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LV Soft Starter Step Up Transformer

LV Soft Starter Step Up Transformer

LV Soft Starter Step Up Transformer

We have a 4160V 600HP air compressor that we need to provide power to and only a 480V source available. It was determined that a soft starter will be required in order to start the compressor. In the interest of saving money, would it be possible to utilize this configuration:

480V Source -> LV Soft Starter -> Step Up Transformer -> MV Compressor

Thank You and I appreciate the help.

RE: LV Soft Starter Step Up Transformer

Yes, done all the time. But you will have to deal with the transformer being properly designed for being back fed and the starter being sized properly because it will now include the transformer losses. Also if you use fuses on the MV side going to the motor, be sure to incorporate Blown Fuse Indication back to the control circuit of the soft starter to shut it down. Most soft starters will detect a phase loss on the output, but the magnetizing current of the transformer may fool that.

Personally, I would put the transformer on the line side and use an MV soft starter.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: LV Soft Starter Step Up Transformer

Thanks and I appreciate you responding jraef. You know, for this configuration being done all the time, I couldn't find anything on the web about it. I think I will go with the MV soft starter route. I wouldn't even know where to start in properly designing the transformer and sizing the soft starter.

RE: LV Soft Starter Step Up Transformer

Jraef, out of curiosity, how would one properly design the transformer for being back fed and size the starter? Is the worry from the motor back feeding? For the soft starter sizing, would I just calculate the transformer losses and add that on top of the whatever is required for the motor? Thanks in advance.

RE: LV Soft Starter Step Up Transformer

The issue it, at least here in the US (an I assume you are here because of the 480/4160 reference), the NEC requires that any transformer used to step up voltage, i.e. "reverse feed", must be specifically listed by the transformer mfr for that purpose. Without going into too many potentially irrelevant details, the usual issue is that, depending on how the windings are done, there can be extremely high inrush currents on the low voltage windings because they were not meant to be energized first, they were meant to have a voltage INDUCED onto them from the (originally primary) higher voltage. Transformer mfrs know all about this and will say, up front, if their design will have a problem or not. I was merely pointing out that you will need to ensure that whatever you use has been vetted for that purpose because it's now codified in the NEC.

And yes, you then need to determine the losses in the transformer because the soft starter will need to supply them. So because the transformer feeds the reactive power for the motor too, the kVA = motor kW/PF and most likely you will end up needing a 750kVA xfmr. Losses on a 750kVA dry type MV transformer could be as high as 15kW, or roughly another 30A at 480V, so you need to make sure your current rating on the soft starter is not "on the ragged edge" of the motor nameplate FLA.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: LV Soft Starter Step Up Transformer

I can't see a problem if you use a 480V delta : 4160V wye transformer designed for step-up duty, because then it is not being back-fed. You can also resistance or solidly ground the neutral and then implement some form of ground fault protection. All off the shelf step-down transformers in that size range will be 4160V delta : 480V wye which means the wye and delta backwards to how it should be which makes them not very suitable for step-up duty. So, getting one purposely built the right way is the best path to take.

You have a soft-starter so it will also soft-energize the transformer or limit the inrush current anyways. Just make sure to add the transformer losses to the soft-starter size, which probably means you have to upsize to a 700hp soft-starter.

RE: LV Soft Starter Step Up Transformer

From my understanding, the inrush issue on a back-fed transformer is primarily due to the winding configuration/positioning. The winding expected to be energized first (wouldn't necessarily be the pri but we'll assume that it is), is physically on the outside of the secondary winding. I'm sure there are other factors and that someone can provide a better explanation.

jraef; in the configuration of 480 -> soft starter -> step up -> motor, is there no issue with over-fluxing the core owing to the low frequency during initial starting? Or do all soft starters work as VFD's, keeping the V/Hz ratio constant during ramp up? Not all that up on soft starters.

RE: LV Soft Starter Step Up Transformer

Soft-starters always work at the line frequency (50 or 60 Hz) so there are no lower frequencies during starting.

RE: LV Soft Starter Step Up Transformer

Another issue with back fed transformers is that most transformers modify the turns ratio slightly so that the secondary voltage is correct while under load. When back fed this will result is a twice as great voltage reduction on the output.

RE: LV Soft Starter Step Up Transformer

Thanks Lionel.

djs - I'm under the impression that only very small transformers use this technique. I've measured the ratio of transformers from 500 KVA to 200 MVA with accurate instruments (eg TR-800) and they're often within 0.1% of calculated using nameplate data. Can anyone confirm this?

RE: LV Soft Starter Step Up Transformer

Yes, that's generally true, compensation windings are most often found at 3kVA and below, for something like this if you wanted it, you would have paid through the nose for it.

And yes, as LionelHutz said, a soft starter does not change the frequency, only VFDs do. A soft starter simply delays the gating (turn-on time) of the SCR in each phase to reduce the net RMS voltage within each 1/2 cycle, but the number of cycles in whatever is fed into it.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: LV Soft Starter Step Up Transformer

Being from outside US, I am not familiar with the Term - transformer being back fed. Can any one explain a bit? Is it step up ? Reverse feed and back feed the same? Then why it is said with delta-star, there is no back feed?
What FrozenE said is correct. Inrush will be more if inside winding is first energized. That way irrespective of the position of transformer always LV will be the first to energise. Will the cost of LV soft starter and MV soft starter the same?

RE: LV Soft Starter Step Up Transformer

Consult an expert from your local rewind shop is there a possibility of redesigning the motor winding from 4160 V to 480 V.
It may be cheaper.

Motor winding design

RE: LV Soft Starter Step Up Transformer

Yes prc.
Back fed equals reverse fed. The proposal is to use a transformer designed for step down duty for step up duty.
Also, if the chosen transformer happens to be a delta:wye transformer, it will become a wye:delta transformer with the issues unique to a wye:delta transformer.

"Why not the best?"
Jimmy Carter

RE: LV Soft Starter Step Up Transformer

What is the number of poles (or rpm & Hz) and number of slots?
A larger number of poles and slots - the greater ability for previously mentioned redesign.
Also, in many cases even an interleaved winding can be applied.


RE: LV Soft Starter Step Up Transformer

Only the people answering the question are proposing to use a step-down transformer reverse-fed or as a step-up transformer. The OP clearly posted step-up transformer in the initial question and only asked about back-fed after it was posted in a response.

RE: LV Soft Starter Step Up Transformer

There is no reason to not mention the other option if could be technically better ( avoiding the new transformer) and less expensive in practice (probably).
It would be useful to consider this possibility.

Winding Design and Motor Repair

RE: LV Soft Starter Step Up Transformer

Comparing typical frame sizes used by a couple of mfrs of 600HP 460V and 4160V motors, the 4160V (frame 5010) appears to be smaller than the 460V (frame 5810). It is close, only off by an inch in length and diameter, but I would be surprised if you could stuff the larger 460V windings into the smaller frame nonetheless. It would be an expensive experiment, especially considering the down time involved compared to just installing an off the shelf transformer and moving on.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

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