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  1. Quote Originally Posted by m_c View Post
    Only the input to the transformer should be fused.

    Fusing the output is more likely to blow up your drivers, than protect anything.
    Is this true!? If so I may be heading in the wrong direction! I already have fuse holders for my drives - could put Neutral Links into them I suppose and convert them into switches! or just take them off... I have seen both sides of this argument and haven't got a clue which is right...

    Also when selecting fuses for the VFD and the 24v supply I'm not sure what I should be getting in terms of current or speed. Apologies - clueless.

  2. #32
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 11 Hours Ago Has been a member for 9-10 years. Has a total post count of 2,125. Received thanks 233 times, giving thanks to others 5 times.
    When modern drivers decelerate motors, the extra energy gets returned back into the power supply. With an unregulated power supply, the energy gets absorbed by the smoothing capacitor(s), which under even hard deceleration of your typical machine during normal use, might lead to a few volts increase.

    If you add a fuse, and that fuse blows for any reason while the motor is moving, there is now nothing to absorb the return energy, which leads to a voltage spike on the drivers input, which can be more than enough to cause the drive to go overvoltage and release magic smoke.

    Plus fuses are not likely to protect the drivers. By the time enough current has flown to blow a fuse, the chances are the electronics in the drive have already failed. Fuse blowing times are measured in milliseconds upwards. Electronic failure will happen in microseconds.

    Fuse selection is generally best to see what manufacturers recommend. For anything with a high start up surge, you really want to use time delay fuses.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  3. #33
    Quote Originally Posted by m_c View Post
    When modern drivers decelerate motors, the extra energy gets returned back into the power supply. With an unregulated power supply, the energy gets absorbed by the smoothing capacitor(s), which under even hard deceleration of your typical machine during normal use, might lead to a few volts increase.

    If you add a fuse, and that fuse blows for any reason while the motor is moving, there is now nothing to absorb the return energy, which leads to a voltage spike on the drivers input, which can be more than enough to cause the drive to go overvoltage and release magic smoke.

    Plus fuses are not likely to protect the drivers. By the time enough current has flown to blow a fuse, the chances are the electronics in the drive have already failed. Fuse blowing times are measured in milliseconds upwards. Electronic failure will happen in microseconds.

    Fuse selection is generally best to see what manufacturers recommend. For anything with a high start up surge, you really want to use time delay fuses.
    Great explanation - it's obvious once you think about the BEMF issue.

  4. #34
    you can have fuses in the DC suppy wiring to stepper drivers provided you have diodes wired across the fuse holder to maintain the path for the returned energy

    Click image for larger version. 

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    John

    PS

    the Gecko G540 has an internal 10A fuse in the positive supply line !
    Last edited by john swift; 05-07-2017 at 10:54 AM. Reason: add PS

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  6. #35
    John,

    What's the formula to workout what size Diodes should be used ?

    I find this one intresting because surly the diodes need to be rated such that they dont negate the use of a fuse in the first instance but can also still handle what may get thrown at them?

    Any clarification is appreciated.
    .Me

  7. #36
    Quote Originally Posted by Lee Roberts View Post
    John,

    What's the formula to workout what size Diodes should be used ?

    I find this one intresting because surly the diodes need to be rated such that they dont negate the use of a fuse in the first instance but can also still handle what may get thrown at them?

    Any clarification is appreciated.
    Lee the diodes will not conduct in normal use but if the fuse does blow and the motors is decelerating it would conduct to dump the BEMF into the caps and it would only happen for a few m/secs a diode handling 1-2A would probably be ok
    ..Clive
    The more you know, The better you know, How little you know

  8. #37
    Hi Lee

    Clive is correct , when a fuse blows the diode will only conduct
    while the motor is generating a higher voltage than the power supplies output

    just because its what I have in my spares box I would use
    a 1N5408 - 1000V PIV 3A diode
    ( I think I used them to rectify the 250V mains )

    with a 70 to 80V DC power supply a 1N5401 - PIV 100V 3A diode would be OK

    John
    Last edited by john swift; 05-07-2017 at 08:06 PM.

  9. Quote Originally Posted by john swift View Post
    Hi Lee

    Clive is correct , when a fuse blows the diode will only conduct
    while the motor is generating a higher voltage than the power supplies output

    just because its what I have in my spares box I would use
    a 1N5408 - 1000V PIV 3A diode
    ( I think I used them to rectify the 250V mains )

    with a 70 to 80V DC power supply a 1N5401 - PIV 100V 3A diode would be OK

    John
    How would you implement this in practice? Solder the diode across the ferrules on the cartridge (I think it would fit back into the holder)?

  10. #39
    Hi Joe

    I would solder the diode across the fuseholders terminals

    Click image for larger version. 

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    John

  11. Thanks John, I have the din mounted flip down type fuse holder so not sure without looking at it if this would be possible but I understand the principal and I'm sure there would be a way!


    Sent from my iPhone using Tapatalk

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