Overload relay — Principle of operation, types, connection

 There is a staggering variety of motor applications, each different, and each with particular overload protection requirements. Exactly how relays achieve their performance to match these requirements is what is critical and what, essentially, separates conventional relays from the newer, electronic models.

What is an electronic overload relay?

The electronic overload relay is a microprocessor-controlled device designed to protect three-phase and one-phase motors against excessive temperature rise from overload and phase failure.

Internal current transformers continuously measure the actual motor current in each phase. When the preset operating value is reached, a trip occurs. After restoring voltage, the relay reverts to its position before the interruption.

Electronic overload relays are the alternative to thermal overload relays. An electronic overload relay offers reliable and fast protection for motors in overload or phase failure.

The electronic overload relays are three pole electromechanical devices. The motor current flows through build-in current transformers, and an evaluation circuit will recognize an overload (overcurrent). The electronic overload relay is self-supplied, which means no extra external supply is needed.

The overload relays have a setting scale in Amperes, which allows the direct adjusting of the relay without any additional calculation. In compliance with international and national standards, the setting current is the rated current of the motor and not the tripping current (no tripping at 1.05 x I, tripping at 1.2 x I; I = setting current). The relays are constructed so that they protect themselves in the event of an overload. The overload relay has to be protected against short circuits.

Advantages of electronic overload relays

Using self-powered electronic overload relays has many advantages for the users, such as:

• With external CTs, they can protect motors up to 1500A
• Reliable, accurate, electronic motor protection
• Easy to select, install and maintain
• Compact size
• Selectable trip class (10A, 10, 20, 30)
• Trip status indicator
• Remote monitoring capabilities
• Status LEDs
• Manual and remote reset