How to Correctly Size a Generator for Motor Starting

Routine Generator testing

A common problem we are seeing on site is when generators are sized incorrectly for the loading profile, resulting in poor performance or worse; overloading, tripping or equipment damage.

Motors, especially large ones, have high inrush currents when they start, which can be several times greater than their normal operating current. Sizing your generator correctly is important to ensure that it can handle both the starting and running demands of the motor without tripping, overheating, or causing damage.

Some of the main factors to take into account are:

1. Understand the Motor’s Electrical Requirements

The motors properties impact on the supply requirements.

• Full Load Amperage (FLA): This is the motor’s current draw when it is running at full load. It is an essential reference point for sizing the generator.

• Motor Voltage: The voltage at which the motor operates is crucial for matching it with the generator's output.

• Motor Power (in kW or HP): This helps in understanding the power requirements of the motor. Larger motors require more power and thus more robust generators.

• Inrush Current: This is the motor's current draw during startup, which is typically much higher than the full load current. Inrush current can be 5 to 8 times the FLA depending on the motor type.

  
  

2. Consider the Type of Motor Starting Method

There are several starting methods that can affect the inrush current:

• Direct-On-Line (DOL) Starting: This is the most straightforward method where the motor is connected directly to the power supply. The generator must handle the high inrush current when the motor starts.

• Star-Delta (Wye-Delta) Starting: This method reduces the starting current by initially connecting the motor windings in a star configuration and then switching to delta once the motor reaches speed. This reduces the demand on the generator but still requires careful consideration during sizing.

• Soft Start or Variable Frequency Drives (VFD): These devices limit the inrush current by gradually ramping up the motor speed. When using these methods, the generator will still need to handle the steady-state current once the motor is running at full speed but may face a lower inrush current.

  
  

3. Account for the Inrush Current

Inrush current during startup can be up to 8 times the motor’s full load current.

To correctly size the generator, contact a suitably qualified electrical contractor or engineer. They will complete the assessment which will include.

• Calculate the Inrush Current: Multiply the motor's full load amperage (FLA) by the inrush multiplier (usually 5-8 times, depending on the motor type). For example, if the motor has an FLA of 20 amps, the inrush current could be 100-160 amps.

• Choose a Generator with Sufficient Strating Capacity: The generator must be able to handle these high inrush currents during motor startup without overloading.

  
  

4. Factor in the Generator's Power Rating

Generators come with two main power ratings: prime power and standby power.

• Prime Power: This is the continuous output that a generator can supply under normal operating conditions. When sizing a generator for motor starting, this should be equal to or greater than the motor’s full load power requirement.

• Standby Power: This is the maximum output a generator can supply for a limited time. For motor starting, the generator’s standby rating is more relevant, as it needs to handle the high inrush current during startup.

  
  

When selecting a generator, ensure that its prime power is capable of supplying the motor's steady-state running power, and its standby power rating should be sufficient to handle the peak inrush current.

5. Consider Other Loads on the Generator

In addition to the motor, you may have other equipment or devices running off the generator. It’s important to consider the total load on the generator.

You should ensure that the generator has sufficient capacity to handle the sum of all these loads, both during motor startup and during normal operation.

6. Use a Generator Sizing Calculator or Consult a Professional

To simplify the process, many generator manufacturers provide online sizing calculators that can help you estimate the generator's size based on your motor's specifications. NPE have a handy chart here Guide to Choosing Generator to Motor Size - www.nationalpump.com.au

We recommend consulting with a qualified electrical contractor, consultant or engineer to confirm generator sizing prior to procurement.

7. Allow Safety Margin Don’t Oversize your generator

It’s always a good idea to provide a safety margin in your generator sizing. Electrical systems can experience fluctuations in demand, and having a slightly larger generator can help accommodate any unforeseen issues like voltage dips or changes in load requirements. However running a generator with too little load can damage the motor resulting in Bore Glazing.

Conclusion

If you need any assistance with generation for your site, or some advice, get in touch with LECE by sending an email to service@lecegroup.com.au. We can assist with information, assistance to develop critical spare plans, complete an assessment or assist with the assessment.