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Old March 3rd, 2017, 04:05 AM   #1
Salman S.
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Confusion regarding power supply rating for CompactLogix

I have a very amateur confusion regarding the interpretation of power supply rating for compactlogix power supplies.

Initial assumption
Any module added to the power supply will function as long as the power rating is met and its within the distance rating. So I expected the controller rating to also contribute to the power consumption.

Confusion
In the datasheet of the power supply (pg. 3) it stated that the controller converts the voltages and supplies to the modules. Given that is the case, what is the power rating on the controller referring to? The value (controller power rating) matches the current requirement of each additional module, as you can see in the attached image. So 4 modules for lets say 1769-L32E would consume 2.64A at 5V(Including controller)? Also, wouldn't the consumption vary depending on the module, so what is this generic rating used for?

Any help is greatly appreciated.

http://literature.rockwellautomation...d008_-en-p.pdf
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Old March 3rd, 2017, 03:38 PM   #2
mdim
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Maybe you can check here:
http://literature.rockwellautomation...d005_-en-p.pdf

But my advice is that you check what power supplies you need as you are talking about 1768 power supplies on pg.3 and then referring 1769 controllers.

Check also in your file down where you see 1769 power suplies
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Old March 5th, 2017, 09:19 PM   #3
Salman S.
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Sorry for the wrong reference, but I was referring to 1769 power supply. Basically I am confused what the backplane current rating on the processor means. Maybe a simple scenario will help convey my confusion.

Scenario
Lets say I have two IO modules that are rated 500mA at 5V DC, a processor with rating 0.66A at 5V DC, and a power supply with 2A capacity at 5V DC. Is my thought process correct to assume given that both IO modules are below 660mA, I only need to be concerned if the total current from both the IO modules should be less than 2A? So essentially I should be able to add 4 of the said hypothetical IO modules.
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Old March 5th, 2017, 09:53 PM   #4
ryangriggs
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You would take the power requirements of ALL modules (CPU included) into consideration when choosing a power supply.

If the CPU draws 660mA, and each I/O module draws 500mA, that's a total of 1.66A needed. So a 2A power supply is adequate, but you couldn't add a third I/O module drawing 500mA, as that would exceed the 2A power supply maximum rating.
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Old March 5th, 2017, 11:31 PM   #5
Salman S.
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Ryangriggs that was my initial assumption but then if you observe the image I attached in my original post, you would see its stated that the total current (no. of modules x modules current requirements) should be less than power supply capacity.

Hence, my confusion. . . .
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Old March 6th, 2017, 01:36 PM   #6
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CompactLogix Platform - Calculating Power Consumption...

It is very confusing, isn't it?

Hopefully this will clear some of it up for you...

Note: I am referring here specifically to the older but still supported 1769 Modular Controllers and in particular I am going to focus on the popular 1769-L32E. The reason I am doing so is twofold...

1. The attached screenshot in Post#1 is from...

Publication 1769-UM011-EN-P -1769 CompactLogix Controllers User Manual (Page 83)

...which refers specifically to the older 1769 L3x Modular Controllers.

2. You have also specifically mentioned as an example a Controller with a current rating of "0.66A at 5V DC". A rating of 660 mA @ 5V DC matches only the 1769-L32E or 1769-L35E Controllers.

So moving on...

The 1769-L32E Controller has a current draw rating of 660 mA @ 5V DC and 90 mA @ 24V DC. We know that it has to able to draw that power from somewhere. That somewhere would obviously have to be the Power Supply. So, and regardless of what one might be reading or misreading, one should know that the Controller is and must be included in the total power consumption calculation.

So, all components that may draw current from the Power Supply should be included in the calculation. That means, both the Controller for Bank 1 plus any Modules which may also be added to this same Bank must be included in the power consumption calculation for the Power Supply in Bank 1. Any Modules which may be added to subsequent Banks, if they exist (Bank 2 or 3) must be included in the calculations for the Power Supply in those Banks.

The general rule is that no matter which Power Supply is used we are only allowed a maximum of 2.0 A @ 5V DC on either side of the Power Supply and 1.0 A @ 24V DC. Let's say we are using a 1769-PA2 which provides 2.0 A (2000 mA) @ 5V DC and 0.8 A (800 mA) @ 24V DC.

The Controller must reside in Slot 0 so it is always on the left side of the Power Supply from the perspective of looking in from the front. The 1769-L32E will automatically consume 660 mA @ 5V DC and 90 mA @ 24V DC from the left side of the Power Supply. This leaves us with a remaining 1340 mA @ 5V DC and 710 mA @ 24V DC. This is before any other Modules have been added.

As the Controller must reside in Slot 0, and it has a distance rating of no more than 4 Slots from the Power Supply; you can only add a maximum of 3 Modules between the Controller and the Power Supply on the left side. So a maximum of 3 Modules may consume some or all of remaining 1340 mA @ 5V DC. Let's say we add 2 x 1769-IQ32T @ 170 mA each @ 5V DC and 1 x 1769-OB32T @ 220 mA @ 5V DC. This has now consumed a further 560 mA @ 5V DC, leaving a remaining 780 mA @ 5V DC at the Power Supply. The total current consumption on the left side is now 1220 mA which is well under the permissible 2000 mA (2.0 A). As no more Modules can be added to the left side of the Power Supply the only way that the remaining 780 mA @ 5V DC may be consumed is by adding Modules to the right side of the Power Supply. If the remaining current is adequate enough a maximum of 8 Modules may be added to the right side of the Power Supply.

Note: These Modules do not draw any current @ 24V DC. It is only the 5V DC that we are concerned with here.

Regardless of documentation, the above is a given. So now let's look at how you are supposed to decipher the confusing information shown on Page 83...

