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Detecting lost phase with current monitoring.
8

Detecting lost phase with current monitoring.

Detecting lost phase with current monitoring.

(OP)
Hi,
for vibrating equipment we used to use a 3-phase current relay from Carlo Gavazzi (type SM190).
The relay would indicate when a symmetrical current was fowing thru 3 phases. If a wire came off in the terminal box on the motor, the relay would switch off.
The problem is this: The particular relay has been discontinued by Carlo Gavazzi, and no similar substitute has been put in its place.
Notice that the ordinary 3-phase VOLTAGE relay will not do. It cannot sense that a wire has come loose at the motor. It must be CURRENT (via current transformers).

Does anybody know of possible makers of similar monitoring relays ?

RE: Detecting lost phase with current monitoring.


A general term for this type of protection is ANSI function 46 — negative-sequence overcurrent relay.

A utility-grade version is described at www.basler.com/downloads/udj7bull.pdf
   

RE: Detecting lost phase with current monitoring.

2
Hello JesterMP

There are a number of options, and it depends on what level of protection that you are after.
Most modern thermal overload relays include differential bar overload protection and this will give you a good measure of current imbalance protection provided that they are set up correctly. This works by mechanically increasing the trip sensitivity based on the imbalance in the phase current.
If you are protecting against an interruption in the circuit to the motor that is electrically close to the motor, then the magnitudes of the currents is all you need to monitor. If you are looking for protection when there is a phase loss on one of the main feeders, then you can get the situation where the motor takes current in on two phases and generates current out the third phase to power other electricla circuits. In this situation, current magnitude alone will not always offer sufficient protection.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Detecting lost phase with current monitoring.

(OP)
Thanks to all for their contributions :)

I need to explain a little bit more what I have to protect against.
There are always two agitators on the sides of each vibrating conveyor. The two agitators rotates completely syncronised and counter to each other when all is well.

If one phase is lost in the terminal box of one motor (something that will happen in a certain percentage of all cases), then the motor will drop a small fraction in speed - but enough to be out of syncronisation with the other motor. The result is wild movements of the conveyor - which in turn will cause damages if not interrupted in time.

About "differential bar overload protection": This has in practice shown not to provide protection against phase loss at the motor. Reducing the overload trip level is not enough (because each motor is normally not significantly loaded).

Most of the suggested devices seems to be out of the question costwise. The SM190 that I am looking for a replacement for was approx 150 EURO (200 USD) for the relay AND a nifty three-phase transformer. But thanks for the suggestions anyhow.

And it IS strange that they didnt have a replacement. But I figure that they didnt sell enough of them (we used to use a lot of them).

RE: Detecting lost phase with current monitoring.

(OP)
Hurrah - sorted !

I was looking at doing the same thing in the same way, I should have been looking at doing it in a slightly different way.
Rather than getting a 3-phase relay with 3-phase current transformer combo, then the simplest/cheapest/smallest solution is to use three singlephase current transformers with builtin current level trip.
There is the EIS type from Carlo Gavazzi that does the trick, and at only 35 EURO / 45 USD per relay !
Its less than half of the SM190 type with CT, and much less than any other solution.

RE: Detecting lost phase with current monitoring.

Nice is nice. As in the above suggestion sometimes all that is needed is three cheap, simple GO/NO GO self powered relays similar to www.rke.com  model MCDR-10 with their contacts wired in series.   A product like this should be available where you are.   I've used these in shaker applications to detect a broken wire.  If there is a PLC in the application, current sense transformers (that can actually generate some voltage) can monitored.

RE: Detecting lost phase with current monitoring.

Jesper - I'm not sure I understand what you're suggesting. I don't understand how three single-phase relays can accomplish current unbalance sensing. Can you explain a little more?

=====================================
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RE: Detecting lost phase with current monitoring.

(OP)
electricpete,

the original 3-phase AC monitoring relay with 3-phase CT DID monitor 3 phase unbalance monitoring.
But all I need is to detect a broken wire as OperaHouse mentions.
Thats why the simple use of three singlephase current-monitoring relays will do the trick.

RE: Detecting lost phase with current monitoring.

Comment: Three single CTs can accomplish a trip on the current unbalance above the trip point (the same for all three CTs).

RE: Detecting lost phase with current monitoring.

(OP)
jbartos,
your post is a bit cryptic, what do you mean.

Just so that you know what I have found:
EIS datasheet: http://62.110.11.151/pdf/UK/EISNNOA2.pdf

RE: Detecting lost phase with current monitoring.

I also don't follow how three single phase relays are going to do the job.  Then too, I am "leery" of new solutions to old problems...that already have plenty of proven fixes.

TimeMark Corporation makes several low cost current unbalance relays that will work.  I think their website is www.time-mark.com

RE: Detecting lost phase with current monitoring.

(OP)
PWR,
I dont need unbalance monitoring !
I need broken wire detection at the motor.

