Calculating motor starting load
Motor starting load is the demand imposed while a motor accelerates from rest. It can exceed the normal running load and cause a temporary voltage or frequency change. A useful assessment considers the starting method, the motor’s acceleration time, and the loads already on the generator.
Understand the input
Starting watts may be listed as total starting demand or as extra watts above normal running demand. Establish which convention a chart or calculator uses. For an illustrative motor rated at 800 W running and 2,400 W total starting demand, the additional starting amount is 1,600 W.
If all selected running loads already total 3,000 W, the corresponding single-start planning scenario is 3,000 + 1,600 = 4,600 W. This arithmetic assumes that the motor is included in the running total and no other relevant start overlaps. It does not validate the motor’s behavior on a particular generator.
When current and kVA matter
A motor may instead provide locked-rotor amps or manufacturer starting-current data. Locked-rotor current describes a stationary rotor condition; the current changes during successful acceleration. Starting kVA is V × A ÷ 1,000 for a single-phase load. For a balanced three-phase load it is √3 × line-to-line V × A ÷ 1,000. [2]
Starting power factor can differ substantially from running power factor. Consequently, multiplying starting current by voltage establishes apparent demand, not automatically real starting watts. Motor size, starting method, driven equipment, and available voltage influence acceleration. A universal multiplier applied to running watts can miss these differences. [2]
Evaluate the whole event
The generator must tolerate the initial event and recover while the motor reaches operating speed. The relevant performance includes voltage dip, frequency response, and duration, alongside kW and kVA. Equipment that appears adequate from a peak-watt label may still have an unsuitable response. [1] [3]
Evaluate each likely sequence, including automatic restarts after transfer to generator power. Enforced delays or compatible starting controls can change the requirement, but guessed reductions should not be entered as verified data. For difficult motors or installed systems, use manufacturer sizing support and measured or documented equipment information.
Sources and further reading
- Cummins — Motor-starting specifications ↗mart.cummins.com
- Cummins — Application manual T-030 ↗powersuite.cummins.com
- Cummins — Transient performance overview ↗www.cummins.com
Use the manual and nameplate for the exact model. Local requirements and site conditions take precedence over general examples.