LV Soft Starter Step Up Transformer
LV Soft Starter Step Up Transformer
(OP)
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.
480V Source -> LV Soft Starter -> Step Up Transformer -> MV Compressor
Thank You and I appreciate the help.





RE: LV Soft Starter Step Up Transformer
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
RE: LV Soft Starter Step Up Transformer
RE: LV Soft Starter Step Up Transformer
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
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
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
RE: LV Soft Starter Step Up Transformer
RE: LV Soft Starter Step Up Transformer
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
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
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
It may be cheaper.
Motor winding design
RE: LV Soft Starter Step Up Transformer
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.
Bill
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"Why not the best?"
Jimmy Carter
RE: LV Soft Starter Step Up Transformer
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.
@ACWindings
RE: LV Soft Starter Step Up Transformer
RE: LV Soft Starter Step Up Transformer
It would be useful to consider this possibility.
Winding Design and Motor Repair
RE: LV Soft Starter Step Up Transformer
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington