Backup power for PLC

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cdenk
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Backup power for PLC

Post by cdenk » Thu Apr 21, 2005 1:51 pm

I Have a PLC (Tri-PLC T100MD888) that will be managing a Tri-Fuel standby generator. Normal power for the PLC will be a 120 VAC to 12VDC linear power supply. The 120 VAC will either be power company or from the generator. There is a delay between switch overs of up to a minute or more. The generator power supply is a 12VDC lead acid battery that will be charged and maintained by the generator's charging system and a 120 VAC charger. The issue is, the brief period while the generator comes up to speed, or switching back to power company power, the PLC needs to remain up and running, otherwise when it would reset, the generator would stop, and oscillation between power sources would result. Have thought about parallel diodes, but the lead acid battery and charges could frequently have voltage above the 12VDC linear power supply. How can I keep the PLC from resetting?
:)

upsmaster
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Re: Backup power for PLC

Post by upsmaster » Thu Apr 21, 2005 5:56 pm

Maybe a dc to dc converter with a input range of
8 to 16 volts dc and 12 out, some plc have internal battery back up option but not sure how long they are designed to hold up for
joe

cdenk
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Re: Backup power for PLC

Post by cdenk » Thu Apr 21, 2005 6:08 pm

The 12 volt coming out of the battery is useable as is by the PLC. But problem is switching over to the battery when 120 volt AC goes down. I guess the theoretical would be a double throw relay to switch between the 2 supplies, with the coil on the 120 volt supply, when the 120 volt drops out the relay switches to the battery, but don't think it will switch quick enough. An electronic circuit like used on a UPS probably would work though. I am able to build circuits, but this is a design area out of my expertice. :)

upsmaster
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Re: Backup power for PLC

Post by upsmaster » Thu Apr 21, 2005 6:24 pm

let the plc run on your battery all of the time.
a ups uses a static switch to slect between com power or inverter power to the output to the load.
not really fast switching just that both sides of the switch are on for one quarter cycle for no break
when slecting one or the other source.
joe

bridgen
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Re: Backup power for PLC

Post by bridgen » Thu Apr 21, 2005 7:34 pm

This scheme would work.

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cdenk
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Re: Backup power for PLC

Post by cdenk » Fri Apr 22, 2005 5:25 am

Then I need a battery charger of say at least 6 amps, 3 amps for the PLC, and 3 amps max. to charge/maintain the battery. Haven't bought the charger yet, so that's a possible. If I understand the UPS's use a "make before break" mechanical relay to shorten the switch over time? The PLC 120v power supply is "Linear", maybe the latent time from loss of 120V to drop in 12VDC is long enough for 120VAC coil relay to switch, will have to experiment.

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