Output Reactor vs dV/dt Filter vs Sine Wave Filter

Long motor cables can increase voltage reflections, leakage current, insulation stress, and electromagnetic interference in VFD applications. This technical guide explains how Output Reactors, dV/dt Filters, and Sine Wave Filters work and how to select the appropriate solution.

Introduction

A Variable Frequency Drive does not supply the motor with a pure sinusoidal voltage like a conventional AC power source.

The VFD first converts the incoming AC voltage into DC voltage. It then uses high-speed electronic switches to create a variable-voltage and variable-frequency output using Pulse Width Modulation, commonly known as PWM.

Although the motor current may appear relatively smooth, the instantaneous voltage at the VFD output consists of fast pulses with steep rising and falling edges.

When the distance between the VFD and the motor increases, these fast pulses interact with the electrical characteristics of the motor cable and the motor.

This interaction may result in:

  • High voltage peaks at the motor terminals
  • Motor insulation stress
  • Increased leakage current
  • Ground fault trips
  • Electromagnetic interference
  • Bearing currents
  • Reduced motor service life

Output Reactors, dV/dt Filters, and Sine Wave Filters are commonly used to reduce these effects.

However, the correct solution cannot be selected based on cable length alone. The VFD, motor, cable, switching frequency, voltage level, grounding method, and manufacturer recommendations must all be considered.

How Does a VFD Produce Motor Voltage?

The output of a VFD contains three important electrical characteristics:

  • Output Frequency
  • Carrier Frequency
  • dV/dt

These terms describe different parts of the VFD output and should not be confused with each other.

Output Frequency

Output Frequency is the fundamental frequency produced by the VFD to control the motor speed.

Typical values include:

  • 10 Hz
  • 25 Hz
  • 50 Hz
  • 60 Hz

Increasing the output frequency increases the rotational speed of the motor magnetic field.

For a standard induction motor, the synchronous speed is determined approximately by:

Synchronous Speed = 120 × Frequency ÷ Number of Motor Poles

For example, a four-pole motor has an approximate synchronous speed of:

  • 300 RPM at 10 Hz
  • 750 RPM at 25 Hz
  • 1500 RPM at 50 Hz

The actual motor speed is slightly lower because of motor slip.

Output Frequency is therefore the main VFD parameter that determines motor speed.

Carrier Frequency

Carrier Frequency is also called:

  • Switching Frequency
  • PWM Frequency

It represents how frequently the VFD power switches turn on and off to produce the PWM output.

Typical carrier frequencies may include:

  • 2 kHz
  • 4 kHz
  • 8 kHz
  • 12 kHz
  • 16 kHz

For example, a 4 kHz carrier frequency means that the PWM switching cycle occurs approximately 4,000 times per second.

The VFD controls the motor voltage mainly by changing the width and timing of these pulses.

At a low output voltage, the pulses are generally narrower.

At a higher output voltage, the pulses become wider.

The height of each switching pulse remains mainly related to the VFD DC bus voltage rather than the commanded motor speed.

Increasing the Carrier Frequency may provide:

  • Smoother motor current
  • Reduced audible motor noise
  • Improved low-speed current waveform

However, it may also cause:

  • Higher VFD switching losses
  • Increased VFD temperature
  • Increased leakage current
  • Higher electromagnetic interference
  • A possible requirement to derate the VFD

The Carrier Frequency does not directly determine the motor speed.

What Is dV/dt?

The term dV/dt means:

Change in Voltage ÷ Change in Time

It describes how quickly the voltage rises or falls at the edge of a single PWM pulse.

Its unit is commonly:

  • V/µs
  • kV/µs

For example, if the voltage rises from 0 to 600 V within 0.2 microseconds:

dV/dt = 600 ÷ 0.2 = 3000 V/µs

If a filter causes the same voltage transition to occur within 2 microseconds:

dV/dt = 600 ÷ 2 = 300 V/µs

The final voltage is approximately the same, but the slower voltage transition produces less electrical stress on the motor insulation.

Therefore:

  • Output Frequency determines motor speed
  • Carrier Frequency determines how often PWM switching occurs
  • dV/dt determines how steep each voltage pulse edge is

Increasing motor speed does not automatically increase dV/dt.

The dV/dt value depends mainly on:

  • DC bus voltage
  • Power semiconductor switching speed
  • VFD design
  • Motor cable characteristics
  • Cable length
  • Motor impedance
  • Output filter design

How Does Motor Cable Length Affect the VFD Output?

A motor cable is not only an electrical conductor.

