Overload Relay | What is Overload Protection?

Introduction

In the realm of electrical systems, the concept of overload protection is paramount to ensure the safe and efficient operation of equipment, particularly electric motors. At AEC Switcher, as electrical dealers, we recognize the significance of safeguarding motors and electrical circuits from excessive currents that can lead to overheating and damage. This blog aims to shed light on what overload protection entails, how it works, and the different types of overload relay devices used in electrical systems.

Demystifying Overload Protection

Overload protection comes into play when an electric motor draws an excessive amount of current. This condition is aptly termed an overload, and if left uncontrolled, it can result in the motor overheating and potentially damaging its windings. The primary objective of overload protection is to safeguard the motor, its branch circuit, and associated components from the adverse effects of excessive heat generated during an overload.

Overload Relays: The Guardians of Motors

Overload relays are instrumental in preserving the integrity of motors and electrical circuits. These relays are an integral part of motor starters, which comprise a contactor and an overload relay. The role of overload relays is to monitor the current flowing in the circuit supplying power to the motor. If the current exceeds a predetermined threshold over a specific time frame, the overload relay will trip. When it trips, it activates an auxiliary contact, breaking the motor control circuit and de-energizing the contactor. This action essentially cuts off the power supply to the motor, preventing it from overheating and causing damage.

Overload relays typically offer a manual reset option, while some are equipped with an automatic reset feature that re-establishes the circuit connection after a predefined cooling-off period. Once reset, the motor can be restarted for continued operation.

Mechanics of Overload Relays

The overload relay is wired in series with the motor, meaning the current that flows to the motor also passes through the overload relay. It is set to trip when the current exceeds a certain level. This trip action opens the circuit between the motor and the power source. After the cause of the overload is identified and addressed, the overload relay can be reset manually or automatically, allowing the motor to resume operation.

Exploring Different Types of Overload Relays

Bimetallic Overload Relay: Many overload relays utilize bimetallic elements or strips, often referred to as heater elements. These strips are made from two distinct metals with varying coefficients of expansion. The strip is heated by a winding carrying current, causing both metals to expand. However, the metal with the higher coefficient of expansion expands more, causing the strip to bend. This bending triggers an auxiliary contact mechanism, opening the normally closed contact in the overload relay. The heat generated is determined by Joule's Law of Heating (H ∝ I²Rt), where I represents the overcurrent, R is the electrical resistance, and t is the time the current flows through the winding. This design is often chosen when automatic reset is required, but repeated tripping and resetting should be monitored to prevent damage to the motor.

Eutectic Overload Relay: Eutectic overload relays feature a heater winding, a mechanical activation mechanism, and a eutectic alloy. This alloy melts at a specific temperature, and when the heater winding heats the alloy, it melts, releasing a ratchet wheel that activates the tripping mechanism, opening the auxiliary contacts.

Solid State Overload Relay: Solid-state overload relays, also known as electronic overload relays, measure current electronically and are heaterless. They offer easy adjustment of trip times and set points and provide protection against phase loss. Their insensitivity to ambient temperature changes reduces nuisance tripping.

Overload Relay Tripping Mechanism

Overload relays employ a trip class that defines the time taken to open in an overload condition. Common trip classes include 5, 10, 20, and 30, indicating the seconds it takes to trip when the motor is running at 720% of its full load. The choice of trip class depends on the motor's thermal capacity and application requirements.

Conclusion

Overload protection is a critical component in the safe and reliable operation of electrical systems, particularly electric motors. The selection of the appropriate overload relay and understanding its operation is crucial for preserving the integrity of motors and associated electrical circuits. At AEC Switcher, we are committed to providing expert guidance and top-quality electrical solutions to meet your specific needs. It's essential to ensure that your electrical systems are equipped with the right overload protection to prevent damage and maintain operational efficiency.