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.
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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
plz send me all type of cntroling diagrams and explanation
thnks for giving this information to me
u guys are good thanks somuch
dear sir
what is formula of curve of overload protection class 10?
in circuit breaker siemens 3wn6173
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.
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…………
Hi Rajendratiwari,
In my blog,i already discuss about electrical calculation,Please find my post.I also create Electrical Formulas Categories for basic formula and calculation.
Please click link for Voltage drop and Cable sizing calculation.I hope it can help you to understand basic calculation.
How to check the functionality of Over Load relay without connect the load (May be by control circuit testing)
Good question…It hard to tested of overload relay functioning without testing equipment like SIMOCODE pro from siemens.This process available only at Lab test.
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