How Inrush Current Affects VFDs and How to Protect Your Electrical System
Inrush current is one of the most common causes of nuisance trips, blown fuses, and unexpected shutdowns in Variable Frequency Drive (VFD) installations. While VFDs are designed to provide soft motor starting, the drive itself can still create high input current spikes during power-up and charging of internal components.
If not properly managed, inrush current can stress breakers, damage components, and shorten the life of your inverter drives and connected equipment.
In this guide, we explain what inrush current is, why it affects VFD systems, and how to protect your electrical network using correct sizing, reactors, and installation best practices.
What Is Inrush Current?
Inrush current is the brief surge of current drawn when electrical equipment is first energised.
For VFDs, this occurs when:
- Power is applied to the drive
- DC bus capacitors charge
- Magnetic components energise
This surge may last only milliseconds but can be several times higher than normal operating current.
Why Do VFDs Create Inrush Current?
Inside every VFD is a large DC bus with electrolytic capacitors. When the drive is powered on, these capacitors behave like a short circuit until charged.
This causes:
- Very high instantaneous current draw
- Stress on upstream breakers and fuses
- Voltage dips on weak supplies
Although most drives include pre-charge circuits, high inrush can still occur — especially with larger drives.
Common Problems Caused by Inrush Current
- MCB or fuse tripping at power-up
- Contactor welding or wear
- Voltage dips affecting other equipment
- Nuisance overcurrent faults
- Reduced capacitor lifespan
These problems are more noticeable on larger industrial drives or installations with multiple drives switching simultaneously.
VFD Inrush vs Motor Starting Current – What’s the Difference?
It’s important not to confuse the two:
- Motor starting current (DOL): 6–8× rated current
- VFD motor starting current: controlled/soft start
- VFD inrush current: occurs when the drive powers up, not the motor
Even though VFDs eliminate motor inrush, they still have input-side charging surges.
When Is Inrush Current Most Severe?
- Large drives (15 kW+)
- Multiple drives powered at once
- Generator or weak supply systems
- Long cable runs
- Undersized breakers
Installations with several drives starting together often experience supply dips or breaker trips.
How to Protect Your System from Inrush Current
1. Use a Line Reactor (AC Reactor)
A line reactor limits the rate of current change and smooths incoming current spikes.
- Reduces inrush stress
- Protects breakers and fuses
- Extends drive lifespan
- Reduces harmonics
Line reactors are one of the most effective protections for VFD installations.
2. Stagger Power-Up Timing
Avoid energising multiple drives simultaneously.
- Use time delays
- Sequence start-up
- Power drives one at a time
This dramatically reduces total surge current.
3. Correct Breaker and Fuse Selection
Standard MCBs may trip due to inrush. Choose protection devices rated for motor/drive loads:
- Type C or D breakers
- Time-delay fuses
- Manufacturer-recommended protection
4. Use Drives with Built-In Pre-Charge Circuits
Most modern drives include controlled capacitor charging to limit surges.
Higher-quality industrial drives often have better inrush management than low-cost units.
5. Improve Overall System Design
- Keep supply cables short
- Avoid undersized wiring
- Ensure stable supply voltage
- Separate sensitive electronics
Real-World Example
Problem: 3 × 18.5 kW drives tripping the main breaker at power-up
Cause: Simultaneous capacitor charging inrush
Solution:
- Add line reactors
- Stagger startup by 5 seconds
- Upgrade breaker to Type D
Result: Stable startup with no nuisance trips.
Do Gearboxes or Motors Affect Inrush?
Mechanical components such as gearboxes or motors do not directly affect electrical inrush, but:
- Higher loads may require larger drives
- Larger drives create higher charging current
Correct mechanical sizing helps keep electrical demand reasonable.
Common Mistakes
- Assuming VFDs eliminate all inrush current
- Using standard domestic breakers
- Powering multiple drives simultaneously
- Skipping reactors to save cost
Frequently Asked Questions
Do VFDs eliminate inrush current?
They eliminate motor inrush but still have input capacitor charging surges.
Are line reactors necessary?
Not always, but highly recommended for medium-to-large drives or unstable supplies.
Why does my breaker trip when the drive powers on?
Likely caused by capacitor charging inrush.
Can multiple drives share one reactor?
Generally no — each drive should have its own protection.
Does oversizing a breaker fix the issue?
It may mask the problem but proper protection devices are safer.
Need Help Protecting Your VFD Installation?
At Drive Outlet Megastore, we supply reliable inverter drives, electric motors, geared motors, and protection solutions for stable, long-life operation.
If you’re experiencing trips or supply issues, contact our team and we’ll help you select the right drive and protection setup for your system.
