Thermal overload relays sense motor current by converting this current to heat in a resistance element. The heat generated is used to open a normally closed contact in series with a starter coil causing the motor to be disconnected. Thermal overload relay has many advantages and disadvantages. Let’s check them one by one.
Advantages of thermal overload relay
The benefits of a thermal overload relay are:
- It protects the motor from overheating. It can be used in 1 or 3-phase motors.
- Some overload relays have internal phase loss protection.
- It has an effective operating principle.
- It is a low-cost device.
- It can be mounted directly to a contactor or it can be mounted to the panel with a DIN rail adaptor.
- It has an internal test button used for troubleshooting.
- Some overload relays have an internal trip class selection button.
- It has a wide and adjustable current range.
- It has a reset function. (manual or automatic)
- It has a temperature compensation feature.
- It has a trip-free mechanism.
Disadvantages of thermal overload relay
The drawbacks of a thermal overload relay are:
- It does not have a direct breaking function. It should be used with other switching or protection devices to disconnect the circuit.
- It does not have short-circuit protection.
- It is slow in operation.
- It does not have a high switching frequency. Cooling is necessary after numerous tripping.
- It does not perform well in heavy-duty starting.
- It does not withstand vibration and shock. (Only some special types can perform well)