How long a direct-on-line 90 kW motor takes to bring a large fan up to speed, and why the answer must be compared with the motor's locked-rotor withstand time.
| Motor | 90 kW, 4-pole, rated speed 1485 rpm, direct-on-line start |
| Inertia | Motor rotor 1.2 kg·m² + fan impeller 45 kg·m² (direct drive) = 46.2 kg·m² |
| Average motor torque during run-up | ≈ 1.4 × rated torque (from the motor's speed–torque curve) |
| Average fan load torque during run-up | ≈ 174 N·m (fan torque rises with speed², averaged over the run-up) |
The average is taken over the whole run-up. With the actual motor and load curves, a step-by-step integration gives a more exact result; the average-torque method is the usual first check.
During run-up the motor draws close to its starting current, so the run-up time must be safely shorter than the motor's permissible locked-rotor (stall) time, from the motor data sheet. A common practice is to require the run-up time to be no more than about 80% of the hot stall time, and to set the motor protection between the two.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Acceleration time | Calculated | 11.3 s (classed as a slow start) | ✓ PASS |
| Margin to stall time (example: 15 s hot stall time) | t ≤ 0.8 × 15 = 12 s | 11.3 s — only just within | ✓ PASS |
Key insight: Run-up time grows in direct proportion to inertia. A heavy fan impeller can make the start several times longer than the same motor driving a pump, even when the steady-state power is identical.
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Open Motor Acceleration Time Calculator calculator →Multiply the load inertia by the square of the speed ratio (load speed ÷ motor speed) before adding it to the motor inertia.
Yes. Motor torque falls roughly with the square of the voltage, so a 20% voltage dip during starting cuts torque by about 36% and lengthens the run-up.