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3Phase Transformer Connection Selection

05/16/2017 7:16 PM

I know there are advantages to each but i wanted to get a consensus from some of you on a simple 3P transformer for an induction motor.

I have 2 induction motors that will run from a VFD

480 LL through a transformer then split and go to 2 VFD,s.

One for each motor.

Those are the only connections that will be made. Nothing else will come off the transformer.

I was thinking about wye wye but am concerned about 3rd harmonics.

If i do delta star or star delta then i have a phase shift to deal with.I dont see the phase shift being to big of an issue because im not trying to parelle trasnformers together. I know that can be an issue for that setup.

Motor will be solidly grounded so at the moment im thinking Delta Delta

Anyone have any suggestions or can recommend what they have commonly seen in industry?

Thanks

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#1

Re: 3Phase transformer Connection selection

05/16/2017 7:42 PM

Doesn't the VFD rectify the incoming power to DC before it inverts it back into a variable frequency AC to the motor? I might be missing something here, but I cannot see where the transformer connection that feeds the VFD would make any difference as long as the voltage is appropriate for the VFD.

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#2
In reply to #1

Re: 3Phase transformer Connection selection

05/16/2017 8:15 PM

Well that is partly what I was thinking also but was looking for confirmation

Assuming the transformer is rated for the load I don't know if it matters either?

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#5
In reply to #2

Re: 3Phase transformer Connection selection

05/17/2017 3:30 AM

The point is that no transformer is needed.

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#16
In reply to #5

Re: 3Phase transformer Connection selection

05/18/2017 12:14 PM

Unless you'd like to isolate power system grounding, perhaps

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#3

Re: 3Phase transformer Connection selection

05/16/2017 9:40 PM

As the transformers are “dedicated” to individual motors the phase angles hardly matter.

From experience a standard Dyn11 is the cheapest option.

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#4
In reply to #3

Re: 3Phase transformer Connection selection

05/16/2017 9:45 PM

But there are not multiple transformers.

Its one transformer powering two VFD,s in parelle

Im thinking that the VFD,s over isolation given that all the transformer will see is the two loads of the VFD. The VFD,s are also the same make and model.

not sure what implications this has

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#7
In reply to #4

Re: 3Phase transformer Connection selection

05/17/2017 6:03 AM

<...not sure what implications this has...> None seen from here yet. However, what is curious is that the VFDs to hand would seem to be unsuitable for the supply voltage. How that happened would be of great interest to the organisation as a learning exercise for the purposes of sharpening-up existing procedures. One might consider replacing them, as they are only for small loads, with the correct ones and eliminate the transformer altogether.

The incorrect VFDs might have value somewhere else at the facility, or even have a trade-in or secondhand sale value, perhaps.

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#6

Re: 3Phase transformer Connection selection

05/17/2017 3:35 AM

<...Motor will be solidly grounded...> Naturally - one must ensure that the installation is carried out in compliance with local electrical installation regulations.

Grounding/earthing doesn't affect the power attachment to the load; grounding/earthing is there to operate (a) circuit protection device(s) in the event of a fault, for safety of (the) individual(s) who happen to be in the vicinity of the equipment at the time of the fault.

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#8

Re: 3Phase Transformer Connection Selection

05/17/2017 9:37 AM

Most if not all your worries are superfluous.

By stating you have a 480 V 3 ph transformer where the LL voltage is 480, then it is already a Delta on the secondary side. Wye-delta configuration is pretty much industry standard for 3 ph supply to the MCC cabinets.

Thus you are coming off the MCC (circuit breakers, etc) to the VFD's, correct?

The VFD takes in the supply power and rectifies it to DC, then provides step 3 phase of PWM (pulse width modulated output) to the motor that responds to that accordingly at a speed controlled by the waveform incident upon the motor coils and the load.

As long as the VFD's in question are sufficient for each motor load separately, and the sum total of current draw from the MCC is below the circuit breaker value, it should work.

