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When would a 13A fuse blow over a motor overload?

Discussion in 'Electrics UK' started by flyingsparks, 20 Jan 2016.

  1. flyingsparks

    flyingsparks

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    Got a bit of an odd one here. Have a customer with a water pump which is plugged into a socket with a 13A plug. It is controlled by a float switch and pressure switch which operate a contactor which brings on the pump.
    Pump is 2.2kW 10A FLC
    He phoned to say it was blowing fuses every so often (perhaps one every 2 days)
    He has changed the plug with a new one. All OK inside that.
    I get there check the motor - all OK there nothing down to earth and windings seemingly ok. The only thing was the line and neutral in the terminal box had got hot and blackened so re-terminated these.
    I also fitted a 10A motor overload as there wasn't one previously.
    Replaced fuse 13A and all OK, drawing between 8-9A.
    Customer phoned this morning to say the pump had stopped again, but the fuse had blown, and the overload stayed ON.

    Why would this happen? Surely a motor overload set to 10A is going to operate before a 13A fuse (unless a dead short appeared perhaps) but I can't see that has happened. He replaced the fuse again and its all running again.

    Ideas what to look at? Thanks.
     
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  3. bernardgreen

    bernardgreen

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    Motor overload breakers are slow acting to allow the motor to "overload" for a few seconds while running up to speed. Wire fuses aren't delayed and will burn out quickly on a heavy overload.

    Does the motor come up to speed quickly or slowly. What liquid is it pumping, is the liquid thicker ( more difficult to pump ) due to cold or other reason ?

    Can you do an impedence measurement on the windings at 50 Hz ? A shorted turn in the winding may not be detected by a DC resistance check but can caused the motor to take a lot more current than normal.
     
  4. flyingsparks

    flyingsparks

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    Thanks Bernard. It comes up to speed quickly - sounds perfectly normal in that way. It is pumping water, but I don't think it is anything like the water freezing as it seems to be completely random times it blows fuses. The pump runs (when working normally) every 5-10 minutes for about 1 minute, well it was when I was there so assume it is like that most of the time. Not sure how I would do an impedance test at 50 Hz? The only thing I noticed as I left yesterday was the pump started squeaking a bit. Couldn't see any grease nipples or anything on the bearings.
     
  5. bernardgreen

    bernardgreen

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    What sort of pump is it. Is it submerged so the pump is always full of water or is it above the water level and sucks water up from the tank. If it sucks water up then it needs to have water in the pump to lubricate and prime it when it starts. There may be a one way valve that prevent the pump draining back into the tank. ( often called a foot valve ) If this valve is leaking and the pump is dry when it starts then the load on the motor will be very high. Also the pump will wear very quickly. Asign of this is that it takes longer for the pump to bring the level back down to that where the float switch turns the pump off

    It could be a worn bearing, instead of running smoothly round the bearing the balls are a loose fit and vibrate, this can put a lot of extra load on the motor.

    A basic impedance check is to measure the current when an AC voltage is applied to the winding and compare this to the current that a known good motor will take. Some motor manufacturers will provide that information either as an impedance in ohms or as a current at a specific voltage.
     
  6. flyingsparks

    flyingsparks

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    OK Thanks for this. It is an above ground pump. My customer has a company who deal with the pump side of things, so I think I may suggest he get's them in to take a look. He had initially called them about the fact it was blowing fuses and they said to get an electrican to check the motor first. Perhaps it is a pump problem after all? It just seems odd that it could be an intermittant fault as when I was there it was only pulling 8-9A and sounded fine. Even when the squeaking started the current didn't change.
    I assume the impedance check needs a special meter?
     
  7. JohnW2

    JohnW2

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    It sounds as if that has effectively already been done. We are told that the pump is drawing 8-9A with ~230V applied - which sounds about right for a running 2.2kW motor.

    Kind Regards, John
     
  8. JohnW2

    JohnW2

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    Do we know whether the fuses always blow at start-up? If the running current is 8-9A, a start-up current of, say, 2-3 times that (for a very brief period) might be enough to blow a 13A fuse but not to operate a ('slow acting') overload.

    Kind Regards, John
     
  9. stillp

    stillp

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    It'll draw maybe 6 times that on startup. Is the starter rated for frequent starts? The fuse doesn't have much time to cool off between starts.
     
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  11. JohnW2

    JohnW2

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    Indeed - per my last post.

    Kind Regards, John
     
  12. flyingsparks

    flyingsparks

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    I'm not sure if it blows at start up or whilst it's running. The odd thing is this set up has been fine for all the time it has been in (at least 12 months) and only just started giving problems. I know it's not ideal having on a plug perhaps I should wire a dedicated circuit for it see if that helps.
     
  13. JohnW2

    JohnW2

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    As has been suggested, if mechanical factors have resulted in it taking longer to get up to speed, the start-up current may now be persisting for longer, hence increasing the risk of the fuse blowing.
    If you can avoid having a 13A fuse (in plug or FCU), that will obviously make the problem of 'the fuse blowing' go away :) (and any sort of 13A protective device is probably "pushing one's luck" as far as the start-up current of a 2.2kW motor is concerned). What OPD would you use if you put it on a 'dedicated circuit'?

    Kind Regards, John
     
  14. flyingsparks

    flyingsparks

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    This is the thing, I'm not sure if there is any mechanical factors causing an overload every so often when no one is looking at to see what happens!?

    I could stick it on a C16 MCB and do away with the 13A fuse.
     
  15. JohnW2

    JohnW2

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    I would imagine that, if that's what's happening, it would probably being mechanical factors (lubrication/bearing problems, wear, maybe even just semi-blocked waterways) slowing down start-up. One would have to get quite imaginative to think up ways in which a 2.2kW motor could blow a 13A fuse once it was running at speed.
    The motor start-up didn't ought to trip that! I presume that the pumps cable is up to 16A?

    Kind Regards, John.
     
  16. flyingsparks

    flyingsparks

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    Quite, yes it's all on 2.5mm.
     
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