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I have run into a number of instances where my 12A DC brush motor (linear actuator) is being overloaded. I have tried fusing it with a 10A Blade Fuse as well as a 15A Blade Fuse. In both instances, the fuses failed to blow. I was under the impression that under-rating the fuse would cause it to blow sooner, and over-rating it would allow for a bit more amperage draw (or current spike) and blow.

Is there a reason why the fuses are not blowing in either situation?

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  • \$\begingroup\$ What does "overloaded" mean? \$\endgroup\$
    – DKNguyen
    Dec 12 '19 at 2:30
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It takes some time for a fuse to blow. A 10A fuse will blow for sure at current of 14.1 A, but when it depends on fuse characteristics.

You would need a motor protection switch (motor thermal switch), like bimetal. It simulates the temperature rise in the copper winding. When cold it can sustain a large overcurrent, but then when hot, it disconnects.

enter image description here

you have to make sure that this kind of switch has the same current rating as your motor.

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Is there a reason why the fuses are not blowing in either situation?

One reason might be that you aren't actually exceeding the fuse rating. You have two choices to work out the stall current:

  1. Wire an ammeter in series, power it up and stall it briefly to measure the stall current.
  2. Measure the motor's resistance between the power terminals. Then calculate the stall current from Ohm's law, \$ I = \frac {V}{R} \$. You may find that at the voltage you are running at that the stall current is lower than you expected.
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