Overview
What are Overload Relays?
Overload relays are electromechanical or electronic protection devices designed to protect electric motors from overheating due to prolonged overcurrent. Unlike fuses or circuit breakers, which primarily protect against short circuits, overload relays monitor the current flowing to the motor over time. When the current exceeds the pre-set limit for a specific duration, the relay trips, opening the control circuit and de-energizing the contactor to prevent insulation damage or motor failure.
Main Manufacturers
PALM Parts Solution stocks a comprehensive range of overload relays from the industry’s leading brands to ensure seamless integration with your existing motor control centers (MCCs):
- Allen-Bradley: Specializing in the E-series electronic and bimetallic designs for NEMA and IEC applications.
- ABB: Known for the TF and EF series, offering high-precision protection for international industrial standards.
- Siemens: Featuring the SIRIUS line of modular protection components designed for high-density control cabinet environments.
Popular Product Families
We provide direct replacements and new components for the following widely utilized series:
- Allen-Bradley 193 Series: Includes the 193-ED and 193-EE E1 Plus electronic overloads, as well as the advanced E300 sensing modules for EtherNet/IP integration.
- ABB TF & EF Series: The TF42 bimetallic series and EF19/EF45 electronic relays provide reliable protection for contactors like the AF series.
- Siemens SIRIUS 3RU & 3RB: The 3RU21 bimetallic relays and 3RB30/3RB31 electronic versions offer integrated auxiliary contacts and manual/auto reset options.
Selection Guide
Choosing the correct overload relay requires matching the device to both the motor’s electrical characteristics and the physical contactor:
- Full Load Amps (FLA): Identify the motor nameplate FLA and ensure the relay’s adjustment range encompasses this value.
- Mounting Style: Determine if you need a direct-mount relay (plugs directly into the load side of a contactor) or a stand-alone relay (requires a separate terminal block or rail mount).
- Reset Method: Decide between manual reset (requires operator intervention) or automatic reset (resets after a cooling period), based on safety protocols.
- Special Requirements: For high-inertia loads, ensure the relay supports Trip Class 20 or 30 to prevent nuisance tripping during long startup sequences.
Replacement & Compatibility
When replacing legacy overload relays, it is often possible to migrate to newer electronic versions for better accuracy. For example, older Allen-Bradley 193-EA units are frequently replaced by the 193-ED or E300 series. If a specific brand is unavailable, many ABB and Siemens units serve as equivalent functional replacements, provided the mounting footprint and trip curves are verified against the original motor starter specifications.
45 products · Allen-Bradley
Frequently asked questions
What is the difference between thermal and electronic overload relays?
Thermal overload relays use a bimetallic strip that bends when heated by excess current, eventually tripping a mechanical switch. Electronic overload relays (EORs) use current transformers and microprocessors to monitor current digitally. Electronic versions offer higher accuracy, wider adjustment ranges, and additional protection features like phase loss detection.
How do I size an overload relay for my motor?
Select an overload relay based on the Motor Full Load Amps (FLA), not the horsepower. Most relays have an adjustable dial; set this to the actual FLA rating found on the motor nameplate. Additionally, match the mounting style (direct mount vs. standalone) to your specific contactor series, such as the Allen-Bradley 100-C or Siemens SIRIUS 3RT.
What do Trip Classes 10, 20, and 30 mean?
Trip classes (Class 10, 20, 30) define the time it takes for a relay to trip under a locked-rotor condition. Class 10 is standard for general-purpose motors (trips in 10 seconds or less at 600% current). Class 20 or 30 is used for high-inertia loads, such as large fans or centrifuges, that require more time to reach operating speed.
Can overload relays communicate with a PLC?
Yes, many modern electronic overload relays, such as the Allen-Bradley E300 or Siemens SIRIUS 3RB24, support communication modules for EtherNet/IP, PROFINET, or Modbus. These allow for remote monitoring of current levels, diagnostics, and energy consumption data via a PLC like a ControlLogix or SIMATIC S7-1500.
How do I adjust the settings for a motor with a high service factor?
If a motor has a Service Factor (SF) of 1.15 or greater, set the relay to the nameplate FLA. If the SF is 1.0, some engineers recommend setting the relay at 90% to 100% of FLA to ensure tighter protection against overheating, though you should always consult the manufacturer’s specific lookup tables for the 193-ED or TF42 series.