Books on Calculating Transformer Loads etc...

[dustin23]

Hello,
I was wondering, I need to increase the size of a transformer/power supply in a enclosure. I am adding a 120Vac, 150Watt enclosure heater. The transformer that is there is for VFD circuit control. Its rated at 150 VA. I dont think this is big enough to handle its job along with the new fan heater. Is there a book that i can buy or some such that would help me with this. The primary would be 480 single phase with of course 120 secondary. sorry i didnt see something like this during my search. Im an electronic engineering technologies trying to learn all th EE stuff for my job. Any help would be greatly appreciated. Thank you.

 

[DanDel]

A 150W resistance heater would add 150VA to your load, so, yes, the existing 150VA transfomer would be severely overloaded.

Why not just add another unit dedicated to the heater, with some breathing room, at least 150VA x 125% = 187.5VA? Just use a 200 or 250VA unit.

Or replace the existing unit with a 350-400VA unit, if you like.

 

[dustin23]

The enclusures are not big enough to keep adding dedicated PS's to the mix. I am also trying to think ahead alittle as well. I was going with the replace existing one with a bigger one to handle the 150 VA already there, plus my fan heater. I didnt mention it but the heater is mounted on top of a 120 vac fan. The fan is rated at 120/.10 amps. I was worried that the inrush from the fan would ruin my asumtion of just adding 150 VA to the existing 150 VA.

 

[DanDel]

The inrush is no problem for the transformer, but possibly for the transformer fuses. You may have to use slower operating fuses if there is a problem.

 

[dustin23]

Im not trying to be anal. How would you calculate the enrush for this application. I keep finding tables for transformer picking that show the kva and inrush characteristics. Sometimes the Kva is enough but the inrush is too high so you have to pick a higher VA. Thanks for your help DanDel. I realy appreciate it. From what im calculating it looks like i can go with a 350 VA transformer and be okay. Then use time-delay fuses rated by code for the transformer. thanks agian for the help.

 

[BJC]

You need to power the heater from a different source. the time you most need the heater is when the VFD is off. If the VFD is running and then shut off ( along with the heater ) condensation can occur and some electrical insulation start sucking up water vapor.

 

[waross]

If this is for condensation avoidance, you only have to stay above the dew point. Often an energised control power transformer will be enough heat.
If this is to keep the equipment above the minimum operating temperature in the arctic, then you will definitely need the heater.
respectfully