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 ?

 

[jbartos]

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.\\\

 

[Rodmcm]

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

 

[JesperMP]

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.

 

[Marke]

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

 

[jbartos]

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

 

[Rodmcm]

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?

 

[jbartos]

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

 

[ScottyUK]

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.

 

[Marke]

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

 

[Rodmcm]

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.

 

[JesperMP]

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.

 

[digitrex]

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.

 

[JesperMP]

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.

 

[jraef]

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"

 

[aolalde]

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?

 

[JesperMP]

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.

 

[JesperMP]

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%.

 

[jraef]

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"