Starting an Electric Motor from a Generator with a VFD: Complete Practical Guide

Why a Generator Fails to Start a Motor: The Inrush Current Problem

An asynchronous electric motor draws current at 5-7 times its rated value during direct-on-line (DOL) starting. For a generator, this represents a critical load: the current surge causes voltage sag, frequency instability, and protection tripping. A 10 kVA generator cannot directly start a 5.5 kW motor, even though the running power requirement is well within capacity.

In practice, engineers face three main problems when powering motors from generators:

• Voltage sag of 30-40% during motor startup, which damages sensitive automation equipment on the same supply line
• Generator frequency fluctuations in the 45-55 Hz range under varying load, while the motor is designed for stable 50 Hz
• Generator protection tripping due to peak current — the generator shuts down and production stops

These problems can be solved using three approaches: direct start with an oversized generator, a soft starter, or a variable frequency drive (VFD). Below is a detailed comparison of each option.

Comparison of Motor Starting Methods from a Generator

The choice of starting method directly affects the required generator size, system cost, and operational reliability. Here are the three main approaches:

As the table shows, a variable frequency drive provides the greatest advantage when working with a generator: minimal starting current, complete equipment protection, and immunity to generator frequency fluctuations.

How a VFD Works with a Generator: Operating Principle

A variable frequency drive contains three main sections: a rectifier, a DC bus (with filter capacitors), and an inverter. This architecture makes VFDs ideal for generator-powered applications:

• The rectifier converts AC to DC. At this stage, the generator can output anywhere from 45 to 65 Hz — the VFD does not care, because the rectifier output is DC voltage regardless of input frequency
• The DC bus smooths out fluctuations. Capacitors on the DC bus compensate for momentary voltage dips and spikes from the generator
• The inverter produces clean output voltage. IGBT transistors generate a PWM signal at the desired frequency (0-400 Hz) and voltage — the motor receives stable power regardless of input supply quality

Thanks to this design, the VFD effectively isolates the motor from the generator. Even if the generator operates unstably — frequency variations, voltage sag under load — the motor receives a clean sinusoidal signal. For more details on VFD operation with generators, see our article Starting an Electric Motor from a Diesel Generator with a VFD.

Generator Sizing When Using a VFD

Using a variable frequency drive fundamentally changes the generator sizing equation. Instead of 3-4x power headroom, 1.1-1.5x is sufficient. Here is how it works in practice:

Generator Power Calculation Formula with VFD

P generator (kVA) = P motor (kW) x K load x K VFD x K margin

• K load = 1.1-1.25 (depends on load characteristics)
• K VFD = 1.1-1.3 (compensates for harmonics and power factor of the drive)
• K margin = 1.1-1.15 (for future expansion)

Calculation Example

A 7.5 kW pump motor needs to be started from a diesel generator:

• Without VFD (direct start): 7.5 x 3.5 = 26.25 kVA — minimum 30 kVA generator required
• With soft starter: 7.5 x 2.0 = 15 kVA — 15-20 kVA generator
• With VFD: 7.5 x 1.15 x 1.2 x 1.1 = 11.4 kVA — 12 kVA generator is sufficient

The cost difference between a 30 kVA and a 12 kVA generator often exceeds the cost of the VFD itself. This means the VFD not only protects equipment but saves money from the moment you purchase the generator.

Harmonics and THD: What You Need to Know When Running a VFD from a Generator

A variable frequency drive draws current in short, high-amplitude pulses (through its front-end diode rectifier). This creates total harmonic distortion (THD), which is more critical for generators than for industrial mains supply. Generators have limited short-circuit capacity, so harmonics have a greater impact on voltage waveform quality.

Recommendations for harmonic reduction:

• Line reactor (choke) at VFD input — reduces current THD from 80-100% to 35-45%. This is mandatory when operating from a generator
• DC choke — installed in the DC bus, provides additional pulsation smoothing
• Passive RLC filter — reduces THD to 8-12%, recommended for low-capacity generators
• Active Front End (AFE) — available in premium VFDs, reduces THD to 3-5% and achieves power factor close to 1

IEEE 519 standard requires voltage THD below 5% and current THD below 8%. When operating from a generator, meeting these requirements is particularly important — otherwise, generator winding overheating, false protection trips, and reduced service life may occur.

