Describe Your Ideal Overload Relay

Describe Your Ideal Overload Relay

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 electronic overload relays.

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.

Other protection features that should be considered for proactive motor and machine protection include Ground Fault Current, Jam, Stall, Underload, Imbalance, Voltage, and Power protection. Large, expensive motors can benefit from these additional protection features, which can stop the electric motor to help prevent damage before the overload protection algorithms. Only electronic overload relays can provide these additional protection features.

Another design feature you should consider is the diagnostic information that is provided by the overload relay. Electronic overload relays are capable of displaying or communicating information to a control system or operator. At a minimum, the electronic overload relays should communicate Percent Thermal Capacity Utilization (%TCU) and Percent Full Load Amps (%FLA). These two diagnostic parameters report the real-time heat content and current draw of the electric motor and can provide the motor operator with an indication as to when the overload might trip. When the %TCU reaches 100%, the electronic overload will force an overload trip.

Higher end electronic overload relays can provide Time to Trip, Time to Reset, Current RMS, Ground Fault Current, Voltage RMS, Power, Historical, and Energy diagnostic information. This information is useful when protecting expensive or process critical motors. This diagnostic information can project when the overload will trip to allow operators to make critical business operation decisions, and it can be used to provide useful information to maintenance staff to minimize repair time when motor problems un-expectantly occur.

The final design feature you should consider are the integration features overload relays provide. Electronic overload relays have a wide current range, which helps minimize the number of part numbers needed to protect a wide range a motors. Some overload relays mount directly to the load side of a contactor, helping save installation time, space, and wiring. Some overload relays offer removable terminal blocks to minimize replacement time. Some electronic overload relays are modular, allowing the user to pick the specific protection and integration features required for a specific motor starter application. And some overload relays offer mechanical accessories to help reduce the additional control wiring needed for a full voltage starter.

Electronic overload relays with communication and I/O options can easily integrate with their brand's corresponding control system with just a few mouse clicks. These systems use the electronic overload relay as a distributed I/O device to control the contactor coil using a communications network command.

How do you describe your ideal overload relay? The answer to this question really depends on the specific electric motor application. For simple motor control systems controlling non-process critical operations, basic bimetallic or eutectic overload relays will work just fine. For large, complex motor control applications that control process critical operations, a designer should consider electronic overload relays to help reduce the complexity of the control system and provide diagnostic information.

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Bill Martin
Posted October 20, 2014 By Bill Martin, Product Manager, Electronic Overload Relays, Rockwell Automation
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