Motor inrush current is a critical parameter affecting electrical system design and protection coordination. Calculating it accurately ensures reliable motor starting and system stability.
This article explores motor inrush current calculation methods per IEEE and IEC standards, providing formulas, tables, and practical examples. Engineers will gain comprehensive insights for precise motor current analysis.
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- Calculate inrush current for a 50 HP, 460 V, 3-phase induction motor.
- Determine starting current for a 15 kW motor with a locked rotor current of 6 times full load current.
- Find inrush current for a 100 kW motor using IEC standard locked rotor current values.
- Estimate motor starting current for a 30 HP motor with a service factor of 1.15 and rated current of 40 A.
Common Motor Inrush Current Values – IEEE and IEC Standards
| Motor Type | Rated Power (kW) | Voltage (V) | Full Load Current (A) | Locked Rotor Current (Inrush Current) (A) | Locked Rotor Current Multiple (x FLC) | Standard Reference |
|---|---|---|---|---|---|---|
| Squirrel Cage Induction Motor | 5 | 400 | 9.2 | 55 | 6.0 | IEEE Std 141-1993 |
| Squirrel Cage Induction Motor | 15 | 460 | 22 | 132 | 6.0 | IEC 60034-12 |
| Slip Ring Induction Motor | 30 | 415 | 50 | 150 | 3.0 | IEEE Std 141-1993 |
| Synchronous Motor | 50 | 600 | 60 | 180 | 3.0 | IEC 60034-12 |
| DC Shunt Motor | 10 | 220 | 45 | 135 | 3.0 | IEEE Std 141-1993 |
| Squirrel Cage Induction Motor | 100 | 690 | 110 | 660 | 6.0 | IEC 60034-12 |
Fundamental Formulas for Motor Inrush Current Calculation
Motor inrush current, also known as locked rotor current (ILR), is the current drawn by the motor at the instant of starting when the rotor is stationary. It is typically several times higher than the motor’s full load current (IFL).
1. Locked Rotor Current (Inrush Current) Calculation
The locked rotor current can be estimated using the formula:
- ILR: Locked rotor current (inrush current) in amperes (A)
- K: Locked rotor current multiple (typically 5 to 7 for squirrel cage motors)
- IFL: Full load current of the motor in amperes (A)
The value of K depends on motor design and standards:
- IEEE Std 141-1993 recommends K values between 5 and 7 for squirrel cage motors.
- IEC 60034-12 provides locked rotor current values based on motor rating and type.
2. Full Load Current Calculation
Full load current can be calculated from motor power and voltage using:
- IFL: Full load current (A)
- P: Motor rated power (kW)
- V: Line-to-line voltage (V)
- η: Motor efficiency (decimal, e.g., 0.9)
- PF: Power factor (decimal, e.g., 0.85)
This formula assumes a three-phase motor.
3. Starting Current with Service Factor
Some motors have a service factor (SF) that affects starting current:
- Istart: Starting current (A)
- SF: Service factor (typically 1.0 to 1.15)
4. Motor Starting Current from Locked Rotor Current and Starting Method
Starting current depends on the starting method (direct-on-line, star-delta, soft starter). For star-delta starters:
For soft starters or VFDs, starting current is controlled and typically less than locked rotor current.
Detailed Real-World Examples of Motor Inrush Current Calculation
Example 1: Calculating Locked Rotor Current for a 50 HP Squirrel Cage Induction Motor
A 50 HP (37.3 kW) squirrel cage induction motor operates at 460 V, 60 Hz, with an efficiency of 92% and power factor of 0.88. Calculate the full load current and locked rotor current using IEEE standards.
- Rated power, P = 37.3 kW
- Voltage, V = 460 V
- Efficiency, η = 0.92
- Power factor, PF = 0.88
- Locked rotor current multiple, K = 6 (typical for squirrel cage motor)
Step 1: Calculate full load current (IFL)
Substitute values:
Step 2: Calculate locked rotor current (ILR)
The motor will draw approximately 326 A at startup, which is about six times the full load current.
Example 2: Estimating Starting Current for a 15 kW Motor Using IEC Locked Rotor Current Values
A 15 kW squirrel cage motor rated at 400 V has a full load current of 28 A. According to IEC 60034-12, the locked rotor current multiple for this motor size is approximately 6. Calculate the starting current.
- Rated power, P = 15 kW
- Voltage, V = 400 V
- Full load current, IFL = 28 A
- Locked rotor current multiple, K = 6
Step 1: Calculate locked rotor current (ILR)
Step 2: Adjust for star-delta starting method
The motor starting current with star-delta starter is approximately 97 A, significantly reducing the inrush current impact.
Additional Technical Considerations for Motor Inrush Current
- Impact on Electrical Network: High inrush currents can cause voltage dips, nuisance tripping of protective devices, and mechanical stress on motor components.
- Starting Methods: Direct-on-line (DOL) starting results in the highest inrush current, while star-delta, autotransformer, and soft starters reduce starting current.
- Standards Compliance: IEEE Std 141 (Red Book) and IEC 60034-12 provide guidelines for motor starting currents and locked rotor current values, ensuring consistent design practices.
- Motor Design Influence: Rotor type (squirrel cage vs. wound rotor), motor size, and construction affect locked rotor current magnitude.
- Thermal Effects: Prolonged inrush current can cause thermal stress; hence, motor protection relays must be selected accordingly.
Summary of Locked Rotor Current Multiples by Motor Type and Size
| Motor Type | Power Range (kW) | Locked Rotor Current Multiple (K) | Reference Standard |
|---|---|---|---|
| Squirrel Cage Induction Motor | 0.75 – 15 | 5.5 – 7.0 | IEEE Std 141-1993, IEC 60034-12 |
| Squirrel Cage Induction Motor | 15 – 100 | 5.0 – 6.0 | IEEE Std 141-1993, IEC 60034-12 |
| Slip Ring Induction Motor | All Sizes | 2.5 – 3.5 | IEEE Std 141-1993 |
| Synchronous Motor | All Sizes | 2.5 – 3.5 | IEC 60034-12 |
| DC Shunt Motor | All Sizes | 2.5 – 3.5 | IEEE Std 141-1993 |
References and Further Reading
- IEEE Std 141-1993 (Red Book) – IEEE Recommended Practice for Electric Power Distribution for Industrial Plants
- IEC 60034-12 – Rotating Electrical Machines – Starting Performance of Single-Speed Three-Phase Cage Induction Motors
- NEMA MG 1 – Motors and Generators Standard
- Eaton Motor Protection and Starting Solutions
Understanding motor inrush current per IEEE and IEC standards is essential for electrical engineers designing motor control and protection systems. Accurate calculations prevent equipment damage and ensure system reliability.