MicroLogix1500 v/current monitoring

[mto ind]

Hi all, great forum. I've been programming PLCs for a number of years but it has always been straight forward digital I/O nothing fancy or complicated. I've got a project where I need to monitor the voltage and amperage for 12, single phase motors (4-120vac, 8-220vac and range from 4-12amps) and output the readings to a Panel View 550. I have a MicroLogix 1500 and a several SLC 5/02 - 04s but have not decided which I will use since this is a rather small project and we have no budget and I don't have an Net AIC yet but by answering my question might help me determine which one to use.
My question: are there any I/O modules out there for any of these PLCs that I'm not seeing in their catalog for directly taking voltage or current readings... or what would be the easiest and cheapest way to obtain these readings so I can port them to my PV550.
Any advice would be great

 

[jstolaruk]

For current and voltage sensing, check out page 665 in the McMaster-Carr catalog. They have some din rail mount sensors with 0-5Vdc outputs that could go into a SLC Analog input card. I'm using them in a upcoming project. They're inexpensive ($200) and readily available (customer requirements). I'm not sure of their accuracy but for us anything was close enough.

 

[mto ind]

Thanks for the post. This should do the trick. I was just hoping maybe there was an all-in-one I/O card out there

 

[Lancie1]

mto ind,

I do not know of any PLC modules for current or voltage monitoring.

For the voltage readings, I would use one 0-to-10 vdc analog input module, with potential transformers and rectifiers to get the voltage at each motor down to the 0-10 vdc range. Then use PLC Outputs to switch each voltage input in a timed sequence (1 every 2 seconds should be fast enough) to the analog input, read and save it in the PLC for tramsittal to the PV550.

For the current readings, you cannot easily switch the currents rapidly without problems. You don't want to leave a current transformer with an "open" circuit, so it would require switching in a shorting resistor before you switch your PLC input off. It is probably not worth the trouble, so you probably will need 12 analog current inputs, and a current transformer and rectifier for each motor to get its current down to a 4-to-20 miliamps dc level.

Doing it with a PLC will take a lot of extra components and wiring. A better option might be a power monitoring system, such as the Square-D POWERLOGIC power monitor.

 

[jstolaruk]

Thanks for the post. This should do the trick. I was just hoping maybe there was an all-in-one I/O card out there

The analog input cards can be set for voltage or current but the levels are low. And you want some isolation from those motor circuits.

 

[rsdoran]

I dont understand why you would need to monitor voltage, that overall should be a constant. Using 120vac inputs the voltage could be a discrete input....ie POWER OK
This can not be low budget if you need current readings for 12 motors, that means 12 analog input channels, if you do voltage for each motor then you are talking 24 analog inputs.

They also make current sensing relays that can give a discrete signal if current goes too high or falls too low etc
http://www.crmagnetics.com/newprod/ProductView.asp?ProdName=CR4395

1746-NI16I 16 point analog module is approx $1000 (actually more)
Cuurent transformers with +/- 4-20ma 12@ $200 each $2400

If you want to keep it low budget then forget using analog, put voltage in as a discrete signal. Use something like the CRMagnetic sensing relays to send a discrete signal for current.

 

[leitmotif]

OK so you have 12 single phase motors each drawing an assumed max 12 amp.
That puts these in ballpark of $200 per motor. Current sensing will run $100 per motor - ballpark. Kind of expensive at first glance.

SO the question is this
WHY do you want or need to monitor current on these small motors
AND is current the best parameter ie if a pump current will work but a pressure switch would probably be better indication pump is pumping and flow ideal (with a hefty price tag).

CTs have advanced since the simple ones with 5 amp secondary. Now you can get them
1. in a relay version SPDT and selectable for over or under current.
2. voltage output take your pick
3. 4 to 20 mA output - at least that is what the one in my hand says.-- made by Veris Industries Portland Or 1 800 354 8556

Dan Bentler

 

[mto ind]

I agree voltage monitoring is worthless and I will probably get my boss to drop it due to pricing, but he won't budge on the currents switches.
This is what we are trying to accomplish. This system is part conveyor part monorail on an existing system that we are combining. The production Engineer who has no idea on how any of this stuff works has convinced managment to incorporate a real time predictive maint section in the panel view, monitoring current levels on all the motors is a small portion that. These motors are geared down to achieve the correct speed so single phase and no VFDs is what I have to work with. There is much more I will be monitoring but I pretty much have that covered. Since I am just the electronic tech in Maint I don't have much say so in the matter I just have to make it work, believe me I've voiced my opinion on all of this. Thanks again on all of your input.

 

[leitmotif]

By monorail are you meaning overhead conveyers on a single overhead track?

Current monitoring may have a point ie jams and other mechanical problems. From what I have seen of these systems the motors are the least of the problems, most problems are mechanical, worn rollers, collectors misalign with the bus work, etc etc.

Dan Bentler

 

[Tom Jenkins]

I have had good luck with amp transmitters from NK Technologies http://www.nktechnologies.com/

 

[Andybr]

If this is a new project you might consider using intelligent overloads in your MCC. The AB E3 Plus units are excellent, sit on DeviceNet and provide current readings as well as all sorts of other data.