Voltage Stabilizer for VFD Loads: Sizing and Selection Guide

Variable frequency drives (VFDs) are used in many industrial applications, including pumps, fans, compressors, conveyors, HVAC systems and production machines.

A VFD can control motor speed very well, but it cannot fix an unstable power supply. If the incoming voltage is too low or too high, the VFD may show an alarm, trip, or stop the motor.

In this situation, a voltage stabilizer for VFD can help keep the input voltage within a more stable range.

But selecting an AVR for a VFD is a little different from selecting one for a normal load. You need to consider the VFD input current, voltage range, harmonics and the lowest supply voltage.

This guide explains the main points in simple terms and shows how to choose the right voltage stabilizer size.

Do You Need a Voltage Stabilizer Before a VFD?

Not always.

Most VFDs can work within a certain input voltage range. If your factory power stays inside this range, you may not need an external voltage stabilizer.

The problem starts when the voltage regularly becomes too low or too high.

For example, a factory may have a nominal 400 V supply, but the actual voltage may drop to 340 V when many machines are running. A generator-powered factory may also have voltage changes when the load goes up or down.

In this case, the basic connection is:

Utility / Generator → Voltage Stabilizer → VFD → Motor

The stabilizer keeps the AC voltage more stable before it reaches the VFD.

It is important to understand that the two devices have different jobs:

• Voltage stabilizer: keeps the supply voltage stable.
• VFD: controls motor speed and torque.
• Line reactor: can help reduce input current distortion and electrical stress.
• Harmonic filter: reduces harmonic distortion.
• UPS: provides backup power or short-term ride-through.

How Voltage Fluctuation Affects a VFD

A VFD does not need exactly 400 V all the time. It normally has an acceptable input voltage range specified by the manufacturer.

The real problem is when the supply stays outside that range or gets very close to the limit.

Common causes include:

• long power cables;
• weak utility networks;
• undersized transformers;
• large motors starting on the same power system;
• generator power;
• remote factories;
• large changes in plant load.

When the input voltage drops, the VFD has less voltage available for its DC bus. If the motor still needs the same power, the input current can increase.

If the voltage becomes too low, the VFD may eventually show an undervoltage fault.

High voltage can cause the opposite problem. If the input voltage is too high, the VFD DC bus voltage also rises and may cause an overvoltage fault.

So before buying an AVR, it is a good idea to measure the voltage at the VFD input. Knowing the actual minimum and maximum voltage is much more useful than only knowing the nominal voltage.

What Happens Inside a VFD?

A typical VFD first converts AC power into DC power. The DC bus then supplies the inverter section, which creates variable-frequency AC for the motor.

The basic process is:

AC Input → Rectifier → DC Bus → Inverter → Motor

For a conventional three-phase diode rectifier, the DC bus voltage can be roughly estimated as:

V_DC ≈ 1.35 × V_LL

For example, with a 400 V three-phase supply:

400 × 1.35 ≈ 540 VDC

If the supply drops to 340 V:

340 × 1.35 ≈ 459 VDC

These are approximate values. The actual DC bus voltage depends on the VFD design, load and input power quality.

The main point is simple: a large drop in AC input voltage also reduces the available DC bus voltage.

That is why severe undervoltage can cause VFD trips, especially when the motor is heavily loaded.

Do VFDs Create Harmonics?

Yes. A standard VFD with a diode rectifier does not draw perfectly smooth current from the AC supply.

The input current is drawn in pulses as the DC bus capacitors charge. These current pulses contain harmonic components.

For one small VFD, this may not be a major problem. In a factory with many large VFDs, however, harmonics can become important.

High harmonic current can increase heating in:

• transformers;
• cables;
• switchgear;
• generators;
• voltage stabilizers.

It can also contribute to voltage distortion in the electrical system.

Does a Voltage Stabilizer Remove VFD Harmonics?

No, not normally.

A standard servo or static voltage stabilizer is designed mainly to regulate voltage. It is not a harmonic filter.

If harmonics are too high, the system may need additional equipment such as:

• AC line reactors;
• DC chokes;
• passive harmonic filters;
• active harmonic filters;
• active front-end VFDs.

In simple terms, the AVR controls voltage, while a harmonic filter controls harmonics.

How to Size a Voltage Stabilizer for a VFD

One common mistake is sizing the stabilizer only from the motor kW.

For a VFD application, it is better to start with the VFD input current.

1. Check the VFD Input Current

Look at the VFD nameplate or datasheet and find:

• input voltage;
• rated input current;
• maximum input current, if available;
• frequency;
• input power factor;
• overload rating.

If the VFD is already running, measured current can also be useful.

2. Calculate the Required kVA

For a three-phase system:

kVA = √3 × V × I / 1000

For example, if a VFD operates at 400 V and draws 90 A:

kVA = 1.732 × 400 × 90 / 1000 ≈ 62.4 kVA

So the load is about 62 kVA under these conditions.

This gives you a much better starting point than using the motor kW alone.

3. Check the Lowest Input Voltage

This point is very important for an AVR.

Suppose a 60 kVA load needs a stable 400 V output, but the input voltage can fall to 320 V.

