Diesel Generator Sizing for Motor Starting

Diesel generator sizing for motor starting is one of the most critical steps in electrical design. A wrong selection can lead to system failure, overheating, voltage dips, and frequent generator shutdowns. The process requires technical precision and a good understanding of motor behavior during start-up.

Electric motors demand a high starting current. Typically, this current ranges between 5 to 7 times the full-load current. When multiple motors start simultaneously, the total starting kVA can become very high. This starting surge must be supported by the generator without causing voltage drops beyond acceptable limits.

Correct diesel generator sizing for motor starting ensures the system remains stable, avoids blackouts, and protects both the generator and the motor. This process becomes even more important in mission-critical applications like hospitals, data centers, and industries where downtime is unacceptable.

The type of motor starting method also impacts generator sizing. DOL (Direct-On-Line) starting causes the highest inrush current. Star-delta and soft starters reduce this surge but still require adequate generator support. The presence of VFDs (Variable Frequency Drives) further changes the game, as they help limit starting current significantly.

It is crucial to understand generator characteristics. The generator’s alternator, engine, excitation system, and AVR (Automatic Voltage Regulator) must handle the transient conditions during motor starting. The generator should not experience a voltage dip beyond 15% or frequency drop more than 10% during start-up.

Let’s break down the technical considerations for diesel generator sizing for motor starting.

Motor Starting Current and Its Impact

When a motor starts, it pulls a large amount of current. For a 3-phase induction motor, this starting current is known as Locked Rotor Current (LRC). It’s usually 600% to 700% of the full load current.

Suppose you have a 50 HP motor with a full load current of 65A. The starting current can be as high as 455A. This current puts stress on the generator, especially if it’s undersized.

The duration of this high current can range from a few seconds to over 10 seconds depending on the motor and load characteristics. The generator must be able to supply this starting power without tripping or causing a system failure.

Voltage Dip Consideration

Voltage dip is one of the key factors in diesel generator sizing for motor starting. If the generator cannot maintain voltage during start-up, other sensitive equipment may malfunction.

The table below shows acceptable voltage dips for different applications:

If voltage dips below these values, relays may trip, and electronics may reset or get damaged.

Generator kVA Sizing Formula

To calculate the required generator kVA, the following formula is used:

Generator kVA = (Motor Starting kW) / (Generator Motor Starting Capability)

Suppose a motor has a starting requirement of 75 kW, and the generator has a starting capability of 0.25 (per unit), then:

Generator kVA = 75 / 0.25 = 300 kVA

You may also need to derate the generator if you are operating at high altitudes or in hot environments. Check the manufacturer’s data for derating factors.

Step Load Capability

Step load is the sudden application of load. Motors starting together represent a step load. Generators must have sufficient inertia and power to absorb this step load. Otherwise, frequency and voltage drop quickly.

Diesel generator sizing for motor starting should account for step load capacity. Large motors starting together may need a generator with an oversized engine and a strong excitation system.

For example, if three motors each require 30 kW at start, the generator must handle a 90 kW step load. A typical generator may need to be rated around 400 kVA to accommodate this requirement safely.

Sizing with Soft Starters or VFDs

Soft starters and VFDs can reduce the starting current drastically. A soft starter may lower it to 2.5 to 3.5 times the full load current. A VFD can bring it down even more.

This reduces the demand on the generator. However, VFDs introduce harmonics, which can affect generator voltage waveform. Therefore, harmonic filters may be required, and the generator must have a low reactance alternator.

Using a VFD allows for more optimized diesel generator sizing for motor starting. It can reduce the required generator size by up to 30%, depending on the application.

Practical Sizing Example

Let’s consider the following scenario:

• Motor size: 100 HP (75 kW)
• Starting method: Direct-On-Line (DOL)
• Starting current: 6.5 x Full Load Current
• Full Load Current: ~100 A
• Starting Current: ~650 A
• Starting kVA: ~500 kVA (estimated)

Now, if the generator has a motor starting capability of 0.3 per unit, then:

Required Generator kVA = 500 / 0.3 = ~1670 kVA

This is a simplified approach. Real-world sizing involves checking transient voltage and frequency response curves provided by generator OEMs.

Use of Generator Excitation Systems

Excitation systems play a big role. Permanent Magnet Generators (PMG) or Auxiliary Winding systems help supply strong initial current for motor starting. Without them, the generator voltage may collapse.

For diesel generator sizing for motor starting, choose a generator with PMG excitation if high inrush loads are involved. It helps maintain voltage and avoid AVR instability.

Parallel Generators and Motor Starting

In large systems, parallel diesel generators are used. Synchronization adds complexity, especially during motor start. Motors should ideally start after the generators are synchronized and stabilized.

Starting motors during sync-up can destabilize frequency. Sequenced starting or soft starters are recommended in parallel generator setups.

Generator Capacity Margin

Always include a capacity margin. Motors can draw slightly more current due to mechanical load variations. A 10-15% margin above calculated generator size is advisable.

This avoids overload during worst-case scenarios. For example, if calculation shows 600 kVA, choose a generator of at least 660 to 700 kVA.

Conclusion

Diesel generator sizing for motor starting is not just about matching kW to kW. It involves understanding motor behavior, starting methods, generator dynamics, and acceptable voltage dips.

Each application may need a slightly different approach. Always review OEM data, run simulations if possible, and consult with experts during design.

Proper diesel generator sizing for motor starting guarantees long-term reliability, minimizes maintenance, and keeps critical operations running smoothly. It may cost more upfront, but it saves significantly in downtime and damage costs over time.

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