VFD Configuration for Generator Operation

Standard factory VFD settings are optimized for industrial mains at 380V/50Hz. When powered by a generator, several key parameters need adjustment:

Required Settings

• Increase acceleration time (ACC) to 15-30 seconds (factory default is 5-10 s). Slower acceleration reduces peak current draw and gives the generator time to stabilize voltage
• Widen the acceptable input voltage range — set the lower limit to 320-340V (instead of 380V -15%). Generators can sag significantly under load
• Increase undervoltage trip delay — instead of instant shutdown, allow the generator 2-5 seconds to recover
• Enable automatic restart function after power restoration. See detailed instructions in our article Automatic VFD Restart After Power Failure

Additional Recommendations

• Limit maximum current to 110-120% of motor rated value — this protects the generator from overload
• Configure soft deceleration instead of hard stop — during braking, energy returns to the DC bus and can overload the generator through regeneration
• Disable dynamic braking or install a braking resistor — without this, excess energy can damage DC bus capacitors

Practical examples of VEICHI VFD configuration for generator operation with pressure switches are described in our article VFD Setup for Pump Operation from a Generator.

Common Mistakes When Connecting a VFD to a Generator

Over years of field experience, we have compiled the most frequent mistakes made when setting up VFD power supply from generators:

• Generator without AVR (Automatic Voltage Regulator). Budget generators produce unstable voltage with fluctuations up to plus or minus 15%. The VFD may operate, but DC bus capacitor lifespan is shortened. Use generators with digital AVR
• Missing line reactor. Without a reactor, VFD capacitor charging peak currents can reach 10-15 times the rated value. For a generator, this is critical — installing a 3-5% reactor is mandatory
• Acceleration time too short. Factory-default 5 seconds is acceptable for mains power, but generators need 15-30 seconds. Every additional second of acceleration means lower peak current
• Connecting multiple VFDs to one generator without sequential starting. If three motors start simultaneously, the generator sees triple the starting current. Configure cascaded starting with 10-20 second intervals
• Ignoring grounding. The generator must have a separate ground connected to the VFD ground. Potential differences between grounds cause parasitic currents and malfunctions

Real-World Scenarios: VFD with Generator Power

Construction Site: Concrete Mixer and Cranes

On construction sites without grid connection, a generator is the only power source. An 11 kW concrete mixer requires a 40-50 kVA generator with direct starting. With a variable frequency drive, a 15-20 kVA generator is sufficient. The additional benefit is the ability to smoothly adjust the mixer drum rotation speed.

Emergency Water Pump Station Power

When mains power fails, water supply switches to generator backup. A 15 kW pump starts smoothly through a VFD without water hammer in the pipeline. The VFD also maintains set pressure via PID control, which is impossible with direct starting. For setup details, see our article Starting Electric Motors from a Generator Using a Compressor Example.

Mobile Production: Woodworking and Stone Cutting

Machine tools with 3-7.5 kW motors at remote job sites. A VFD enables starting equipment from a compact generator and also allows tuning optimal RPM for specific materials — for example, reducing speed when processing hardwood.

Agriculture: Irrigation and Ventilation

Irrigation systems and livestock facility ventilation fans in remote areas without grid power. VFDs provide smooth pump starting, dry-run protection, and automatic capacity adjustment based on demand. For more practical answers, see our article TOP Questions About VFDs and Soft Starters.

When a Soft Starter Is Better Than a VFD for Generator Applications

Despite VFD advantages, there are situations where a soft starter is a sufficient solution:

• Motor runs at constant speed — no speed control needed, just soft starting. For example, a conveyor belt or ventilation fan
• Budget is limited — a soft starter costs 2-3 times less than a VFD of equivalent power rating
• Generator has adequate headroom — if the generator is 2-2.5 times more powerful than the motor, a soft starter will handle the start
• Simplicity of connection — a soft starter does not generate harmonics and does not require line reactors or filters

However, if speed control is needed, the generator has limited capacity, or sensitive equipment shares the same supply line — a variable frequency drive remains the only reliable solution.

Summary and Recommendations

A variable frequency drive solves three critical challenges when operating from a generator: it reduces starting current by 4-6 times, stabilizes voltage and frequency for the motor, and allows using a smaller generator. For reliable system operation:

• Use a generator with digital AVR rated at minimum 1.2 x motor power
• Install a 3-5% line reactor at the VFD input
• Increase acceleration time to 15-30 seconds
• Widen the acceptable input voltage range in VFD settings
• For multiple motors — configure sequential cascaded starting

Need help selecting a variable frequency drive for generator operation? Our engineers will help calculate the optimal configuration for your application.