The approximate current at 320 V is:

I = 60,000 / (1.732 × 320) ≈ 108 A

At 400 V, the current is only:

I = 60,000 / (1.732 × 400) ≈ 87 A

So when the input voltage is low, the stabilizer has to handle more current.

Always check the stabilizer capacity at the lowest input voltage, not only at the nominal voltage.

4. Leave Some Capacity Margin

It is usually better not to run the stabilizer continuously at 100% of its rated capacity.

A preliminary margin of around 20–30% can be considered for many industrial applications. The actual margin depends on the load, temperature, harmonics, operating time and future expansion.

Calculated Load	Possible Preliminary AVR Size
30 kVA	About 40 kVA
50 kVA	About 60–65 kVA
80 kVA	About 100 kVA
100 kVA	About 125 kVA

These are simple preliminary examples. The final AVR size should be checked against the actual site conditions.

Can One AVR Supply Several VFDs?

Yes. One three-phase voltage stabilizer can supply several VFDs if its capacity is large enough.

For example:

• VFD 1: 40 kVA
• VFD 2: 30 kVA
• VFD 3: 25 kVA
• VFD 4: 20 kVA

Total load:

40 + 30 + 25 + 20 = 115 kVA

With a preliminary 25% margin:

115 × 1.25 ≈ 144 kVA

A 150 kVA voltage stabilizer could therefore be considered as a starting point.

But first check whether all four VFDs really run at full load at the same time. If they do not, the actual required capacity may be lower.

For large plants, measured load data is always better than making assumptions from motor nameplates.

Servo vs Static Voltage Stabilizer for VFD

Both servo and static stabilizers can work with VFD systems. The better choice depends mainly on how quickly the input voltage changes.

Feature	Servo Stabilizer	Static Stabilizer
Voltage regulation	Automatic	Automatic
Response speed	Slower	Much faster
Moving parts	Yes	No mechanical regulating parts
Maintenance	Some mechanical maintenance	Generally lower
Large capacity	Widely available	Available depending on design
Cost	Usually lower	Usually higher

When Should You Choose a Servo Stabilizer?

A servo stabilizer is often a good choice when the voltage changes slowly and the main problem is long periods of undervoltage or overvoltage.

For example, if a 400 V factory supply regularly drops to 350 V during peak production, a servo AVR may be a practical solution.

When Is a Static Stabilizer Better?

A static stabilizer is worth considering when faster voltage correction is important.

It can be a good fit for sensitive industrial equipment, automation systems and applications where the supply voltage changes quickly.

When comparing products, check the actual response and correction time rather than relying only on the word “fast” in a product description.

Do You Need a Line Reactor with a VFD?

A line reactor is sometimes installed between the voltage stabilizer and the VFD:

Utility → AVR → Line Reactor → VFD → Motor

A line reactor adds impedance to the input side of the VFD. It can help reduce current peaks, input current distortion and some electrical stress.

However, not every VFD needs an additional reactor. Some drives already have an AC reactor or DC choke built in.

Check the VFD manufacturer’s recommendation before adding one.

When Is a Voltage Stabilizer Not Necessary?

If the incoming voltage is already stable and stays comfortably within the VFD’s input range, you may not need an AVR.

For example, if your 400 V supply normally stays between 390 V and 410 V and the VFD is designed to accept this range, installing a stabilizer may not provide much extra value.

An AVR also will not normally fix:

• VFD harmonic problems;
• motor bearing current;
• poor grounding;
• motor insulation problems;
• incorrect VFD settings;
• regenerative overvoltage;
• complete power outages.

For example, if a motor produces regenerative energy during fast deceleration, the VFD DC bus may rise too high. An upstream AVR cannot remove this energy. A braking resistor, braking unit or regenerative solution may be needed instead.

What Information Do You Need for an AVR Quote?

Example: Choosing an AVR for a VFD Pump System

VFD Voltage Stabilizer Selection Checklist

Before selecting an AVR, check:

• What is the nominal voltage?
• What is the lowest voltage?
• What is the highest voltage?
• What is the VFD input current?
• How many VFDs will be connected?
• Will they run at the same time?
• Does the VFD have an AC reactor or DC choke?
• Is the system powered by a generator?
• Is the load continuous?
• Is the stabilizer installed indoors or outdoors?

These details are usually enough for a manufacturer to make an initial AVR recommendation.

FAQ

Conclusion

A voltage stabilizer for VFD is mainly useful when the incoming power supply has regular and significant voltage fluctuations.

Before choosing an AVR, check four things first:

• Nominal voltage
• Minimum voltage
• Maximum voltage
• VFD input current

Then consider the number of VFDs, simultaneous load, harmonics and the required capacity margin.

Remember that an AVR and a harmonic filter solve different problems. The AVR stabilizes voltage, while reactors and harmonic filters are used to deal with harmonic current.

Need a voltage stabilizer for your VFD system? ZHENGXI supplies three-phase servo and static voltage stabilizers for industrial applications. Send us your VFD datasheet, input current, nominal voltage and measured minimum/maximum voltage. Our engineers can help check the required kVA, input range and suitable stabilizer configuration for your project.