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#9
In reply to #8

Re: 3Phase Transformer Connection Selection

05/17/2017 10:48 AM

Can you please tell me what makes you assume its a delta by me saying its 480 LL?

Because i dont actually know what the source connecton is so im curious what makes you think that

Its not really from an MCC. The The feeder to the transformer will be coming from a small distribution block from a power cabinet. As far as the load of the motors go, the only information i have is that they are .5HP each 230V 3P

The VFDs are AB power flex 525

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#10
In reply to #9

Re: 3Phase Transformer Connection Selection

05/17/2017 10:54 AM

IF the LL is the same for all three phases for 480, then it is Delta.

If LL is √3 480, then it is Wye. Phase to ground in a Wye set up is 480 ( if it is the secondary).

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#11

Re: 3Phase Transformer Connection Selection

05/17/2017 1:07 PM

You do not mention what the secondary voltage is. As to your question, it will make no difference to your VFDs if they are powered from a Wye or Delta winding on the secondary side.

Biggest concern I see is how will you ground the secondary? Your choices are to use a Wye secondary and ground the middle of the Wye, use a Delta secondary and corner ground one phase or use a zig zag to create a pseudo grounding point on the delta secondary.

I would Use a Wye secondary.

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#12
In reply to #11

Re: 3Phase Transformer Connection Selection

05/17/2017 2:04 PM

230V

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#13
In reply to #12

Re: 3Phase Transformer Connection Selection

05/17/2017 2:10 PM

That will be a hard beast to find. You may be stuck with a delta/delta. Though I would still try to find a delta/wye.

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#14

Re: 3Phase Transformer Connection Selection

05/17/2017 8:50 PM

All VFDs are designed to operate on a solidly grounded Wye secondary. Period. In all industrialized countries of the world, EXCEPT the US and Canada, there is no such thing as a Delta service, they ONLY use Wye in one form or another. So the entire concept of needing to use components that are rated for a Line to Line potential is foreign to them.

Inside of the VFD there are Surge Protective Devices and Common Mode Noise suppression devices that are connected line to line, then tied to ground in a Wye configuration. If you feed a VFD with a Delta service, and there is a fault or surge on the line side, those devices attempt to become the Wye point of your Delta, for about 16 milliseconds, the time it takes for them to vaporize and coat the inside of your VFD with conductive debris. After that, the rapid decline of the rest of the components can often be spectacular, but even if not, always expensive. The same is true by the way for a resistance Grounded Wye system.

f you have a Delta or NGR Wye service, it's always recommended that you use a Delta-Wye Drive Isolation Transformer to feed your drives with a solidly grounded Wye service. You don't need to run the neutral to the VFD, 3 wires is fine, you just need the Wye point of the SOURCE to be referenced to ground.

Some drives, such as Allen Bradley, that are built for the North American market (not adapted from drives built for Europe and Asia), provide a simple way of making it POSSIBLE to use them on a Delta or NGR Wye system, by removing a jumper on the ground reference point for those SPD and CM noise filter components. That will prevent them from blowing up, but also removes much of the protection that those systems provided. In the case of A-B if you do remove the jumpers, you void the UL listing. So even thought they make it possible, they warn you not to do it. Asian drives simply tell you that they can ONLY be used on solidly grounded Wye systems, some European drives allow you to disconnect the internal ground reference points, but make it a nightmare to do so (Siemens is what I have in mind).

Since you have a choice, chose wisely, stick to a Wye secondary. You will have a hard time finding a 240/139V Wye secondary because it has no other practical use, but they are out there if you insist, specifically for this purpose. Much more common in this voltage "class" is to use a 208Y120V secondary.

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#15
In reply to #14

Re: 3Phase Transformer Connection Selection

05/18/2017 9:54 AM

JRaef...

As always, I find your contributions to this forum to be of exceptional quality. Thank you for the information concerning the internals of VFDs and the implication to them of a deltas vs a wye based supply.

I was not aware that the use of delta wound secondaries is not a common practice in other portions of the globe other than in NA. Is this knowledge based on anecdotal evidence or is their a document to that effect that can be read?

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