Is a constant voltage transformer necessary for electronic cabinet installations? 1

[bdn2004]

On a PLC installation they are specifying a constant voltage transformer 120VAC/120VAC ahead of the 24VDC power supply for the PLC, the scale electronics it serves, the HDMI, and a receptacle. Is this standard practice to put these in? Aren't DC power supplies regulated anyway? It adds heat and costs to the job...I just question the purpose.

 

[itsmoked]

I cannot imagine that being needed with a modern 90~260V switching supply making the 24Vdc. Constant voltage transformers are indeed expensive, and generate a lot of waste heat.

I could see an isolation transformer if the scale is idiotically sensitive and the "receptacle" has an odd usage.

Sounds like a call to whoever wrote that requirement is in order.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rtronics]

What is the "scales"?
Is it say, a weighbridge in the middle of an industrial site, with welders, compressors etc. scattered around.
In that case an isolating and regulating transformer may be required. What does the HDMI feed? Is there optically isolated communications?
The bare minimum of power supply may not be enough.
Ray.

 

[bdn2004]

It's a truck scale on the outskirts of the plant. The raw material from the plant is delivered into some large bins via plant piping. Trucks drive under the bins, insert a spout, and fill up based on the weight. This is an industrial site. The PLC cabinet is located in a conditioned room. The PLC input/output cards are optically isolated. I'm sure there is welding occasionally here, but not a part of operations. The HDMI will show the levels, weights etc. This whole operation is a backup plan to the main operation...so it's not a critical and won't be used that often.

 

[rtronics]

Thats what I would call a weighbridge.
Those things reward careful and thorough installation with reliable operation.
Ray.

 

[itsmoked]

I'd suggest you design it to take a constant voltage supply under a plan to not add the expense or energy waste unless it "really proves necessary". Then proceed with just a standard switcher and it will likely work fine. If supply noise has to be exceptionally low then you will need a standard regulated linear supply instead of a switching supply. No difference really except it will dissipate more heat and will need to be set to 120V input or whatever you're supplying the supply with.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[bdn2004]

So - the MCC guy is providing a 480-280Y/120V transformer with a small panelboard. This is the same panelboard that would feed a small ac unit, fluorescent lights in the small control room (15'x8') and the outputs to the solenoid controlled valves. The MCC will have two feeds into it at 480V and a manual transfer switch for maintenance and reliability. So is this constant voltage supply you suggest - is the feed from a branch breaker good enough? Or are you suggesting something between the two? This constant voltage xfmr was just to feed the PLC power supply. The PLC power supplies in themselves are quite costly I've noticed. What do get for your money if not a well regulated output?

 

[itsmoked]

So bdn2004 this is the letter/question you're asking of whoever specified the Constant Voltage Transformer?

Not sure what the question is here. I'd continue with my original suggestion. A truck scale is pretty nice to detect if it's having power supply issues. You have huge numbers on a display somewhere. That's all it does. You install everything turn it ON and look at the display. If the numbers are steady you calibrate the scales with a standard weight provided by your county or a commercial calibration service.

If there is a problem it will be readily apparent on that big display to anyone who looks at it with their eyes open. The lower digits will be dithering about and jumpy the results will be unstable. Watch the scale while someone turns ON the air conditioner and any other thing that might be 'noisy'. If things remain stable thru those load changes I wouldn't want the added expense and energy waste represented by a CVT.

Of course there is always the chance the scale unit is a complete piece of garbage sold by a rinky-dink company with no competency that hides behind ridiculous requirements like needing a CVT and when their garbage product craps-out they use the lack of something (absurd) that was specified being missing as a reason to deny a warranty claim. That's something we here at Eng-Tips can't assess for you or your client. To avoid this kind of issue you generally need 'approval' from the rinky-dink company and that kind of approval is only available while the rinky-dink company's salesman is drooling over a pending sale that won't happen unless he agrees a pointless CVT is not needed.

So that said, you should probably get approval from the scale company's tech department that you don't need no stinkin CVT or the company wanting the scale installed needs to make the warranty battle verse no CVT call themselves. Best would be the tech support waiver.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[bdn2004]

I guess I'm a little confused on your answer - which I think is to just hit it with 120VAC and be done with it. I didn't really think about the power to the truck scale electronics themselves as being an issue. The only specification we had is to install this like another similar installation on site. And that one had the constant voltage transformer 120V/120V that I was questioning the need for. I'm planning at this point to leave it out. The truck scale hasn't purchased either, I'll look to see if it requires such - it won't be a junky one.

 

[Skogsgurra]

There may be another problem; switching transients that easily kill an SPS.

"In the outskirts of the plant" is a combination of words that may indicate a possible problem. You never can tell what else will be connected to that 208/120 V, I guess there is one or two outlets as well. And I have seen such outlets being used for angle grinders, little welders and whatever. They are being run until a fuse blows or a magnetic trips and then, you have a lot of stored magnetic energy (in the transformer core) looking for somewhere to go. Any SPS nearby is the most likely victim and if the transformer energy is large, which it is when the fuse blows, then a standard SPS can/will get hurt. Been there, seen that.

