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Overload Relays (OLR) Protect motors - Working Principle
Overload Relays (OLR) protect electric motors from overheating by monitoring current consumption and breaking the control circuit when excessive, prolonged current is detected. They commonly use bimetallic strips that bend upon heating, triggering a contactor to stop the motor. This prevents insulation damage, burning out, and prolonged overloading.
Key Aspects of Working Principle
- Current Monitoring: The OLR is installed in series with the motor (typically under the contactor) and carries the motor current.
- Thermal Element (Bimetal): In thermal OLRs, heater elements surround bimetallic strips. When the motor draws excessive current, these strips heat up.
- Bending/Tripping: The bimetallic strips consist of two different metals that expand at different rates. Excessive heat causes them to bend, moving a trip lever.
- Circuit Interruption: The lever opens the Normally Closed (NC) contact (
) in the motor control circuit, which de-energizes the contactor coil and cuts power to the motor.
- Resetting: Once the bimetal cools, the relay can be manually or automatically reset to restart the motor.
Key Features
- Time-Delayed: They allow for safe, short-duration high currents (like starting inrush current) but trip on continuous, harmful overloads.
- Protection Type: They safeguard against motor overload, phase failure/loss, and phase imbalance.
- Adjustable: Most thermal OLRs have a dial to set the current limit according to the motor's full-load ampere (FLA) rating.
- Trip Classes: Defined by the IEC Standard 60947-4-1, trip classes (5, 10, 20, 30) indicate the time in seconds the relay takes to trip at 600% of the full-load current.
- Ambient Compensation: Advanced thermal relays use a second bimetallic strip to adjust for external temperature, preventing "nuisance trips" in very hot or cold environments.
- Phase Loss Protection: Many modern OLRs (especially electronic ones) can detect when one phase of a three-phase system fails, preventing motor burnout.
- Reset Modes: Relays can be set to Manual Reset (requiring an operator to push a button after cooling) or Auto-Reset (restarting once the element cools).
For more detailed specifications, you can consult technical guides from manufacturers like Schneider Electric or ABB.
Core Components & Functions
- Heater Elements/Bimetal Strips: Sense current heat.
- Current Setting Dial: Allows adjusting the relay to the motor's full-load amperes (FLA).
- NC Contact (95-96): Breaks the contactor coil circuit to stop the motor.
- NO Contact (97-98): Used for alarm signaling.
- Reset Button: Switches the relay back to operational mode after cooling down.
Common Types & Mechanisms
| Type |
Working Mechanism |
Best Use |
| Thermal (Bimetallic) |
Uses a bimetallic strip (two bonded metals with different expansion rates). Excess current generates heat via a coil, causing the strip to bend and trigger the trip lever. |
Small-to-medium motors with stable loads. |
| Electronic (Solid-State) |
Uses internal current transformers and microprocessors to digitally monitor current. It trips instantly once a digital threshold is reached. |
Large motors or variable-load applications. |
| Magnetic |
Operates on electromagnetic force rather than heat. A magnetic field pulls a plunger to interrupt the circuit when current spikes. |
Quick response needs, like high-speed protection. |
| Eutectic (Melting Alloy) |
A heater coil warms a tube filled with a eutectic alloy. When the alloy melts, it releases a spring-loaded ratchet to trip the relay. |
Applications requiring strictly manual resets. |
Why OLR is Essential
- Overload Protection: Prevents burning from, for example, a jammed conveyor.
- Phase Failure Protection: Protects 3-phase motors if one phase fails.
- Thermal Memory: Simulates motor temperature, ensuring the motor cools down before restarting.
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