Sizing a diesel generator for motor starting requires calculating both the motor's normal running demand and its temporary starting power. Large induction motors can draw several times their rated current during startup, causing voltage dips and engine speed fluctuations. The correct generator size depends on motor starting kVA, starting method, existing electrical loads, and the generator's transient response capability.
For industrial applications, simply adding motor nameplate kW ratings is not sufficient. Engineers must verify that the alternator can support the starting current and that the diesel engine can provide enough mechanical power to accelerate the motor without excessive frequency reduction.
1. Why Do Electric Motors Require More Generator Capacity During Startup?
An induction motor draws a high current when first energized because its rotor is stationary. This condition is commonly called locked-rotor operation. The starting current can be several times higher than the motor's normal full-load current.
For many direct-on-line (DOL) induction motors, starting current is approximately five to eight times full-load current, although the actual value depends on motor design and nameplate data.
This temporary demand creates two important challenges:
- Voltage dip: High starting current causes a temporary reduction in generator terminal voltage.
- Frequency dip: The sudden increase in real power demand can slow the engine before its governor restores operating speed.
If voltage drops excessively, the motor may develop insufficient starting torque. Contactors can also drop out, and sensitive equipment connected to the same electrical system may malfunction.
The generator must therefore support both the motor's starting requirements and the loads already operating when the motor starts.
2. How to Calculate Motor Running kVA and Starting kVA
Motor nameplate information provides the starting point for generator sizing. Engineers should collect rated power, voltage, full-load current, efficiency, power factor, and starting characteristics.
For a three-phase motor, approximate running apparent power can be calculated using:
Running kVA = √3 × Voltage × Full-Load Current ÷ 1,000
Alternatively, when motor output power, efficiency, and power factor are known:
Running kVA = Motor Output (kW) ÷ (Efficiency × Power Factor)
Starting apparent power can be estimated from locked-rotor current:
Starting kVA = √3 × Voltage × Starting Current ÷ 1,000
When only the running current is available, a preliminary estimate may use a starting-current multiplier. However, the actual locked-rotor current or motor starting code should be used for engineering calculations.
Worked example: A 30 kW industrial motor
Assume an industrial pump has the following specifications:
- Motor output: 30 kW
- Voltage: 400 V, three-phase
- Efficiency: 92%
- Running power factor: 0.85
- Starting current: Six times full-load current
First, calculate the approximate running kVA:
30 ÷ (0.92 × 0.85) = 38.3 kVA
The full-load current is approximately:
38,300 ÷ (√3 × 400) = 55.3 A
Estimated starting current:
55.3 × 6 = 331.8 A
Estimated starting kVA:
√3 × 400 × 331.8 ÷ 1,000 = 229.9 kVA
The motor therefore requires approximately 38.3 kVA during normal operation but may demand around 230 kVA during direct-on-line startup under these assumptions.
This illustrates why motor starting can determine the alternator size even when the motor's running power appears relatively small.
3. How Different Motor Starting Methods Affect Generator Size
The motor starting method has a major influence on the electrical demand placed on a generator. Reducing the applied voltage or controlling motor acceleration can lower starting current, although the available starting torque must remain sufficient for the driven equipment.
| Starting Method | Starting Current | Generator Sizing Impact |
|---|---|---|
| Direct-on-line (DOL) | Often around 5–8 times full-load current | Usually creates the highest instantaneous starting demand |
| Star-delta | Typically around one-third of DOL line starting current under comparable conditions | Reduces starting demand but also reduces starting torque |
| Soft starter | Controlled by the configured current limit | Can reduce starting kVA and voltage dip |
| Variable-frequency drive (VFD) | Generally much lower than DOL starting | Controls acceleration but introduces harmonic considerations |
| Autotransformer starter | Reduced according to the selected voltage tap | Reduces starting current while retaining more flexibility in starting torque |
For the 30 kW pump in the previous example, a suitable reduced-voltage starter could substantially reduce starting kVA compared with DOL operation.
However, reduced-voltage starting also reduces motor torque. For a centrifugal pump starting under certain conditions, this may be acceptable. For a heavily loaded compressor or conveyor, insufficient torque may prevent successful acceleration.
VFDs can provide controlled acceleration and reduce direct motor inrush. However, their harmonic currents may require additional alternator capacity or suitable harmonic mitigation.
The selected starting method should therefore match the mechanical load, process requirements, and generator characteristics.
4. How to Determine the Generator Size for Motor Starting
The final generator capacity must satisfy both the normal running load and the largest starting event. These are separate checks because engine kW capability and alternator kVA capability impose different limitations.
Follow this engineering process:
- List all connected loads: Record the running kW, kVA, and power factor of each machine.
- Identify the largest motor: Obtain its full-load current, locked-rotor current, and starting method.
- Calculate the existing load: Determine which equipment remains operating when the motor starts.
- Calculate starting demand: Establish the motor's starting kVA and starting power factor.
