In every industry or domestic implementations, it is important to have adequate protection for your motor. There are other options such as fuse and circuit breakers that are also alternatively used as protection against thermal overload or phase failure. However, these options protect the circuit and not your motor. Unlike fuse and circuit breakers, overload relays detect the overload on the motor and respond by interrupting the power supply. This can save you several a significant amount by keeping the motor and its winding safe. Here is a complete overview of overload relays for better understanding.
- Understanding the principle
An overload relay consists of a bimetallic strip and relies on its electro-thermal properties for its function. The strip is connected in the motor circuit so that the current flows to the motor through its poles. As the current passes, this strip heats up. If the heat exceeds a set value, the strip bends and breaks the circuit to save your motor.
The relays work in conjunction with the contactors. When the strip is heated, it breaks the power supplied to the contactors. This de-energizes the contactor leading to interruption in power supply to the motor. The tripping time is inversely proportional to the flow of current. This means the more current flow the faster it goes down.
- Types of overload relays
While the principle explained above is generic, it slightly differs for different types of overload relays. Here we discuss various types and their function.
- Thermal OLR
The principle explained above is ideal for a thermal overload relay. It uses a bi-metallic strip through which the entire current to the motor flows. It directly heats the strip and causes a trip in case of overheating. In an alternative method, the strip is placed close to the current-carrying conductor. As the conductor heats up it heats the strip as well.
- Magnetic OLR
Magnetic overload relay works by detecting the strength of the magnetic field generated by the current flowing to the motor. Within the coil holding the motor current, placed a variable magnetic core. The core is dragged up by the flux arrangement within the core. As the displacement reaches a set value the relay trips. Magnetic relay is not affected by ambiance temperature and is used at places with significant temperature variations.
- Electronic OLR
This overload relay uses microprocessor-based protection for your motor. It typically responds to a heat sensor with a current transformer. In the event of overload faults, these relays sense the temperature and trip the circuit to prevent damage. These relays are precise and do not cause energy loss within them.
- Trip class of overload relays
Trip class of an overload relay is typically the time it takes to open the connector and break the circuit in case of overloads. You can find variants as class 5, class 10, class 20, and class 30 which trips in 5, 10, 20, and 30 seconds of 600% of the full load to the motor respectively.
- Applications of overload relays
- Protecting motors
- Declaring trip commands for protective devices by detecting overload or fault conditions
- Developed in solid-state electronics and microprocessors systems
- Deactivating devices when extreme currents are drawn by them
