ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades Phase loss and consequences

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.