It also has:

  • Resistance
  • Inductance
  • Capacitance
  • Characteristic impedance

When a fast PWM pulse travels through a long cable, the cable behaves like a transmission line.

The impedance of the cable and the motor may not be equal. When the pulse reaches the motor terminals, part of it may be reflected back through the cable.

This phenomenon is known as:

Reflected Wave

The incoming pulse and the reflected pulse may combine, producing a temporary voltage peak at the motor terminals that is higher than the voltage measured near the VFD output.

The severity of this effect depends on:

  • Motor cable length
  • Cable type
  • Pulse rise time
  • System voltage
  • VFD switching technology
  • Motor impedance
  • Motor insulation quality
  • Grounding and shielding

Common Problems Caused by Long Motor Cables

High Voltage Peaks at the Motor Terminals

Reflected waves may create high voltage peaks at the motor terminals.

These repetitive peaks place additional stress on the insulation between motor winding turns.

Older motors or motors not designed for inverter operation may be more sensitive to this stress.

Motor Insulation Damage

Continuous exposure to fast voltage pulses may gradually weaken the winding insulation.

The problem may begin as an intermittent fault and later develop into:

  • Turn-to-turn short circuits
  • Ground faults
  • Complete winding failure

Increased Leakage Current

Longer cables have greater electrical capacitance between:

  • Phase conductors
  • Conductors and earth
  • Conductors and cable shield

PWM pulses repeatedly charge and discharge this capacitance.

This may increase:

  • Leakage current
  • Common-mode current
  • Ground conductor current
  • Ground fault trips
  • Electromagnetic interference

Electromagnetic Interference

The fast voltage transitions at the VFD output can produce electrical noise.

The motor cable may transfer this noise to nearby equipment, including:

  • PLC systems
  • Measuring instruments
  • Pressure sensors
  • Flow sensors
  • 4–20 mA signals
  • RS-485 networks
  • Encoder signals
  • Communication cables

Proper shielding, grounding, cable routing, and output filtering may be required.

Bearing Currents

Common-mode voltage may cause unwanted current to pass through the motor bearings.

Over time, this current may produce:

  • Bearing surface pitting
  • Fluting
  • Increased vibration
  • Abnormal noise
  • Reduced bearing life

Bearing currents are not caused by cable length alone.

They are also affected by:

  • Motor construction
  • Grounding method
  • Shaft grounding
  • Motor size
  • Bearing type
  • VFD switching characteristics

Warning Signs of a Motor Cable Problem

The motor output system should be reviewed when one or more of the following conditions appear:

  • Repeated ground fault trips
  • Abnormal motor temperature
  • Repeated motor insulation failure
  • Unstable sensor signals
  • Communication errors
  • High leakage current
  • Excessive motor noise
  • Repeated bearing damage
  • Operation becomes unstable with longer cable length
  • High voltage peaks are measured at the motor terminals

These symptoms do not always confirm a motor cable problem.

The motor, load, VFD parameters, wiring, grounding, and incoming supply should also be checked.

Output Reactor

What Is an Output Reactor?

An Output Reactor is an inductive component installed between the VFD output and the motor.

Typical connection:

VFD Output → Output Reactor → Motor Cable → Motor

The reactor adds impedance to the output circuit and reduces rapid changes in current.

Benefits of an Output Reactor

An Output Reactor may help:

  • Reduce output-current ripple
  • Limit rapid changes in current
  • Reduce short-circuit current
  • Reduce some voltage peaks
  • Improve stability with some long cables
  • Reduce stress on the VFD output stage
  • Support certain multi-motor applications
  • Reduce some electrical noise

Limitations of an Output Reactor

An Output Reactor does not convert the PWM voltage into a sine wave.

It also does not reduce voltage rise time as effectively as a dedicated dV/dt Filter.

It may not provide sufficient protection when:

  • The motor cable is very long
  • The motor insulation is weak
  • Voltage peaks are severe
  • The motor is not inverter-duty
  • Low electromagnetic interference is required
  • A near-sinusoidal output is required

When Should an Output Reactor Be Considered?

An Output Reactor may be considered when:

  • The cable length exceeds the VFD recommendation without accessories
  • Several motors are connected to one VFD
  • Additional output impedance is required
  • Rapid current changes must be reduced
  • A cost-effective intermediate solution is required
  • The VFD manufacturer recommends an Output Reactor

dV/dt Filter

What Is a dV/dt Filter?

A dV/dt Filter is designed to reduce the voltage rise rate of the PWM pulses.

