Control Transformer Issue

makr8100

Member
Join Date
Jan 2018
Location
USA
Posts
31
Hi everyone,

I'm having issues with a control transformer (120V to 24V). Our conveyor system is set up to protect against product jam-ups through a probe fault system. If product hits a probe (metal rod) it shorts the probe to the frame of the conveyor (ground), and ground then brings the voltage over the relay to 0V, which de-energizes the coil.

What's actually happening now is I get ~6VAC from ground to 24V neutral. When a probe hits the frame (or I jumper ground to the probe circuit) the coil stays energized. Can I tie 24V neutral to 120V neutral, and would that solve my problem? Or since this started occurring recently should I suspect the transformer went bad and needs to be replaced?


Thanks,
Mark
 
this procedure is not allowed in my opinion !
you have the potential for electrical shock / electrocution.
all someone has to do is be good and sweaty and touch one end of the frame when the probe is touched and another steel structure to complete the loop.

since the problem started recently, I suspect that you are getting feedback voltage / current from another circuit.
you typically connect the x2 terminal of the transformer to panel ground.

in any case, in my opinion, you are asking for trouble.
but I will say that I do not know your entire setup.
james
 
I have used something similar years ago I don’t know the brand.
On water level system both potable and waste water, very reliable for many years.
I don’t think you can even used them anymore due to shock hazard.
The problem I think you are running into is the resistance of the circuit is varying to much, high resistance will cause the relay not to work.
Without actually seeing your layout it would be hard to help you much
But I would try a photo beam across the conveyor at the height of your probes it would give a much safer more reliable system.
A light certain would be the best option, you could easily program it for different height products on the conveyor as needed.
 
This is a 15 year old system I am trying to repair, not something I'm trying to create. The issue I'm having only started a few weeks ago. We have 3 systems like this and have never had an incident due to electrical shock. At the moment X2 is not connected to ground (to me it makes sense to connect it to 120V neutral), but that's what I'm asking is if it's ok to do that. Is there a standard on whether I'd connect to ground vs neutral?

As for the transformer output I'm getting 25.69V, so the output itself is good, but the ~6V difference is what's preventing the relays from functioning properly.

We're a pizza factory. The only product we run through this is pans of uncooked pizza crusts, so product variation is of little concern here. We're just looking for pans that got stuck, and this finds them because they stack when jammed.

I've attached pictures to clarify the setup. There are too many levels for a light curtain, and individual sensors would be a wiring nightmare not to mention it's a harsh environment (90 deg, 90%RH is typical), most sensors wouldn't survive well, and those that would are even more expensive.

Side note- if anyone familiar with this relay block has a suggestion for a replacement I'd love to hear it. I can't find documentation and I'm 90% sure they're discontinued.

The diagram shows how the inputs are sent to the relay block. We are only using 2 channels on this one (diagram is from another similar system as this one's documentation is missing). 1A (far left red wire) goes to a PLC input, open on the PLC input triggers the alarm condition. Ground to the probes (yes it's a ground loop from 1B to 1C) is supposed to de-energize the coil. 5 (wire 200) is the 24VAC hot, 8 (wire 201) is 24VAC neutral, 6 (wire 1A) is 120VAC hot, and 7 is control reset.

Spiral Convyor
Probes
Relay Block
Diagram
 
For safety I would run +24VDC to a relay coil and let the probe ground out, providing the 0VDC to the other side of the coil.

That is how it's done in stamping presses when they want to know if a probe is touched. It is also used for level control of conductive (non-RO) water, where there is no potential for power to the probe if someone touches it.
 
Right, that's the theory behind this, but the voltage difference between the neutrals is what's preventing it from functioning. Is the transformer to blame? And can I correct it by tying X1 to H1 or should I just replace it?
 
For this to work one side of the 24V transformer output will have to be grounded. What you are saying sounds like a broken ground wire if it used to work.

I had a similar issue putting in 24VDC timers that were activated by a rack put in a station that grounded out a probe. I had erratic operation until I found that each module had it's own ground wire going to the control panel, but in the panel all the ground wires were coiled in a loop and a couple of them had wires beyond the insulation that happened to be touching the enclosure side - the line had been running for years without any grounds hooked up to each module.
 

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