Delta 3 Phase and VFD problems 1

[EdDanzer]

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?

 

[DickDV]

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.

 

[jraef]

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
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[EdDanzer]

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.

 

[waross]

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

 

[mc5w]

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

 

[mc5w]

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

 

[mc5w]

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

 

[EdDanzer]

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.

 

[jraef]

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:

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
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[ozmosis]

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.

 

[thewellguy]

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.

 

[jraef]

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
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[ozmosis]

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."

 

[gcaudill]

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.

 

[DickDV]

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.