Protecting the VFD and motor from power surges and transients is one of the most critical priorities while maintaining a drive system. There are several sub-components within the VFD and motor which are sensitive to irregularities in power such as voltage spikes, line distortions, current surges or harmonics.

Different types of reactors and filters are employed to regulate these distortions. They are as follows:

• Line Reactor
• Dv/Dt Reactor
• Harmonic Filter
• Dv/Dt Filter
• Sine Filter

By using these devices either separately or in combination, the service life of the motor and VFD can be increased. It also improves the system performance and reliability of the drive system.

Depending on the output voltage of the motor drive, the cable length and layout are also important factors that influence voltage spikes. The appropriate reactor or filter chosen for the specific cable length can help mitigate this issue.

We shall be covering each of these devices in detail in a series of blogs. To begin with, in this first part of the series, let us understand the functions, differences, and applications of the different types of reactors and filters used.

Different types of Reactors and Filters

Reactors
Reactors – are basically inductances. They are electromagnetic devices that consist of steel core electrical grade laminations and copper inductor coils. The coil forms a magnetic field as current flows through it, which limits the rate of rise of current. Depending on whether they are installed at the input side or output side of the VFD, they are known as line or load reactors.

Filters
Filters – are a combination of inductance and capacitance. They are electromagnetic devices which consist of a passive L-C circuit that offer protection from harmonic disturbances and transients. They may have longer range protection compared to reactors.

Line Reactors for VFD

• They are also known as input reactors as they are installed at the input side of the VFD.
• Their primary role is to protect the VFD from incoming line voltage distortion such as spikes and surges.
• Switching transients can peak over 1000 volts. The inductive characteristics of the Line Reactor restricts this sudden surge and eliminates nuisance tripping.
• By limiting the inrush current to the rectifier in the VFD, the line reactor can minimise harmonic distortions.
• Commonly used in Motor Drives, Automotive, Industrial Automation, and power Quality applications.

 

Dv/Dt Reactors

• They are also known as load reactors as they are installed at the output side of the VFD, before the motor.
• They derive the name Dv/Dt Reactor because they provide efficient reduction of high output voltage dv/dt from IGBT motor drives.
• The high design inductance of the Dv/Dt reactor will smoothen the rough voltage spikes and slow down the distortion as the voltage is provided to the motor.
• They are effective in protection of motor coil insulation from premature aging and destruction.
• Common applications include Servo drives, close loop vector drives, motor drives with short motor cables, Robotics and Industrial Automation.

 

Harmonic Filters for VFD

• There are 2 types of harmonic filters –passive harmonic filters and active harmonic filters.
• Passive harmonic filters utilize components like inductors, capacitors, and power resistors to filter noise on single variable speed drives before electricity reaches equipment with an electromagnetic field.
• Active harmonic filters can work with many variable speed drives and actively reduce noise by constantly monitoring electricity and injecting currents to mitigate harmonics.
• They are L-C combination electromagnetic devices installed at the input side of the VFD.
• Their primary role is to protect the VFD and regen unit from line side disturbances and transients.
• Designed in a way that the filter passes current at the fundamental frequency but blocks current at higher frequencies.
• Provides excellent harmonic mitigation performance and helps to achieve IEEE 519 standard.
• Common applications include Motor Drives, Automotives, Industrial Automation and power Quality applications.

 

Dv/Dt Filters

• They are L-C combination electromagnetic devices installed at the output side of the VFD, before the motor.
• Their primary role is to protect the motor by regulating drive output voltage and preventing spikes to the motor.
• Limits the peak voltage and protects the motor insulation windings from premature wear and tear.
• Low pass filters with low inductance, limited reduction in the control dynamic.
• Minimizes interference propagation towards neighbouring equipment or lines.
• Output current ripple is lower, but the voltage is still PWM pulse pattern shaped.
• Have a longer range of motor protection compared to load reactors.

 

Sine Wave Filters for VFD

• They are L-C combination electromagnetic devices installed at the output side of the VFD, before the motor.
• Sinusoidal output filters with high inductance and capacitance, helps to optimise the output signal.
• Protects the motor by regulating the PWM wave distortions coming out of the drive so that it resembles an AC sine wave.
• They help to minimise motor drive losses by reducing the high frequency leakage currents, reducing bearing currents and eliminating the pulse reflections in the motor cable.
• Common applications include Motor drives with long motor cable, motor drives with multiple motors in parallel and retrofitting installations.

 

The table below summarises the features and functions of the various reactors and filters discussed above for quick reference.

Summary of Reactors & Filters for VFD and Motor Protection

Device	Position	Purpose
Line Reactor (also called Input Reactor) INDUCTANCE ONLY	Before the VFD	Protects the VFD from incoming line voltage distortion such as spikes and surges
Load Reactor (also called Dv/Dt Reactor or Output Reactor)	Between the VFD and motor	Protects the motor by regulating drive output voltage
Harmonic Filter INDUCTANCE & CAPACITANCE	Before the VFD	Protects the VFD and regen unit from line side disturbances and transients
Dv/Dt Filter INDUCTANCE & CAPACITANCE	Between the VFD and motor	Protects the motor by regulating drive output voltage and preventing spikes to the motor
Sine Filter INDUCTANCE & CAPACITANCE	Between the VFD and motor	Protects the motor by regulating the PWM wave distortions coming out of the drive so that it resembles an AC sine wave