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Commentator

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Choosing a Fuse

05/04/2013 11:56 AM

I don't like to ask questions before I do a proper research, but I'm pressed for time and have to come with a sensible explanation for one of my clients.

It's a contour measuring station ($100k range) that perennially blew fuses in one of the power supply that provide power for two drives (1.5kW each) driving two motors of, so far, unknown specs.

After finding a magnetic break intermittent I thought I hit it, but after that the fuse kept blowing. Checked the drive, did a signature components comparison against the good one, one drives the motor moving camera on Y axis, the other turns the measured piece (the fuse was blowing when the camera was trying to move).

Didn't find anything, check the motor, no abnormalities, so I put 10A fuse (drive commissioning calls for 10A in power supply, application has 6A, and.. everything started working just fine. Lowered the fuse value to 8.5A, works fine for a month now.

There are two more machines like that blowing fuses as well just not that often, I suspect fuse value is underrated

I know without the motor spec it's difficult to rate a fuse. I just need a general good designing approach how to find a proper value. Slow blow, then what % of the max current for starters.

TIA

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Guru

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#1

Re: choosing a fuse

05/04/2013 12:21 PM
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Guru
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#2

Re: choosing a fuse

05/04/2013 1:03 PM

WHAT VOLTAGE?

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Guru

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#3

Re: Choosing a Fuse

05/04/2013 11:14 PM

Unless you have the means to investigate, you are out of luck.

If many examples behave the same way, 10A it is. Sloppy way it is, consider overheating. Please measure at least some parameters.

This really is not a way to do real engineering.

Contradictory? Yes, short of real measurements.

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#4

Re: Choosing a Fuse

05/05/2013 3:19 PM

Are you using fuses with the correct trip curve or just some fast blow types that are partially melting over time due to the high inrush currents which degrade their trip characteristics causing premature fuse operation.

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Commentator

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#5

Re: Choosing a Fuse

05/05/2013 4:48 PM

thanks gents for pitching in. I know without measuring the nominal , max and in-rush current not much can be said, I'll take some measurements next week and follow links that were posted to figure out what should be there..

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Power-User

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#6

Re: Choosing a Fuse

05/06/2013 11:06 AM

May I make a suggestion that you check the wiring, since if the wires are undersized you could be overheating the circuit.

Try putting new larger temporary wires to the machines and see if they still blow the fuses, if they do then my suggestion is moot, otherwise it could be part of the problem.

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Commentator

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#7
In reply to #6

Re: Choosing a Fuse

05/06/2013 11:46 AM
  1. wires are oversized right now. The nominal current that I measured is well below the what wires can handle. Thanks for suggestion though.
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#8

Re: Choosing a Fuse

05/06/2013 3:42 PM

I have found in the pass where fuse continuously blow but everything seem to be to spec to investigate the fuse holder. Not being able to see the fuses myself. Is it truely blown due to over current. Or is it possible that the element separated from the cap. Poor contact in the holder can some times cause the solder to melt. If you can't see the element to tell, cut the fuse apart.

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#9
In reply to #8

Re: Choosing a Fuse

05/06/2013 4:11 PM

thanks, but that's not the case. As I wrote in the beginning, I put 8.5A instead of 6A and unit works for last months without a hitch.

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#10

Re: Choosing a Fuse

05/06/2013 8:28 PM

"...After finding a magnetic break intermittent I thought I hit it..." Assuming that you're referring to a magnetic "brake" you may have "hit on it" and not realized it. Since you posted in an electronics forum, everyone naturally assumed it was an electrical problem, but perhaps it's more mechanical in nature.

You also said that "...the fuse was blowing when the camera was trying to move...", are you sure that the brake is releasing completely, that the slides are not binding, and that there is nothing inhibiting the motion of the camera and/or the workpiece? Without knowing the type of motor or braking mechanism, it's possible that there's enough binding or delayed release to cause more than normal current to flow continuously and pop the fuse on switching transients or the like. After all, it's a system, and a fault in one subsystem can have an effect on the other seemingly unrelated ones.

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#11

Re: Choosing a Fuse

05/09/2013 10:48 AM

Um, what does it say on any of the following:

  • Equipment manufacturer's manual
  • Amtech cable calculation records
  • "For construction" wiring schematics
  • Commissioning records
  • "As built" wiring schematics
  • Handover documentation, or failing all of that
  • British Standard 7671 in relation to the cable size, length and method of installation?
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