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

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