Detecting lost phase with current monitoring. 8

[JesperMP]

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 ?

 

[electricpete]

I am most familiar with the old electromechanical relays such as:

http://www.geindustrial.com/cwc/products?pnlid=6&famid=31&catid=222&id=ijc&lang=en_US

I am sure there are newer model current balance relays available.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[Dopler]

Try

http://www.selinc.com

Literature

http://www.selinc.com/appguide/9620.pdf

Or

http://www.geindustrial.com/cwc/products?pnlid=6&famid=31&catid=216&id=735737&lang=en_US

Or

http://www.geindustrial.com/cwc/products?pnlid=6&famid=31&catid=218&id=239&lang=en_US

 

[busbar]

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

A utility-grade version is described at

http://www.basler.com/downloads/udj7bull.pdf

 

[MakeItSpin]

http://www.symcominc.com/

has 3 phase current sensing models.

 

[Marke]

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

 

[jbartos]

Suggestion to the original posting: Some manufacturers recommend a replacement/substitute product if they discontinue a product. E.g. visit

http://www.abbus.com/abbrelays/pdf/ILsIBs/Allentwn/46d_H add.pdf

for a proper indication of the replacement/substitute.

 

[JesperMP]

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).

 

[JesperMP]

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.

 

[OperaHouse]

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

http://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.

 

[electricpete]

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?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[JesperMP]

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.

 

[jbartos]

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

 

[JesperMP]

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

 

[PWR]

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

http://www.time-mark.com

 

[JesperMP]

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

 

[electricpete]

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[OperaHouse]

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.

 

[jbartos]

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.\\\

 

[JesperMP]

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