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deanfran
February 15th, 2012, 10:08 AM
This is a simple one I'm sure, but I have never run across the situation before, and don't know exactly how to do this. I have a 3 phase 480 VAC motor, that I want to monitor. What I am trying to do is initiate a 24 VDC input to a Click PLC when the electric feed to the motor is shut off. This shut off would be by one of two switches or a thermal overload. Would a 280VAC coil/24 VDC load relay connected to one leg of the 3 phase be sufficient?

cornbread
February 15th, 2012, 10:27 AM
Not exactly sure what you are asking but is sounds like you want a phase loss relay?

http://www.time-mark.com/SearchResults.aspx?categoryID=1

SCADA_Dude10
February 15th, 2012, 10:54 AM
If I understand what you are trying to do correctly: If you are running the motor with a contactor (mag starter), I would energize a relay with the coil voltage the same as the mag starter. The relay would change state when the contactor pulled in. If you ran 24V through the contacts on the relay to the PLC input you should be able to see if the motor contactor is on or off. If it's on a drive I would use the aux contacts on the drive.
Hope this helps.

Tom Jenkins
February 15th, 2012, 10:54 AM
As I interpret what you want to do, the simplest way is to have an auxilary contact on the starter wired to the PLC input. When the starter drops out, the contact will change state.

brucechase
February 15th, 2012, 10:56 AM
It sounds more like you need a NC contact from the motor starter. When the starter is closed (Motor running) the contact is open. When the starter is Opened (motor not running due to stop PB or Overload), then the contact is closed.

I would suggest the opposite arrangement though. Using a NO contact on the starter for you 24 VDC, then when the starter is closed, you have 24 VDC at the input and when the starter is Open, then you have no voltage at the input. You can then use this in your program to alarm or even turn off the motor output.


***EDIT**** I guess I was last. I must type way too slow.

SCADA_Dude10
February 15th, 2012, 10:56 AM
Tom,
I like your idea better than mine.

deanfran
February 15th, 2012, 11:11 AM
The starter plan would work, but there is one hitch. The first of the two switches is at a control panel that would open the starter contact. The second is a rotary safety switch at the motor. Though it is very unlikely that someone would ever open the rotary switch without first stopping the motor from the control panel first, I would like to be able to capture any of the possibilities. That said, the starter contact may be the best solution.

brucechase
February 15th, 2012, 11:29 AM
The starter plan would work, but there is one hitch. The first of the two switches is at a control panel that would open the starter contact. The second is a rotary safety switch at the motor. Though it is very unlikely that someone would ever open the rotary switch without first stopping the motor from the control panel first, I would like to be able to capture any of the possibilities. That said, the starter contact may be the best solution.


If this is truly a critical application, then there are several ways to tackle this problem. If you want to determine if the motor is turning, then a zero speed switch would take care of all the above situations.

Another way to tackle this is to have a set of auxiliary contacts in the safety switch that is located at the motor and run the start signal through that set of contacts. I normally do this when there is a VFD that controls the motor in the field. I always purchase a SquareD safety switch with auxiliary contacts in the switch (every switch now has them even if I don't wire them up - only 1 part number). This is then hard wired in the start circuit (or through the PLC as an interlock to allow the motor to start). The advantage of running the contacts to the PLC is you can display the status of the safety switch on the HMI.

TConnolly
February 15th, 2012, 11:44 AM
-----] [------------\
\
-----] [--+----------O Motor
| /
-----] [--)----+----/
| |
| |
+--0-+ 480Vcoil relay




It sounds like you want to detect loss of power (such as opening of disconnect or clearing of fuses and not just the opening of the motor contactor.

Relays with 480V coils are common and easy to get. Connect it between legs. Thats the cheap way.

A phase loss relay is a better way.

A 280 VAC coil relay would not be sufficient unless you have a four wire system (which is not common for 480) and connect from one leg to the neutral. I don't recommend this approach. It may not be compliant with NFPA 79 9.1.1

You could also connect a small 25VA control transformer between the two legs and use a low voltage relay.

joseph_e2
February 15th, 2012, 12:28 PM
We've done what Brucechase suggests. The local motor disconnect has an aux contact connected to a PLC input. The PLC controls the motor starter, so if either the disconnect or the motor circuit breaker is open, the PLC drops the contactor and won't allow it to close.

dahnuguy
February 15th, 2012, 02:39 PM
Using an aux contact from the starter will allow you to monitor the starter position , baring any malfunction of the aux contact or starter. ( I have seen both)

I used a device that passes all 3 legs going to the motor through 3 holes. Picture a rectangle block with 3 holes in it. Motor wires going through each hole.

The windings in the monitor would read the voltage and current being pulled by the motor and report the value as an analog output. I think it also had a couple of digital outputs for running, and phase loss.

It was years ago and I am sure they are easy to find.

This is the only way to monitor the actual motor.

I did it to display power used by an agitator to approximate the viscosity of a mixture and to control the agitator speed based on load.

But it should also allow you to monitor the motor in your application as well.

TConnolly
February 15th, 2012, 03:00 PM
I think dahnuguy is referring to one of these
http://www.loadcontrols.com/products/products.html

joseph_e2
February 15th, 2012, 03:06 PM
dahnuguy: We considered something like that for a different application but didn't quite get it done. Do you have make/model info handy?

edit: never mind...too slow ;)

leitmotif
February 15th, 2012, 03:07 PM
The question comes down to this
What are you trying to monitor??