If this is the reason for the CVT spec, I would not let it out. But, I do not like the additional power consumed, either. So I would either make sure that there are no outlets for any temporary tools (they will get connected non-the-less) or put in a sturdy MOV - or a CVT that is being switched in only when that backup thing is in use. That won't reduce the installation cost, but it will certainly reduce the operating cost, which over the years will be higher than the purchase cost.

There might be some concern with the longevity of the MOV. But there are good design aids that help. If the number of transients are known (very few, usually) and the energy in the transient is known (cannot be any higher than the energy stored in the transformer core) then you can find out how many years the MOV lives. Select an MOV that lives at least ten years under worst-case assumptions. That will probably result in a more than twenty years of real lifetime. And, in that time, most industrial operations are either rebuilt, ceased, hit by an atomic bomb or the Comet...



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[GroovyGuy]

Howdy bdn2004,
I would not consider a CVT unless there was a requirement to do so. They are nice to have, but they are expensive in terms of capital costs and energy losses.

For mission critical control systems (ie where things go boom), we generally will install a UPS c/w static bypass switch, and redundant parallel SPSs.

However, what is the lightning incidence level at this facility? Often a truck scale is off by itself and can be the highest structure in the area, often by means of a local building or local lighting poles or bins. If this is the case you might want to consider a TVSS for the facility; at least for the controls etc. From what I recall lightning and load-cells are not compatible.

 

[itsmoked]

I guess I'm a little confused on your answer

So there we're even!

I see. I thought the scale people were demanding the CVT but it's just the facility historic similarity.

Both Skog's and Guy's inputs are valid too.

I'd probably run the 120 out there with an outlet for the inevitable drill and for the scale. Plug a filtered spike suppressor powerstrip into it and power your scale thru that. Block off or mount the powerstrip so those pesky power drills don't get plugged into it and proceed.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The cost of wasted energy to run the constant voltage has been mentioned a few times. In an air conditioned space, multiply that by two. You pay to waste the heat and then pay again to remove the waste heat with the air conditioner.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Yes, that's right.

And it costs a lot more to remove heat (low efficiency) than to create heat. I tried to find a reliable number for cooling efficiency and came up with so different answers that I think that there's a lot more to it than I ever understood.

Anyone got a good rule-of-thumb?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

The efficiency depends also on the difference between the condensing temperature and the evaporation temperature.
It was a long, long time ago and was in regards to heat pump efficiencies.
With a set point of 70 degrees F, a heat pump would deliver three times as many BTUs per kW as electric heating with an outside ambient of 50 degrees F.
When the ambient dropped to 32 degrees F, the BTUs per kW was equal to electric heating.
These figures may not be accurate, but the fact of a wide range of BTUs per kW remains.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bdn2004]

I met the Plant today. Apparently as someone had mentioned above (rtronics) ....years ago when the scale was installed, they asked us to mimic, it was a requirement to put this CVT in. It's a Toledo scale. I was told the idea was that the load cells are so sensitive that any variance in the incoming power could perhaps give a false reading. This project is to extend a local scale nearby - I had thought we were getting a new one. We will likely change out the main controller and more load cells. We meet the Vendor next week. I'll follow up with his recommendations in this post. Thanks for all the good information.

 

[Skogsgurra]

"the load cells are so sensitive that any variance in the incoming power could perhaps give a false reading"

Crap, CRAP CRAP!!!

That's the least I would bother about.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

That's funny bdn2004 that they seem to think fluctuations in the line-voltage could have the slightest affect in the scale readings. That would only matter if they used some ancient non-regulated power supply fed directly from line voltage. I can't imagine anyone doing that in the last 30~35 years.

That's funny stuff there!

And it costs a lot more to remove heat (low efficiency) than to create heat.

That is not correct. That's what the whole COP is about. That's Coefficient Of Performance. Most refrigeration systems are designed to run with a COP of between 2 and 5. A COP of 5 means you can move 5 times the heat energy as you consume doing it, whereas a resistor has a COP of only 1.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

That is true for heating. But is it true for cooling? I once heard that it takes around 3 kw cooling to remove 1 kW losses from an electrical room.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

COP is bidirectional and it doesn't matter whether it is bringing heat or taking heat from a space. I would say your numbers are way off these days. Back a few decades ago they could well have had COPs of 0.4 and such.


COP is also a function of the delta and so is definitely affected by the local outside air temperature. You can have a very badly matched system that puts it in a very poor situation that would screw up the COP but 0.33 is not really a possibility for a nominally functional system these days.

Re: Home type heatpumps. Since the inside temp lives in a very narrow temp range (65~75F) but the outside can range wildly like -40~120F a household HPump will see some very bad deltas that reduce the heat pump's COP down to nearly 1 but you still get the energy you used to try to move heat into the heating space. They often just give up and switch to electric 'supplemental heat'. Because of this HPs are contradicted in locals that drop their COPs very much as being a waste of capital because they my have a poor payback. You wouldn't ever want a HP in, say, Minnesota.

Keith Cress
kcress -

http://www.flaminsystems.com