Motor and Line Chokes for VFDs: Purpose, Selection, and Wiring

Why Chokes Are Needed in Variable Frequency Drive Systems

A variable frequency drive (VFD) operates on the principle of pulse width modulation (PWM), converting mains voltage at 50 Hz into AC voltage with adjustable frequency and amplitude. During this process, high-frequency harmonics, capacitive currents, and impulse overvoltages arise, negatively affecting both the supply network and the electric motor. Two types of chokes are used to combat these phenomena: line (input) reactors and motor (output) chokes.

Chokes are inductive elements that limit the rate of current rise. Installing a choke at the input or output of a frequency converter significantly improves the reliability of the entire electric drive system, reduces electromagnetic interference, and extends equipment service life.

Line (Input) Reactor: Functions and Operating Principle

A line reactor is installed between the power supply and the VFD input. Its primary task is protecting the network from harmonics generated by the VFD and protecting the drive itself from external disturbances.

Key Functions of a Line Reactor

• Suppression of higher current harmonics. The VFD rectifier draws pulsed current containing the 5th, 7th, 11th, and other harmonics. A line reactor reduces the total harmonic distortion (THDi) from 80-130% down to 30-40%, decreasing the load on the transformer and other equipment on the network.
• Overvoltage protection. The reactor smooths out transient voltage spikes (for example, from capacitor bank switching or lightning strikes), preventing them from reaching the DC bus of the VFD.
• Inrush current limiting. When the VFD is powered on, it charges the DC bus capacitors, causing a brief current surge. The reactor limits this surge to a safe level.
• Power factor improvement. By smoothing the shape of the consumed current, a line reactor raises the system cos(phi) from 0.6-0.7 to 0.85-0.95.
• Isolating multiple VFDs on the same bus. When several frequency drives are powered from the same transformer, line reactors prevent mutual interference and resonance effects.

When a Line Reactor Is Mandatory

Installing an input reactor is always recommended, but certain situations make it essential:

• The supply transformer capacity exceeds the VFD power rating by more than 10 times (low network impedance).
• Power factor correction capacitor banks are present on the network.
• Multiple VFDs or soft starters operate on the same bus.
• The cable from transformer to VFD is shorter than 5 metres.
• The supply network is subject to frequent overvoltages or voltage sags.

Motor (Output) Choke: Functions and Operating Principle

A motor choke is installed between the VFD output and the electric motor. It protects the motor from the adverse effects of PWM modulation and ensures reliable operation with long motor cables.

Key Functions of a Motor Choke

• Reducing voltage rise rate (dU/dt). The PWM output generates pulses with rise times of 0.1-0.5 microseconds. In a long cable, these pulses reflect from the motor terminals and can double in amplitude, reaching 1200-1600 V at a nominal 400 V. A motor choke reduces dU/dt to safe values of 200-500 V/microsecond.
• High-frequency harmonic suppression. The output choke smooths the current waveform, bringing it closer to a sinusoid. This reduces additional winding heating by 6-10%, extending insulation service life.
• Bearing current reduction. The PWM signal creates parasitic capacitive currents through the motor bearings, causing electrical discharge machining (EDM) damage. A motor choke reduces the amplitude of these currents by a factor of 3-5.
• Long cable capacitive current compensation. With motor cable lengths exceeding 30 metres (unshielded) or 15 metres (shielded), cable charging currents can reach 10-20% of the motor rated current, causing false overcurrent protection trips on the VFD.
• Motor winding insulation protection. Standard motor insulation is rated for inter-turn voltage amplitudes up to 1000 V. When operating from a VFD without a choke, this value can be exceeded, especially in older motors with degraded insulation.

When a Motor Choke Is Mandatory

• Motor cable length exceeds 30 m (unshielded) or 15 m (shielded).
• The motor does not have reinforced insulation (insulation class below F or H).
• The motor is old or shows signs of degraded insulation.
• The PWM carrier frequency is below 4 kHz.
• Acoustic noise reduction at low rotational speeds is required.

