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how to set up a motor overload

Discussion in 'Electrics UK' started by markymark2406, 2 Sep 2011.

  1. markymark2406

    markymark2406

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    Just learning a bit on motors and setting up a overload, i have looked around on the internet but there is so much conflicting information. So how do u set up a overload say for example a 3 phase motor in star running at 0.65kw (just figures plucked out of the air)?
     
  2. sparkticus

    sparkticus

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    The overload relay setting depends upon where it is located in circuit, what type of starter circuit you have. It's all about allowing the motor to come up to full speed (load plays a big factor here) while still maintaining overload protection. Have a look at this first then come back with any questions.

    http://www.moeller.net/binary/schabu/sb0801g.pdf



    TIP: always look at the motor plate for the motor current per phase but you sometimes need to factor for different loads (inertial loads for example)


    EDIT, just for clarification when I say relay setting I mean the percentage of full load current (FLC) depending where it is in circuit but not more than 100% unless you can justify it based on type of load. You may see all sorts of comments about 1.1% of FLC and even higher but that is bad practice to generalize. You need to characterise the load first if/before you do that or you could end up owing your company for a motor rewind!!
    If a motor is running at higher current than stated at full load then unless that is a very intermittent state you need a bigger motor or lower load. Or as ericmark implies, the motor needs servicing.
     
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  4. ericmark

    ericmark

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    As said there are so many different starting methods. In the main we buy starters ready built for a set size of motor.

    A lot is experience, where for example a motor trips out on start up we would first check the dash pot for oil. If oil level was OK then we would replace with a thicker oil.

    Today the dash pot is rare but with problem motors we can use a soft start or inverter where again we can select the ramp times to suit.
     
  5. TonyS

    TonyS

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    Thermal O/L’s are set at 100% of FLC, dashpot at 115%.
    For a star/delta starters the setting is .57 of the FLC (be careful, some manufacturers will have applied the correction factor)
     
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  7. ricicle

    ricicle

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    If the overload is in a DOL starter then it will be set to 100% of the F.L.C.
    If it is in the most common type of Star/Delta starters then the O/L will not be monitoring line current, it will be monitoring phase current which is 58% of line current. (As touched on by TonyS)
     
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