Overload relay setting and calculation

Overload relay is the one of important device for motor control.It can prevent our motor from overheat or winding burning due overload of ampere.

We need to setting the value of overload relay properly depend on our application and motor full load ampere.If we setting low from FLA,it can cause motor trip c0ntinues and process not running smoothly.

But if we setting to high from FLA,the overload relay cannot protect the motor if overload happen.It can cause the motor failure or winding burns.So we must calculated and get a right setting for the overload relay.

How to setting Overload Relay protection?

Basically to setting the overload value,we refer to this formula :

1) IB ≤ In ≤ IZ

IB = Anticipated operating current of the circuit
IZ = Current carrying capacity of conductor or cable or Motor
In = Rated current of protective device

2) I2 ≤ 1,45  x IZ

Note:
IZ = Current carrying capacity of conductor or cable or Motor

For adjustable protective devices, In corresponds to the value set.
I2 = The current which causes tripping of the protective device under the conditions specified in the equipment regulations (high test current).

Common practice

Normally for overload relay setting depend on FLA ( Full Load Ampere ) of motor.We can see at the NAMEPLATE of motor.Normally setting for overload is 5% until 10 % more than FLA.

But it is depend on operation and functional of motor.For more detail setting,please refer manual guide of motor from manufacture.

We also can setting overload relay value depend on value of service factor motor.Example if service factor 1.15,we can setting 125% from FLA and if service factor is 1.0,we can setting 115% from FLA of motor.

 Manual and automatic reset.

Normally,overload relay have this 2 reset function selection.We can choose manual or automatic reset after it trigger overload from motor.

I recommended for manual reset function because we can know when it trip and we can troubleshooting the root cause for tripping issue.So after we found the root cause,we can reset back the overload relay and can continue the operation.

Comments

  1. hi sir,
    I have one question about the relay settins in real time application. that is
    While changing the relay setting in live air circuit breaker Have we switch off the breaker or no need?
    I will give you one answer please check whether thwt is correct or not?
    While increasing the relay setting of the air circuit breaker no need to switch off the circuit breaker.If we want to decrease the relay setting means we have to switch of the circuit breaker is it correct if this is wrongmeans plese give me correct idea and what is the actual reason and correct answer.
    Regards
    Gopalthulasidoss

  2. chaithu says:

    plz send me all type of cntroling diagrams and explanation
    thnks for giving this information to me

  3. cephaspet says:

    u guys are good thanks somuch

  4. dear sir
    what is formula of curve of overload protection class 10?
    in circuit breaker siemens 3wn6173

  5. Jamie says:

    Hey all,

    I was wondering why my overload relay in an air compressor with a star delta starter is downsized compared to the motor fla?

    • Hi Jamie..
      Your are compressor still running or always stop due to overload relay trip?

      If air-compressor still running :-
      1) check your overload relay condition.Perform test trip to ensure it is function properly,if not change it.
      2) Check your Full Load Ampere using Clamp on meter for actual value and compare with your setting.

      If your compressor always trip :-
      1) Use your actual FLA value and make calculation and set a new setting for your overload relay.

  6. Dear Sir
    i would like to inform you i need electrical easy formula.or all electrical coculletion because my math matik is very week
    1) druop volteg
    2) callculetion of size of cable for equpment ( motol or etc)
    3 from kva to mva 4) motor polarizetion 5) transformer steblity test
    ETC…………

  7. G.Viswanathan says:

    How to check the functionality of Over Load relay without connect the load (May be by control circuit testing)

  8. Hai, I would like to ask,

    I found a single line diagram, overload setting @ current/voltage module is 63A…630A.
    Breaker is 250AF/250AT
    Load is 15.75kW
    ****************************************************
    My calculation: 15.75*1000/sqrt(3)/440V/0.8=25.8A

    My question is
    1) is the overload setting too high? should 25.8A within the overload setting? Can I say minimum value of overload setting cannot lower than 25.8A?
    2) is the MCCB rating too high?current only 25.8A, but the breaker is 250AF?normally, we refer to which value, AF or AT (Because I have seen some stated 160AF/100AT)?why there is two AF and AT for MCCB?

    Thank you

  9. axel musonda says:

    Dear sir
    Am very pleased finding were we can share knowledge about electrical engineering .plis let me receive emails on the email above

  10. AMritpal Singh says:

    Dear sir

    i have a compressor having rating current 0.8A @220volt 5oHz. How can i calculate the overload protection for this compressor .

    • Hi,
      You can used roughly calculation as per below example :-
      Normally I set my overload protection around 115% to 120% from Full Load current.
      For your application :
      (0.8 x 120)/ 100 = 0.96 Ampere –> 1 Ampere

      Thanks

  11. Rajesh Kumar says:

    Dear Sir,

    Please solve my problem ,I want to run a three phase squirrel cage induction motor of 10 HP,415 volts rating by DOL Starter,Star Delta Starter and VFD(30Hz).
    Please tell me the overload relay range and its setting,contactor rating,TPN MCB rating for backup fuse protection and cable size in each case.Sir,please give me the full calculation for this.I shall be highly thankfull for this.
    With Regards,
    Rajesh Kumar

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    found any interesting article like yours. It is pretty worth enough for me.
    In my opinion, if all webmasters and bloggers made good content as you did, the internet will be a lot more useful than ever before.

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