how to size a transformer?

[tingyu]

Hi,
I need to size a 600v, 120/240v three phase dry type transformer for some 120v loads and 240v loads.
I have the wattage and current rating information for all the loads.
How could I calculate the KVA rating for the transformer?
I dont think it is just simply the KVA sum of the all the loads.
Could anyone help me?

tingyu

 

[davidbeach]

Why wouldn't it be the sum of the load kVA values? kVA values add just fine. You will need to watch the amount of 120V loading though and check it against the manufacturer's limits on 120V loading. Your local electrical code may have more to say about transformer loading.

 

[tingyu]

Thank you very much David!
tingyu

 

[Electic]

usually such transformers are rated on basis of heat rejection. That means you can simply add single and three phase loads as a first step.

Then to make sure you don't burn up an internal jumper, add each phase load, convert to amps and make sure you are within the transformer's capability.

Gary

 

[itsmoked]

He has the WATTAGE... He needs the K V A....
Not the same thing! What is the rule of thumb for the tranny calcs;

wattage x ? => transformer KVA?

What are the loads comprized of tingyu?

 

[tinfoil]

1) as previously stated, you need the kVA to figure out the required loading:

kVA = kW / power factor

2) Is EVERY load going to be used simultaneously? Are ANY loads only going to be of very short duration? You may be able to take advantage of load diversity to reduce the required size of your unit.

 

[cflatters]

Just summation the real and reactive power components of the loads should enable the total kW and kVAr (and therefore kVA and PF) to be calculated.

ie S = P + jQ summated for each load

Its a doddle in excel.

The kVA's can only be summated if their power factors are similiar.

 

[Electic]

The question appears to be "how 1 phase and 3 phase loads are added", not a power factor question.

KVA single and three phase loads may be added to determine net load on transformer's cooling capability. Max current also needs to be checked to ensure adequate capacity.

Simply summing in VA (rather than watts) is routine procedure and specified by our local authorities.

Interesting how some people make this very complicated including phase angles etc.

 

[EEJaime]

Tingyu,
The critical element of this type of system, 120/240V, 3ph, is the quantity of single phase, 120V loads. I presume you are using 3-single phase transformers, configured in a delta bank, with a center tapped transformer to provide the 120V power? If you sum all the three phase loads, (kVa), that will give you the minimum size of the three phase transformer bank, (so if your total three phase load is 150kVa, you will have 3-50kVa transformers. Then you need to sum the 120V loads because these will only be on one of the transformers. You take the sum of the 120V loads in kVa, add them to one of the transformers, say you had 20kVa of 120v. load, you will have as your service transformer bank, 2-50kVa and one 75kVa transformer wired in a delta configuration with the 75kVa transformer center tapped to provide your 120V power.

May I ask what your application is? This is an older type of installation which many utilities are moving away from. If it is an internal industrial type application, I understand, but in most applications, one would recommentd a balanced 208Y/120V, 3ph/4w. installation. In any case I hope that is helpful.

 

[mc5w]

A 3-wire delta primary 4-wire delta secondary transformer is a bad configuration because more than a minor amount of neutral current creates a circulating current that burns up the transformer. What a 120 volt load on this type of transformer does is to imitate a turn-to-turn short.

What utilites use nowdays is 3-wire WYE primary 4-wire delta secondary. You still have a neutral imbalance limit because the center tapped secondary acts like a small tertiary winding, but not as severe as mentioned above.

A better transformer for your application would be a 120Y208 volt secondary transformer and then use booster transformer to create 138Y240 volt bransch circuits for those loads that need 240 volts rather than 208 volts. You can use 120 volt by 16 volt buck-boost transformers or use General Electric's packaged 120 by 138 volt autotransformer. The circuit breakers up to 100 amps are cheaper and you can balance the 120 volt load across all 3 phase.

If you have load that genuinely need 240 volts 3-phase and 120/240 volts single phase, why don't you use 2 transformers and panelboards? This may be the most economical solution. Single phase load centers are very economical just as long as you avoid the crummy makes. Single phase lighting would be isolated from some of the voltage drop from starting of 240 volt motors.