weird PLC power supply issue

[petersonra]

So Tuesday I go to a job site to watch a couple of electricians install a new PLC into an existing control cabinet. It was fully tested in my office before it shipped so I figured it would be a walk in the park.

No such luck. Power supply will not work. I finally clal tech support they tell me either the power supply or rack is bad.

Get a new one of both. Same problem.

Electrician's meter reads solid 118V at the terminals of the power supply.

We get another rack and power supply. Same thing.

Electrician powers up first rack and power supply with patch cord from an outlet. Works fine, just like it did in my office when I tested it.

Eventually we found that there was a ferro resonant xfmr in the cabinet that was powering just the two PLC power supplies. One that did not work and one that was working fine. We took the ferroresonant xfmr out of the circuit and all was good.

My guess is either the inrush on the power supply was so high the xfmr could not handle it or there was some weird interaction between the two.

 

[Jraef]

So Tuesday I go to a job site to watch a couple of electricians install a new PLC into an existing control cabinet. It was fully tested in my office before it shipped so I figured it would be a walk in the park.

No such luck. Power supply will not work. I finally clal tech support they tell me either the power supply or rack is bad.

Get a new one of both. Same problem.

Electrician's meter reads solid 118V at the terminals of the power supply.

We get another rack and power supply. Same thing.

Electrician powers up first rack and power supply with patch cord from an outlet. Works fine, just like it did in my office when I tested it.

Eventually we found that there was a ferro resonant xfmr in the cabinet that was powering just the two PLC power supplies. One that did not work and one that was working fine. We took the ferroresonant xfmr out of the circuit and all was good.

My guess is either the inrush on the power supply was so high the xfmr could not handle it or there was some weird interaction between the two.

The main drawback to CVS (ferroresonnant) transformers is that they "flat top" as they reach capacity. But because typical SMPS power supplies, like those for PLCs, draw power at the peaks of each sine wave, that flat topping is exactly what they cannot tolerate. So most likely what happened is that the CVS was sized based on the total load, but was too close to full load, so it flat topped the output sine wave. When the second power supply came on it could not get full input power and its crowbar circuit kicked in to prevent it from giving low voltage to the PLC.

 

[Saturn_Europa]

So Tuesday I go to a job site to watch a couple of electricians install a new PLC into an existing control cabinet. It was fully tested in my office before it shipped so I figured it would be a walk in the park.

No such luck. Power supply will not work. I finally clal tech support they tell me either the power supply or rack is bad.

Get a new one of both. Same problem.

Electrician's meter reads solid 118V at the terminals of the power supply.

We get another rack and power supply. Same thing.

Electrician powers up first rack and power supply with patch cord from an outlet. Works fine, just like it did in my office when I tested it.

Eventually we found that there was a ferro resonant xfmr in the cabinet that was powering just the two PLC power supplies. One that did not work and one that was working fine. We took the ferroresonant xfmr out of the circuit and all was good.

My guess is either the inrush on the power supply was so high the xfmr could not handle it or there was some weird interaction between the two.

No UPS?