On Page 83, or in Post#1, we can see from the page title that the table is supposed to guide the user on how to "Calculate System Power Consumption". "System" should infer that all components that may draw current from the Power Supply be included here. But they are in fact not. The table is actually only referring to Controller current consumption. This is the first piece of misleading information.

Next, the title for the table itself says...

Table 19 - I/O Module Power Consumption Calculation Table

This is the second piece of misleading information. The table is not referring, at all, to Discrete I/O, Analog I/O or Speciality Modules. Again, it is only referring to Controller current consumption.

Within the columns of the table we see references to "Number of Modules" and "Module Current Requirements". Again, these are misleading or confusing descriptions. The only "Module" they are referring to here is the Controller itself. If we use their somewhat confusing terminology of "Module" for a Controller and then fill in the table we will see that for the "Number of Modules" they are actually referring to the "Number of Controllers" that will be used in the System. This, of course, can only be 1 Controller in Slot 0 for the 1769 Modular Systems. So the table is completed by filling in...

Number of Modules: 1 (beside 1769-L32E)

Calculated Current:
at 5V DC: 660 mA
at 24V DC: 90 mA

Total at 5V DC: 660 mA
Total at 24V DC: 90 mA

So now, having completed this very confusing but somewhat pointless table, we have apparently calculated the "System Power Consumption", even though we haven't even mentioned an actual Module yet.

The current ratings shown in that table are just the Controller specific current requirements of which you can find in...

Publication 1769-TD005K-EN-P - CompactLogix Controllers Specifications (Page 54)

They have nothing to do with the current consumption of I/O Modules. You do not add the number of Discrete I/O, Analog I/O or Speciality Modules to this table using the ratings shown. The modules have their own specific ratings which you can look up in...

Publication 1769-TD006_-EN-P - 1769 Compact I/O Modules Specifications

The current draw ratings in that document, for the intended Modules, should be used along with the Controller rating to calculate the total "System" power consumption.

What they have failed to provide here is a clear opening description of what is to be included in the power consumption calculation and then the means to add the actual Modules after the Controller in the table so that one can calculate the actual current draw for the "System".

More to come...
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Old March 6th, 2017, 01:36 PM   #7
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Continued...

If we now move forward and look at the same information provided for the somewhat newer CompactLogix 5370 Controllers. Specifically the 5370 L3 Controllers as these use the same form factor as the older 1769-L3x Controllers...

I refer you to to the earlier linked document again but now at Page 8...

Publication 1769-TD005K-EN-P - CompactLogix Controllers Specifications (Page 8)

Here you will see the Controller current draw ratings for the Standard 5370 L3x Controllers - 500 mA @ 5V DC and 225 mA @ 24V DC. The L3 Safety Controllers consume more but we won't concern ourselves with those here.

So knowing the current draw is the same for all of the Standard 5370 L3x Controllers I will now refer you to the User Manual and Page 239...

Publication 1769-UM021_-EN-P - CompactLogix 5370 Controllers

Page 239 is the start of the section "Calculate System Power Consumption". You can read through the intro or skip down to Page 240 which covers the section "Calculate Power Consumption in Single Bank". This section contains Table 29, which is the equivalent to Table 19 for the older Controllers covered in the older manual.

Here you will see a marked difference in the clarity of information provided...

Firstly, that heading is specific to a "Single Bank" which means Bank 1.

Next, the "Important" note states...

Quote:
One bank requires the CompactLogix 5370 L3 controllers to reside in the leftmost slot. At minimum, you must calculate the power consumption of the controller on the left side of the power supply.

If more modules are installed on the left side of the power supply, you must calculate the power consumption for those modules as well.

If more modules are installed to the right of the power supply, you must calculate the power consumption for that side separately.
This now specifically states what you have been wondering and I have been outlining above.

Next, the first column in the table denotes the "Side of Power Supply" we are referring to with "Left - Required" beside the listed Controllers because you must have one Controller present here. They also have the "1" filled for the Controller under "Number of Modules" as this again is a requirement and because they all now share the same rating. For the older L3x Controllers you had to decide which controller to put the "1" beside because they had different ratings. They also have the ratings filled in for the rest of the columns for the Controller. The only gripe here is they are still referring to a Controller as a "Module" in this table. This is not ideal and may still lead to confusion for some.

Moving on, the next column for the Side is "Left - Optional" beside " I/O Module-specific". This now points out to you that you can optionally add more Modules to the left side (3 max.) and that you must find out the "Module-specific" rating for each one so as to add them to the required and fixed Controller rating of 500 mA @ 5V DC.

They then go on to the "Right" Side column and inform you that you may have up to 8 Modules to the right of the Power Supply and that their ratings are also "Module-specific" and so you will have to look them up and enter them into your list.

This table is now much clearer in what needs to be entered to calculate the total power consumption for the "System". They also point out the important exceptions at the bottom.

Somebody must have realised that the old documentation needed a "little" more clarity and I think they did a pretty good job at bringing it along. Just drop that "Module" reference for the controllers and it may just be perfect?

After teasing all that out I will say this. The RA Integrated Architecture Builder (IAB) is an invaluable tool when trying to configure these systems as it does all this calculating for you, and more. I would advise users to try it. Even if you are not building complicated systems it can make life a bit simpler and also provide a better visual understanding of what it is you are building. It's not always perfect but it is helpful.

Regards,
George





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Old March 6th, 2017, 09:31 PM   #8
Salman S.
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Thanks Geospark your explanation was excellent! I really appreciate you taking the time to write such an elaborate explanation.

I feel such a time-consuming explanation warrants a long response but you really went through and through any doubt I would've had.
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Old March 7th, 2017, 05:07 PM   #9
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Thanks Geospark for the detailed explenation
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