Protection cost of any system is 5% of the total system value. Every machine need protection from faults. Overload Protection is Economic protection for LT (415V) motors, If there is a LT motor then there must be Overload relay. Overload relay protects motor from being heated during overload condition. Every motor is designed to run at it's Full load current(FLC). but if you run motor beyond it's FLC then it must be disconnect after some time from supply to increase life of motor. Overload relay do this work.
General Philosophy for setting of Overload relay:-
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| Motor curve vs Thermal relay curve |
As shown in above graph, Current draws by motor is also flow in overload relay. if motor damage time for that overload current is t1 then relay operating time t2 is always less then t1. (t2<t1).
Types of Overload relays:-
There are mainly two type of overload relays is used in plants.
1. Electromagnetic overload relay
2. Electronic overload relay.
1. Electromagnetic overload relay:-
Overload relay mounted in panel of motor, where it's starter (DOL or Star delta) is mounted. As name suggest this type of Overload relay has bimetalic strips in it, which is made by two different metal so it's properties is also different. This bimetaic strip convert temperature change into mechanical displacement, both metal expand at different rate as they are heated, generally steel and brass or steel and copper is used. We have to set current in overload relay which is FLC of motor.
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| Electromagnetic overload relay |
Overload relay is shown in above image which has 3 power terminal (R-Y-B). For setting it is one type of rotary adjuster which can easily move by tester. NO and NC Contact and to reset the relay reset button is provided. In DOL starter NC contact of overload relay is used to drop off the contactor and NO contact is used for trip indication on panel. Operating time of relay is always depend upon overload percentage which is shown in below graph.
Assume your motor set current is 40A. if your motor draws (1.2 times) 48A continuously then it will trip in 1000sec. if your motor draws (1.5 times) 60A continuously then it will trip in 300sec. if your motor draws (2 times) 80A continuously then it will trip in 50sec. (Here i assume that your motor is just started after long time.Cold curve). Motor cannot be start untill overload relay reset.
2. Electronic overload relay:-
Electronic overload relay or Solid state overload relay consist electronic circuit for tripping. It is costlier than electromagnetic one but its accuracy is better than electromagnetic overload relays.
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Electronic overload relay |
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Electronic overload relay without cover |
Trip class in Overload relay:-
Generally Overload relay tripping known from its class, Trip class specify length of time it will take for the relay to open in an overload condition.
As per standards there are 6 classes most common which is Class 5,10,15,20,25 and 30.
Class 5 Suggest that overload relay trip in 5 sec if current flow will become 6 times of set current.
Class 10 suggest relay trip in 10 sec if current flow will become 6 times of set current.
In Electromagnetic overload relay, trip class is fixed whereas in electronic type overload relay we can change the trip class as per our requirement.
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| Class tripping |
If you have any doubt than write in comment section.
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