Blows occasional fuse

[fred]

Heavy duty dust extractor. Single phase motor. Every now and then it blows a fuse on start up but when fuse is replaced it will start and stop ok for another few days.

Is this likely to be just motor trouble or could it lie elsewhere. While I have a test meter my knowledge of its use is very sketchy.

Its a big heavy motor perched on top of the machine and I don't relish having to remove it so would welcome suggestions of other possibilities I might check before I go down that road.

Thanks to all

[cl]

Is this a new symptom? I.e. did it work for some years without blowing fuses and then start blowing fuses or has it always done this?

If it has always done it then it's just the start-up surge of the motor that's blowing the fuse. It's obviously marginal (the fuse size that is) and each time the motor starts it *almost* blows the fuse and after a few restarts the fuse does blow.

What size (as in horse-power or watts) is the motor and what fuse is it blowing? Is it a 13 amp fuse in a 3-pin plug or is it one in a fuse box?

[Bob Minchin]

It is quite likely to be a 2 pole motor and so needs to accelerate up to

2850rpm and will draw a lot extra power in doing so. Anything over 1.5HP and 2 pole on single phase is likely to blow 13amp plug fuses from time to time. The solution is a dedicated 16amp commando socket wired through a type c breaker which is specifically designed for motor loads and takes longer to trip.

[meow2222]

Odds are you just need a larger or slower fuse, with emphasis more on slower.

NT

[John Rumm]

Induction motors have a startup current draw that is often at least 5x times their nominal run current and may be as much a 9x...

Hence the need for Type C or D circuit breakers for protection.

[fred]

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Many thanks for all the replies. We have had this unit for many years and i t always ran from a 3 pin plug with a 13 amp fuse. Recently the lead was mo ved from one socket to another nearby and the trouble seems to stem from th at. I'll jury rig a 15 amp fuse and try that out

[newshound]

Presumably running on greased rolling element bearings? Does it still turn freely by hand, since bearing stiffness will increase the startup current. (Agree with other comments though)

[John Rumm]

Even a different socket may have resulted in a small reduction in wiring resistance, and hence more inrush current.

What size motor is it?

[Roger Mills]

Or even a bad contact, causing the plug to overheat.

[Brian_Gaff]

Could be just getting stiffer at start due to grot and gunge in the bearings or whatever, just pushing it over the edge. Brian

[meow2222]

ows a fuse on start up but when fuse is replaced it will start and stop ok for another few days.

e I have a test meter my knowledge of its use is very sketchy.

having to remove it so would welcome suggestions of other possibilities I m ight check before I go down that road.

In the not very likely event that its an open wire fuse that blows, tie som e knots in it, every half inch apart or less. This slows breaking, as the k not absorbs heat for a brief time. This assumes the cabling can cope with t he startup surge - I'd be surprised if it couldnt.

NT