In choosing overload protection of your motor, what factor/s we must consider?

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.

-asically to setting the overload value,we refer to this formula :

1) IB ≤ In ≤ IZ

IB = Anticipated operating current of the circuitIZ = Current carrying capacity of conductor or cable or MotorIn = Rated current of protective device

2) I2 ≤1,  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 % 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 factor1.,we can setting% from FLA and if service factor is1.0,we can setting% from FLA of motor.

 Manual and automatic reset.

Normally,overload relay have this2 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.