It is a protection device used to protect motors from overload current (when the
motor draws more current than its rated capacity).
*How OLR Works:
1. Normal Condition:
When the motor is running at normal load, the OLR does nothing. The motor runs
smoothly.
2. Overload Condition:
When the load on the motor increases, the current also increases.
The bimetal strip inside the OLR heats up due to this high current.
3. Bimetal Strip Bends:
As the strip heats, it bends (warps).
This bending activates a trip mechanism.
4. Trip Action:
The trip mechanism disconnects the contactor coil,
cutting off power to the motor - and the motor stops.
5. Reset:
After the OLR cools down, it can be reset manually or automatically to restart the
motor.
Main Parts of OLR:
1. Heater Element- Heats the bimetal strip when current flows.
2. Bimetal Strip - Warps when heated.
3. Trip Mechanism - Breaks the circuit during overload.
4. Reset Button - To restart the system after tripping.
5. NO/NC Contacts - Used in the control circuit for protection.
Example:
Suppose you have a 10A motor and the OLR is set to 10A.
If the motor starts drawing 15A current due to overload,
the OLR will trip within 5-10 seconds, disconnecting the motor to prevent
damage.
Advantages of OLR:
Protects the motor from overheating
Prevents motor burning
Reduces fire and electrical hazards
Increases motor life
Reduces maintenance cost