Overload relays are critical protective devices designed to prevent motor damage due to excessive current. Calculating the correct overload relay setting ensures optimal motor protection and system reliability.
This article delves into the IEC standards for overload relay calculation, providing formulas, tables, and real-world examples. It equips engineers with precise methods to select and set overload relays accurately.
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- Calculate overload relay setting for a 15 kW, 400 V, 3-phase motor.
- Determine relay current for a motor with 22 A full load current and 1.15 service factor.
- Find overload relay trip current for a 7.5 kW motor with 1.25 service factor.
- Calculate overload relay setting for a 30 kW motor operating at 415 V, 50 Hz.
Common Values for Overload Relay Calculator – IEC
| Motor Power (kW) | Rated Voltage (V) | Full Load Current (A) | Service Factor (SF) | Overload Relay Setting (A) |
|---|---|---|---|---|
| 1.5 | 400 | 3.6 | 1.0 | 3.6 |
| 3.0 | 400 | 6.3 | 1.15 | 7.25 |
| 5.5 | 400 | 10.0 | 1.25 | 12.5 |
| 7.5 | 400 | 14.0 | 1.15 | 16.1 |
| 11 | 400 | 20.0 | 1.0 | 20.0 |
| 15 | 400 | 27.0 | 1.25 | 33.75 |
| 22 | 400 | 38.0 | 1.15 | 43.7 |
| 30 | 400 | 50.0 | 1.0 | 50.0 |
| 37 | 400 | 63.0 | 1.15 | 72.45 |
Fundamental Formulas for Overload Relay Calculation According to IEC Standards
Overload relay settings are primarily based on the motor’s full load current (FLC) and service factor (SF). The IEC 60947-4-1 standard provides guidelines for setting overload relays to protect motors effectively.
- Full Load Current (IFL): The rated current drawn by the motor at full load, typically found on the motor nameplate.
- Service Factor (SF): A multiplier indicating the motor’s ability to operate above its rated load without damage.
- Overload Relay Setting Current (IOL): The current setting on the overload relay to ensure proper motor protection.
1. Basic Overload Relay Setting Formula
Where:
- IOL = Overload relay setting current (Amperes)
- IFL = Motor full load current (Amperes)
- SF = Service factor (unitless, typically 1.0 to 1.25)
This formula ensures the relay setting accommodates the motor’s permissible overload capacity.
2. Adjusted Overload Relay Setting for Ambient Temperature
Ambient temperature affects relay trip characteristics. IEC recommends adjusting the relay setting based on ambient temperature correction factors (Kt).
Where:
- IOL,adj = Adjusted overload relay setting (Amperes)
- Kt = Ambient temperature correction factor (typically between 0.9 and 1.1)
Correction factors are provided by relay manufacturers or IEC guidelines based on ambient temperature ranges.
3. Overload Relay Setting Considering Motor Starting Current
Motors draw a high inrush current during startup, often 6 to 8 times the full load current. Overload relays must tolerate this without nuisance tripping.
Where:
- Istart = Motor starting current (Amperes)
- Ttrip = Time delay before relay trips (seconds)
Relay time-current characteristics must be selected to allow motor startup without tripping.
4. Thermal Overload Relay Trip Time Calculation
IEC overload relays operate on thermal principles. The trip time depends on the current magnitude relative to the relay setting.
Where:
- t = Trip time (seconds)
- k = Relay constant (depends on relay design, typically 10 to 20)
- I = Actual current through relay (Amperes)
- IOL = Overload relay setting current (Amperes)
This inverse time characteristic ensures faster tripping at higher overload currents.
Detailed Real-World Examples of Overload Relay Calculation – IEC
Example 1: Calculating Overload Relay Setting for a 7.5 kW Motor
A 7.5 kW, 400 V, 3-phase motor has a full load current of 14 A and a service factor of 1.15. The ambient temperature correction factor is 1.05. Calculate the overload relay setting.
- Step 1: Identify the full load current (IFL) = 14 A
- Step 2: Service factor (SF) = 1.15
- Step 3: Calculate basic overload relay setting:
- Step 4: Apply ambient temperature correction factor (Kt = 1.05):
Result: Set the overload relay to approximately 17 A to ensure proper motor protection.
Example 2: Selecting Overload Relay for a 22 kW Motor with High Starting Current
A 22 kW, 400 V motor has a full load current of 38 A and a service factor of 1.15. The motor starting current is 6 times the full load current. The relay constant k is 15. Calculate the overload relay setting and estimate the trip time if the motor current reaches 50 A.
- Step 1: Calculate basic overload relay setting:
- Step 2: Verify relay setting against starting current:
The relay must tolerate 228 A for the motor start duration without tripping. This is managed by the relay’s time delay setting.
- Step 3: Calculate trip time for actual current of 50 A:
Interpretation: The relay will trip after approximately 48 seconds if the motor current remains at 50 A, allowing for temporary overloads without nuisance tripping.
Additional Technical Considerations for IEC Overload Relay Calculations
IEC standards emphasize the importance of matching overload relay characteristics with motor and application requirements. Key factors include:
- Motor Starting Method: Direct-on-line (DOL), star-delta, or soft starters affect starting current magnitude and duration.
- Ambient Conditions: High ambient temperatures require derating of relay settings to prevent premature tripping.
- Motor Service Factor: Motors with higher service factors can tolerate higher overloads, influencing relay settings.
- Relay Type: Thermal, electronic, or microprocessor-based overload relays have different trip characteristics and setting ranges.
- Coordination with Other Protective Devices: Overload relays must coordinate with short-circuit protection devices to ensure selective tripping.
IEC 60947-4-1 provides detailed guidance on these aspects, ensuring reliable motor protection and system safety.
Authoritative External Resources
- IEC 60947-4-1: Low-voltage switchgear and controlgear – Contactors and motor-starters
- Schneider Electric – IEC 60947-4-1 Overview and Application Guide
- Eaton – Overload Relay Selection and Application
- ABB – Motor Protection and Overload Relays
By adhering to IEC standards and using precise calculations, engineers can optimize overload relay settings, enhancing motor longevity and operational safety.