Odd problem with Fan belts on extract fan.

[Denatone]

This situation has happened at work and I'm wondering if anyone has come across anything similar because i'm a little stumped.

Came in to find HVAC system wasn't running as the BMS was recording an airflow failure in the extract part.
Large belt driven impeller extract fan for the building run from star/delta starter.
Check the control circuit and power circuit for the starter system and the 63A BS88-2 fuses (gG not gM for some reason) in two of the phases had blown.
Check continuity and IR down motor windings everything looks normal
Replace fuses and the motor seems to start and run fine but the BMS is still saying no flow.
Actually physically check the motor and drive-shafts and find that the 3 belts connecting motor to impeller shaft have come off the pulley, not snapped just come off and 2 have big chunks taken out. (spb 4250 belts so not exactly small)
Inspected the shaft, the safety enclosure and the impeller and both are fine and undamaged.

Any ideas as to what caused them to come off and would them coming off cause a sudden increase/decrease in the load that would cause the motor fuses to blow?

Cheers

 

[Marvo]

.........Any ideas as to what caused them to come off and would them coming off cause a sudden increase/decrease in the load that would cause the motor fuses to blow?

Cheers

9 times out of 10 belts would derail off the pulleys due to lack of maintenance. Especially with longer belts they need periodically tensioning to take up the slack and obviously replacing if they show signs of wear and tear. Quite often if they get too slack a single one of the belts derails and it gets dragged around the pulley wheels at high speed by the remaining belts which then also get derailed so it's normal to find that either all the belts are still on or all the belts are off and hence they look like they've been through a war zone.

If you purchase new belts try and get the suppliers to match them as close as possible for length, it makes life easier for setting them up on the fan. When you install them just check the shafts and the pulley wheels are still straight and obviously check the geometry between the two pulley wheels is correct....and don't over tighten them otherwise you'll be replacing bearings next week!!!!!

As for taking out fuses, if the motor tests good and is still running smoothly ie no mechanical damage, the only reason I can imagine is that the derailed belts got tied up around the motor pulley and caused overload or locked rotor. During overload conditions one of the fuses will win the race and immediately the current on the remaining two phases will increase rapidly until a second fuse blows. At this point the current on the remaining phase drops to zero because there's no return path so the third fuse remains in tact.

 

[Denatone]

Yeah that sounds about right. It had me and the boss scratching our heads about it.
Mostly trying to work out if the motor failed and somehow threw the belts or the belts came off and that somehow blew the fuses.
cheers for that i'll make sure the new belts go on proper

 

[Bobster]

Make sure the transition time on the SD starter is set correctly. Too long or too short and it will reduce the lifetime of the belts with undue loading.

 

[Darkwood]

Think Marvo's kind of nailed this on the head and put it to bed ... the fuses blowing is a little bit of a curve ball TBH but plausible as we don't really know what could have shot the current up so fast that the fuses were taken out rather than the overload protection (check this is set up correctly 0.58 x FLC if in Delta leg)... but yes as marvo says it could have locked the rotor long enough if the belts jumped off and jammed between the pulley and housing.

 

[Darkwood]

PS.... I'd be inclined to check manually and electrically the shaft for smooth rotation it could be the case a cracked ball bearing may have locked the rotor jumping the belts off as a consequence, this may have freed itself but could be evident by listening and feeling the shaft while turning it.

 

[Marvo]

As DW rightly added it's worrying that the thermal overload protection should have picked up the fault and tripped before the fuses let go unless the fuses are very closely rated compared to the run current. I'm assuming there is a thermal overload if the fuse type is gG and not aM type. The fact the fuses blew would certainly warrant checking the thermal overload is operational and is correctly set on the current selector and also make sure the thermal isn't set to auto re-close after fault disconnection.

*Edit*
One other thought occurred, it's a long shot but I encountered a large (37kW) forward curved centrif fan many years ago that used to throw belts sometimes when they periodically tested the mechanical fire dampers. When the dampers slammed closed the sudden change in load caused problems. Some fans are more susceptible to overloading under increasing static pressure than others, it's doubtful you'd ever encounter a similar problem with a backward curved fan for example.