Aircon 3ph induction motor (1.5kw) destroyed by phase loss several times, as was a similar compressor motor.
Installed phase loss detectors controlling contactors - latest phase loss caused instantaneous catastrophic failure of the contactor supplying the aircon unit - supply MCBs to the aircon didn't trip but the old rewireables local to the motor did blow on the live phases.
Replaced the burnt bits and the aircon runs fine - why did this happen ??

Not being any sort of expert in power distribution I'm wondering if a recloser chattered the contactor or is this more to do with the motor.

I'm going to replace the simple phase loss detector with a timed unit with over/under voltage detection - am I going the right way ?

Chris

If the phase loss caused the motor to stall then the current on the remaining live phases would increase.

The phase-loss detector would then try to open the contactor so that the latter would be breaking a much higher current than normal.

Could it be that the breaking capacity of the contactor wasn't high enough to cope with this?

Contactor rated at 20HP, motor 2HP

The way I've wired it the 3 phases are fed to the loss detector and the contactor's coil is fed from 2 of the phases - the phase failure occured in one of the phases supplying the contactor coil so it should have dropped out immediately not allowing time for current increase.

(phase appeared to totally drop)

This system works on a compressor which has a larger motor and a smaller contactor.

I'm going to try a Broyce M3PRT loss relay which has time delay and settable voltage dropout to avoid waiting for the contactors coil to dropout in what may be, at the moment, an indeterminate time

(present detector only does phase loss or contactor drops out thro lack of volts)


At least now we are left with a serviceable motor as whatever happened blew the fuses after the contactor - whereas before the motor always died

Chris

PS todays job was fault finding a Matrix display board which was smoking - oh what a joy an Atlas analyser is, it looks like it's aimed at the hobbyist but anyone dealing with semiconductors could benefit from one even if only because it identifies pinouts.

Sounds a curious fault. I must confess that I don't have a lot of practical experience with 3-ph motors.

Any of you industrial guys have any ideas on this one?

Voltage-based phase-loss relays have very limited reliability for motor protection. They should only be relied upon to inhibit motor starting, in what US typically calls "3-wire control" obliging {investigation before} manual restart. Current-balance protection is more reliable, but usually must be inhibited for the interval during starting.

Careful review of claimed protection capabilities is warranted.

Compared to NEMA T-frame motors, sensitivity to and damage from voltage imbalance is typically much greater for hermetic-refrigeration compressors and submersible water pumps, given their usual high current densities and low thermal inertia from compact {induction-motor stator} design.

Thanks everybody

Phase loss relay looked like a simple of the shelf solution to a problem but it seems it's not fullproof.

I'm going to call Broyce about their M3PRT loss relay as it looks better than the one that cooked the contactor in that it is settable for time, voltage and regenerative volts - as usual the quarter page manual is no help at all.

The cause of our problem is twofold, persistant phase failure from a substation and millions of starlings roosting on HV lines - I'm tempted to (ta Bjarney) to not use the reclosure feature of the loss relay and add a manual reset.

Chris

Just as a sideline comment, Chris,
What size and type of MCB was there
protecting the Aircon equipment?,
was it a 3pole interlinked type?

Another related question: What size are the rewireable fuses at the motor?

No details or significant experience here, but what about using a correctly sized IEC-style overload relay to backup existing, with contacts interrupting contactor control? They are sold as being superior for single-phasing protection. Paul?

To be honest I have almost nil experience with these as I get involved with commercial stuff so rarely, but they've certainly got to be better than the old rewireable fuses. I don't why the latter remained popular in the U.K. for so long.

See solid-state overload relays, e.g. http://www.ab.com/power/prodinfo/smp/smp1f.html …then link to "phase loss protection." It is possible a properly sized and set device of this type may be less expensive that a replacement hermetic-refrigeration compressor.



[This message has been edited by Bjarney (edited 08-18-2002).]

