Transformer Question

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Power Transformers | Medium, Large and Industrial Power ...

Contact a supplier of large transformers and ask their technical staff. They want to sell you one so they have a vested interest in getting you one of sufficient size. Remember they want to sell you one so get all the information they have and can provide. You have no obligation to buy from them. If possible contact several suppliers.

Another possibility is to contact your power supplier. Some will assist you but many will not. It is worth a phone call to ask.

Otherwise do as #1 has indicated.

Good Luck, Old Salt

Following IEEE Std C57.96-1999 ch. 6. Basis for calculations of temperatures, loss of life, and loads Equivalent load of a different load and duration may be calculated using formula (3):

LEQ = [(L1^2*t1 + L2^2* t 2 + … + Ln^2* t n)/(t1+ t2 + … + tn)]^0.5

and if will be only one start per hour and no pause then the transformer has to be 253 kVA.

if will be 4 starts per hour and no pause then 300 kVA will be ok.

However, you must check the voltage drop [taking into consideration

the minimum voltage permissible at secondary terminal [usually 0.95 of rated].

The ambient temperature and the elevation have to be considered.

As stated in #2, but, I am sure you have a "regular" source for electrical components which you purchase, get your monies worth from them and give them your requirements and let them do the leg work for you. Why spend your time doing something you can get for no cost to you. If you want to check to see if the price you are going to pay is fair, once you have the model, etc., of what will be supplied you should be able to cross reference to other manufacturers.

It would help if we knew the starting method.

With closed transition Y∆ we started 800HP blowers on 800KVA dedicated 11/.433KV transformers daily.

DOL 800HP crushers needed something a bit bigger. It was pure chance a couple of 4000KVA 11/3.3KV transformers had been decommissioned on another plant so we pinched them.

400kVA or bigger depending on what future loading you may need to account for, what is readily available and the budget of course.

400kVA for that motor is plenty.

if i choose the 400kVA transformer, the secondary current will be 556 Amp. As i state before the current for starting the motor is 3 x full load current = 1047Amp. How the transformer can run the motor if i choose 400kVA transformer? any calculation to prove it?

It's not going to pull 1047A continuously.

Transformers can be overloaded for short times with no ill effect or even longer times (possibly continuously) if the resultant life shortening is acceptable. It's a time and temperature issue when it all comes down to it.

To get formulae or tables then you will have to do your own searching or contact your friendly neighbourhood transformer vendor.

If you are uber paranoid then start the motor with a VFD or soft starter. Then you can "calculate" the starting current waveform harmonic effects...

The public supply grid has many elements that are routinely overloaded to maintain continuity of service.

The maximum voltage drop at start -usually for 200 kW and up-has to be equal or less than 10%-if the starting torque has to be not less than rated torque. In this case, if the transformer is dedicated for this motor supply only then 500 kVA transformer will be suitable [6% short-circuit voltage] or 300 kVA [if short-circuit voltage will be 4%].

A very rough Rule of Thumb is 1KVA per Hp.,and is allowed for certain NEC calculations.

This will put you in the ball park, then other variables must be considered, as mentioned elsewhere, but this will give a good starting point.

As always, consult the transformer manufacturer before purchasing, and let them hold the liability ball.If you spec it, you are responsible if you get it wrong.

Good Luck

Is the transformer to be used only for running a motor?In my experience when ever I plan for installing /purchase of a new transformer the capacity is decided considering the anticipated additional loading expected in the near future. Ambient temperature of the area where you want to install the transformer is another factor to be considered. As per IEEE standards a 24 hour average temperature of 30 ° C is considered as base for transformer loading calculations . The 300 kVA capacity as recommended in earlier replies appears to be sufficient . I have used 250 kVA transformer for pump house motors 240 kVA motor loads. The only difference was it was a combination load of two motors 120 kVA and both runs only for peak hour requirements. So you have to decide depending on your ambient temperature and loading pattern and anticipated future requirement in the same location.

yup...only to run the motor...anyone can show the calculation??

Industry STANDARD KVA ratings 3 phase are 225,300,500---

225 also will do but why not 300?

Here's the source of an excell calculator with formulae.

Download the spread sheet here.

According to this, 400kVA is ok if your locked rotor multiplier is only 3. That would be a type A motor code. What's the plate say?

Permissible/desireable voltage drop can make a difference too. Just the one motor hanging off this so a bigger dip should be fine for the short duration if it is not a code A motor.

If it is a code A motor then the voltage dip is <1%....

500kVA is better but it will cost you more (not a little) and you may not really need it.

The 400kVA transformer will not fail catastrophically if the locked rotor current is higher than you thought and you don't start and stop the motor like a kid playing with a light switch. If the transformer has time to cool it will be fine and in any event there should be thermal protection on that box of metal.

Have a play with the spreadsheet. It's explained therein. Do more reading too. Make a decision.

My invoice for finding this is in the post. You can send the spreadsheet's author a thank you note.

What does this motor drive. Loading what from start? Or no load at start?

Transformer need to know!

Assume locked rotor for starting.

Define LOCKED !

Why plan for unreal?

Max torque.

Nothing unreal about a motor starting from a stationary state.

Any mechanical loads coupled to the motor that also need to be accelerated will not result in a higher peak starting current (locked rotor current) it will prolong the starting current duration and that will need to be considered if the motor starts very frequently.

<will prolong the starting current duration and that will need to be considered if the motor starts very frequently.>

Wal you said IT ALL.

That is why OP should define - even now.

Question needs to be answered -- and settled!

locked rotor 738%

And What is the Loading from Start to Full Speed?

You can describe -if not quantify.