Missing electrical phases

A customer of ours has several 100HP water pumps, three phase, 480 volts at their facilities. In more than one occasion they had experience interruptions with the three phase system and a motor was damaged the last time it occurred due to a missing phase.
When I was visiting the project I noticed, in several of the enclosures, they had installed Amp transducers one each of the three phases.
When I asked why? No one seems to remember the reason they were installed and why there were wired all the way to terminal blocks but not to PLC or any controllers.
Question:
Can I wire these three amp transducers to the PLC analog cards and used them for monitor the loss of any of the three electrical phases? Does anyone see anything wrong with this scenario?
Thanks for your input!!

Yes, you could do that, but one of these might be easier, faster reacting, and less expensive:

http://www.omron247.com/marcom/pdfcatal.nsf/0CBD576FC430C3F48625723D0065C1B6/$FILE/D20K8ABPM1206.pdf

http://www.macromatic.com/products/phase-monitor-relays.html

http://www.squared.com/us/products/...88A85256CB60056D979/$file/phaselossrelays.htm

http://www.benderrelay.com/phaselossrelays_WP_msn.htm

http://www.crouzet-usa.com/catalog/_phasecontrol.shtml

Paul
EDIT: your method might detect more than just a missing input phase, so it might protect against other problems like a faulty disconnect switch.

Surely if the motors had the correct motor protection circuit breakers then the motors wouldn't get damaged?

Three relays to detect might be another way to detect phase failure; maybe connected to a circuit breaker with a shunt trip?

In answer to your question - yes you could use the feedback from the current transducers to monitor the motor condition but if you have a phase failure then there is a fair chance that the PLC will be offline too.

Nick

It would depend on the ct's, too. If they're "old fashioned AC CT's", they won't work in a plc.

There is one current transducer per each phase for one motor or for a group of motors? If is for a group of motors, the current can fluctuate with the load. A voltage transducer will do a better job in my opinion. It will also detect a low voltage problem in the electrical system and can be installed for a group of motors, saving money for the customer.

piscis said:

A customer of ours has several 100HP water pumps, three phase, 480 volts at their facilities. In more than one occasion they had experience interruptions with the three phase system and a motor was damaged the last time it occurred due to a missing phase.
When I was visiting the project I noticed, in several of the enclosures, they had installed Amp transducers one each of the three phases.
When I asked why? No one seems to remember the reason they were installed and why there were wired all the way to terminal blocks but not to PLC or any controllers.
Question:
Can I wire these three amp transducers to the PLC analog cards and used them for monitor the loss of any of the three electrical phases? Does anyone see anything wrong with this scenario?
Thanks for your input!!

Click to expand...

The motors may have been damaged from brief interruption of power when they were reenergized and were out of phase. They should have 3 wire starting (also called low voltage protection) such that they drop out on loss of voltage and require manual restart.

The single phased motor damage could have been prevented by proper overcurrent (overloads) protection in starter. A single phased motor can continue to run but will probably not start - in either case current in the two energized phases will be high which is what overloads will sense and trip motor out.

I think you should verify that these motors controls are properly installed. Installing the volt sense relays without doing this is like putting a bandaid on gangrene.

When you have verified proper motor controls you can add the relays Okie recommended. They are a good idea and can prevent or sense other problems (underloading ie broken drive belt) etc etc.

Dan Bentler

Use a power monitoring relay to turn the motors off - or to input the PLC to turn the motors off. Any reputable maunfaturer has them.
Schneider, Carlo Gavazzi etc.
Make sure you get the one that monitors rotation, power failkure and assymetry. Sometimes the ones that do not monitor assymetry get fooled by lots of motor load according to Schneider here in Ozz.
I use them always in motor control centres. Single phase detection overloads SOMETIMES are not fast enough and a motor or VSD gets damaged.

Bob
Maybe this is what you meant but just to make sure.
Over loads especially the heater type may not be fast enough especially so if the wrong heaters are used. That is one nice thing about the newer units.

I was always taught to put overload protection in at least two of the three phases - ideally all three for proper single phase protection. For sure if only in one phase it may miss single phasing.

Dan Bentler

I always design 3 phase motor controls with a phase monitor relay. They cost less than $200. They are designed to protect motors and they do a good job of it. Most PMRs monitor phase loss, phase imbalance, brownout and reverse rotation.
The current transformers are better than nothing but still not as good as a PMR.
A 3 phase motor will start and run with one phase missing. The big problem is that the windings will overheat quickly. Heat kills motors, maybe not right away but it will shorten the life of the motor. Even a phase imbalance of just a few percent can cut the life of a motor in half.
Most VFDs have the protection built in and don't require external PMRs.
My suggestion is to use the current transformers as well as the PMR. Then everything is covered. What is the cost to replace a burned out 100hp motor??? Several manufacturers make convertors to convert the raw signal from the current transformer into a 4-20 mA signal. Try Weidmuller, Phoenix, etc.

I have never trusted thermal overloads at all. They allow too much current through for too long when a phase is dropped. Even then I would only use them with single phase protection on all 3 phases Dan. I do not believe there is a commercially available overload that does not look at all 3 phases.
These days I use motor circuit breakers exclusively. They are designed for the job with a higher magnetic tolerance component to get motors away.
They also have a high kA rating - I do not wish to get into a discussion re fault levels by the way. Fault levels, cascading, enhanced fault levels are all covered in the manufacurers literature so you can work out the fault level for yourself. Suffice it to say the motor circuit breakers help quite often with fault level.
In my mind the next most important protection item is the power monitor relay, in addition to the maotor circuit breaker.
If all the motors are small I normally use 1 power monitor relay per panel. If the motors are large I use 1 per motor, unless the specification calls for 1 per motor. Have to work to specifications and not add too much cost or you lose the job unfortunately.
This is important

A 3 phase motor will start and run with one phase missing. The big problem is that the windings will overheat quickly. Heat kills motors, maybe not right away but it will shorten the life of the motor. Even a phase imbalance of just a few percent can cut the life of a motor in half.

Click to expand...

and is why I use power monitor relays. The other thing is they are fail safe, 1 phase missing, incorrect rotation, assymetry problem or voltage out of bounds and the contact opens.