Generator Sizing: The Motor-Starting Step Matters as Much as Running Load

Adding the running kilowatts of a plant’s loads is a useful starting point for generator selection. It does not describe what happens when a large motor starts while other equipment is already operating. That event needs its own load and transient-performance assessment.

Describe the starting event

A motor’s starting demand differs from its established running demand. The generator’s voltage and frequency response can affect the motor’s ability to accelerate and the behavior of other connected equipment.

Cummins’s motor-starting discussion separates the ability to carry the running load from the permitted voltage and frequency disturbance during starting. It also considers recovery behavior. A nameplate power comparison alone leaves those questions unanswered.

Put the sequence into the enquiry

The T-030 application manual treats individual starting requirements and combined load steps separately. It discusses staged loading as an option where the system provides suitable sequencing.

Give every supplier the same load schedule. For each important motor, include its electrical data, starting method, driven equipment and the loads that remain connected when it starts. Identify which sequence is enforced by controls and which starts can occur independently during normal operation.

Enquiry item	Why it matters
Running load already connected	Establishes the generator condition before the start
Motor and starting arrangement	Defines the starting event to assess
Permitted voltage and frequency response	Connects generator behavior to equipment tolerance
Acceleration and recovery requirements	Defines how long the disturbance may matter
Restart sequence after an interruption	Captures a different case from normal production

Check the difficult case

Do not assume an orderly initial start is the only demanding event. A large motor may restart later when the generator already carries other loads. Ask the application engineer to identify the governing case and the assumptions behind the selection.

Where sequencing reduces the proposed generator size, document the control dependency. The equipment rating and the sequencing arrangement should be accepted as one system, with the site’s required loading priorities retained.

Commissioning should compare the agreed event with recorded voltage, frequency and equipment response. Successful engine start and stable operation at final load are useful checks, but they do not demonstrate every motor-starting condition. Keep the final sequence and acceptance record with the generator documentation.