IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Issues with Overhead Electric Traveling Crane
Topic Summary:
Created On: 19 February 2013 09:00 PM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 19 February 2013 09:00 PM
User is offline View Users Profile Print this message



rcs2

Posts: 6
Joined: 25 July 2008

Hi,

We've got a few issues with one of our Overhead Cranes. Basically, when in operation the crane is constantly tripping on O/L current when raising with a load attached. The load is around 60t. However, this only happens at the far end of travel and no other place within the facility. I believe it's due to a voltage drop occurring across the BusBar's as they're around 100m long and 'fed' from the end furthest from the problem. We've taken current readings across all 3 phases (415Vac) and they are (reasonably) balanced across the full travel with load attached. Could the distance of run increase the reactive power and cause the tripping. I'm assuming that an increase in reactive power won't show up on a standard clip-on ammeter. Also, I've noticed during testing that the further the crane moves along the busbar when it picks a load up the lights on the crane dim more the further in moves along but the current measured remains the same.

The FLC of the hoist motor is between 80 > 120A depending on lifting speed. It's a Slip-ring (rotor resist) 3-phase motor.

Any advice would be great.

Ray
 19 February 2013 10:56 PM
User is offline View Users Profile Print this message



Jaymack

Posts: 4785
Joined: 07 April 2004

From the symptoms, I'd be looking for faulty connections on the busbars, particularly the downshop busbars. 100m is not too lengthy, I've had busbars about 1000m length in an aluminium smelter, but fed from both ends to counteract the voltage drop for 2 overhead cranes. (Fun and games with thermal expansion on the smelter startup). There is a distinct danger of hoist motors reversing due to "single phasing", i.e. a hoist load may fall with the load rather than raise it, unless single phase protection is fitted.

Regards
 19 February 2013 11:52 PM
User is offline View Users Profile Print this message


Avatar for Legh.
Legh

Posts: 3538
Joined: 17 December 2004

If the problem is volt drop, although I suspect something more since it has worked up until now, you could move the feeder cables and connect them to the centre of the busbars rather than at the end.
But then it depends how the busbars are positioned to allow for the run of the track.....

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 20 February 2013 07:24 AM
User is offline View Users Profile Print this message



rcs2

Posts: 6
Joined: 25 July 2008

Thanks for the response. Forgot to mention that the busbar is a, rather poor, sectional design with joints every 2/3 meters. Therefore, we've got upwards of 40 joints along the length with around 25% being outside with very little weather protection. Thats why i was wondering about voltdrop (across a bad joint) causing an increased current as the motor will be trying to lift the same load.
 20 February 2013 07:51 AM
User is offline View Users Profile Print this message



Jaymack

Posts: 4785
Joined: 07 April 2004

Originally posted by: rcs2
Therefore, we've got upwards of 40 joints along the length with around 25% being outside with very little weather protection. Thats why i was wondering about voltdrop (across a bad joint) causing an increased current as the motor will be trying to lift the same load.

There will be an increase in current for the "healthy" phases, which may result in motor overheating dependent on the load, but the power output will be less from the motor, under "single phasing" conditions.

No signs of sparking at busbar joints? Fault finding should be possible from the crane gantry, assuming the downshop leads are the problem; it could be shoes but is more probably the busbars. A low resistance ohmmeter, or preferably a Ductor tester could be used ......... initially at the suspect points. Finally, I would be looking at the frequency of maintenance checks, and the stocks of spares.

Regards
 20 February 2013 07:55 AM
User is offline View Users Profile Print this message



Jaymack

Posts: 4785
Joined: 07 April 2004

Originally posted by: Legh
If the problem is volt drop, although I suspect something more since it has worked up until now,



Regards
 20 February 2013 10:45 AM
User is offline View Users Profile Print this message



rcs2

Posts: 6
Joined: 25 July 2008

Jaymack,

Thanks for the responses. We changed the shoes as an easy 'quick fix' but, apart form normal ware & tear, they were fine. Our plant operators (no technical training) have reported sparks but upon inspection there was very little evidence of sparking on either the shoes or bars. The system is around 6-7 years old and the maintenance is reasonable good.

Will keep digging.
 20 February 2013 12:11 PM
User is offline View Users Profile Print this message


Avatar for stateit.
stateit

Posts: 2227
Joined: 15 April 2005

This is certainly nowhere within my realm of experience. But...

