Variable frequency drives (VFDs) run pumps, fans, compressors, and conveyors in plants of every kind. They save energy by matching motor speed to the job, which is why so many facilities rely on them.
A drive also changes the electricity around it. It affects the power supply on one side and the motor on the other. Reactors and filters keep those effects in check. This guide covers what they do, when you need them, and how to choose.
The Short Answer: A VFD needs reactors and filters because it creates two different problems. On the input side, it draws current in short pulses, which produces harmonics. On the output side, fast switching can send high voltage peaks to the motor, and long cables make that worse. Reactors and filters reduce both.
What Happens on the Input Side of a VFD?
Most VFDs begin with a diode bridge rectifier that turns AC power into DC. The rectifier only conducts near the peak of the voltage wave. So the drive pulls current in narrow spikes rather than a smooth flow.
Those spikes contain harmonics. These are extra frequencies layered on top of the normal 50 Hz or 60 Hz supply. In a standard six-pulse rectifier, the main ones are the 5th, 7th, 11th, and 13th.
What problems do harmonics cause?
- Overheating of transformers and neutral conductors.
- Nuisance tripping of circuit breakers.
- Early failure of power factor correction capacitors.
- Torque pulsations, extra noise, and added heat in the motor.
What Happens on the Output Side?
The drive switches its output at high frequency. Veikong’s sine wave filter datasheet says this can produce voltage peaks of 1,300 V or more at the motor terminals and coils. That constant stress ages the motor.
It weakens the coils, wears and pits the bearings, and adds heat, noise, and cable interference. The longer the distance between drive and motor, the stronger the effect.
Which Reactors Help, and What Does Each One Do?
Veikong’s range includes three reactors, also called chokes: DC, AC input, and AC output.
1. DC reactor
A DC choke sits between the rectifier and the DC bus capacitors. According to Veikong’s datasheet, it:
- Reduces pulsing current in the DC circuit.
- Helps protect the rectifier bridge and keeps capacitors from overheating due to pulse current.
- Improves power factor and limits grid voltage transients.
- Is rated for DC 500 V to 1000 V, with noise under 65 dB measured one meter away.
- Offers class F or class H heat resistance as an option.
The selection table for 400 V drives runs from 0.4/0.75 kW (3 A) up to 720 kW (1,800 A). Many Veikong drives also include a built-in DC choke as standard, or offer optional external reactors.
2. AC input (line) reactor
The simplest fix on the input side is a 3% or 5% AC line reactor. It raises the source impedance, which smooths the sharp current pulses and cuts harmonic current distortion.
3. AC output reactor
Long motor cables raise capacitive leakage current, which can trip the drive or damage it. Veikong’s sizing guide recommends an output reactor once the cable passes 50 meters.
Which Filters Does Veikong Offer?
The product range lists an AC input filter (EMI), an AC output filter, a sine-wave filter, and a harmonic filter.
1. Sine-wave filter
This filter sits between the drive and the motor. The datasheet lists these benefits:
- Lower voltage peaks, so less wear on motor insulation and bearings.
- Less motor noise and better motor efficiency.
- Reduced interference from the motor cables.
- Residual ripple voltage (THD) below 5%.
- Available for 3×380 V systems, from 1.5 kW to 630 kW.
2. Harmonic filters
Some sites need more than a reactor, such as those with sensitive equipment or many drives. Passive harmonic filters are tuned to trap specific harmonics, typically the 5th and 7th. Active harmonic filters sense distortion in real time and inject opposite currents to cancel it.
How Do You Choose the Right Protection?
Does every VFD need a filter or reactor?
Not always. Veikong notes that simple, low-cost steps often bring a big improvement, so advanced filtering is not always needed right away.
A quick reference
- Harmonics on the input side: A 3% or 5% AC line reactor, or a DC choke.
- Motor cable over 50 m: An output reactor.
- Motor cable over 100 m: A sine wave filter, plus a lower carrier frequency.
- Sensitive equipment or many drives: A passive or active harmonic filter.
- Longest run: Veikong VFDs support up to 500 m of shielded cable with proper filtering.
What should you tell the supplier?
Veikong’s datasheet says all network conditions must be gathered before a filter is chosen:
- Rated values and type of the load on the filter.
- Rated values of other non-linear loads.
- Where the filter will be installed.
- Whether power factor equipment is present, and what type.
- Motor frequency, switching frequency, and fundamental operating frequency.
Veikong’s technical team asks for your motor nameplate details and a description of the application. They say they will reply with a recommendation within 24 hours.
About Veikong
Shenzhen Veikong Electric Co., Ltd. is an AC drive company that researches, makes, and trades high-, medium-, and low-voltage frequency inverters. Veikong was founded in 2004 and cites more than 20 years of experience.
Shenzhen Veikong Electric Co., Ltd. follows the ISO 9001 standard for quality management and that its products have passed CE certification.
Summary
A VFD affects both sides of a system. On the input side, it draws current in short pulses, which creates harmonics. A 3% or 5% AC line reactor, a DC choke, or a harmonic filter can bring those down.
On the output side, fast switching can put high voltage peaks on the motor. Output reactors and sine-wave filters reduce that stress. Veikong recommends an output reactor above 50 meters of motor cable, and a sine wave filter with a lower carrier frequency above 100 meters.
With proper filtering, Veikong drives support shielded cable up to 500 meters. Simple parts often do the job first, so match the choice to your motor, cable run, and application.
Contact Veikong
Need help matching a reactor or filter to your drive and motor? Send your motor nameplate details and application description to the technical team.
- Email: info@veikong-electric.com
- Technical support: Service@veikong-electric.com
- Phone: +86-0755-89587650
- WhatsApp: +86 15989436541
Address: 4F, Building 5, Dongluyang Industrial Park, No. 4, Tengfeng 4th Road, Fuyong Phoenix Third Industrial Zone, Baoan District, Shenzhen, China