Two 45kW fans is 45×2=90kW, which appears lower than the 150kW generator set. However, fans belong to asynchronous motor loads. Three key factors shall be taken into consideration: starting inrush current, power factor and derating of generator sets.

1.Basic Power Calculation

Rated active power of two fans: 90kW， For a 150kW standby power diesel generator set, its continuous sustainable output power is generally 0.8 times the standby power, approximately 120kW (prime power).Since 90kW ＜ 120kW, the power demand for steady-state operation can be satisfied.
⚠ Critical risk: motor starting current. When a fan adopts direct-on-line starting, the starting current reaches 5–7 times the rated current, bringing severe impact. The rated current of a single 45kW fan motor is around 85A, and the instantaneous starting current can hit 420–590A. This will lead to sharp voltage drop of the generator, protective tripping and unstable excitation.

2.Conclusions for Three Working Conditions

①: Direct full-voltage starting for both fans (NOT RECOMMENDED)❌ Simultaneous direct starting is prohibited, and successive direct starting of the two fans within a short interval is also not allowed.The inrush current generated when starting the first 45kW fan directly will cause severe voltage dip, which may result in voltage collapse, AVR regulator alarm, generator set protective trip and starting failure.Condition ②: Reduced-voltage starting (Star-Delta / Soft Starter) with sequential startup✅ Feasible with soft starters or star-delta starters, which limit the starting current to 2–3 times the rated current.

Operating requirements:(1) Start one fan first; start the second fan only after rotating speed and voltage become fully stable.(2) Synchronous startup of two fans is forbidden.(3) The power factor of fans is generally around 0.8, and the apparent power of the 150kW unit can fully meet the demand.
Condition ③: The 150kW unit serves as standby unit operating on construction sites under high temperature or high altitudeHigh altitude and high ambient temperature will cause further derating of the generator set. Soft starters are preferred over star-delta starters under such conditions.

3.Practical Recommendations

（1）. Soft starters are preferred; direct full-voltage starting is not allowed.（2）. Startup sequence: Start one fan, wait 30–60 seconds until voltage and frequency stabilize, then start the second one.（3）. It is strictly forbidden to start both fans simultaneously with one single operation.（4）. Cables and air circuit breakers shall be selected considering the motor starting inrush current.(5). Power margin reservation: The long-term operating load shall be controlled within 70%–80% of the set’s prime power. 90kW falls within a reasonable range.

4.Brief Summary

A 150kW diesel generator set can support two 45kW fans under steady-state operation. However, direct full-voltage starting is absolutely prohibited. Equipped with soft starters or star-delta starters and adopting staggered sequential startup, the system can operate normally.