I'm was sorking on my ladder....and I lite up!!!

Ok let me explain this other issue.

There are 4 DC drives, powered on 220VAC. These drives are currently set in moition by the PLC with two isolated relays in series with a trim pot that limites the speed. The classical thing. Those drives are getting old and I only have one in back up. Eventually I'll have to got with standard AC. As the PLC is about to be changed I might get this settled right away.

Thsese drives rotation FWD and REV can also be controles by a isolated +/- 10 VDC. As I have available a 1746-NO4V I could use it to activate these 4 drives.

It clearely states in the manual that the DC voltage should be isolated.

The 1746-NO4V can also be hooked to a seperate power supply to avoid overloading the PLC's power supply.

Hopefully I provided enough information this far for my question.

If I have all those drives controled by the same card and all working at the same time, are they still isolated? Isolated for me is that the output of the ps is floating with no reference to the neutral or ground, am I correct?

Thanks.

Yes and no. Depending on how the card works, all the drives can be isolated as a group, but not necessarily isolated from one another.

Where this causes problems is say that I have multiple 4-20mA current sources being read by a non-isolated input card (very common with AB processors). An isolated input card would use a device such as an optocoupler to separate the input signals. An optocoupler is nothing more than an LED driven by the input signal which is almost directly connected optically to a phototransistor. The two circuits are electrically isolated by the optical "bridge" in the optocoupler. A common input card design then uses a single analog-to-digital converter with a multiplexer connected to the inputs. A non-isolated card doesn't have the optocoupler and just connects the inputs directly to the multiplexer.

If I have 3-wire current loop transmitters and they have different ground references, I can have severe trouble when I try to connect both inputs to a single card in a non-isolated situation. Each input by itself might work fine. But when both are connected, the differential voltage across the inputs of the multiplexer can "bleed through" (it's a solid state device) and cause it to be impossible to get a reading on the other input because the differences in their grounds (usually an AC voltage) causes elevated voltage readings in the analog-to-digital converter that cause it to go out of range or have a severe error.

The same thing can happen in reverse with non-isolated outputs. If this really becomes a problem, you can buy signal isolators. Some sources are Omega (www.omega.com) and Action IO (www.actionio.com). But, if your drives are all fed from a common power supply and working off a common ground (and the ground is common to the IO card), chances are that none of this will happen and you will be fine even without isolated inputs. Because of the low input density and extremely overprices on Allen Bradley isolated IO cards, I usually just buy a bunch of isolators for a given project and then use them only if I need them.

Hello,

The four drives are in the same enclosure, with the same 15Kw 120/220v transformer from a 600v source. And all share the same ground.

I've checked a more recent oven that uses an 1746-NIO4I to control two Power Flex 70. The speed is controlled by the analog output but the rotation direction is done by two OW16 relay outputs each.

I will check with my local distributor. KB electronics Penta-Power drives (KBRG-225D). In the manual there is a warning: Do not common multiple drives without a signal isolator. What I understand from this is that I cannot hook two or more drives with the same output, all drives in parallel. As I have four outputs and four drives all would be driven separately. KB sells an isolator: SI-4X.

Thank you Paul

I spoke with my local distributor.

It is recommended to place an isolator for each drives. It comes out to be about $115 CND.

Thanks for the help.