OT Siemens LOGO Capicator to maintain power

barryoc

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Join Date
Nov 2009
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Kilkenny
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Hi all,


I am using a logo 12/24 to start a generator when power fails. Initially logo is power from a 24v supply, then when the power fails it switches to the battery on the generator. This all works fine.


My issue is when the logo commands the generator to start the voltage drops due to the load the starter motor is taking and the logo shuts down.


Would it be possible to use capicators to maintain the power for this initial voltage drop? What should i watch out for if doing this and what capacitor size?


I dont want to install a second battery as i will have to link the charging circuit to this battery also.


Any suggestions welcome.


Regards,
Barry.
 
I had similar problem with a custom made lead battery powered hydraulic forklift, when the hydraulic pump motor started then the PLC entered error state.

I solved it with a diode and a electrolitic capacitor, I don't remember the exact size but for sure it was 20000 microF or bigger
 
Last edited:
I found the electrical diagram:

The PLC had a 22000 uF 40V capacitor in parallel with the power terminals and powered through a BY252 diode and a 5 ohm 5W resistor
 
I'd be a little careful here; for a cold engine you could be cranking for at least 5 seconds. You'd need to check that the cap will hold up for that long, and then recharge before your next cranking attempt.

Either fit a DC UPS with small SLA batteries, or find a boost converter that will accept the reduced battery voltage and produce a regulated 12 or 24V output for your PLC.
 
The problem does not last 5 seconds, the voltage drop only takes 20 or 30 milliseconds, until the chemical reaction in the lead battery increases and the voltage returns to normal.

It is a typical behavior of this type of batteries.
 
Thanks all,


LFE can you post the drawing? Going to try this first as it is the simplest and cheapest.


B.
 
You must measure the current consumed by the PLC and recalculate the value of the resistance so that it has a voltage drop of 0.5V approximately.

IMG_20181117_203304-01.jpeg
 
Lfe, it really does depend on the battery CCA and the starter motor. Battery voltage can drop to less than 9V during cold cranking and will last as long as the starter motor is engaged.

By definition, cold cranking amps (CCA) is how many amps a battery can supply for 30 sec while maintaining at least 7.2V.

For a piece of critical infrastructure like a standby generator, I'd want to be designing around those worst case scenario parameters.

A cap is a cheap and simple solution, but I'd hate to be the one trying to troubleshoot an aged electrolytic cap in 10 years time when the genset intermittently faults on startup.
 
Any modern car has its PLC and even a fieldbus that does not fail during engine start and everything is powered by the same battery.

But on the other hand in these recent cars when the battery is old suddenly one day it no longer starts because the electronics fails due to lack of voltage.

To be sure, I propose that Barryoc do a test: measure the voltage of the battery during start-up if it goes below 21V, forget the capacitor system and look for a separate power supply for the PLC,

A small battery is not so expensive either.
 
Any modern car has its PLC and even a fieldbus that does not fail during engine start and everything is powered by the same battery.

But on the other hand in these recent cars when the battery is old suddenly one day it no longer starts because the electronics fails due to lack of voltage.

To be sure, I propose that Barryoc do a test: measure the voltage of the battery during start-up if it goes below 21V, forget the capacitor system and look for a separate power supply for the PLC,

A small battery is not so expensive either.


Probably these have designed so that there is step up converter somewhere, which made allways 12VDC, even if battery voltage fails below. Or designed so that voltage needed is much less than 12V.


OP don't have that or UPS either
 
Thanks all,


Have decided to go with Saffa and bought a boost converter. Accepts 6-30v and give a constant output voltage adjustable between 6-30v for €25. This is a home project so no big issues if it doesn't work. Just trying to save elderly parents the hassle of having to go outside to the garage to manually start if required.


Barry.
 

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