3 Phase motor fault finding Teaser

[AndyD1]

Hi guys,

Sorry in advance for this long winded question but I have just been asked to identify a fault on a 3 phase heating pump in a large commercial premises and would appreciate it if anyone can give me a bit of advice on where to start.

The pump is rated at 15kW (on the plate) and has a full load current of 25.4A it is a soft start pump with invertor and is controlled through a MCCP panel by way of a contactor and BMS.

The issue, apparently, is that the pump runs up ok but within 10minutes or so it is blowing out 2no 40A HRC fuses on two separate phases (red and yellow phases in old money) for no apparent reason. The fuses are not motor rated (they are gG) but with the soft start and length of time to trip in mind I would not think that this was were the issue lies.

The company that installed it (now gone bust) have been back in and are pointing the finger at the contactor, cables and fuses but with no explanation of why (bit of a cop out in my opinion)!!

Any help/ place to start would be much appreciated before my head bad day explode

Many thanks in advance.

Andy D

 

[Andy™]

does the fuses blow when something happens (like a contactor changing or something), or just randomly after a certain amount of time?

id go with some kind of overload issue, expecially if its taking that long to blow. maybe clamp meter on the conductors when its running, see if any phases are using more power than expected, or any differences between the 3

if anything abnormal, IR the windings for any breakdown of insulation

 

[AndyD1]

Cheers Andy,

I haven't seen it happen with my own eyes yet but have been told that it happens after about 10 minutes with no other factors coming into play.

Have already planned the clamp meter check for Monday along with seeing it happen myself. Newish pump so would be surprised if it is the windings but will certainly check with IR test.

Thanks again for the quick reply.

 

[Sidewinder]

Andy, the fuses are upstream of the soft start yes?

Could be the soft start unit?

Other possibility is a control fault.

It may not be the motor at all.

It could be a for example a faulty contactor coil kicking in which is 400V, this then popping the fuses.

Would not be the 1st time I have seen control ccts not fused down on control panels!

Also if these fuses are already taking say 19A each and the control fuses are say 6A it "may" be the case that the 20A HRC's are the weakest link in the chain due to the "base" load?

HTH

 

[revjames]

I would thoroughly check the motor. I have had motors blow fuses because of worn bearings before now. As others have said, check contactor, inverter etc. Remember if a phase is lost then the other 2 will overload straight away. Not always the case with an inverter as a lost phase would bring an error message up but worth checking.

 

[Sidewinder]

rev,

Good point about the brgs but surely you'd hear them before that!

Mind depends on the ambient noise level I spose!

 

[Nicky Tesla]

are the bearings ok, they maybe getting hot after 10 mins and seizing causing the motor to work harder.

 

[Evans Electric]

Suggest you disc. the output to the motor before you IR the windings.

 

[Sidewinder]

Quite Deke!

The soft start may well be vulnerable to IR test voltages "up it"!

 

[Lee321]

what does the soft start say?

 

[oldtimespark]

Is there a wiring diagram available that could be posted.

Some soft starts only control two phase conductor and the third just passes through.

Take a look at the motor info plate to see if it is 415 star 240 delta and check the motor connection to see if it wired correct for the applied voltage.

Is it a inverter (VFD) or a softstart ?

 

[Andy™]

just another thought, if the thermal overload isnt tripping, the fault may be before it?