External Relay causes PLC fault

[pimpim32]

For me it happened exactly how Sergei said.
Diode in parallel connected opposite.
ML1500 24VDC input, relay output, same 24VDC(10A) supply to ML.
PLC closes the output, short on 24VDC, PLC suffer due to surge (not enough supply), flash fault, resets itself and starts over and over.The fuse didn't even blink....
Didn't dare to press manually the relay, however I traced the wrong diode and since then I am telling myself I should not let the PLC to resets itself and to trap the fault.(Which I will do someday).

I will bet some money it is about some surge on the supply of the PLC.

Hook up the scope and see.

 

[triplewhammy]

Attached is the schematic...not the best but hey desparate times call for desparate measures!

-Josh

 

[CharlesM]

Does the output have an LED to show status of the output? If it does and the LED is on and the output is not then you have a bad output.

Next I would put a diode on the coil of the relay. Make sure you have it the correct direction. Also make sure you have the coil of the relay wired correctly. If it has a status light then make sure + is +.

 

[Lancie1]

Josh,
In your schematic, are the two 24 VDC points from the same power supply, or separate power supplies? If they are coming from the same supply, then you could be overloading the supply, or one set of wires may be reversed. For example, is the (-) on the Pump the same point electrically as the (-) on the PLC?

Reading back, you said this was a Water Filtration Plant, and everything runs from 24 VDC. I am thinking now that you have one common power supply. Are other motors starting when this Pump starts? That can cause overloading of the 24VDC supply.

 

[triplewhammy]

Josh,
In your schematic, are the two 24 VDC points from the same power supply, or seperate power supplies? If they are coming from the same supply, then you could be overloading the supply, or one set of wires may be reversed. for example, is the (-) on the Pump the same point as the (-) on the PLC?

Yes the positives and negatives are all on the same 24V, 20A power supply. I did notice when it faults, the LED status light on the power supply blinks off for a split second as it faults...

-Josh

 

[bernie_carlton]

Use separate 24VDC power supplies - case closed.

 

[pimpim32]

You are saying that everything operates on 24VDC.
If this includes the plc then put a new power supply just for PLC, don't remember how much a ML1000 will draw but I guess 2Amp will do.

You are lucky you didn't fry the output first time on direct

 

[triplewhammy]

You are saying that everything operates on 24VDC.
If this includes the plc then put a new power supply just for PLC, don't remember how much a ML1000 will draw but I guess 2Amp will do.

You are lucky you didn't fry the output first time on direct

I'm not quite understanding what you're saying. Yes, all these run on the same power supply but I figured 20A would be plenty to source the current. Why would the PLC output be fried?

-Josh

 

[Lancie1]

Yes the positives and negatives are all on the same 24V, 20A power supply. I did notice when it faults, the LED status light on the power supply blinks off for a split second as it faults.

Gotcha! A 20 Amp power supply, and a 13.5 Amp pump motor that probably pulls 25 Amps for a split second when it starts, pulling down the voltage so low that the PLC goes off, and even the LED on the power supply blinks off!

It is amazing that it works at all! You need to get a larger power supply, or separate power supplies, or put an electronic Soft starter on the motor.

 

[pimpim32]

Because relay output on ML1000 stands for 2.5A and if I do not remember how much inrush (7.5??)

You put 15A fuse for pump, but inrush is sure over 20A as you are saying that supply led is blinking when pump starts.

 

[triplewhammy]

Well glad we seem to have the problem sorted out! Any reasons why the fuse doesn't blow if it's pulling 25A for a split second...?

-triple

 

[Tharon]

Oh, so the PLC itself runs off of the same 24V power supply? I assumed it was powered by 120Vac.

Does your 24V Power Supply have a LED on it? You can usually watch the Power Supply LED grow dim as the load gets too great and the supply starts to shut off.

I don't know about other people, but I usually try to keep Power and Control circuits seperate.

Well glad we seem to have the problem sorted out! Any reasons why the fuse doesn't blow if it's pulling 25A for a split second...?

-triple

The fuse is probably a time delay. Since coils draw a large inrush current, you don't want to raise a fuse so high to get by that, because then you wouldn't be protecting the normal operation of the coil.

That and the 25 total amps that the power supply is trying to produce, doesn't all go through that one branch, you get what... a couple amps for the micrologic, and some on the relay coil.

 

[Lancie1]

Well glad we seem to have the problem sorted out! Any reasons why the fuse doesn't blow if it's pulling 25A for a split second...?

Yes, it is called a "time delay". Fuses rated for motor starting must have a built-in delay that allows a large current for a short period of time, to get the motor started and up to speed. That would be a normal expected action for a motor-starting fuse. Otherwise every motor would blow its fuse while starting. The fuse must be rated to blow at 125% of normal running current, not starting current.

Welcome to the "real" world, as opposed to the theoretical one that your ivory-tower professors have been teaching you the past 4 years! There are a few differences!

 

[pimpim32]

If you try harder you can blow it don't worry...

Just see the specs for your fuse and compare with your notes about current protective devices ( time vs amps)

 

[triplewhammy]

Yes, it is called a "time delay". Fuses rated for motor starting have to have a built-in delay that allows a large current for a short period of time, to get the motor started and up to speed. That would be a normal action for a fuse.

Welcome to the "real" world, as opposed to the theoretical one that your ivory-tower professors have been teaching you the past 4 years!

Haha! Nice! Trust me, I know I have a lot of learning to do for the "real world" hence the reason I'm doing this project, I'm learning a lot! The fuse is a 15A car fuse, would this have the same delay-effect?

Also, let me clue you in on the bigger picture...this setup is merely the lab test. The final will be powered by solar panels and a backup battery bank. With this batter bank, we will run into the same issue..the controls and power components will be powered by the same source. We are getting 4 AGM batteries with a 183 amp-hour discharge (20 hr rate). These will be paired, then wired in parallel (for a total of 4). Would this suffice? After all, like I said the power supply issue is merely for in-lab testing. Thanks!

-Josh