a drive whose EMC filter is not disconnected is installed on a corner grounded TN system, the drive will be damaged.

So another "maybe" is that this is only an issue for drives with built-in EMC filters, those without them don't care and those with them who have never thought of the issue don't realize it (yet).

JRaef.com
"Engineers like to solve problems.  If there are no problems handily available, they will create their own problems."   Scott Adams  
For the best use of Eng-Tips, please click here -> FAQ731-376

RE: Delta 3 Phase and VFD problems

This is to do with the grounding of the Y capacitor network both within the drive and also within the EMC filter. Drives with an EMC fitted will either need the filter removing or disabling on an IT network. Drives with no EMC filter will still have, sometimes, a Y capacitor network internally to provide some level of filtering. Where this is possible, then the star point needs disconnecting. Siemens Micromaster drives (MM4) have a similar arrangement to the ABB drives whereby the Y cap network can be disconnected.
It is often good practice to fit an output choke when this is done to minimise the effects of a ground fault if this were to occur.

RE: Delta 3 Phase and VFD problems

I have used the ABB asc-550 series drives on deta systems without a problem.  The manual does address the issue and you have to wiring it alittle different(remove 2 ground screws).  Other than than that they have being working well.

RE: Delta 3 Phase and VFD problems

Thanks sed2, that explains a lot. So it appears to be mostly an issue with the filters and some drive manufacturers (like Siemens and ABB) provide the ability to modify for ungrounded systems and others (like Teco) don't have built-in filters so it's not an issue for them either.  But the ones who have filters and no means to modify (as is apparently the case with Baldor and maybe Benshaw) can't be used on ungrounded delta systems. Interesting. You would think that this issue would be better explained.

JRaef.com
"Engineers like to solve problems.  If there are no problems handily available, they will create their own problems."   Scott Adams  
For the best use of Eng-Tips, please click here -> FAQ731-376

RE: Delta 3 Phase and VFD problems

jraef
not all drives have a grounded internal Y cap and so in theory may not fall into this problem area. I notice the new Danfoss drives FC102 have a statement:
"Do not connect 400 V frequency converters with RFI-filters to mains supplies with a voltage between phase and earth of more than 440 V. For IT mains and delta earth (grounded leg), mains voltage may exceed 440 V between phase and earth."

RE: Delta 3 Phase and VFD problems

For what it's worth, I have used Baldor, ABB, AB, and Delta drives in a facility with both 230V and 460V, delta connected, ungrounded substations.  I have used the said drives on both voltages.  I have not had any drive failures related to the delta connected substation nor have I had to make any special preparations for the drives.  

RE: Delta 3 Phase and VFD problems

gcaudill, as an ABB application specialist, you're just plain lucky.  Probably your power supply has had the good fortune of staying reasonably balanced with very little leakage to ground from any one phase.

I doubt that any of the drives you mention would be happy on a corner grounded source or a floating source with significant, balanced leakage to ground.

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