Motor RTD Monitor

Hello,

I have a motor with 6 rtd's in it. What's the cheapest monitor I can slap on there to give me a contact closer, high temp alarm. Instead of having a 6 channel rtd card in my PLC. Or converters/isolators for each.

There are mutiplexing RTD scanners out there that are fairly inexpensive. They only look at one RTD at a time and cycle through them, so they are not as real-time and not as quick as something like a Motor Protection Relay, but it does the job. Here's one.
http://electrical-controls.springercontrols.com/category/nt-series-temperature-controls

A quick search turns up lots of choices:

http://www.omega.com/pptst/CN606_612.html

http://www.motortronics.com/pages/motor_rtd.html

I think using an RTD input card is going to be pretty cost effective. I did it that way all the time. Note also that winding RTDs are two per phase, with the intent that you can monitor three, one per phase, and still pick up over temp from I2R heating. Because the RTDs are wound into the motor stator the second RTD per phase is a backup in case the other one has a broken lead or such.

I'm curious as to the answers, because what you're asking for is an RTD input, converter, some HMI/display to make alarm adjustments, and relay output(s). Your PLC already has everything but the RTD inputs. I suspect the alternatives are close in cost to an RTD card, without needing external space/footprint.

Precision Digital has an indicating, 8 ch mux that will feed a panel meter the RTD signals, one at a time at a configurable cycle rate and dwell time.



Their scheme for 3 wire RTDs is shown below:



Lots of info here:
https://www.predig.com/pd138

Please let us know what you find.

Dan

if the monitor module have capability to record/recall the latest alarm which zone are fault
it's more convenient to operator/maintenance guys.

Well, I contemplated the PLC input, but the Control logix L18 uses Point-IO.

The 1734-IR2 is around $240 per card and its only 2 channel.

If I am to monitor both RTD's per phase, that's alot of IR2 cards.

If I only want contact closer on High temp, maybe one the other solutions may be better.

http://www.automationdirect.com/adc...ature_Transmitters_(RTD_Input)/DIN_Rail_Mount

Different signal condioners to convert to 4-20MA.

If you are happy with 2nd hand, IR2's can be purchased for around £40/card on ebay.

Even new, 3 x 240 = 720 which is more expensive than, for example, the omega scanner linked by Tom. However if you currently have no spare IO, then you'd need to add a DI as well to have the HI alarms recognized by the PLC.

If you do bring them into the PLC then you can monitor temperature over time and look at trends as opposed to just having a high alarm function.

Or converters/isolators for each.

Click to expand...

You can convert RTD to 4-20ma for around £50/channel. So for £300 + a 2 channel analog current input point IO module (off the top of my head around £170), and some CO relays you could roll your own.

A 1794-IR8 is an 8 point RTD monitor unit for Flex I/O, in case you didn't know. You said ControlLogix L18, I think you meant CompactLogix L18, but either way, you can use Flex I/O modules via Ethernet.

If 4 are enough put them in a wheatstone bridge, and then you only need 1 simple analog input.
if a bit in electronics, you can put them all in one line and use one opamp to light an optocoupler that will fire a digital input on PLC.
max current is 1 mA so with 6 of them the supply is 0.6 Volt.

another way is to use converters to 4-20 mA (around 5 $) and a few relays to switch channel, that way you can see each sensor apart. (for a big motor this will be important)