External Relay causes PLC fault

[kittydog42]

You could always hook up a 24V rechargeable battery to the system now as a test. It may provide enough additional capacity to keep the PLC online and test your theory.

 

[pimpim32]

Sure it will suffice.

Before I go I advice check again the specs for your relay.
I guess it is rated for 15A at 24V.Check the inrush as well ( use a scope to make sure you got it right).
Inrush specs for the relay must be at least 1.5 bigger than pump inrush, and if you are starting/stopping often then your relay is due for an early death.

If you are starting let's say once a day then don't worry.

Good luck with your project

 

[CharlesM]

If you have an O-scope you can look at the power supply and see if the voltage drops. I think it would have to be a scope because a meter is not fast enough. Maybe an analog meter might show it.

 

[Mickey]

Use separate 24VDC power supplies - case closed.

Ditto:

Reread this post and DO IT!

 

[Lancie1]

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?

Fuses have different time-current characteristic curves. These are plots of the time it takes to trip on the X-axis, and the triping current on the Y-axis. Fuses rated for motor-starting duty generally have a higher initial tripping current, but all fuses will trip at a higher current for a short time period, as opposed to the normal tripping current for a long time period.

Can you explain why your relay is cycling on-off? If you see that, you have learned another valuable trouble-shooting tool for future use.

Your batteries may provide more starting current for the motor than the 20 Amp power supply. Many solid-state "switching" power supplies have a current limiter that only allows the rated current to flow, with no excess capacity, whereas your battery ouput will only be limited by battery capacity.

What are some solutions to get the pump motor started without increasing battery size (or power supply size)? Because this is a DC circuit, what about adding a capacitor in parallel with the power supply output (not the motor)?

Scenario: Turn on main power, then DC capacitor charges up, storing power. PLC switches relay, applying power to motor. Motor starts to pull more current than power supply can handle, lowering the voltage. Now the capacitor voltage is higher than circuit voltage, so capacitor starts to discharge, keeping voltage high enough to prevent PLC from dropping off-line, and also spinning the pump. Voltage gradually recovers as pump gets up to speed, then the capacitor recharges, ready for the next start cycle. I leave it to the students to calculate the size of the capacitor! Hint: ANything is better than nothing, and you only need to figure a discharge time constant of about 10 seconds to go from 24 volts down to say 18 volts (or whatever your PLC minimum voltage).

 

[Paullys50]

.....The fuse is a 15A car fuse, would this have the same delay-effect?

As simple as a fuse seems, fuses can react very differently. During my internship I had the joy of "fuse testing" in which I tested about 10 different fuses by various manufacturers (10 each 2 different types of tests), all were rated the same.....if I remember they were rated 120VAC 1 Amp, anyway using a scope to record the inrush currents, and recording the time it took for the fuse to open....there were surprsing time differences between the fuses. Granted this was milisecond differences, but in some cases I was saw around (vaguely remembering) a quarter second difference, in some cases that could be just enough time for the fuse not to blow and cause a problem somewhere else.

 

[triplewhammy]

OK folks Good news! PLC is now able to turn on the relay when I hit FORCE on (previously unable). This is probably a no-no in the industry, but I wired two older power supplies in series together to get my 24V (they each output around 20A). You guys were correct, the issue was that we were over-working the single power supply...THANK YOu!!

The only problem now is that my code won't work automatically! One thing after another, sheesh! Here's the problem I'm having... After the timer hits 10, it should latch the bit "done" which in turn should kick on the pump and solenoid. Once, the tank drains (this is a water filtration system), the low level should open up, causing everything to unlatch, and the process repeats as it fills up. Here's the issue, the pump and solenoid are NOT turning on. In fact, after 10, it "attempts" to turn these on (all outputs light up for a second) but then turns everything off, including the UV bulb which was previously on! Any help now?

-Josh

 

[Lancie1]

Josh,

For a timer Done (DN) bit to stay on, the Timer logic rung must stay true, or "on". Use the Timer Enable bit to latch the timer on (self-latching), so that it does not self-reset. In other words, some condition starts the timer, but once started, it latches itself in with its own EN bit, until reset by some other condition (low level switch in this case).

If you are doing that already, then I would need to see the complete program (at least the section controlling the pump) for further troubleshooting. Could you post it here?

Your pump control should look something similar to this:

PS: I trust that you wired the power supplies in PARALLEL, instead of series. A series connection would give you 48 volts, instead of 40 Amps.

 

[triplewhammy]

Josh,

For a timer Done (DN) bit to stay on, the Timer logic rung must stay true, or "on". Use the Timer Enable bit to latch the timer on (self-latching), so that it does not self-reset. In other words, some condition starts the timer, but once started, it latches itself in with its own EN bit, until reset by some other condition (low level switch in this case).

If you are doing that already, then I would need to see the complete program (at least the section controlling the pump) for further troubleshooting. Could you post it here?

Your pump control should look something similar to this:

PS: I trust that you wired the power supplies in PARALLEL, instead of series. A series connection would give you 48 volts, instead of 40 Amps.

OK GUYS I HAVE EVERYTHING WORKING NOW THANKS A LOT!!!

The final error was due to the addition of the second power supply setup. I wired up the PLC with this new voltage (approx 24.6V) BUT did not wire the INPUTS to that same voltage (they were still on the old one). Thus, when it was time to kick everything on, there was an internal short (I assume) in the PLC causing faulty logic of one of the inputs. Inputs are now wired to same voltage level as PLC, and everything works! Thanks a lot guys especially for the quick responses, you have really helped a student here!

-Josh

 

[Lancie1]

Great, glad to be of some small service. Pass it on when you get to be my age!