Sizing Transformer

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excited_electrone

excited_electrone

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Electrical E.I.T
Hello all,

I am estimating a transformer for a package of 3 motors, it's a hydraulic unit, two 250Hp & one 60Hp, rated at 460V. Incoming line is 4160V. The application is a hoist operation.

Now, referring to table 44, C22.1-18, current ratings for three phase ac motors, I have got total load of 302A*2+77A*1 = 681A at 460V. So, calculating kVA demand for this load is = 681*1.732*460 = 542,566 VA. Now, allowing 1.5 multiplication factor for the hoist operation, I was told by seniors that hoist operations are different than normal cases so we use 1.5-1.7 multiplication factor for estimating, I come to 814kVA. And I sized transformer for 750kVA, which might get changed!

So, my question is what about the inrush current? What things I have to take into consideration while sizing a transformer with inrush current available?

Thank you!
 
If the motors are on VFDs there is no real inrush to worry about. The inrush is all very low power factor and the transformer will never see it for the most part because it is all on the load side of the VFD.

The other issue you may face is most POCOs have a rule about how large a motor can be started across the line.

If you are starting across the line, you can calculate what the voltage drop will be when you start up one or more motors. You know the impedance of the transformer and thus can calculate how much VD will happen when you pull starting load. You don't want the VD to cause the starters to drop out on low voltage. For modern high efficiency motors the startup current can be close to 10X the run current.

Personally, if I was sizing an operation like this with ATL starting I would probably make it whatever standard size transformer is about 2X the normal FLC of all the motors for estimating purposes. It is just a best guess and does not need to be perfect at the estimate stage.
 
excited_electrone said:
Hello all,

I am estimating a transformer for a package of 3 motors, it's a hydraulic unit, two 250Hp & one 60Hp, rated at 460V. Incoming line is 4160V. The application is a hoist operation.

Now, referring to table 44, C22.1-18, current ratings for three phase ac motors, I have got total load of 302A*2+77A*1 = 681A at 460V. So, calculating kVA demand for this load is = 681*1.732*460 = 542,566 VA. Now, allowing 1.5 multiplication factor for the hoist operation, I was told by seniors that hoist operations are different than normal cases so we use 1.5-1.7 multiplication factor for estimating, I come to 814kVA. And I sized transformer for 750kVA, which might get changed!

So, my question is what about the inrush current? What things I have to take into consideration while sizing a transformer with inrush current available?

Thank you!
Click to expand...
Are you talking about inrush current from energizing the transformer or the motor loads?

Sent from my BE2028 using Tapatalk
 
petersonra said:
If the motors are on VFDs there is no real inrush to worry about. The inrush is all very low power factor and the transformer will never see it for the most part because it is all on the load side of the VFD.
Click to expand...
Yea, that makes sense!
petersonra said:
Personally, if I was sizing an operation like this with ATL starting I would probably make it whatever standard size transformer is about 2X the normal FLC of all the motors for estimating purposes. It is just a best guess and does not need to be perfect at the estimate stage.
Click to expand...
Thank you for your advise!
petersonra said:
The other issue you may face is most POCOs have a rule about how large a motor can be started across the line.
Click to expand...
There is no Poco involved as it is running from a genset!

So, I am just worried about the inrush current only, suppose it's ATL, as it is going to be 10(approx) times higher than FLA, how would transformer manage to keep up with that? Let's say, 750kVA transformer gives around 1200A on 480V side, inrush might be around 381*10A. I know I am missing something, may be it's a basic but it is really irritating that I can't able to digest this!

Please educate me there!

Thank you.
 
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excited_electrone said:
So, I am just worried about the inrush current only, suppose it's ATL, as it is going to be 10(approx) times higher than FLA, how would transformer manage to keep up with that? Let's say, 750kVA transformer gives around 1200A on 480V side, inrush might be around 381*10A. I know I am missing something, may be it's a basic but it is really irritating that I can't able to digest this!

Please educate me there!

Thank you.
Click to expand...

Transformers, similar to other devices, can supply more than their rating momentarily. Think of the kVA rating as what it can supply continuously. It can go higher than that momentarily when the motor is starting, and you'll see the voltage drop because of it, but the transformer will generally be ok to supply a good bit more than its rating during the motor starting.

Was that your question?

Side note: 750kVA @480V is ~900A
 
bwat said:
Transformers, similar to other devices, can supply more than their rating momentarily. Think of the kVA rating as what it can supply continuously. It can go higher than that momentarily when the motor is starting, and you'll see the voltage drop because of it, but the transformer will generally be ok to supply a good bit more than its rating during the motor starting.

Was that your question?

Side note: 750kVA @480V is ~900A
Click to expand...

Yea, 902A it is! I don't know how I came up with 1200A, must be calculated for a 1000kVA :unsure:,1202A!

But, yea you explained it what I asked for! Do you know any theory/book where I can refer this problem? Want to dig more...

Thank you everyone!
 
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