PHASE MONITORS VS 3 POLE CIRCUIT BREAKERS

[kwired]

A lot of my work is with large refrigeration racks in supermarkets and you have nailed it.

Auto restart is a huge advantage.

I agree to an extent. Automatically restarting something over and over that does have a problem isn't necessarily doing the equipment any favors. If there is regular cycling of a lost phase condition it may be well worth knowing about it and why it is happening. It is nice to automatically reset such equipment if the phase loss was just an occasional utility problem. GEtting more advanced with controls and somehow recording the incident and frequency of the incident can also be useful in preventative maintenance.

 

[iwire]

I agree to an extent. Automatically restarting something over and over that does have a problem isn't necessarily doing the equipment any favors. If there is regular cycling of a lost phase condition it may be well worth knowing about it and why it is happening. It is nice to automatically reset such equipment if the phase loss was just an occasional utility problem. GEtting more advanced with controls and somehow recording the incident and frequency of the incident can also be useful in preventative maintenance.

You just can't let it go.


It happens that any shutdown of the equipment I have been talking about is logged and a message sent out to the right people immediately and automatically.

 

[kwired]

You just can't let it go.


It happens that any shutdown of the equipment I have been talking about is logged and a message sent out to the right people immediately and automatically.

Well we didn't know that until you mentioned it. Not all installations are going to be that way.

I know you have worked in many larger grocery stores or similar facilities. I have worked in grocery stores also, but in some instances the entire store may fit inside a cooler within some of the stores you have worked in. Some similarities, yet like being on a different planet.

Some places phase loss isn't all that common of an event either. Many small municipal distribution systems are pretty clean and reliable power sources. Most activity that ends up causing line problems is limited to acts of nature or someone maybe driving their car into a pole, the rural systems even though there may be less customer density, are more prone to phase loss, brown out, transient voltages, etc. mostly because there is so much more line length for something to happen somewhere.

 

[SceneryDriver]

Many small municipal distribution systems are pretty clean and reliable power sources. Most activity that ends up causing line problems is limited to acts of nature or someone maybe driving their car into a pole, the rural systems even though there may be less customer density, are more prone to phase loss, brown out, transient voltages, etc. mostly because there is so much more line length for something to happen somewhere.

Oddly enough, where I live and work, the city-owned power company is terrible. They have several phase-loss events per year, on average. Last December, a squirrel met its demise on the HV side of the substation transformer, and half the city lost one leg. I guess two-outta-three phases was good enough.

That phase loss event took out our freight elevator; the motor itself was protected as it is run from a VFD, but the 480v-to-120v control transformer was connected to the dropped leg on one side. This lowered the control power's voltage to somewhere (I'm guessing) around 70vac or so. This in turn cooked the coil on the 100A e-stop contactor ahead of the VFD. The switchmode power supplies didn't care about the low voltage, but that contactor sure did.

After replacing the contactor coil, I added phase loss protection to the control power circuit. If the PLM sees anything wonky with the incoming 480v power, it opens the 120v side of the control power circuit; no control power, no worry about damaging the contactor or other 120v loads.

We could have saved four days of downtime - and the $150 cost of a new contactor coil - if the original panel builder had included a $60 PLM relay. Needless to say, I'm a fan of PLM's.


SceneryDriver

 

[templdl]

That phase loss event took out our freight elevator; the motor itself was protected as it is run from a VFD, but the 480v-to-120v control transformer was connected to the dropped leg on one side. This lowered the control power's voltage to somewhere (I'm guessing) around 70vac or so. This in turn cooked the coil on the 100A e-stop contactor ahead of the VFD. The switchmode power supplies didn't care about the low voltage, but that contactor sure did.

After replacing the contactor coil, I added phase loss protection to the control power circuit. If the PLM sees anything wonky with the incoming 480v power, it opens the 120v side of the control power circuit; no control power, no worry about damaging the contactor or other 120v loads.

We could have saved four days of downtime - and the $150 cost of a new contactor coil - if the original panel builder had included a $60 PLM relay. Needless to say, I'm a fan of PLM's.
SceneryDriver

Great forensics. As a former application engineer I certainly appreciate your understanding about the actual application of control circuits. Thanks for chiming in.

 

[meternerd]

OK...my 2 cents. We use PM's on all of our pump stations. But when we have a station wired Delta at the service, and we lose a primary phase, the phase monitors will not trip because the voltages are still matched pretty well, since it is basically running as an open Delta. Another problem with them is that they trip on low voltage. Sounds like a good thing, but on Wye-Wye services, when starting 400 or 500 HP motors using soft starts, the inrush sags the voltage so much the PM trips. If we adjust the low voltage trip to prevent it, we then won't trip when we lose a primary phase. We ended up putting a time delay in the PLC logic to bypass the PM until the motor is up to speed. Wish it was a perfect world, but it ain't.

 

[GoldDigger]

OK...my 2 cents. We use PM's on all of our pump stations. But when we have a station wired Delta at the service, and we lose a primary phase, the phase monitors will not trip because the voltages are still matched pretty well, since it is basically running as an open Delta.

The effect you describe does somewhat resemble an open delta, but there is a big difference between two pots feeding three wires, which can still provide halfway decent voltage regulation as long as the load and the two pots are properly sized. If you go to zero load or heating lighting loads the voltage will still be there, and motor starting inrush can still be handled, although not as well as with a closed delta.

But when you open one phase conductor, then with no loads the voltage on that phase line inside the plant will go to zero. If there are running motor loads, they will see a single phase condition because the missing phase wire was required for two sides of the delta. But, as you said, already running motor loads will produce a wild voltage on the third phase wire. This can provide a non-zero voltage on the open phase wire, but will not have the ability to start a motor that is bigger than ones that are already running. And running motors will be only capable of partial power output and may overheat.

A properly designed open delta, on the other hand, will work just fine for all of the different load cases.

 

[meternerd]

The effect you describe does somewhat resemble an open delta, but there is a big difference between two pots feeding three wires, which can still provide halfway decent voltage regulation as long as the load and the two pots are properly sized. If you go to zero load or heating lighting loads the voltage will still be there, and motor starting inrush can still be handled, although not as well as with a closed delta.

But when you open one phase conductor, then with no loads the voltage on that phase line inside the plant will go to zero. If there are running motor loads, they will see a single phase condition because the missing phase wire was required for two sides of the delta. But, as you said, already running motor loads will produce a wild voltage on the third phase wire. This can provide a non-zero voltage on the open phase wire, but will not have the ability to start a motor that is bigger than ones that are already running. And running motors will be only capable of partial power output and may overheat.

A properly designed open delta, on the other hand, will work just fine for all of the different load cases.

One thing some folks are not aware of is that utility underground padmount Wye-Wye transformers are usually "common core" type, because they're cheaper to build. All three windings share the same core. It's basically two horizontal sections and three vertical sections, with one set of windings using one vertical section. Primary and secondary windings are wound together on one core section. Makes for better inductive coupling. During a primary single phase condition, the two Hot phases will induce voltage into the Dead phase and keep the three phase output somewhat balanced. Depending on load, of course. It has "faked" out our phase monitors on more than one occasion. We can set imbalance tighter, but then we get false trips, which can impact the reliability of the system. Always a tradeoff of some sort.