Is a constant voltage transformer necessary for electronic cabinet installations?
Is a constant voltage transformer necessary for electronic cabinet installations?
(OP)
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.






RE: Is a constant voltage transformer necessary for electronic cabinet installations?
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
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
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.
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
Those things reward careful and thorough installation with reliable operation.
Ray.
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
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
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
"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
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
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.
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
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
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
Bill
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"Why not the best?"
Jimmy Carter
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
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
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
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
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
Crap, CRAP CRAP!!!
That's the least I would bother about.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
That's funny stuff there!
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
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Is a constant voltage transformer necessary for electronic cabinet installations?
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