Selecting the correct contactor for motor applications ensures optimal performance and safety in electrical systems. This calculation involves matching motor ratings with IEC standards for reliable operation.
This article covers detailed formulas, tables, and real-world examples for IEC-compliant contactor selection. It guides engineers through precise motor contactor sizing and verification.
Artificial Intelligence (AI) Calculator for “Contactor Selection for Motors Calculator – IEC”
- ¡Hola! ¿En qué cálculo, conversión o pregunta puedo ayudarte?
- Motor power: 15 kW, Voltage: 400 V, Frequency: 50 Hz, Starting method: Direct On Line (DOL)
- Motor power: 7.5 kW, Voltage: 230 V, Frequency: 60 Hz, Starting method: Star-Delta
- Motor power: 30 kW, Voltage: 690 V, Frequency: 50 Hz, Starting method: Soft Starter
- Motor power: 5.5 kW, Voltage: 400 V, Frequency: 50 Hz, Starting method: Direct On Line (DOL)
Common Values and Tables for Contactor Selection According to IEC Standards
IEC Motor Rated Current (IN) vs Motor Power (P) at 400 V, 50 Hz
| Motor Power (kW) | Rated Current IN (A) | Typical Starting Current (A) | Contactor Rated Operational Current (A) | Recommended Contactor Model (IEC) |
|---|---|---|---|---|
| 0.75 | 2.0 | 6.0 | 6 | LC1D09 |
| 1.5 | 3.5 | 10.5 | 9 | LC1D12 |
| 2.2 | 4.8 | 14.4 | 12 | LC1D18 |
| 3.7 | 7.0 | 21.0 | 18 | LC1D25 |
| 5.5 | 10.0 | 30.0 | 25 | LC1D32 |
| 7.5 | 13.5 | 40.5 | 32 | LC1D40 |
| 11 | 18.0 | 54.0 | 40 | LC1D50 |
| 15 | 24.0 | 72.0 | 65 | LC1D65 |
| 18.5 | 28.0 | 84.0 | 80 | LC1D80 |
| 22 | 34.0 | 102.0 | 95 | LC1D95 |
| 30 | 43.0 | 129.0 | 115 | LC1D115 |
| 37 | 53.0 | 159.0 | 135 | LC1D135 |
Typical Contactor Rated Operational Current (Ie) According to IEC 60947-4-1
| Contactor Model | Rated Operational Current Ie (A) at 400 V AC-3 | Rated Operational Current Ie (A) at 690 V AC-3 | Mechanical Endurance (Operations) | Electrical Endurance (Operations) |
|---|---|---|---|---|
| LC1D09 | 9 | 5.5 | 10 million | 1 million |
| LC1D12 | 12 | 7.5 | 10 million | 1 million |
| LC1D18 | 18 | 11 | 10 million | 1 million |
| LC1D25 | 25 | 15 | 10 million | 1 million |
| LC1D32 | 32 | 18 | 10 million | 1 million |
| LC1D40 | 40 | 22 | 10 million | 1 million |
| LC1D50 | 50 | 28 | 10 million | 1 million |
| LC1D65 | 65 | 36 | 10 million | 1 million |
| LC1D80 | 80 | 45 | 10 million | 1 million |
| LC1D95 | 95 | 55 | 10 million | 1 million |
Typical Motor Starting Current Multipliers for Different Starting Methods
| Starting Method | Starting Current Multiplier (x IN) | Typical Starting Time (seconds) | Comments |
|---|---|---|---|
| Direct On Line (DOL) | 6 – 8 | 0.5 – 2 | Highest starting current, simple control |
| Star-Delta | 2 – 3 | 2 – 5 | Reduced starting current, more complex wiring |
| Soft Starter | 1.5 – 2.5 | 3 – 10 | Controlled ramp-up, reduced mechanical stress |
| Auto Transformer | 3 – 4 | 2 – 5 | Intermediate starting current reduction |
Essential Formulas for Contactor Selection for Motors (IEC)
1. Calculating Motor Rated Current (IN)
The rated current of a three-phase motor can be calculated using the formula:
- IN: Rated current (Amperes, A)
- P: Motor power (kilowatts, kW)
- U: Line-to-line voltage (Volts, V)
- η: Motor efficiency (decimal, typically 0.85 to 0.95)
- cos φ: Power factor (decimal, typically 0.8 to 0.95)
This formula assumes balanced three-phase supply and steady-state operation.
