What information should be assessed together when selecting a motor that will run on a generator?

The rated power of the motor alone is not enough information; the moment that strains the generator is the moment the motor starts. On a direct-on-line start, an induction motor takes a multiple of its rated current for a few seconds, and the bulk of that current is reactive. If the generator cannot absorb this surge, the voltage drops, the motor fails to run up, and other devices fed from that line suffer as well.

Data to request from both the motor and the generator

Points that are often overlooked

• Starting sequence: With several motors, it is usually easier to start the largest one first, while the generator is lightly loaded.
• Frequency deviation: If the speed governor of the diesel engine driving the generator reacts late to a load step, the frequency drops momentarily; motor speed follows the frequency.
• Altitude and temperature: Both the generator and the motor lose power at high altitude and in hot surroundings; derating information should be obtained separately from the documentation of each.
• Drive-fed loads: A frequency inverter softens starting but produces harmonic currents; this should also be stated when selecting the generator.

Who should do the sizing?

Whether the generator capacity is sufficient for the motor start becomes clear with the generator supplier's starting calculation. For this calculation, it is enough to pass on the motor's nameplate data and starting method in full. You can read how starting current arises in the article on starting and run-up current, and review candidates by power and speed in the three-phase motor shop.