Thermal relay setting calculation is critical for protecting motors from overload and ensuring operational safety. Accurate settings prevent damage and optimize motor lifespan.
This article explores the IEC standards for thermal relay settings, providing formulas, tables, and real-world examples. Learn to calculate and apply settings effectively.
Artificial Intelligence (AI) Calculator for “Thermal Relay Setting Calculator – IEC”
- Motor rated current: 15 A, Thermal relay setting: ?
- Motor rated current: 30 A, Service factor: 1.15, Setting: ?
- Motor rated current: 45 A, Ambient temperature: 40°C, Setting: ?
- Motor rated current: 60 A, Starting current: 6x rated current, Setting: ?
Common Thermal Relay Settings According to IEC Standards
IEC 60947-4-1 and IEC 60255 provide guidelines for thermal overload relay settings. The relay setting is typically based on the motor’s rated current and service factor.
| Motor Rated Current (A) | Service Factor (SF) | Thermal Relay Setting (A) | Typical Application |
|---|---|---|---|
| 5 | 1.0 | 5.0 | Small pumps, fans |
| 10 | 1.15 | 11.5 | Conveyor belts, compressors |
| 20 | 1.0 | 20.0 | Industrial mixers, blowers |
| 30 | 1.15 | 34.5 | Large pumps, crushers |
| 50 | 1.0 | 50.0 | Heavy-duty motors, mills |
| 75 | 1.15 | 86.25 | Industrial presses, crushers |
| 100 | 1.0 | 100.0 | Large compressors, pumps |
Additional Thermal Relay Settings for Various Motor Types
| Motor Type | Typical Service Factor (SF) | Recommended Thermal Relay Setting | Notes |
|---|---|---|---|
| Squirrel Cage Induction | 1.0 – 1.15 | Rated Current × SF | Most common motor type |
| Slip Ring Induction | 1.0 – 1.25 | Rated Current × SF | Higher starting torque motors |
| DC Motors | 1.0 | Rated Current | Thermal relays less common |
| Servo Motors | 1.0 | Manufacturer specified | Often use electronic protection |
Fundamental Formulas for Thermal Relay Setting Calculation – IEC
Thermal overload relays protect motors by tripping when current exceeds a set threshold for a specific time. The setting depends on motor rated current and service factor.
- Thermal Relay Setting (Iset): The current at which the relay trips to protect the motor.
- Rated Motor Current (Irated): The nominal current specified on the motor nameplate.
- Service Factor (SF): A multiplier accounting for motor overload capacity, typically 1.0 to 1.15.
- Ambient Temperature Correction Factor (Kamb): Adjusts setting based on ambient temperature deviations.
Basic Thermal Relay Setting Formula
Where:
- Iset = Thermal relay setting current (Amperes)
- Irated = Motor rated current (Amperes)
- SF = Service factor (dimensionless, typically 1.0 to 1.15)
- Kamb = Ambient temperature correction factor (dimensionless)
Ambient Temperature Correction Factor (Kamb)
IEC standards recommend adjusting relay settings for ambient temperatures different from 40°C (standard reference). Typical values:
| Ambient Temperature (°C) | Correction Factor (Kamb) |
|---|---|
| 20 | 0.85 |
| 30 | 0.93 |
| 40 (Reference) | 1.00 |
| 50 | 1.07 |
| 60 | 1.15 |
Time-Current Characteristic Curve Considerations
Thermal relays trip based on both current magnitude and duration. The trip time decreases as current increases beyond the setting.
Where:
- t = Trip time (seconds)
- k = Constant depending on relay design (typically 10 to 30 seconds)
- I = Actual current (Amperes)
- Iset = Thermal relay setting current (Amperes)
This inverse time characteristic ensures protection against sustained overloads while allowing short surges.
Real-World Application Examples of Thermal Relay Setting Calculation
Example 1: Setting Thermal Relay for a 30 A Motor with Service Factor 1.15 at 40°C
A squirrel cage induction motor has a rated current of 30 A and a service factor of 1.15. The ambient temperature is 40°C. Calculate the thermal relay setting.
- Given: Irated = 30 A
- SF = 1.15
- Kamb = 1.00 (at 40°C)
Using the formula:
The thermal relay should be set to 34.5 A to protect the motor effectively.
Example 2: Adjusting Thermal Relay Setting for a 50 A Motor at 50°C Ambient Temperature
A motor rated at 50 A with a service factor of 1.0 operates in an environment at 50°C. Calculate the adjusted thermal relay setting.
- Given: Irated = 50 A
- SF = 1.0
- Kamb = 1.07 (from ambient temperature table)
Calculation:
The thermal relay setting must be increased to 53.5 A to compensate for the higher ambient temperature.
Additional Technical Considerations for Thermal Relay Settings
- Starting Current Impact: Motors often draw 5-7 times rated current at startup. Thermal relays tolerate short surges without tripping.
- Service Factor Variability: Some motors have service factors up to 1.25; always verify motor nameplate data.
- Ambient Temperature Effects: High ambient temperatures reduce relay trip time; correction factors are essential.
- Relay Calibration: Periodic calibration ensures accuracy and compliance with IEC standards.
- Coordination with Other Protective Devices: Thermal relays should coordinate with short-circuit and earth-fault protection for comprehensive motor safety.
IEC Standards and Guidelines for Thermal Relay Settings
The International Electrotechnical Commission (IEC) provides comprehensive standards for motor protection devices, including thermal overload relays. Key standards include:
- IEC 60255 – Electrical relays for protection, monitoring, and control.
- IEC 60947-4-1 – Low-voltage switchgear and controlgear – Contactors and motor starters.
- IEC 60034 – Rotating electrical machines (motor rated current and service factors).
Adhering to these standards ensures reliable motor protection and compliance with international best practices.
Summary of Best Practices for Thermal Relay Setting Calculation
- Always use the motor’s rated current and service factor from the nameplate.
- Apply ambient temperature correction factors based on installation environment.
- Verify relay trip characteristics to match motor starting and running profiles.
- Coordinate thermal relay settings with upstream and downstream protective devices.
- Regularly test and calibrate thermal relays to maintain protection integrity.
By following these guidelines, engineers can optimize motor protection, reduce downtime, and extend equipment life.