Sizing a control transformer?

Status
Not open for further replies.

Huntxtrm

Senior Member
Location
Cleburne
hello all,
I am trying to size a control transformer,480ac/120vac, and I have run into a snag or two. Some of the equipment I'm using does not have inrush VA in the specs. But instead, inrush amps/volts. For instance I have a 60watt power supply that is 120vac/24vdc it is listed at an maximum inrush current of 50amps @ 200vac. is should I just use 10000VA? Or, do I even worry about inrush on a powersupply? I also have some 120v relays in the mix that show only .5va sealed, but shows nothing inrush? which route should I go. I've never done one of these before, and I want to do it right, so I learn and continue on the right path. Any advice would be appreciated. Thanks
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
hello all,
I am trying to size a control transformer,480ac/120vac, and I have run into a snag or two. Some of the equipment I'm using does not have inrush VA in the specs. But instead, inrush amps/volts. For instance I have a 60watt power supply that is 120vac/24vdc it is listed at an maximum inrush current of 50amps @ 200vac. is should I just use 10000VA? Or, do I even worry about inrush on a powersupply? I also have some 120v relays in the mix that show only .5va sealed, but shows nothing inrush? which route should I go. I've never done one of these before, and I want to do it right, so I learn and continue on the right path. Any advice would be appreciated. Thanks
I think you missed seeing a decimal point on the inrush value of your 60W power supply! It's more likely to be 5.0A at worst case.

CPTs have the ability to deliver high current for a short time to handle some inrush current, that's what makes them different from general distribution transformers. Different designs provide different performance, so I find it best to actually consult the catalogs of your favorite mfrs and see how they tell you to determine the size. You will see that it varies. Some will tell you a simplistic formula of largest inrush plus total sealed, which is a safe bet, but you might end up with more transformer than you need, which is good for them, maybe not for you. Other more complex methods involve doing two calculations and picking the larger result. Here's one common version of that:

1. Add up the total sealed VA
2. Multiply that by 125%
3. Calculate the maximum inrush VA for all devices that could close at the same time.
4. Add that value to the sealed VA value of step 2,
5. Divide that value by 4

Select the transformer VA that is the larger between steps 2 and 5.

As a gross general rule, if the device has a coil but you don't know the inrush value, only the sealed, then you assume the inrush is 10x the sealed, that is the worst case scenario. That's why I think you missed a decimal point on your power supply inrush. 60W at 120V is 0.5A, inrush would be somewhere in the 5A range, not 50A, which would be 100x the rated amps!
 

Huntxtrm

Senior Member
Location
Cleburne
I think you missed seeing a decimal point on the inrush value of your 60W power supply! It's more likely to be 5.0A at worst case.

CPTs have the ability to deliver high current for a short time to handle some inrush current, that's what makes them different from general distribution transformers. Different designs provide different performance, so I find it best to actually consult the catalogs of your favorite mfrs and see how they tell you to determine the size. You will see that it varies. Some will tell you a simplistic formula of largest inrush plus total sealed, which is a safe bet, but you might end up with more transformer than you need, which is good for them, maybe not for you. Other more complex methods involve doing two calculations and picking the larger result. Here's one common version of that:

1. Add up the total sealed VA
2. Multiply that by 125%
3. Calculate the maximum inrush VA for all devices that could close at the same time.
4. Add that value to the sealed VA value of step 2,
5. Divide that value by 4

Select the transformer VA that is the larger between steps 2 and 5.

As a gross general rule, if the device has a coil but you don't know the inrush value, only the sealed, then you assume the inrush is 10x the sealed, that is the worst case scenario. That's why I think you missed a decimal point on your power supply inrush. 60W at 120V is 0.5A, inrush would be somewhere in the 5A range, not 50A, which would be 100x the rated amps!

Good info, thanks. You have taught me some valuable information. I looked at the values given, again, for the power supply.. It says "50 amps max at 200vac" must be a misprint. But, the next larger one says "70amps max at 200vac". That's why I was questioning it. Plus, I am very green at this.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Good info, thanks. You have taught me some valuable information. I looked at the values given, again, for the power supply.. It says "50 amps max at 200vac" must be a misprint. But, the next larger one says "70amps max at 200vac". That's why I was questioning it. Plus, I am very green at this.
Who is the mfr? Idec? If so, I think I might know what's going on there (and even if not Idec, it's something that happens with cheap power supplies like them). It's really a semantics issue, partly because of translation problems.

These are "Switch Mode Power Supplies" (SMPS), as are most on the market now. An SMPS has a simple transformer in front, feeding a rectifier, then the DC is fed into a DC/DC converter to drop and regulate the DC voltage to what you need. Between that rectifier and the DC/DC converter, there are capacitors that store energy for the converter, plus they smooth out the ripple in the DC that comes from the rectification of AC. Capacitors charge almost instantly, so when first energized, there is an "surge" of current to charge those caps, limited only by the available current in the line source, and the impedance of the little transformer. So what most GOOD power supply designs use is a "pre-charge circuit" that adds a resistor in series with the capacitors. The resistor slows down the rise of that current surge and limits the peak so that the capacitors are not stressed by that surge. Once that is done, the resistor is no longer needed, so on large systems there is a relay that closes to short across the resistor, on small systems they use what is called an "NTC" resistor; it has a "Negative Temperature Coefficient" of resistance meaning that when cold, the resistor has high resistance, but as it heats up, the resistance drops until it is almost not there any more. That resistor is more expensive, but saves the cost and space of the relay.

The cheaper the power supply, the less attention paid to "details" like that. So you end up with a design that has NO pre-charge circuit at all, the result of which is a very high "surge" of current when it is started initially. So in that case, 50A is possible. The result of that is that your power supply was cheaper, but will destroy itself in time. There is no free lunch.

But here's where the semantics issues comes to play. That is a cold start surge current, not really "inrush" current in the sense we normally think of it for the purposes of sizing a CPT. That's because it ONLY happens once when you FIRST apply power to the line side of the power supply, i.e. in a typical control panel only when you close the main disconnect switch. When you do that, you typically are NOT going to have anything else come on at the same time, because your power supply is likely supplying power to whatever is going to TELL that other device to come on, which it wouldn't do until AFTER that cold start surge is over with. In addition, even IF the surge causes a drop in the CPT output voltage for that instant, the voltage drop will actually HELP with the surge current problem within the power supply design. It would be similar to the effect of using an autotransformer starter for a large motor.

Bottom line, don't even factor it in to the sizing of your CPT, just use the maximum current draw at the power supply's peak output, probably about 1.6A at 120V, so 192VA.
 
Status
Not open for further replies.
Top