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So, this is something that I shouldn't need to do, but I do. As far as I understand, it should be trivial, bit I'm hazy on the details.

I have a 2kW mains AC power tool with a rotary output. Its reliable, works well, does its job.

Unfortunately it seems to have skipped some kind of surge suppression or energy dump in the electrics, because if the rotary spindle jams in the material it's working on,it blows the mains fuse in the plug. Then I have to stop using it, hunt down a spare 13A cartridge fuse (UK mains style), replace, and continue.

It happens pretty easily, its usual use makes it that way, and I consider the tool safe but poorly designed from that point of view - other rotary tools I own don't blow fuses if they briefly jam.

In an ideal world, I'd switch to a competing model but there isn't an affordable competing model for this specific tool, and the tool itself is quite usable. It just runs through 13A fuses at a rate of 5-10 a day on a busy day.

Now, I routinely strip down, repair, and at times fix and update my equipment. I build my own electrical items. It occurs to me that adding some kind of appropriate surge handling inline between tool and mains plug, would be a tiny item, maybe 2-3 components, to handle it better. I'd trust myself to do this safely, and robustly, but I have no idea of a suitable design.

What would I need to do, so that the transient surge from a jammed output spindle abruptly and temporarily stopping the motor, wouldn't blow the fuse, but current draw from other fault conditions would still blow it, so that it would still perform its safety function correctly?

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  • \$\begingroup\$ You need a motor protection. A well sorted hardware shop or electrical supplies shop should have one. It need to match the motor specs, so show the shop a photo of the sticker on the motor and they should find you a matching one. \$\endgroup\$
    – KristoferA
    Jun 18, 2021 at 9:02
  • \$\begingroup\$ Buy rather than make oneself? Happy to, if I underestimated the task. I didn't think it would need a shop bought item, just a handful of discreets such as a varistor or thyristor, capacitor, transient suppression diode, or whatever. I'd be happy to shop buy, if needed, and it would help to know what aspect of the motor specs they need to know (I only know mains voltage and stated motor power: 2200W @ 230V AC), and a budget example of such a device. But if it is easy with discreet components, I'd still like to know how one would approach it. \$\endgroup\$
    – Stilez
    Jun 18, 2021 at 9:10
  • \$\begingroup\$ If I need more motor info I could probably ask the tool manufacturers tech team, but I'm not sure what I'd need to ask. (Apart from "why doesn't your tool handle surges properly if/when it jams, like everyone else's does", that is ;-) ) \$\endgroup\$
    – Stilez
    Jun 18, 2021 at 9:13
  • \$\begingroup\$ The basic things you need to know to pick the right motor protection is: 1) number of phases, 2) voltage, 3) running current. With that, no need to build something custom, these things are standard components, and they are usually not too expensive. \$\endgroup\$
    – KristoferA
    Jun 18, 2021 at 9:16
  • \$\begingroup\$ It runs off UK mains, which is 2 phase nominal 230V, and is 2200/230 the appropriate value for running current, or should I run it free and under load with an AC ammeter in line? Are those the figures needed? \$\endgroup\$
    – Stilez
    Jun 18, 2021 at 9:24

2 Answers 2

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forget using the terms transient surge, energy dump, surge suppression etc, the problem you describe is a simple motor overload.

The obvious solution is to avoid stalling the motor. A thermal circuit breaker or a class D rated thermal/magnetic circuit breaker might be a possible solution.

A mechanical solution would be a torque limiting clutch.

You've been eager to suggest it is trivial and can be fixed with 2,3 components but not given us the critical piece of information - what sort of power tool is it? And, if it is a common problem encountered with this type of tool, surely there is a common solution for it?

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  • \$\begingroup\$ Its an electrical concrete saw. 300mm rotary blade, for cutting deeply in stone and masonry. Most of these are 2 stroke fuel motors, so there are very few that are safe indoors (fumewise) and most other electrical cutters are sold/priced heavily premium. It is common, but for liquid fuel you'd restart the motor anyway, obviously there's no fuse to blow and no need for a "usual solution". The closest equivalent would be a 230mm angle grinder with a concrete blade, and the angle grinders I know, don't blow the fuse if the blade jams in a cut, so presumably they have overload protection built in. \$\endgroup\$
    – Stilez
    Jun 18, 2021 at 9:20
  • \$\begingroup\$ But its a pretty common tool for anyone needing to cut bricks, blocks, kerbstones, concrete lintels, masonry, and so on. \$\endgroup\$
    – Stilez
    Jun 18, 2021 at 9:22
  • \$\begingroup\$ Avoiding stall is tricky, you don't always know what's in a block of old concrete (hidden steel?) and they often need pressure (hard stone, old flint concrete, dense material). I don't have a way to rebuild the tool itself, to update the clutch. So I'm limited to something that is between the wall socket and the tool itself - inline in the cable from plug to tool, replaces the plug, or which the tool plugs into and itself plugs into the mains. \$\endgroup\$
    – Stilez
    Jun 18, 2021 at 9:28
  • \$\begingroup\$ Use a lighter touch on the feed. Easy for me to say - some jams build catastrophically. \$\endgroup\$
    – glen_geek
    Jun 18, 2021 at 14:27
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The obvious solution would be to use a resettable circuit breaker that trips at a lower current than that which blows the fuse.

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  • \$\begingroup\$ Inline fast RCD? I considered it, but I don't know what spec I'd need to look for, or break time/current etc. Would need to not trip on usual use, but trip fast enough before the fuse can melt - I assume everyday cartridge fuses have typical minimum break time/current curves. I also considered replacing the cartridge fuse by a 20mm antisurge fuse and modifying to accommodate, but I don't believe that taking a safety device out of circuit is anything I'd want to do. That fuse is there as it is, for good reasons, and its staying. Just needs a way to not blow upon a simple overload transient \$\endgroup\$
    – Stilez
    Jun 18, 2021 at 9:39

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