I can't tell you how many times I've gotten a frantic call from a site manager at 2 AM. The grid just dropped, the backup diesel fired up beautifully, but the facility's main HVAC compressor didn't kick in. The breaker tripped, or worse, the motor just sat there humming and overheating until the thermal overload popped.
The culprit? They sized the genset for 'running' kilowatts, completely ignoring the massive spike of 'starting' the motor. It's the oldest trap in the book. Let's talk about how to actually size a generator for heavy inductive loads without buying a 2-megawatt machine for a 500kW facility.
The Math Behind the Madness: FLA vs. LRA
Cummins 4B3.9-G2 Engine
When you look at a motor nameplate, you see the Full Load Amps (FLA). That's what the motor draws once it's up to speed and doing its job. But to get from zero to 1800 RPM, the motor needs to overcome inertia.
During that first fraction of a second, the motor draws Locked Rotor Amps (LRA). Depending on the motor design and starting method, LRA is typically 6 to 8 times the FLA.
Let's look at a real-world example. Say you have a 100HP, 480V 3-phase motor.
- Running (FLA): Roughly 110 amps (about 90 kW).
- Starting (LRA) Direct-On-Line (DOL): Easily 750 amps.
That starting surge requires nearly 600 kVA of apparent power just to get the rotor spinning. If your generator's alternator can't supply that transient current, the voltage collapses. The engine might have the raw mechanical torque to handle it, but if the electrical side fails, the whole system fails.
If you are using reduced voltage starters like Star-Delta, soft starters, or VFDs, you can chop that LRA down significantly. But if you're running DOL across a massive facility, you need to plan for that massive kVA spike.
Voltage Dip and the 15% Rule
When a massive motor starts, it pulls so much current that the generator's voltage takes a nosedive. This is called voltage dip.
Why do we care? Because if the voltage dips too low, you get a cascade of failures:
- Contactors drop out: Any relay or contactor holding a circuit closed might chatter or open if voltage falls below 70-80%.
- VFDs fault out: Variable frequency drives are incredibly sensitive. A deep voltage dip will instantly throw an 'Undervoltage' fault and trip the drive offline.
- Other motors stall: If you have other motors already running, a severe dip robs them of torque. They slow down, draw even more current, and eventually trip on thermal overload.
The industry standard rule of thumb is to keep the voltage dip below 15% (and absolutely no lower than 20% for very short durations).
How do you achieve this? You need an alternator with low subtransient reactance (X''d). More importantly, you need the right excitation system. For motor-heavy loads, I always spec a PMG (Permanent Magnet Generator) excitation system. A PMG gives the Automatic Voltage Regulator (AVR) the independent power it needs to force the alternator field to recover voltage in under 2 seconds. A standard shunt excitation system will struggle and drag its feet, letting the voltage dip linger too long.
Step Loading and the Sequence of Operation
Here's a secret: you don't actually have to start all your motors at the exact same millisecond. That's what load shedding and sequencing are for.
If your facility has three 200kW chillers, starting them all simultaneously via DOL would require a generator the size of a small house. Instead, we use step loading.
We program the PLC or the ATS control panel to sequence the loads.
- Second 0: Grid fails. Genset starts and stabilizes.
- Second 5: Essential life safety loads and IT servers transfer.
- Second 10: Chiller #1 starts.
- Second 20: Chiller #2 starts.
Staggering the starts by just 5 to 10 seconds lets the engine governor and alternator AVR catch their breath and recover the voltage before the next massive hit. It's the cheapest way to downsize your generator. If you can sequence your loads, you can often drop your required genset size by 20% to 30%.
Don't Forget the Alternator: Engine vs. Standby Rating
Here's a trap I see procurement officers fall into all the time. They look at the engine's standby rating and assume the whole genset can handle that load during a motor start.
The engine and the alternator are two completely different beasts. The engine provides the mechanical torque (kW), while the alternator provides the electrical capacity (kVA).
For motor starting, the alternator is almost always the weak link. You might have a massive, high-torque engine like our Cummins K38 or SDEC large blocks that can easily spin the rotor, but if the alternator is undersized, the voltage will still crater.
A standard trick we use at Jinjia for motor-heavy applications is oversizing the alternator relative to the engine. We might pair a 400kW engine with a 550kVA alternator. The engine handles the running kW load perfectly, while the oversized alternator provides the massive transient kVA needed to handle the LRA without dipping below that critical 15% threshold. Always check the alternator's standalone motor-starting kVA rating, not just the engine's continuous kW.
Bottom Line
Motor starting is where the rubber meets the road. It's an art and a science, and guessing your loads will only lead to expensive callbacks and frustrated facility managers. You have to calculate the LRA, manage the voltage dip with the right excitation, sequence your step loads, and ensure your alternator isn't bottlenecking your engine.
If you're spec'ing a new project and dealing with heavy motor loads, don't guess. Reach out to the engineering team at Jinjia Power Equipment. We'll run the transient analysis, size the alternator correctly, and build a genset that actually performs when the grid drops. Contact us today at jinjiadongli.com to get your project moving.