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Using VFDs With Generators & Weak Power Supplies

Using VFDs With Generators & Weak Power Supplies

Using VFDs With Generators and Weak Power Supplies: What You Must Know

Running a VFD from a generator or weak electrical supply introduces challenges that don’t exist on a stable grid connection.

If these issues aren’t understood and planned for, they can cause nuisance trips, unstable operation, DC bus faults, or permanent damage to the drive — even when the VFD is correctly sized for the motor.

This guide explains how VFDs interact with generators and weak supplies, the most common failure points, and how to design a reliable system.

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Why Generators and Weak Supplies Are a Problem for VFDs

VFDs draw current differently from direct-on-line motors.

Instead of smooth sinusoidal current, a VFD’s input rectifier pulls current in pulses. On weak supplies, this causes:

  • Voltage dips
  • Frequency instability
  • DC bus undervoltage or overvoltage
  • Unpredictable trips

These issues are often misdiagnosed as “faulty drives” but are actually supply-related.

Inrush Current & Rectifier Stress

When a VFD powers up, the DC bus capacitors charge rapidly.

On a generator or weak grid, this causes:

  • High inrush current
  • Generator voltage sag
  • Rectifier stress

Repeated stress during power cycling shortens the life of input rectifiers and capacitors.

Capacitor ageing is covered in: VFD Maintenance Checklist

DC Bus Instability Explained

The DC bus is the heart of a VFD. Stable DC voltage is critical.

Weak supplies can cause:

  • Undervoltage trips: generator can’t respond fast enough
  • Overvoltage trips: regeneration during decel with nowhere for energy to go

Fast deceleration worsens this problem:

What Is Deceleration Time in a VFD?

Harmonics & Voltage Distortion

VFDs introduce harmonic distortion into the supply. Generators are far more sensitive to harmonics than utility grids.

Problems include:

  • Generator overheating
  • Voltage waveform distortion
  • Interference with other loads

Harmonics are covered in depth here: How to Reduce Harmonic Distortion in VFD Systems

How to Size a Generator for a VFD

Generator sizing rules for VFDs are very different from DOL motors.

General guidelines:

  • Generator kVA rating should typically be 2–3× the VFD input kVA
  • Allow additional margin for other connected loads
  • Account for starting and transient conditions

Undersized generators cause exactly the same issues as undersized VFDs: Oversized vs Undersized VFDs

Critical VFD Parameter Settings for Generators

Correct parameter setup is essential when running from generators.

  • Increase acceleration time to reduce current spikes
  • Increase deceleration time to avoid regeneration
  • Disable aggressive torque boost
  • Set conservative current limits

Control mode selection also matters: VFD Control Modes Explained

Line Reactors, Filters & Protection

Additional components dramatically improve reliability.

Line Reactors

  • Reduce inrush current
  • Stabilise DC bus voltage
  • Protect rectifiers

EMC & Input Filters

  • Reduce harmonics
  • Improve generator waveform stability

Related compliance guidance: VFD Compliance Explained

Common Mistakes to Avoid

  • Using a generator sized for motor kW only
  • Fast accel/decel ramps on weak supplies
  • No line reactor on generator-fed systems
  • Assuming all VFDs behave the same on generators

Many of these mistakes appear in: VFD Installation Mistakes That Cause Drive Failures

Recommended Categories for Generator Applications

Example Drives Known to Perform Well on Generators

FAQ

Can I run any VFD from a generator?

No. Some drives are more tolerant of voltage and frequency variation than others.

Do I always need a line reactor?

Strongly recommended for generator-fed or weak supplies.

Why does my VFD trip when stopping on a generator?

Regenerated energy raises DC bus voltage. Increase decel time or add braking.

Need Help With a Generator-Fed VFD?

If you’re running a VFD from a generator or weak supply, we can help size the generator, select protection, and configure parameters correctly.

Contact us: https://driveoutletmegastore.com/contact-us/

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