2. Starting Current (Istart)
Starting current depends on the starting method and is a multiple of rated current:
- Istart: Starting current (A)
- Kstart: Starting current multiplier (from starting method table)
- IN: Rated current (A)
3. Contactor Rated Operational Current Selection
The contactor must be selected with a rated operational current (Ie) greater than or equal to the motor’s rated current and capable of handling starting currents:
- Ie: Contactor rated operational current (A)
- IN: Motor rated current (A)
- Istart: Motor starting current (A)
- Safety Factor: Typically 1.2 to 1.5 to account for transient conditions
4. Power Factor and Efficiency Typical Values
| Motor Power (kW) | Typical Efficiency (η) | Typical Power Factor (cos φ) |
|---|---|---|
| 0.75 – 3.7 | 0.80 – 0.88 | 0.75 – 0.85 |
| 4 – 11 | 0.88 – 0.92 | 0.85 – 0.90 |
| 15 – 30 | 0.90 – 0.94 | 0.90 – 0.95 |
| 37 – 75 | 0.92 – 0.96 | 0.92 – 0.96 |
Detailed Real-World Examples of Contactor Selection for Motors (IEC)
Example 1: Selecting a Contactor for a 15 kW Motor at 400 V, 50 Hz, DOL Starting
Given:
- Motor power, P = 15 kW
- Voltage, U = 400 V
- Frequency = 50 Hz
- Starting method = Direct On Line (DOL)
- Efficiency, η = 0.92 (typical for 15 kW motor)
- Power factor, cos φ = 0.90
- Safety factor = 1.3
Step 1: Calculate rated current IN
Step 2: Determine starting current Istart
From the starting method table, DOL starting current multiplier Kstart = 6 (conservative value)
Step 3: Calculate minimum contactor rated operational current Ie
Step 4: Select contactor model
From the contactor table, the closest standard IEC contactor rated operational current ≥ 120.5 A is LC1D115 (115 A) or LC1D135 (135 A). Since 115 A is slightly below 120.5 A, select LC1D135 for safety and reliability.
Final selection: LC1D135 contactor rated for 135 A at 400 V AC-3.
Example 2: Selecting a Contactor for a 7.5 kW Motor at 230 V, 60 Hz, Star-Delta Starting
Given:
- Motor power, P = 7.5 kW
- Voltage, U = 230 V
- Frequency = 60 Hz
- Starting method = Star-Delta
- Efficiency, η = 0.90
- Power factor, cos φ = 0.88
- Safety factor = 1.3
Step 1: Calculate rated current IN
Step 2: Determine starting current Istart
From the starting method table, Star-Delta starting current multiplier Kstart = 3 (conservative value)
Step 3: Calculate minimum contactor rated operational current Ie
Step 4: Select contactor model
From the contactor table, the closest IEC contactor rated operational current ≥ 54.9 A is LC1D50 (50 A) or LC1D65 (65 A). Since 50 A is below 54.9 A, select LC1D65 for safety.
Final selection: LC1D65 contactor rated for 65 A at 230 V AC-3.
Additional Technical Considerations for Contactor Selection
- Voltage Rating: Ensure the contactor voltage rating matches or exceeds the motor supply voltage, including transient overvoltages.
- Utilization Category: IEC 60947-4-1 defines categories such as AC-3 for squirrel cage motors, which is critical for contactor selection.
- Mechanical and Electrical Endurance: Consider the expected number of operations to ensure contactor longevity.
- Auxiliary Contacts: For control and signaling, auxiliary contacts may be required; verify their ratings and configurations.
- Coil Voltage: Select coil voltage compatible with control circuit voltage (e.g., 24 V DC, 230 V AC).
- Ambient Conditions: Temperature, humidity, and installation environment affect contactor performance and must be considered.
- Short-Circuit Withstand: Verify contactor’s short-circuit rating and coordinate with protective devices.
Standards and References
- IEC 60947-4-1: Low-voltage switchgear and controlgear – Contactors and motor-starters
- NEMA Standards for Motor Control
- Schneider Electric LC1D Contactor Catalog
- Eaton Motor Control Products
By following these guidelines, engineers can accurately select IEC-compliant contactors that ensure motor protection, operational reliability, and compliance with international standards.