RE: Detecting lost phase with current monitoring.

Ok. I think I see how you are using it Jesper.  You set the overcurrent setpoint below your minimum (no-load) current.  Output contact should be closed during operation.   Wire three output contacts in series.   If any of the three monitored phase currents drops to below the setpoint, there is interupption of the output circuit to indicate that one phase is lost.

It sounds like you are not providing comparable level of protection to what you had previously (full unbalance protection).  Whether or not you need the full protection is up to you.   As you know unbalanced voltages can still cause motor overheating even if the phase currents remain below overload setpoints.  This is particularly a concern for larger motors where negative sequence affects the rotors. I don’t see unbalance protection on small motors.  

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RE: Detecting lost phase with current monitoring.

That is basically the idea, but these are available from other sources with 5A, 250VAC relay contacts.  Unless you are set up with low voltage, these would be a pain.   They are generally 10A or over, but have a large CT that allows you to make 5 or more turns to get the required amps down.  These are nothing more than a CT, rectifier, filter cap and relay. If they have a pull in current of 10A, the hold in is less than 3A.  The starting current of most small motors can easily pull in the relay without a lot of additional turns.

RE: Detecting lost phase with current monitoring.

Suggestion to JesperMP (Electrical) May 4, 2004 marked ///\\\
jbartos,
your post is a bit cryptic, what do you mean.
///Too imposing on an eng-tip volunteer.\\\
Just so that you know what I have found:
EIS datasheet: http://62.110.11.151/pdf/UK/EISNNOA2.pdf
///Actually what I meant would be to have all three units normally open contacts, each aligned with the indicator/recorder, in tripped status, e.g. normally open contact would be kept closed and provide an indication. If one line conductor happen to open its circuit or cause the higher resistance or impedance, the normally open contact that was tripped closed would open to Normally Open position. This would indicate faulty line. Three pen indicator/recorder would indicate this as the first one having been tripped.\\\

RE: Detecting lost phase with current monitoring.

(OP)
Electricpete, the motors are relatively small. Max 5kW.
I really need only to check for broken wire at the motor.

OperaHouse, the type that my link pointed to starts at 2A and maxes out at 50A. Thats about perfect for my application. For smaller motors you can make more turns as you suggest. I dare say that the type you linked to has an enormous hysteresis, and the setpoint is fixed. The EIS has only 7-13% hysteresis and the setpoint is adjustable. But it is good to see that others are doing the same thing.

jbartos, I am still not sure what you mean. There is no recorder or such, only ONE input to my PLC from all three relays. I could connect each relay to an individual input to determine which phase that tripped. But I have no need for that.

Thanks to all, lets finish this thread now ;)

RE: Detecting lost phase with current monitoring.

Suggestion to JesperMP (Electrical) May 5, 2004 marked ///\\\
jbartos, I am still not sure what you mean. There is no recorder or such, only ONE input to my PLC from all three relays. I could connect each relay to an individual input to determine which phase that tripped. But I have no need for that.
///There are normally more ways feasible to implement monitoring. It may very well happen that what is discussed in here is available on the market at a reasonable cost, with the better principle of operation than discussed, with the better quality, etc.\\\

RE: Detecting lost phase with current monitoring.

Why not a simple ring Ct around all three phases and an overcurrent relay. If you get 'broken wire' then the CT will see current and trip

RE: Detecting lost phase with current monitoring.

(OP)
rod,
you cant use a single CT. It will give zero signal with 3 phases or 2 phases, so it cannot detect broken wire.
The motor will continue to run with only 2 phases. The magnetic field that the CT feels, will still be zero.
If you put a clamp meter on 2 phases on a motor running on 3 phases, then it will see something.
If you put a clamp meter on 2 phases on a motor running on 2 phases, then it will see nothing.

I think that using an EIS relay on each phase like I suggested before will work perfectly and will be quite cheap too.

RE: Detecting lost phase with current monitoring.

Hello Rodmcm

A single CT around all phases would read zero current provided that all current paths were through the CT. This is true for three phase current flow, or two phase current flow.
If there was current flow through a path or conductor that did not pass through the CT, then this would be measured by the CT.
This is the principle that some earth leakage protection relays operate. What you describe would give earth leakage protection, but not phase loss protection as required.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Detecting lost phase with current monitoring.

Comment:
Three phase currents flowing in three conductors will form:
Ia + Ib + Ic = 0+j0 amps
if the load is balanced.
Currents in two phase conductors with the third conductor open:
Ia + Ib + 0+j0 = Id in amps different from 0+j0 amps
since
Id=|Ia|/_0° + |Ib|/_120° = |Ia| + |Ib|xcos120 + j|Ib|xsin120=
=|Ia|-0.5x|Ib|+j0.866x|Ib|
If |Ia|=|Ib|=|I|
then
Id=|I|/_60° in amps

RE: Detecting lost phase with current monitoring.