It makes the pulse edges less steep and helps limit voltage peaks at the motor terminals.

Typical connection:

VFD Output → dV/dt Filter → Motor Cable → Motor

Benefits of a dV/dt Filter

A dV/dt Filter may help:

  • Reduce voltage rise rate
  • Reduce motor-terminal voltage peaks
  • Reduce winding insulation stress
  • Reduce the effect of reflected waves
  • Improve motor insulation life
  • Reduce some bearing-current effects
  • Reduce electromagnetic interference compared with an unfiltered output

Does a dV/dt Filter Produce a Sine Wave?

No.

The VFD output remains a PWM waveform, but the voltage edges become slower and less stressful.

A dV/dt Filter therefore provides an intermediate level of protection between an Output Reactor and a Sine Wave Filter.

When Should a dV/dt Filter Be Considered?

A dV/dt Filter may be suitable when:

  • The motor is located far from the VFD
  • Motor insulation stress is a concern
  • The motor is old
  • The motor is not fully inverter-duty
  • Voltage peaks appear at the motor terminals
  • A complete sine-wave output is not required
  • A Sine Wave Filter is unnecessary or too expensive

Sine Wave Filter

What Is a Sine Wave Filter?

A Sine Wave Filter is installed at the VFD output to convert the PWM voltage into a waveform that is close to sinusoidal.

It normally contains inductive and capacitive components that reduce high-frequency PWM components.

Typical connection:

VFD Output → Sine Wave Filter → Motor Cable → Motor

Benefits of a Sine Wave Filter

A Sine Wave Filter may provide:

  • A near-sinusoidal motor voltage
  • Significant reduction in dV/dt
  • Reduced motor-terminal voltage peaks
  • Reduced motor insulation stress
  • Lower motor audible noise
  • Lower electromagnetic interference
  • Improved operation with long motor cables
  • Improved conditions for older motors
  • Reduced bearing-current effects in some applications

When Should a Sine Wave Filter Be Considered?

A Sine Wave Filter may be considered when:

  • The motor cable is very long
  • The motor is not inverter-duty
  • The motor insulation is sensitive
  • Low motor noise is required
  • The motor is difficult or expensive to access
  • Strict EMC performance is required
  • A transformer is connected between the VFD and motor
  • A near-sinusoidal motor voltage is required
  • The manufacturer recommends a Sine Wave Filter

Limitations of a Sine Wave Filter

A Sine Wave Filter normally has:

  • Higher cost
  • Larger physical size
  • Greater weight
  • Additional heat losses
  • Output voltage drop
  • Specific current-rating requirements
  • Maximum output-frequency limits
  • Minimum Carrier Frequency requirements
  • Compatibility limitations with some VFDs

The filter and VFD manufacturer instructions must be reviewed together.

Output Reactor vs dV/dt Filter vs Sine Wave Filter

Output Reactor

Primary function:
Adds output impedance and reduces rapid current changes.

Motor protection level:
Moderate.

Output waveform:
PWM.

Typical cost:
Lowest of the three solutions.

Typical application:
Moderate cable-length issues, output-current control, and some multi-motor systems.

dV/dt Filter

Primary function:
Reduces voltage rise rate and motor-terminal voltage peaks.

Motor protection level:
Good.

Output waveform:
PWM with slower voltage edges.

Typical cost:
Medium.

Typical application:
Protecting motor insulation from high dV/dt and reflected-wave effects.

Sine Wave Filter

Primary function:
Removes most high-frequency PWM components and produces a near-sinusoidal output.

Motor protection level:
Highest of the three solutions.

Output waveform:
Close to a sine wave.

Typical cost:
Highest.

Typical application:
Very long cables, sensitive motors, low-noise applications, and demanding EMC requirements.

How to Select the Correct Output Solution

The filter should not be selected based only on the motor power or cable length.

The following factors should be reviewed.

VFD Data

  • VFD model
  • Rated output voltage
  • Rated output current
  • VFD power
  • Carrier Frequency
  • PWM technology
  • Maximum permitted motor cable length
  • Compatible output filters
  • Maximum output frequency
  • Derating requirements

Motor Data

  • Rated power
  • Rated voltage
  • Rated current
  • Rated frequency
  • Motor insulation class
  • Inverter-duty capability
  • Motor age
  • Bearing design
  • Permitted motor-terminal voltage
  • Permitted dV/dt

Cable Data

  • Cable length
  • Cable cross-sectional area
  • Cable insulation type
  • Shielded or unshielded cable
  • Cable capacitance
  • Installation method
  • Number of parallel cables
  • Shield-grounding method

Application Conditions

  • Motor speed range
  • Maximum output frequency
  • Load type
  • Starting and stopping frequency
  • Ambient temperature
  • Number of motors
  • Required noise level
  • EMC requirements
  • Maintenance accessibility

Output Filter Installation

Output filters should be installed between the VFD and motor.

