Understand why pumps, compressors, fans and other motors can determine generator size. This guide supports early planning; the final generator and electrical installation should be checked against the actual equipment, site conditions and duty.
Running power is only the start
A motor can draw several times its normal current while accelerating. The generator must support the resulting voltage and frequency dip.
Starting method matters
Direct-on-line, star-delta, soft starter and variable-speed drive arrangements impose different demands. Obtain the equipment data rather than applying a generic allowance.
Load sequence matters
Starting one motor at a time can materially reduce the peak step. Controls should prevent an uncontrolled simultaneous restart after mains failure.
Driven equipment matters
A lightly loaded fan and a fully loaded positive-displacement compressor can accelerate very differently even with similar motor nameplates.
Confirm with modelling or testing
For critical projects, use alternator and engine transient data, motor acceleration curves and the acceptable voltage-dip criteria. Commission the actual sequence on site.
Collect the right motor information
Record rated kW, full-load current, efficiency, power factor, starting method, starting current or code letter, acceleration time and the driven machine. The highest-running motor is not always the hardest to start. A loaded compressor or pump may require sustained torque, while a fan with a variable-speed drive may present a much gentler input ramp.
Set acceptable transient limits
The generator engine and alternator respond together to a sudden load. Define the maximum permissible voltage dip, frequency dip and recovery time for other connected equipment. Contactors, controls and electronic systems may drop out even when the generator itself continues running. The allowable disturbance therefore comes from the site loads, not a generic rule.
Reduce demand through sequence and controls
Where operations permit, prevent simultaneous motor starts and reconnect essential loads in a deliberate sequence. Soft starters and variable-speed drives can reduce starting current, but their settings, harmonics and bypass arrangements must be understood. Verify the proposed sequence during commissioning and document it for future controls changes.
Information to send with an enquiry
Share the site location, required voltage and phase, intended prime or standby duty, expected running hours and a schedule of the equipment to be powered. Include motor starting methods, UPS or variable-speed-drive data, the largest load step, fuel-runtime target, noise constraints and the proposed installation position where known. Photographs, electrical drawings and measured load information help the team distinguish firm requirements from early assumptions. FW Power can then discuss an appropriate generator, control arrangement, delivery, installation, commissioning and ongoing service scope without treating a guide calculation as a finished design.