A thermal imaging camera would surely help in inspecting the busbar joints.

-------------------------
S George
http://www.sg-electrical.com
 20 February 2013 01:42 PM
User is offline View Users Profile Print this message



Jaymack

Posts: 4785
Joined: 07 April 2004

Originally posted by: stateit
A thermal imaging camera would surely help in inspecting the busbar joints.

Perhaps, but after the fault position, if the camera was following the crane shoes. No current flowing - no thermal image.

Regards
 20 February 2013 02:00 PM
User is offline View Users Profile Print this message



Jaymack

Posts: 4785
Joined: 07 April 2004

Originally posted by: rcs2
We changed the shoes as an easy 'quick fix' but, apart form normal ware & tear, they were fine.

These are shrouded bars no doubt (and expandable to boot, pardon the pun); in which case, sparking at high resistance connections may not be evident. The continuity requires to be tested across each suspect joint, and co-incidently by shaking the busbar. Alternatively 3 voltmeters connected across the incoming phases, but located and viewed on the crane or from below, should detect the phase/s and position of the faulty connection/s, as the crane travels.

Regards
 20 February 2013 05:58 PM
User is offline View Users Profile Print this message



whjohnson

Posts: 750
Joined: 24 January 2009

If it's rotor resistance, then are the resistances and their internal connections ok on each phase?
Is the crane remotely operated via a pendent (and timed relays switching out the resistance banks as the motor speed is increased?)

Or is there an human operator up in the cab 'driving' the thing?

I used to work on some very old Vaughn O/H cranes dating back to the 1940's with exposed round section trolley wires running the full length of the plant for the supplies, and brass shoe collectors (pick-ups) which fed the crane directly from the wires. The crane had to be 'driven' from the cab via controllers which were like the old tramcar controllers with a big handle on top.

Basically, if the operator wound the controller to full speed too quickly, the motor trip would operate (old Allen West dashpot set-up). No problem with the load because there was the biggest eff-off brake solenoid (3 starred coils lifting a massive laminated lever against a spring - they knew how to build em in them days!) which failed 'safe' with no supply.

Just wondering if the rotor resistances are being switched out too quickly, especially as there is a load weight hanging on the hook.
I'd also be checking to see if the hoist motor brake is working properly and not dragging too much at the far end of the run.
Whilst up there, get the fitters to check any drive shaft bearings and gearboxs too!

-------------------------
Political correctness is a doctrine, fostered by a delusional, illogical minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.

Edited: 20 February 2013 at 06:05 PM by whjohnson
 22 February 2013 10:45 AM
User is offline View Users Profile Print this message



jcm256

Posts: 1941
Joined: 01 April 2006

rcs2: What does the Report of Thorough Examination; say about this fault, I am sure this fault would be on the report for further investigation or immediate repair.


The Lifting Operations and Lifting Equipment Regulations 1998 (LOLER) require the user to hold a current Report of Thorough Examination. This equipment requires thorough examination at least every 12 months

Just note: be careful around lifting equipment, no need to tell you that.

http://www.healthandsafetyatwo...ering/wilfred-williams
 24 February 2013 02:09 PM
User is offline View Users Profile Print this message


Avatar for rh2577.
rh2577

Posts: 164
Joined: 31 March 2006

Too much speculation here for a very dangerous item of equipment (I should know I'm involved with crane maintenance and repair). Get a professional crane maintenance company in to check it out.
HTH

-------------------------
Rob Heywood MIET

As you get older, three things happen, the first is that your memory goes and..........I can't recall what the other two things are!

Norman Wisdom
 24 February 2013 05:09 PM
User is offline View Users Profile Print this message



Jaymack

Posts: 4785
Joined: 07 April 2004

Originally posted by: rh2577
Too much speculation here for a very dangerous item of equipment (I should know I'm involved with crane maintenance and repair).

Having been a shift maintenance electrician for many years in the steel industry, the most probable cause of this fault is fairly obvious. An industrial maintenance electrician, requires broader technical knowledge than say one who is only exposed to house wiring.

Get a professional crane maintenance company in to check it out.

Perhaps pertinent for smaller organisations with no skilled maintenance staff, but quite frankly, the interviewing techniques could be to blame, in that the correct calibre of personnel has not been selected.

Regards

I'm just a poor country boy trying to earn a crust!
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.