The filter is normally installed close to the VFD unless the manufacturer specifies another arrangement.

Typical connection:

VFD Output → Output Filter → Motor Cable → Motor

Power-factor-correction capacitors must not be connected directly to the VFD output.

Standard surge protection devices and components not designed for PWM voltage should also not be connected to the VFD output without technical verification.

Output contactors should not normally be opened or closed while the VFD is producing output unless the complete system is specifically designed for this operation.

Carrier Frequency and Output Filter Performance

Carrier Frequency affects:

  • Motor audible noise
  • VFD temperature
  • Switching losses
  • Leakage current
  • Electromagnetic interference
  • Cable-capacitance current
  • Output-filter performance

A higher Carrier Frequency may reduce the audible sound from the motor, but it can increase:

  • VFD switching losses
  • VFD temperature
  • Leakage current
  • Common-mode current
  • Cable charging current
  • Filter heating

When using a dV/dt Filter or Sine Wave Filter, the Carrier Frequency must remain within the operating range specified by the filter manufacturer.

Correct Motor Cable Installation

An output filter cannot compensate for poor cable selection or installation.

Recommended practices include:

  • Use a cable suitable for VFD applications
  • Use a low-impedance protective-earth conductor
  • Use appropriate cable shielding
  • Terminate the cable shield correctly
  • Use 360-degree shield termination when required
  • Avoid long, thin shield-grounding wires
  • Separate motor cables from signal cables
  • Separate power cables from communication cables
  • Cross signal and motor cables at right angles when necessary
  • Avoid coiling excess motor cable
  • Use EMC-compatible cable glands when required
  • Ground the motor frame correctly

Common Installation Mistakes

Selecting the Filter Based Only on Motor Power

Filter current, voltage, output frequency, Carrier Frequency, cable length, and application conditions must also be reviewed.

Using One Cable-Length Limit for Every Application

There is no universal cable-length limit that applies to every VFD, motor, and cable.

Using an Output Reactor When a Sine Wave Filter Is Required

An Output Reactor can reduce some electrical stress, but it does not produce a sinusoidal voltage.

Incorrect Carrier Frequency

An unsuitable Carrier Frequency can cause filter overheating or poor filtering performance.

Switching an Output Contactor While the VFD Is Running

Connecting or disconnecting a motor while the VFD is producing output may cause faults and stress the VFD power stage.

Incorrect Grounding

Poor motor and cable-shield grounding may allow interference and leakage-current problems to continue even when an output filter is installed.

Applying an Insulation Tester to the VFD

The motor and cable must be disconnected from the VFD before insulation-resistance testing.

Insulation-test voltage must never be applied directly to the VFD output terminals.

Engineering Checklist

Before selecting an output filter, confirm:

  • VFD manufacturer and model
  • VFD rated voltage
  • VFD rated current
  • Motor nameplate data
  • Motor insulation type
  • Inverter-duty capability
  • Motor cable length
  • Motor cable type
  • Cable-shield grounding method
  • Carrier Frequency
  • Maximum output frequency
  • Number of motors
  • Load type
  • Motor age
  • Actual problem being investigated
  • VFD manufacturer recommendations
  • Filter manufacturer recommendations
  • Filter voltage drop
  • Filter heat-loss requirements
  • Installation and ventilation requirements

Technical Conclusion

Long motor cables can increase the effects of PWM voltage produced by a VFD.

Possible consequences include:

  • Motor-terminal voltage peaks
  • Reflected waves
  • Increased leakage current
  • Motor insulation stress
  • Electromagnetic interference
  • Bearing-current problems

An Output Reactor adds impedance and reduces some rapid current changes.

A dV/dt Filter reduces the voltage rise rate and provides improved protection against reflected-wave and motor-insulation problems.

A Sine Wave Filter provides the highest level of voltage filtering and produces a motor voltage that is close to sinusoidal.

There is no single output-filter solution suitable for every application.

The correct selection should be based on the complete system, including:

  • VFD
  • Motor
  • Cable
  • Carrier Frequency
  • Output Frequency
  • Load characteristics
  • Grounding
  • Manufacturer instructions

Related Posts