Motor inrush current — what it is and why it is so high
Inrush current (locked rotor current) is the current flowing through induction motor windings at the moment of switch-on, when the rotor is stationary. It is 5-8 times the rated current. For a 22 kW motor that means 250-360A instead of 45A running current. Duration is 0.5 to 10 seconds depending on load.
Calculation formula
- I_start = I_rated x K_start
- I_rated = P / (sqrt3 x U x cos_phi x eff)
- Example: 15 kW, 380V, cos_phi = 0.86, eff = 0.90. I_rated = 29.4A. I_start = 29.4 x 7 = 206A.
Inrush current table for standard motors
| Power | I_rated | I_start (K=7) | I_start (K=5) |
|---|---|---|---|
| 1.5 kW | 3.7 A | 26 A | 18.5 A |
| 4 kW | 9 A | 63 A | 45 A |
| 7.5 kW | 17 A | 119 A | 85 A |
| 15 kW | 32 A | 224 A | 160 A |
| 22 kW | 45 A | 315 A | 225 A |
| 37 kW | 75 A | 525 A | 375 A |
| 55 kW | 110 A | 770 A | 550 A |
How to reduce inrush current
| Method | Inrush current | Cost (22 kW) |
|---|---|---|
| Direct on line | 500-700% | ~500 UAH |
| Star-delta | 200-300% | ~2,000-3,000 UAH |
| Soft starter | 200-350% | ~8,000-15,000 UAH |
| VFD | 100-150% | ~20,000-35,000 UAH |
Why you need to know inrush current
- Circuit breaker selection (D curve for motors)
- Generator sizing (2.5-3x motor power for DOL start)
- Cable cross-section selection
- Starting method selection
Frequently Asked Questions
Where to find the exact starting current multiplier?
On the motor nameplate (NEMA Code Letter) or in the manufacturer catalogue. If unavailable, use K=6-7 for standard motors.
Is inrush current dangerous for the motor?
For the motor itself — no, it is designed for it. Dangerous for the supply (voltage sag), breakers (nuisance tripping) and mechanical drive train (shock).
How long does inrush current last?
From 0.5 to 10 seconds depending on load. No-load motors: 0.5-2 seconds. Heavy start (compressor, crusher): up to 10 seconds.
Does a soft starter reduce starting time?
It increases start time (5-30 second ramp) but reduces peak current. Total starting energy is roughly the same but the peak is much lower.
Which circuit breaker for a motor?
D curve, rated at or slightly above motor I_rated.
Summary
Inrush current = I_rated x 5-7. To reduce it use a soft starter or VFD.