Just as a sideline comment, Chris,
What size and type of MCB was there
protecting the Aircon equipment?,
was it a 3pole interlinked type

30A type 'C' 3 pole interlinked which also supplied the control gear and chillers/heaters and another 1.5kw motor which has (strangely) never failed.

Another related question: What size are the rewireable fuses at the motor?

20A which blew nicely (on the reamaining live phases)and left the motor intact.

Previously the motor had died.

Allow me to interject a note for our American friends here:

Type C breakers are usually specified for commercial use, such as for motors as we're discussing here. In general, the tripping characteristic means that the instantaneous trip value is somewhere in the range of 5 to 10x rated current, as opposed to 3 to 5x for the type B breakers which are generally used in domestic panels.

Type C breakers are usually specified for commercial use...

We also have Type 'D' (amongst others) which break at something like 10-14 times rated current - I was looking to replace and old rewireable but Type B + C wouldn't take the inrush - was warned off Type D as it was unlikely that it would trip in the required time as we couldn't supply the fault current so it was back to BS1361 cartridge fuses.

Chris

Just wanted to add some stuff to the great information already posted this thread! Feel free to pass it by if already covered.

The problem of Phase Loss on AC Induction Motors [Polyphase Motors for this discussion] can be disasterous!
It requires "More Than Simple" Detection Devices to eliminate the resulting effects.

For instance, if one Phase is lossed while an Induction Motor is running, the Motor will of course continue running in a "Single Phasing" mode. The Motor will create Voltage and Amperage on the Lossed Phase, since the Motor is similar to a Transformer.
This makes it difficult to use Voltage Detection for Phase Loss while a Motor is running.

When the running Motor begins Single Phasing, it will draw higher current from the intact Phases.

Closely matching the FLA of the Motor with either Overload Relays in all 3 Phases, or fuse size, is the simplest and least expensive way to protect against Phase Loss.

As known, a Polyphase Motor will not start from a non-rotating Rotor [Rotor is completely still] with only Single Phase available. A connected problem here is that if enough Motors are already running and can deliver enough KVA to start a Polyphase Motor during a Phase Loss situation, it will start!

This problem is really complex and has many factors to be considered.
It would make a great topic of discussion in the Theory area if someone would like to discuss it more [hint hint]

I'll check back to see what's up.


Scott S.E.T.

P.S. sorry to jump in here so late!

Hi Scott
I'm a bit tied up with Health & Safety at work at the moment plus it not being phase loss season now - demand is low and the Thrushes haven't arrived to roost on the HV lines yet.

Quote....
For instance, if one Phase is lossed while an Induction Motor is running, the Motor will of course continue running in a "Single Phasing" mode. The Motor will create Voltage and Amperage on the Lossed Phase, since the Motor is similar to a Transformer.
This makes it difficult to use Voltage Detection for Phase Loss while a Motor is running
........

Thanks for that - I hadn't (obviously) given enough thought to what exactly was going on when the phase was either flapping or down completely - the worst of this is that the protection equipment is (as you say) powered and, under fault conditions, back supplied by the circuits I'm trying to protect.
Would be ideal if I had 3 phase in the workshop and could look at the characteristics of the loss relay - it's supposed to drop out smartly at a set voltage but noone knew exactly when it would pull in....the contactor it feeds chatters very badly from 200-300V.
Thinking simplistically (good at that)if the loss relay actually drops out on phase loss I might use the N/O aux contacts of the contactor in parallel with a N/O push button to make a manual start no-volt release system.

As I mentioned it's Safety Policy/Risk Assesment time again and a lot of PAT testing - in the UK we don't have Code Inspectors - here everyone who thinks they have some authority makes up rules on the fly and the PoCo's have no rules - or that's the way it appears...in my job I'm most afraid of the health & Safety people so I go by their interpretations - whatever they are.
I'll try to get the phase loss bits back in their box and see what happens as it's a lot more interesting than I imagined.

Chris