13-02-2015

Describe Your Ideal Overload relay

By Bill Martin - Product Manager, Electronic Overload Relays

Electric motors are the backbone of today’s modern industry. They provide the mechanical energy needed for most manufacturing processes. Government agencies have imposed strict requirements for implementing some form of thermal overload protection for all electric motors over 1 horsepower. With so many different options available, how are you supposed to choose the right overload relay for your motor starter application?

To describe the ideal overload relay for a full voltage motor starter, you need to consider the following aspects of overload relays.

• Protection Features
• Diagnostic Features
• Integration Features

At a minimum, an overload relay needs to provide thermal overload protection. This form of protection measures the current being consumed by the electric motor and applies it to a thermal overload model to simulate the heat content inside the electric motor. There are two types of overload models: I2T and Two Body Model. I2T is the most common model, and it is used by most bimetallic, eutectic, and C A Two Body Model takes into an account the starting and running characteristics of an electric motor. This model provides a more accurate simulation of the heat content inside an electric motor, especially for large or medium voltage electric motors.

The second most important protection feature to consider is Phase Loss protection. Phase loss is the leading reason for motor failure. Phase loss occurs when one phase of current equals 0 Amps, and it occurs due to a blown fuse. It takes 30 minutes or more for an overload protection algorithm to detect phase loss. Long term exposure to phase loss will cause damage to the electric motor, and only electronic overload relays can detect a phase loss condition within three seconds or less.