Blown fuse at motor startup

Discussion in 'General Electronics Chat' started by cncman, Apr 3, 2013.

  1. cncman

    Thread Starter New Member

    Apr 3, 2013
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    Hi everybody,
    I´m new to this forum and needing some advise as I´m trying to troubleshoot a problem with a cnc milling machine.
    The milling machine has 2 pcbs, the control stage and the power stage; the power stage feeds the control stage, 3 steppers (X,Y,Z) and it also controls 3 relays (milling motor and auxiliary pump).

    Basically, when I turn the machine on everything seems to be normal. It even has an autocheck to check the steppers and optoswitches. I tried the autocheck and everything is fine but when I start a job, as soon as the milling motor starts, the 2A "slow blow" fuse blows, turning everything off.

    At first, I thought that the 1/4HP motor was the culprit but I checked the windings and the capacitor and they were fine. As I had another motor and cap I swapped them but the problem still exists. Motor is not the problem.

    As I also have a spare control stage I swapped them, even though I thought that the former control stage was ok, and the problem continues.

    So far, I narrowed the search to the power stage.
    I then disconnected the motor and connected a 100W bulb instead.
    When I turn on the machine it starts up normally, steppers move back and forth as usual and when I start a job the bulb lights. So...
    The power stage seems to be working?!

    The only bad thing I found was a value shifted resistor from a snubber (showing 133 ohms instead of 120). I have already replaced it.

    I thought that maybe the main motor relay is failing to contact properly, maybe it can still handle a 100w bulb but not the current draw at motor startup? Any ideas before I start replacing components?

    I would very much appreciate your input.
    Regards.
     
  2. PackratKing

    Well-Known Member

    Jul 13, 2008
    850
    215
    Considering that a 1/4 horse motor will pull close to 30 amps while starting, is your 2-amp fuse that keeps failing, even adequate for the job ???

    You may consider upgrading to a slightly larger value fuse, and see if the problem persists
     
  3. #12

    Expert

    Nov 30, 2010
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    I can't even believe a 2 amp fuse is in series with a quarter horse motor. That seems so wrong! Check again.
     
  4. cncman

    Thread Starter New Member

    Apr 3, 2013
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    0

    Hi PackratKing and #12, thank you very much for your answers.
    Indeed, I mistyped: To make matters worse it is not 1/4HP but 1/2HP!

    This is what I get from the motor plate:

    Mercadal S.A. Barcelona
    Motor type: BMO 70/2
    1- MOT # 369359
    220V 2.7A
    1/2 C.V. / 0.37 kW / S1/ cos phi: 0.92
    2870 rpm / 50 Hz
    Class B insulation /IP 00 / VDE 0530/72
    Cap. 16uF@450V

    The fuse is correct, I checked the service manual and it clearly states T2A@250V slow blow fuse only.
    Any ideas?
     
  5. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
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    I still think the fuse is too small.
    Has this machine been running with a 2 amp fuse?
    Or, is the machine new to you and you don't know if it ran with a 2 amp fuse?
    How can a 2 amp fuse support a 2.7 amp motor?
    Something seems mismatched. Have you connected 220v directly to the motor to see if it runs? If so, have you measured the current?
     
  6. cncman

    Thread Starter New Member

    Apr 3, 2013
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    Hi gerty, the machine has been running with that fuse until it started to blow it at motor startup.
    I have a second identical milling machine that has been running with the original 2 amp fuse since 1995.
    I checked the motor bearings and they are ok, I swapped motors and the "good motor" blows the fuse too. That´s how I discarded a motor issue.
     
  7. #12

    Expert

    Nov 30, 2010
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    Theory: The fuse is not in series with the motor. Is there another way to get power to the motor, or is that 2 amp fuse right at the beginning, with no possible routes around it?

    Very strange problem. It runs a light bulb, but not a motor. Can you post a schematic?
     
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Are you in the states or somewhere with 60hz vs 50hz AC?
     
  9. cncman

    Thread Starter New Member

    Apr 3, 2013
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    Hi mcgyvr, I´m in Uruguay. 220v/50hz AC.
     
  10. Lundwall_Paul

    Member

    Oct 18, 2011
    220
    19
    Use a slow blow fuse.
     
  11. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The OP stated that they have a "slow blow" type in use.
     
  12. cncman

    Thread Starter New Member

    Apr 3, 2013
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    Yes, the fuse is in series with the motor and it is right at the beginning. I will make a drawing and post it tonight.
     
  13. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    with no load, or a very low inertia load, the slow blow's characteristics could start your motor, although code provides for 1.75 times FLA. Partially loaded, you'd have no problems.

    One question would be your source voltage. If you were running below nameplate voltage, expect your current to rise.
     
  14. tuneup1969

    New Member

    Apr 2, 2013
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    0
    All slow blow fuses are not the same. Just because it fits and the rating is the same does not ensure that the time and amperage at which it blows is the same. What are the markings for the blown fuse including mfr. and it's twin in the other machine? Is everything identical including mfr.? If not look them both up online and check the specs of each.
     
  15. tuneup1969

    New Member

    Apr 2, 2013
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    Just had a thought. Since it has a cap it's a more than likely it is a single phase motor. More specifically a split phase motor. In that case it could be that the centrifugal switch is not kicking out the capacitor after start up and your fuse is blowing before the capacitor does. In this case I'd disassemble the motor and check the condition of the switch plate and clutch assembly.
     
  16. strantor

    AAC Fanatic!

    Oct 3, 2010
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    So you've replaced the motor, still blows. replaced the board, still blows. Without seeing it for myself, I assume that the only other component left is the wiring between the two, and that has to be the problem.

    Or, if the machine has installed a 1/2hp motor but only needs (ex.) 1/16 hp to do it's job, that could be the reason for the low value fuse, and perhaps there is some mechanical condition (other than the bearing you've already checked) causing it to actually use the full 1/2hp.
     
  17. gerty

    AAC Fanatic!

    Aug 30, 2007
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    In one of his earlier posts he replaces the motor with a light bulb , and the bulb lights up when the motor is supposed to start. Which would indicate, to me, the wiring is ok.
    I'm still stuck on the 2amp fuse. Motor rated 2.7 amps has to draw several amps at start up.
    I asked if he metered the current while powering the motor without the driver, no reply. And if he's not comfortable doing that, we understand.
     
  18. gerty

    AAC Fanatic!

    Aug 30, 2007
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    In his first post he claims the fuse blows as soon as the motor tries to start. What you're talking about could definitely be a problem, but the motor would run probably for several minutes before getting hot/blowing fuse.
     
  19. cncman

    Thread Starter New Member

    Apr 3, 2013
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    Right, the original fuse is slow blow.
     
  20. cncman

    Thread Starter New Member

    Apr 3, 2013
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    Hi GetDeviceInfo, many thanks.
    Even at full load the 2A fuse should not blow. I have a second identical milling machine that has been running with its original T2A@250 fuse since 1995. Source voltage is correct.
     
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