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motor overload

Discussion in 'Electrics UK' started by ade2k, 9 Mar 2012.

  1. ade2k

    ade2k

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    On a 3 phase motor which has a overload which had tripped could it have tripped due to the motor getting hot and drawing more current ?

    Went to job today where the overload had tripped and needed the button reset on the overload. Reset and the motor ran fine.

    This is the second time it has tripped in 3 mths is this a sign the motor could be on the way out ?

    How you test a overload.
     
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  3. Bolty

    Bolty

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    A little more info is needed here methinks.

    What kind of load does the motor drive (pump, conveyor belt, gearbox etc).

    Overloads will trip usually after one of the following :-
    sustained overload, loss of an incoming supply phase or bad connection / open cct across a contactor, motor winding failure or faulty overload unit.
    Depending on the type of overload, all you usually check is the resistance across the input and output legs of the overload and see if theres any significant difference or open cct. if it's an electronic type, it may be trickier to test, like i said a little more info may help us.

    You may be best measuring the current drawn by each phase to the motor using a clamp meter and seeing if the current drawn is exceeding the motor nameplate.

    I'm called out to stuff like this all the time (usually in the early hours) and twice in 3 months isn't that uncommon but it depends on what the motor is driving.

    Chances are the motor will be fine but please do the usual motor tests to make sure.

    Bolty.
     
  4. ade2k

    ade2k

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    The motor is driving a oil burner nu-way nol 25

    The burner is from 1988 so motor has no data badge left on it.

    The overload was set at 1.5 amp and the other burner in the boiler house was set at 1.2 amp .

    So I think the last person just upped the amps on the overload.

    When reset the burner ran sweet . I have lost my clamp meter as I did not use it in years and got trashed in the van so looks like I will have to invest in a new one.

    Me is thinking new motor time ....
    Are overloads very accurate or can they give problems
     
  5. Bolty

    Bolty

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    motor overloads tend to be fairly accurate (well as accurate as you can be i suppose with a sliding scale for adjustment).

    Like i said before, don't assume the motor is duff. do your checks and watch it when it's running and see if it sounds ok etc. If the motors in a hostile enviroment (excessive heat etc), this will all effect the motors behavour and so the current drawn.

    overloads can and do fail from time to time. i'm maintaining bits of kit with control gear and overloads from the 60's (allen west starters with dash pots & obsolete mte stuff :LOL: ) to brand new kit, and i've had no pattern of increased failure with age. The same can be same with the motors as well.

    If you can go back - do your motor tests, measure the current drawn per phase when running & check the control gear and you'll build a better picture before condeming the motor.

    Intermittent tripping can be bad contactor(s),loose connections at the panel or motor or a dodgy overload unit too. worth bearing in mind.

    Bolty.
     
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  6. Taylortwocities

    Taylortwocities

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  7. conny

    conny

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    Assuming motor is only small unit, (say 0.55kw or 0.75kw),so it will be connected in star at the terminal block. With motor isolated and locked off check the following;
    IR to earth at 1,000v
    Resistance between each phase and starpoint.
    Resistance between phases, (should obviously be approximately double the previous test)
    Remove star point links and test resistance of each phase individually. If there is a significant difference in this test then in all probability motor has suffered damage to windings and best to replace.
    (May be a problem if no nameplate but if you decide to do this post again and I will give you some guidelines to follow to determine probable size).
    From a mechanical point of view can you turn the motor shaft easily? Does it feel smooth and lubricated or do the bearings feel 'dry'? Is there any vertical or axial movement,(backwards and forwards), in the shaft? How is it connected to the pump? (Belts, chains, direct coupling?) Are both units securely fastened to their bases/each other?
    It may have only tripped twice in three months but if not investigated this will happen more frequently until it fails completely. Regular inspection/maintenance can save a fortune in repair costs.
     
  8. ericmark

    ericmark

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    In the main I have found that overloads which habitually trip out over a longish time have a non electrical cause at the root.

    I have had all sorts from over greasing to faulty bearings but normally not an electrical fault

    The problem is measuring the torque. Theory is easy measure the speed and torque and work out horse power required and see if it matches the motor but in real terms you can't measure the torque.

    Removing the motor and checking bearing are free is likely a good start. I had one pump problem were it was first in line and there was water getting into the tank so this pump was often pumping a little water.

    You say you have a number of pumps I would run suspect motor disconnected from pump so no load and measure amps and then one of the other motors again no load and compare results. That should show if it's the motor or what it's driving. Could just swap motors.

    You say old. So have they got dash pots and if so are these filled with oil? You could try a slightly thicker oil in case the hole is worn and allowing it to work a little too quick.
     
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