Why motor starting changes the calculation
A motor can draw several times its normal running current while starting. A generator selected only from the steady operating load may trip, overheat, perform poorly or contribute to equipment damage when that short peak arrives.
Generator selection therefore needs to cover the complete loading profile rather than the motor nameplate power alone.
1. Record the motor requirements
The article notes that starting current can commonly be five to eight times full-load current, depending on the motor and starting arrangement.
- Full-load amperage at the expected operating point
- Motor voltage and phase
- Rated power in kilowatts or horsepower
- Expected inrush current and acceleration time
2. Account for the starting method
- Direct-on-line starting places the full starting demand on the generator.
- Star-delta starting reduces the initial current before changing configuration at speed.
- Soft starters and variable-frequency drives can limit inrush by ramping the motor, but their own supply and steady-state requirements still matter.
3. Match running and peak capacity
Check that continuous or prime power can carry the normal load and that the generator can tolerate the starting peak without unacceptable voltage or frequency disturbance.
Include every other load that may be connected when the motor starts. Starting sequence can be just as important as total connected load.
4. Allow a sensible margin
A margin can accommodate changing loads and voltage dips, but substantially oversizing is not automatically safer. A generator that operates for long periods at too little load can develop its own reliability problems.
Use manufacturer data and have a qualified electrical contractor or engineer confirm the selection before procurement.