Cost of intelligent MCC's vs hard-wiring

J

Thread Starter

John Scarborough

I was after comments regarding the cost difference between using intelligent MCC's versus hardwiring to the PLC.

I am specifically using a Siemens S7-400 system but am interested in comments about other brands too.

Does the reduced cost of cable and labour compare with the increased costs of fieldbus type devices?

The increased functionality of these devices is not an issue. The issue is lowest possible cost (as usual).
 
The hard wired PLC controlled system is cheaper.
Here are some pros and cons.

Intelligent MCC usually use a slow speed signal bus. If you are going to control a few VFD the response is very slow and I personnely prefer using a direct 4-20 ma from PLC than the bus.
You do away with the intelligent modules which reduces the cost to somewhere $600 to $800 per cubicle. But you loose many information from the MCC that could be tapped into your Computer.
MCC is some thing that people do not touch once well installed. ie do not bother until anual maintenance.

Non intelligent MCC are also easy to repair, modify,upgrade and cheaper. You can get many vendors to quote for a type tested MCC whereas Intelligent MCC is specific to a few manufactures. You can always use a vertical section in a normal MCC and put your PLC there.

Best regards,
Sekar
 
B

Bob Peterson

I have run the numbers a couple of times using devicenet. it always ends up that even accounting for reduced wiring costs, the end result is that the devicenet solution is more costly.

Bob Peterson
 
J
The number of signals between a PLC and a MCC varies greatly. It can be as little as just one (run/stop), or it can be dozens if you have different speed settings, interlocks, fault feedback etc. For hardwiring more signals means more wires. I.e. the more signals the more networking makes sense.

Reduced wiring is not the only saving and benefit of fieldbuses. Long term operational and maintenance benefits include the capability remotely reconfigure, diagnose and completely operate the drive over the network. This simplifies many tasks and it makes it possible to proactively avoid some failures of motors etc. by monitoring temperatures, currents and torque etc.

Take a look at the book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" (buy online in hardcopy or download immediately in softcopy), particularly chapter 2 for the benefits. Mind the line wrap, make sure to get the complete path all the way to the 6: http://www.isa.org/Template.cfm?Section=Shop_ISA&Template=/Ecommerce/Product Display.cfm&ProductID=3036

You can even download chapter 1 for free in softcopy form. Mind the line wrap, make sure to get the complete path all the way to the 5: http://www.isa.org/Template.cfm?Section=Shop_ISA&template=/Ecommerce/Product Display.cfm&ProductID=4585

Jonas Berge
==================
[email protected]
www.smar.com
 
Jonas Berge said --

"Reduced wiring is not the only saving and benefit of fieldbuses. Long term operational and maintenance benefits include the capability remotely reconfigure, diagnose and completely operate the drive over the network. This simplifies many tasks and it makes it possible to proactively avoid some failures of motors etc. by monitoring temperatures, currents and torque etc." Jonas Berge ================= [email protected]

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I wanted to add some more to this topic. I have installed both. I have used profibus DP (on deltaV and Devicenet network on AB500 series) Neither had the ability to store/backup parameters files. What other options are there? I read recently of a SquareD Altivar with Ethernet and a Web Page for config. I don't know if I can Upload/Downlaod with thier web app, SquareD, please comment on this...

Main reasons to use hardwired are: Ability to easily swap spares among manufacturers. Easy Debug for non network savvy electricians. Lower Cost.

I have found the same issues arise with Solenoid Valve Stacks which are Network Connected. Usually you pay double the cost of the I/O to put it on the network, and add complexity to overcome during startup and comissioning. Remeber, if you have to start it up, and the data isn't needed for process requirments, K.I.S.S.

S. Landau
 
S

ScienceOfficer

Bob---

I'll agree that I have regularly encountered systems that were overall less expensive to do with home-run wiring than with DeviceNet. I'll also agree that simple motor starters upgraded with fieldbus electronics can seriously drive up the price of an MCC.

That said, MCCs in my experience generally turn out to be cost-effective with a fieldbus if the CPU is external to the MCC. Just a few drives with analog control can make the economic benefit dramatic.

I'm not doubting your experience, but mine is different. I routinely propose multiple architectures to my clients so they can make this decision for themselves. In our organization, I don't get the credit if my fieldbus stuff goes out in an MCC, and my colleague that does is getting a lot of wins these days.

I win if you are right, and I'm telling you ruefully that I am actually losing!

Hope this helps!

Larry Lawver Rexel / Central Florida
 
J
Your case clearly illustrates that to benefit from the networking the host computer must have an engineering tool that allows the user to take advantage of the network capabilities. E.g. in the case of Profibus-DP your master is most likely only class 1, pretty much only permitting the desired speed to be communicated to the drive. I.e. no access to drive parameters. To do driver parameterization you also need a master class 2. To me the only viable Profibus class 2 master is the Siemens PDM tool thanks to the support of EDDL (Electronic Device Description Language), the file that explains the drive parameters to the host computer software. The drive must be supplied with EDDL file, just GSD file is not enough for useful interoperability. DDL makes that tool interoperable with lots of DP and PA devices. I.e. it takes more than a Profibus-DP module in the PLC to get the full benefit of the networking. At the moment there are actually three forms of host interoperability for Profibus supported by different hosts and devices: Profile (supported by E-H host), EDDL (supported by Siemens host in an extended manner), and FDT/DTM (supported by ABB host). Combining the host and drive may therefore not be entirely simple. I do not have much experience with DeviceNet, but I believe they use a device support file similar only to the Profibus GSD file, i.e. only basic communication settings, not a complete explanation of all parameters. This also means that access to all device parameters from the host computer tool may not be possible.

The web technology solution should guarantee that you can access all parameters online provided the device is connected. However, most likely you will not be able to do anything before the device is connected. I.e. I suspect there may know way to prepare a configuration in advance at the engineering stage, and download it later during commissioning.

So what is the ideal bus? I think FOUNDATION(tm) Fieldbus has the best solution for interoperability among devices and with the host computer.
It uses only DDL as host interoperability mechanism, supported by all hosts and all devices. Unfortunately no variable speed drive yet supports FOUNDATION(tm) Fieldbus.

Jonas Berge
==================
[email protected]
www.smar.com
 
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Bob Peterson

I tend to agree that if you have a lot of drives it can push the equation in the direction of a bus based system. I find that it is usually cost effective (or close to it) to use the RIO adaptors for AB VFDs IF there are a couple analog points involved. The only thing you have to be mindful of is that RIO racks get used up pretty quickly this way.

We almost always put the I/O in the MCC cabinet, and prewire it in our shop. This is a fairer comparison than one where you are routing hundreds of wires from a remote I/O panel.

Bob Peterson
 
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