By Bill Martin, product manager, Electronic Overload Relays, Rockwell Automation
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 hp. With so many options available, how do you choose the right overload relay for your motor starter application?
To describe the ideal overload relay for a full-voltage motor starter, consider the following aspects of overload relays:
• Protection features.
• Diagnostic features.
• Integration features.
Protection Features
Thermal Overload Protection. At a minimum, an overload relay must 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.
Two types of overload models are available: I2T and Two Body Model.
I2T is the most common model. It's used by most bimetallic, eutectic and C A.
Two Body Model takes into account an electric motor's starting and running characteristics. It provides a more accurate simulation of the heat content inside an electric motor, especially for large or medium voltage electric motors.
Phase Loss Protection. The second protection feature to consider is phase loss protection. Phase loss is the leading reason for motor failure. It occurs when one phase of current equals 0 amps and results from 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 damage the electric motor, and only electronic overload relays can detect a phase loss condition within three seconds or less.
Other Protection Features. Other protection features to consider 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.
Diagnostic Information
Another design feature to consider is the diagnostic information provided by the overload relay. Electronic overload relays can display or communicate 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 about 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 data is useful when protecting expensive or process-critical motors, and it can project when the overload will trip, allowing operators to make critical business operation decisions. It also can provide useful information to maintenance staff to minimize repair time when motor problems occur unexpectedly.
Integration Features
The final design feature to consider is the integration ability overload relays provide. Electronic overload relays have a wide current range, which helps minimize the amount of part numbers needed to protect a range of 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 integrate easily with its 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.
What is the ideal overload relay? The answer to this question depends on the specific electric motor application. For simple motor control systems controlling nonprocess-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.
Learn more about Rockwell Automation Industrial Control Components.
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