I assume jbartos that you support my idea? To the others: the phase vectors do not balance with 2 currents and hence you will have loss of phase protection wont you?


RE: Detecting lost phase with current monitoring.

Comment on the previous posting: Yes, at least, that is what the mathematics says.

RE: Detecting lost phase with current monitoring.

2
jB and Rodmcm,

If you lose one of three phases, you lose the 120 degree relationship that jB's equations incorporate. You have a single-phase supply at line-line voltage. The windings which had a connection to the missing phase now appear connected in series across the remaining two phases. Thus, the two remaining phases are loaded by the original winding plus the other two phases in a series string, which may be reduced to a single R-L load across two phases.

Draw up the vector diagram for this situation and you will see that the core balance CT 'sees' the same current going through it on one of the two phases as it does returning on the other. This method will only detect a differential in the currents, such as might exist if an earth fault were detected, and absolutely will not detect a lost phase.






-----------------------------------

Ask a silly question and you are laughed at for a moment.

Don't ask the question and you might be laughed at for eternity.

RE: Detecting lost phase with current monitoring.

hello jbartos

I totally agree with ScottyUK. If you lose the third phase in a three wire installation, you now have a single phase system with a higher voltage. Your calculations are correct in a four wire situation where there is neutral connected to the star point on a star connected motor. i.e. a four wire installation.
Generally speaking, installations of induction motors are three wire for both star and delta connected.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Detecting lost phase with current monitoring.

Good point I give up! forgot about the loss of neutral reference! What about just using  an ordinary 3phase overload be it via CTs or direct, modern elements trip for single phasing.

RE: Detecting lost phase with current monitoring.

(OP)
rod,
The "differential" part of an overload relay reduces the trip level only, it doesnt cause it to trip instantly.
The agitators on the conveyors in question are normally only lightly to medium loaded. So it will take too long time or forever for a "normal" overload to trip.

RE: Detecting lost phase with current monitoring.

Hi Jesper,

Why not install 3nos of single-phase undercurrent relays? When a wire is broken, it will trip as long as the current drops to a set value.  Try see this: http://www.crompton-instruments.com/protector262.pdf

However, you need a separate control circuitry to block the undercurrent trip during initial start-up of motor.

We have implemented similar undercurrent relays in our plant on a few unit of liquid oxygen pumps, with the intention to stop the motors if the pumps cavitate.

RE: Detecting lost phase with current monitoring.

(OP)
Hi digitrex,

well that is exactly what I am planning to do allready. See earlier posts.
My "control circuitry" is a PLC, so I am applying a timeout alarm for the signals from the current relays. The alarm will be generated 0.5 seconds after startup, when the relays do NOT indicate minimal current.
And I doubt anyone can beat the total price of approx 100 USD / 80 EURO per motor for the devices I have found.

RE: Detecting lost phase with current monitoring.

JesterMP,
Your solution is a good one for your application, however you are neglecting the cost of the overload relay, which although necessary, could be considered redundant against a solid state overload relay with phase current loss and/or current imbalance protection. Many offer this as a standard feature now. In addition, many soft starters are also building this kind of protection into their digital front end controls, which then adds the motor on-off control into the mix as an all-encompassing device.

"Venditori de oleum-vipera non vigere excordis populi"


RE: Detecting lost phase with current monitoring.

JesperMP : Thinking about the phase loss consequences “If one phase is lost in the terminal box of one motor (something that will happen in a certain percentage of all cases), then the motor will drop a small fraction in speed - but enough to be out of synchronization with the other motor. The result is wild movements of the conveyor”

Could you install a limit switch close enough to the conveyor to send a trip signal?

RE: Detecting lost phase with current monitoring.

(OP)
jraef.
Thats a very good idea !
The price is approx the same as the CT relays.
But as this also eliminates the normal overload for each motor, it is almost free (so you beat my price ! :)).
And there will be no extra space needed.
All in all, this is the solution I was looking for.

aolalde.
Our experience is that it is a lot more problematic and much less reliable, to try and put a sensor or switch somewhere at the conveyor.

RE: Detecting lost phase with current monitoring.

(OP)
jraef.
Upon futher investigation I must conclude that your suggestion will NOT work !
These electronic overload relays have a charachteristic that are the same as the old bimetallic types with differential sensitivity.
An example is the 3RB10 from Siemens. It will trip upon phase failure or unbalance, IF the current draw is above 85% of the set motor rating.
The 3RB12 has a more advanced characteristic, but it is basically the same.
My requirement is that the relay must trip, even if the conveyor is running idle with a motor current around 50%.

RE: Detecting lost phase with current monitoring.

JesperMP,
I posted a response with several suggested manufacturers who offered a solution, but that post has been removed, probably because it appeared too commercial. Suffice it to say that what you need is available from several other sources, including the one you already looked at. You just need to focus on electronic phase current loss protection.

"Venditori de oleum-vipera non vigere excordis populi"


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