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

[OkiePC]

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]

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]

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]

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]

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]

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]

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