Automation Control Power

Your PLC will still likely require DC power that is reasonably regulated. Thus you will now require two supplies instead of one. You can probably get I/O modules that instead switch and control line voltage (230 V AC 50 HZ) instead of a different AC voltage much more easily. This will mean only one supply and probably even easier to access to contactor, etc.

I generally use 24 VDC for inputs as they generally require minimal power and DC sensors are the most commonly available However for solenoid controlled valves and other higher wattage outputs I use 115 VAC as in the US this is the most common primary control voltage. In these cases I use relay or AC output modules on my PLC.

Good answer! You beat me to it. If speed is a factor, AC is not the answer.

The saying goes.."penny wise, pound foolish" . Did anyone come up with a total cost saving? Is that number credible against the overall project cost?

The probable millions of DC supplied systems in world came about from practical implementation over a long time [from before there was Google], and by some very bright engineers. I don't see real advantages in changing to AC.....

24VAC not so common. Never heard its been used as far as my experience. But good idea if you want to have your product a exclusive after sales & services and quite a smart marketing strategy, but could also be worst option, customer knowing this, might not consider to purchase your product if finding spares is troublesome.

I came from sales, service and maintenance company and they did also like that in the case of simple I/O modules and controllers are individualized and sell by different modules not available on industrial types of products on the market. The more the parts, the more to replace in the future and they were smart doing like this, pitfall to customers and using public. When I can thought of replacing the whole controller with simple industrial PLC, I/O modules and vfd for ease of the customer. But perhaps business is such like that- they all went crazy for the money.

We use a large quantity of equipment with 24V AC & DC PLC systems.

While the initial cost is acceptable, the maintenance issues and costs as well as excessive downtime cost associated with 24V field mounted devices is extremely high where exposed to moisture, rain, dust, chemicals, and vibration.

The 24V systems work well in a clean, well controlled environment however the voltage level is not high enough to overcome the resistance introduced by oxidation, dust, and/or low contact pressure in the external devices providing critical feedback to the PLC.

The most reliable equipment I have ever worked on throughout my career are the 120V and 48V systems whether AC or DC.

Bell Laboratories developed the 48V "Bell Standard" after extensive field and lab tests that revealed the conduction issues in circuitry powered at less than 48V. (While tedious, it is well worth reading.)

Based on personal experience I would not use a 24V PLC system for controlling any process unless the process is located in a clean, well maintained environment.

Agreed... we used to hate 24 volt systems in a wash down food industry application. Water would get in... pull the voltage level low enough that the I/O devices would not function properly yet not blow the fuses.

When we got stuck with low voltage DC and AC systems... we routinley changed the I/O modules and field devices to 120 volt AC. When water got in... it blew the fuses making trouble shooting much easier.

Suggesting to someone in NZ to use 120VAC controls is a little less than sensitive to the fact that not everyone does things the way we do in North America... It could take him a month or more to get a replacement 120V coil for a starter there!

Beside the slight difference in timing using 24VAC controls and the fact that you will still need a DC power supply for your PLC anyway, there are other issues. Many 24V high density I/O systems are DC only, because they use solid state outputs because those are now selectable between sink or source in order to be flexible around the world, but that then makes it so that they CANNOT be used on 24VAC. So you will find yourself limited on your selection of I/O modules that you must use if you switch to 24VAC, in fact in some cases your outputs must be relays.

Where in this thread is that suggestion made?

Safety in the workplace should be a concern that must also be incorporated in the operations as well as automation of equipment.

Working with AC controlled and powered devices will most likely increase the leakage currents in the work area(s), both via capacitive and inductive couplings... Leakage currents that may go beyond acceptable safety limits for any operators / persons present in the work area. Having said that, it would be then advisable to use a more expensive "low Leakage" isolation transformers designed for your applications...

Good luck

If you all of your installed machinery is currently 24VDC, changing to any other AC voltage incurs a cost penalty because you then have hold both AC and DC spares. Any savings made will be wiped out. You will need to train maintenance staff on both systems rather than one. Your time would be better spent rationalizing the design of new machines so that all new and existing machines use a common set of components. By standardizing on one PLC, much of the software for a new application can be cut and pasted from existing applications saving both time, cost and risk (because you know that the sub routine you are pasting in is fully proven).

Thanks to all who responded.

There may be little that comes of this, but we are getting pressed to find any worthwhile economies that we can, our competitors certainly seem able to find them. We are going to do some cost study on this over the next few weeks.

Our equipment goes onto appliance factories in many places around the world, and typically it is our customers that specify the brands of equipment that we must use and spares are sourced locally where possible. We have learned to work with whatever brand of PLC the customer wishes - AB, Siemens and GE have been the last 3 projects.

I think that we need to stick to 24V as a safe working voltage (ELV here in NZ), but wouldn't have many issues technical issues with using AC where possible if the savings made it worthwhile. By keeping to this low control voltage, we can segregate the power cabinets (460VAC, 380VAC ...) from the controls cabinets (24V max) with power-off-to-unlock access to the power cabinets and just-open-the-door access for the controls cabinets. We have used 115VAC for control of some safety contactors that needed these ac-operated electronic interfaces for PLC operation, but this wiring was kept in the power cabinet.

The timing jitter on switching may be a cause for concern. Thanks for the heads-up.

I wonder too, if there are not further delays introduced by using relay-output modules as opposed to solid-state?

Cheers,

Mark aka Paulusgnome

Depending on environmental conditions, using open contact relays may not be suitable due to possible contacts arcing. Which will be a non-issue when using solid state relays..