- Check voltage dip: Compare the predicted voltage dip with the limits of the motor and other connected equipment.
- Check frequency response: Confirm that the engine can accept the required real-power load without excessive speed reduction.
- Verify the complete generator set: Use manufacturer motor-starting curves and transient performance data.
Do not simply add the motor's starting kVA to the facility's total running kVA and treat the result as the final generator rating. Starting and running loads can have different power factors and transient characteristics.
The alternator must support the electrical current and reactive power, while the engine must provide sufficient real power throughout motor acceleration.
For accurate selection, request the manufacturer's motor-starting capability curves and confirm the expected voltage and frequency response for the complete generator set.
5. Voltage Dip, Frequency Dip, and Generator Transient Response
Voltage dip describes the temporary reduction in generator output voltage when a large motor starts. Frequency dip describes the temporary reduction in electrical frequency caused by engine speed slowing under sudden real-power demand.
These two responses are related but are not identical.
Voltage performance depends on:
- Alternator subtransient and transient reactance.
- Excitation system and AVR response.
- Motor locked-rotor kVA and starting power factor.
- Other connected electrical loads.
- Permissible voltage reduction of the connected equipment.
Frequency performance depends on:
- Diesel engine power reserve.
- Governor response and engine inertia.
- Magnitude of the sudden real-power demand.
- Motor acceleration time.
- Existing generator loading.
There is no single voltage-dip percentage that suits every application. The acceptable limit depends on motor contactors, control equipment, mechanical load, and project specifications.
Cummins discusses motor starting, voltage dip, and generator recovery in its Liquid-Cooled Generator Set Application Manual. Its engineering guidance emphasizes evaluating both initial voltage dip and recovery behavior rather than relying solely on running kVA.
6. Practical Ways to Reduce Generator Requirements
When motor starting demand makes a generator excessively large, engineers can often improve the electrical arrangement without increasing generator capacity unnecessarily.
Consider these measures:
- Sequential starting: Start large motors individually rather than simultaneously.
- Soft starters: Limit motor starting current while maintaining adequate starting torque.
- VFDs: Control motor acceleration and operating speed where the process permits.
- Load management: Temporarily disconnect nonessential loads during major motor starts.
- Generator paralleling: Use multiple synchronized generator sets when project capacity and redundancy requirements justify it.
- Appropriate alternator selection: Select an alternator with suitable motor-starting and voltage-regulation performance.
For industrial sites with multiple pumps or compressors, sequential starting can significantly reduce the maximum simultaneous demand. This can help avoid purchasing a generator sized for an unnecessarily severe startup condition.
For additional capacity calculations, consult the diesel generator sizing guide. It explains how connected loads, power factor, and site conditions influence generator selection.
When reviewing ZLN Power diesel generator solutions, provide the motor nameplates, starting methods, existing running loads, and acceptable voltage-dip requirements. These details allow the proposed engine and alternator configuration to be evaluated against the actual application.
Conclusion
Sizing a diesel generator for motor starting requires more than calculating the motor's rated kW. Engineers must evaluate locked-rotor current, starting kVA, existing loads, voltage dip, frequency response, and the generator's transient capabilities.
The most reliable approach combines motor nameplate information with manufacturer generator performance curves. Suitable starting methods and sequential load management can also reduce peak demand and improve overall system reliability.
Frequently Asked Questions
How much larger should a generator be for motor starting?
There is no universal oversizing multiplier. The required capacity depends on the motor's locked-rotor kVA, starting method, existing load, acceptable voltage dip, and the generator's transient performance. Manufacturer motor-starting data should determine the final selection.
How many times the full-load current does a motor draw during startup?
Many direct-on-line induction motors draw approximately five to eight times their full-load current during startup. The exact starting current depends on motor design and should be verified using the motor nameplate or manufacturer documentation.
Does a soft starter reduce the required generator size?
It can. A soft starter limits motor starting current and may reduce the required alternator capacity. However, sufficient starting torque must remain available for the mechanical load.
Why does generator voltage drop when a large motor starts?
High motor starting current creates a sudden electrical demand that causes voltage to fall across the alternator's internal impedance. The AVR then responds to restore voltage, while the engine governor manages the associated speed and frequency changes.
Can a VFD solve motor starting problems with a diesel generator?
A VFD can reduce direct motor inrush by controlling acceleration. However, harmonic currents and drive compatibility must also be assessed to ensure stable generator operation.
Reference Sources
- Cummins – Liquid-Cooled Generator Set Application Manual. Covers motor starting loads, locked-rotor kVA, voltage dip, and generator sizing.
- Cummins – Specifying and Validating Motor Starting Capability. Explains motor-starting performance, alternator excitation, and voltage recovery requirements.
- Kohler – Sizing Generators for Motor Starting. Discusses locked-rotor kVA, transient voltage dip, and motor acceleration.
Technical note: All numerical examples are illustrative. Final generator sizing must use actual motor starting data, alternator capability curves, engine load-acceptance data, and project-specific voltage and frequency limits.
