Isolation transformer.. Math!

[edmundopt]

Hello,

 I have some work to do with a 320W Switching power supply, it is a 240VAC(max) mains input.
 I want an isolation transformer to work with it, so I need a new one, I am looking at this :

 https://pt.rs-online.com/web/p/transformadores-de-seguridad/4369808/

 Wich is BLOCK TIM 500 rated at 230AC with 500VA, two output of 2x115VAC, wich I will join to make 230VAC

 Will it be near the limit ? Maybe a 800VA one will be better?
 This will be a one time only job, so, saving € is important!
 What is the math on isolation  transformers for power output ?
 

[Fsck]

Are you fully loading that PSU? And, do you have the specs of the PSU?

[edmundopt]

MeanWell SP-320-24, 24volts output,  stepper motor driver up to 8Amps, and resistive dummy load up to 10amps.

[CaptnYellowShirt]

If its any kind of modern PSU, its power factor should be really close to 1.00 (~0.95). In that case it should be drawing something very close to 320VA. 500VA should be just fine:

  320     /    0.95   /     0.92   =  366 VA
power   /    PF      /     trans eff  =  peak VA from wall

[edmundopt]

Aren't there any pratical maths for transformers?

for example, relays can switch :
75% on resistive loads
40% on inductive loads
20% on motor
10% on Incandescent bulbs

[Zero999]

Does the switched mode power supply have power factor correction? If so a 500VA unit will do, otherwise it may need to be larger, depending on the actual current drawn.

How long do you intend to operate it for? If it's just for test purposes, it may be fine for a short period of time, say under 10 minutes.

[edmundopt]

yes, it has active power factor correction.  About Meanwell, they do pretty good PS, this one is 87% or better, 0.95PF.
All of their PS beat  the specifications mentioned.

Testing, periods of 1 to 2 min, 5max.

[Zero999]

Then I think a 500VA transformer is ample.

[Fsck]

Then I think a 500VA transformer is ample.

I get: (320W / .82 psu efficiency [they spec 87%])/(0.90 PF [their specs are >0.95 for "typical"]) / (0.90 transformer efficiency [again, assuming you may have a slight dud]) = 464VA.

Meanwell's specs are incomplete, efficiency and PF appear to be specced at 230VAC and 25C ambient. I think you'll probably be fine unless one or both item don't meet their basic specs.

[megajocke]

The transformer is probably rated for output power so its efficency shouldn't matter.