Simple question re: motor current/voltage go no go to PLC

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?

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

Motor Monitor

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.

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.

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.

Tom,
I like your idea better than mine.

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.

deanfran said:

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.

Click to expand...

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.

-----] [------------\

                     \

-----] [--+----------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.

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.

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.

I think dahnuguy is referring to one of these
http://www.loadcontrols.com/products/products.html

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

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

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.