How to Calculate Motor Starting Surge for Generator Sizing
The number one reason industrial backup generators fail during an outage is not mechanical breakdown—it is generator stalling due to motor starting surge. When an induction motor is first turned on, it acts almost like a short circuit, drawing a massive spike of electricity known as Inrush Current or Locked Rotor Amps (LRA). If your generator is not sized to handle this spike, the voltage will collapse, magnetic contactors will chatter, and the engine will stall.
Locked Rotor Amps (LRA) vs Full Load Amps (FLA)
Look at the nameplate of any industrial motor (a chiller compressor, a conveyor drive, a massive exhaust fan). You will see two critical numbers:
- FLA (Full Load Amps): The continuous current the motor draws while running at its rated horsepower.
- LRA (Locked Rotor Amps): The massive instantaneous current the motor draws in the first few milliseconds of starting, before the rotor begins to turn.
A standard NEMA Design B motor started "Direct-On-Line" (DOL) will typically have an LRA that is 6 times higher than its FLA. This means a motor that runs on 50 Amps will demand 300 Amps to start. Your generator must be capable of providing that 300 Amp spike without letting the voltage drop below acceptable limits.
NEMA KVA Code Letters
If your motor nameplate does not list LRA, it will list a "NEMA Code Letter" (from A to V). This letter dictates the kVA required per horsepower to start the motor.
| NEMA Code Letter | Starting kVA per Horsepower | Surge Multiplier (Approx) |
|---|---|---|
| Code F | 5.0 - 5.59 kVA / HP | 5x |
| Code G | 5.6 - 6.29 kVA / HP | 6x |
| Code H | 6.3 - 7.09 kVA / HP | 7x |
| Code J | 7.1 - 7.99 kVA / HP | 8x |
Motor Starting kVA (SkVA) = Motor HP × NEMA Code Multiplier
Example: A 50 HP Code G motor requires: 50 × 6.0 = 300 kVA just to start, even though it only requires roughly 40 kVA to run.
How to Mitigate Motor Surge
Oversizing a generator just to handle a 3-second motor surge is extremely expensive. Instead, engineers use motor starters to reduce the inrush current, allowing them to purchase a smaller generator.
- Variable Frequency Drives (VFDs): The best solution. A VFD ramps the frequency up slowly, reducing the starting surge to 100% - 150% of FLA. A motor on a VFD barely registers as a surge to a generator.
- Soft Starters: Uses thyristors to reduce the voltage during startup. Reduces the surge to 250% - 300% of FLA.
- Staggered Starting: If you have 5 massive motors, do not start them all simultaneously. Use timers in your PLC to start Motor 1, wait 10 seconds, start Motor 2, etc. The generator only has to handle one surge at a time.
Frequently Asked Questions
What happens if the generator voltage dips too low?
If the starting surge pulls the generator voltage down by more than 30%, the magnetic fields in your facilitys contactors and relays will weaken causing them to drop out ("chatter"). This shuts off the equipment entirely and can weld contactor pads together.
Do VFDs cause problems for generators?
While VFDs solve the starting surge problem, they introduce Harmonic Distortion (THD) back into the generator alternator. If too many VFDs are used, the generator alternator must be oversized or fitted with harmonic filters to prevent overheating.