Selection Calculation: A Three-Step Process
Selecting a thermal relay (thermal overload relay) involves three steps: matching the rated current, setting the trip current, and coordinating with the contactor.
Industry experts note that the calculation process follows this sequence: rated current (selecting the specification range), trip current (motor rated current × 0.95–1.05), and contactor coordination (matching current ratings).
Since thermal relays protect motors from overload, the setting value is critical.
Three-Step Process
- Rated Current: Select the thermal relay specification (ensure the current range covers the motor’s rated current);
- Trip Current: Setting value = motor rated current (fine-tuned by × 0.95–1.05) for accurate overload protection;
- Contactor Coordination: Match the thermal relay with the contactor (current ratings must align) for a complete assembly;
- Wiring Method: Direct connection or via current transformer (for high currents); select the method based on current;
- Reset Mode: Manual or automatic reset;
select based on the application requirements.
Calculation Key Points
First, ensure accurate settings.
Trip current = motor rated current (for overload protection) to prevent nuisance tripping or failure to protect.
Second, ensure range coverage.
The specification range must cover the motor current (including the adjustable range) to provide a sound basis for selection.
Third, ensure coordinated assembly.
Coordinate with contactors/circuit breakers (protection coordination) for a complete system design.
Fourth, apply environmental corrections.
Adjust settings based on the environment (e.g., temperature or frequency derating).
Pitfalls to Avoid
One common issue is setting the trip current too high.
Setting is too high (fails to protect against overload);
the setting should match the rated value.
Second is “range mismatch”: the specification range does not cover the requirements (leading to protection failure), so the range must be properly matched.
Third is “improper coordination”: incompatibility with the contactor (regarding installation or current);
a coordinated, integrated setup is required.
Trends and Reminders
Motor protection is moving towards “intelligence,” driven by trends in electronic overload protection (offering precise settings) and smart motor protection (IoT integration).
For those selecting equipment, the recommended approach is: “calculate in three steps, set precisely, and ensure coordinated integration.”
Accurate calculation for thermal relays ensures stable motor protection.