What does this motor drive?
Is it important to know if driven load is not running?

Why is it important to know if the motor is told to run (line contactor shut)

You can get current sensing relays that sense motor line current and if too high or too low (broken belt is one application) it trips and activates alarm etc etc. This is about as close to putting a speed sensor (or output sensor ie pressure switch on pump) on driven load as it gets and costs about 100 for parts.

Dan Bentler

BryanG
February 16th, 2012, 04:02 AM
Auxiliary contacts on switching devices are simple but just because a relay/contactor is closed doesn't mean that a drive is moving.

Checking the voltage going to a drive by purpose built relay or placing a standard relay across the motor phases is one step better, but voltage doesn't mean current and so doesn't mean a drive is moving.

Looking at the current taken by a drive by using detectors with relay or analogue outputs (Google: current transducer 4-20mA 3 phase) tells you that the drive is pulling current and you can be fairly certain that if a current is within range that the drive is healthy.

Putting a motion sensors on the drive will tell you that the drive is running but not necessarily that the drive is doing any work.

If you put the motion sensor on the driven machine you can see that the machine is moving but it may not be doing work.


It always comes down to how important it is to know that something is moving and the price the job can afford. If you are at NASA you put in a lot of redundant systems. But you must also remember that the more devices you fit the more chance that one will fail and give a false reading.

OkiePC
February 16th, 2012, 06:30 AM
The further you can go to the opposite end of the driven equipment, the more proof you will have that it is indeed running. The motor current/phase relays are very good. But what if the gearbox output shaft loses a key? What if the driven conveyor belt (or other) slips or breaks?

In cases like those, in the past, we have used motion sensing devices on the tail (undriven) roller of a conveyor, for example. It can be as simple as a prox switch and 180 degree flag and using the PLC to check for stall conditions. In some cases, it ends up being easier, as well as more effective, to do something like that than to pull new pipe and wires to an an existing local disconnect.

brucechase
February 16th, 2012, 07:34 AM
What I am trying to do is initiate a 24 VDC input to a Click PLC when the electric feed to the motor is shut off. This shut off would be by one of two switches or a thermal overload.


Based on the requirements given by the OP - turning off the motor by a switch or tripping by thermal overload - the easiest solution is to use the aux contact off the starter.

Even though most of the ideas presented are great for some systems, I would rarely use a phase loss relay or even motion sensing on the hundreds of motors in my facility. The cost and information provided just isn't worth it.

OkiePC
February 16th, 2012, 07:58 AM
Based on the requirements given by the OP - turning off the motor by a switch or tripping by thermal overload - the easiest solution is to use the aux contact off the starter.

Monitoring the starter OL aux. is certainly a great place to start, but I think one of his switches you highlighted, is actually a disconnect switch local to the motor. Monitoring that with an aux. contact is also often very simple to do, and could be wired in series with the OL contact if he is hurting for spare inputs.

DickDV
February 16th, 2012, 08:25 AM
Using the aux contact on the starter and adding an electronic overload in place of the conventional overload would get you on/off as well as single phase and undervoltage.

To me, that's a lot of basic info with very little extra paraphenalia. As has been already covered, going beyond that gets more involved.

dahnuguy
February 16th, 2012, 08:50 AM
The question really needs to be associated with a budget and a level of critical functionality,

"What can I do for under a hundred dollars to have some idea of whether this system is running?"

or

"People will die quickly if this stops moving, make sure I know when it stops."

There is a lot of ground between those two.

I prefer to be specific when discussing these types of things, especially with people who assume so much.

That's why I say "An Aux contact on the motor starter will tell you when the starter is energized, baring any malfunction of the starter or Aux Contact."

Just so everyone is clear on what they are getting for 100 dollars.

But what if the motor is disconnected from the wiring? or the gearbox breaks? or the output shaft from the gearbox shears? , or the convener belt breaks? or the parts are slipping on the conveyor? or the parts get stuck at a transition? or fall off the conveyor?

We can add a sensor for every scenario and we can program for a long list of issues and we can add an alarm message for each one and add all the recovery and resets etc.............if that's what they really want.

The point is to identify the level of functionality that the end user requires and then target the correct area to monitor.

One could argue that monitoring the end result is more important. Trust the overloads to do their job, and look at the actual process for signs of movement.

If the box on the conveyor hasn't moved for 30 seconds while the logic indicates that it should, then you have an issue.

If you want more detail, spend more money and add more sensors and code until you have the information that suits your budget and curiosity.

deanfran
February 16th, 2012, 02:14 PM
Thanks you for all the replies. As has been mentioned, the level of monitoring is certainly contingent on how critical the application whether from a process or safety viewpoint. This particular case is not critical from either. This motor is on a pump that is part of a water distribution system. What I am trying to control is have certain other pieces of auxillary equipment shut down if the pump shuts down, either by human intervention or malfunction. If these auxillary pieces for some reason continue to run nothing bad happens to people or equipment, so the starter option will be enough. That said the Power Cell unit looks like it would be pretty cool. Not $650 cool, but...

leitmotif
February 16th, 2012, 02:57 PM
In this case I would put a pressure switch on the pump discharge. Could also do a flow switch but more expensive than a good $75 pressure switch

Dan Bentler