Re: AB PLC 5 and Clary UPS crashes

I need the UPS on this equipment because of the frequent power bumps and the remote location of the pump-station. When the power-supply crashes it takes out the communication function of the PLC until I can reset the power-supply. When this happens the pumps shut down and cannot be restarted. There was a time when the UPS was sent in for service and a differnt UPS was used, but the power-supply would still crash with the new UPS.

Ben,

I'm not familiar with a Clary UPS but a *real* UPS supplies the load all the time. When the utility power disappears, there is absolutely no bump. All that happens is that the battery starts running down. When the power comes back, the battery starts to recharge. The only time there can be a bump of any kind is when the UPS itself fails and you are switched to utility power. To make this absolutely bumpless, the frequency of the UPS is synchronized to the utility power.

In your case, it seems the UPS is a backup supply only. It cuts in only when the power fails. You have no simple way of knowing what happens unless you put on some instrumentation and watch your power before and after. For example:

1 - Is the voltage the same before and after the transfer? It could be that your UPS is undersized and starts to current limit. When that happens, the voltage will drop. If your main load senses a voltage drop and disconnects, the voltage will come back up. It would require a fast recorder to catch the momentary dropout.

2 - Is the frequency before and after the same? It is less likely that this is a problem since most electronics will accept a very broad frequency range (47 to 63 Hz).

3 - Is the waveform the same before and after. It is supposed to be a pure sine wave. Making a pure sine out of DC is expensive for power applications and it is much cheaper to make a square wave. Unfortunately power supply transformers react very poorly to square waves.

Walter.

Ok, I have never had to use remote comms on a PLC.

I would still reccomend checking the output of the UPS for noise or harmonics during power sags or outages. You may need to go as far as a variac on the UPS input.

There could be another source of problems, I have a 386 based display term>I need the UPS on this equipment because of the frequent power bumps

Walter,

Thank you for the suggestions and I will try them this week. This Clary UPS I have does supply 120 volt ac to the load all of the time from it's batteries and inverter, no switching from utility to battery with the power bumps. That is why I have been pulling out my hair over this problem. The power bumps should in no way have any effect on this power supply.

Thanks, Ben

Hi Ben, to clarify what Walter said, there are three main types of UPS:

1. True On-Line - mains supply charges the battery and the output power is supplied off the battery all (100%) of the time (ie. Walter's "real" UPS). These units are the most expensive.

1. Line Interactive - output is fed from mains supply during normal operation, but smart surge supression and voltage regulation maintains corrects output voltages regardless of the state of the incoming supply. Switches to battery when mains fails.

2. Off-line - output is fed from mains supply during normal operation. UPS only runs off battery on mains failure. There is typically no voltage regulation, nothing.

I suspect your Clary unit is on 'off-line' unit. These are the cheapest and most common UPS's around. If you are having major problems with power dips, I suggest you toss the Clary UPS in the bin and buy a 'true on-line' type - it is not worth spiking your expensive PLC by going cheap on the power supply.

Cameron:-)

Now you got me too. Have a quick look to confirm that there isn't some component of your system that is NOT on the UPS. Could it be that part of the PLC is actually plugged into utility?

Walter.