Line Reactor vs Motor Choke: Comparison

How to Select a Choke by Power Rating

Choke selection is based on the rated current of the VFD or motor. Key rules:

• Rated choke current must equal or exceed the VFD rated output current.
• Choke voltage rating must match the supply voltage (for line reactors) or VFD output voltage (for motor chokes).
• Inductance is chosen according to power: 2-4% impedance for line reactors, 1-3% for motor chokes.
• Insulation class of the choke should be at least equal to the motor insulation class (typically F or H).

Most frequency converter manufacturers provide choke selection tables matched to VFD model and power rating. If you are using a drive from the Chastotnik catalogue, our specialists will help you select the appropriate choke.

Choke Wiring Diagrams for Variable Frequency Drives

Line Reactor Wiring

A line reactor is connected in series between the circuit breaker (or contactor) and the VFD power input terminals L1, L2, L3 (R, S, T) often. Connection procedure:

1. Disconnect power and verify zero voltage at all terminals.
2. Connect the reactor input terminals to the circuit breaker output terminals.
3. Connect the reactor output terminals to the VFD power inputs (L1/R, L2/S, L3/T).
4. Check all terminal connections are torqued to manufacturer specifications.
5. Ensure a minimum ventilation clearance of 100 mm above and below the reactor.

Motor Choke Wiring

A motor choke is connected in series between the VFD output terminals (U, V, W) and the motor terminal box:

1. Disconnect power and verify zero voltage.
2. Connect the choke input terminals to the VFD output terminals (U, V, W or T1, T2, T3).
3. Connect the choke output terminals to the motor terminal box.
4. Ensure the cable from VFD to choke is as short as possible (preferably under 1 m).
5. Bond the choke enclosure to the common earth bus.

For more details on three-phase motor connections, see our article Connecting a three-phase motor to a 380 V supply.

Common Mistakes When Using Chokes

During installation and operation of chokes in frequency converter systems, several mistakes commonly reduce protection effectiveness or lead to equipment damage:

• Confusing line and motor chokes. These chokes have different parameters and are not interchangeable. A line reactor installed at the VFD output will not provide adequate dU/dt protection for the motor.
• Undersizing the current rating. A choke with insufficient current rating overheats, loses inductance, and may fail. A minimum current margin of 10% is recommended.
• Inadequate ventilation. Chokes dissipate significant heat. Without proper cooling, their temperature can exceed the permissible 120-155 degrees C, leading to insulation degradation.
• Ignoring VFD manufacturer recommendations. Some frequency converter models have built-in DC bus chokes, and an additional external choke may be redundant or even detrimental.
• Long cable between VFD and motor choke. The choke should be located as close as possible to the VFD output to minimise pulse reflections in the cable between VFD and choke.

Chokes vs Sine-Wave Filters: What Is the Difference

In addition to chokes, sine-wave (LC) filters are used to protect motors from PWM pulses. A sine-wave filter consists of a choke and capacitor forming an LC circuit that converts the PWM signal into a nearly pure sinusoid. Key differences:

• A motor choke reduces dU/dt and partially smooths the current waveform but does not convert PWM into a sinusoid.
• A sine-wave filter fully converts the PWM signal into sinusoidal voltage, allowing standard motors to be used without cable length restrictions.
• The cost of a sine-wave filter is 3-5 times higher than a motor choke, so it is typically used for cable lengths exceeding 100 m or for motors with non-standard insulation.

For most industrial applications (ventilation, pumps, conveyors), a motor choke is sufficient. Read more about VFD applications in HVAC systems in our article Frequency converters for ventilation.

Practical Recommendations for Selection and Operation

Based on years of experience implementing variable frequency drives, the following practical recommendations can be highlighted:

• For motors up to 7.5 kW with cable lengths under 20 m, a choke is often unnecessary if the VFD has a built-in DC bus choke.
• For motors from 11 kW and above, a line reactor should always be installed to protect the supply network.
• When multiple VFDs operate on the same bus, a line reactor on each drive is mandatory.
• A motor choke should be installed when cable length exceeds 30 m or when operating motors without reinforced insulation.
• Check the choke temperature during initial startup and after one week of operation — overheating indicates incorrect selection.
• Regularly check terminal connection torques — thermal expansion can loosen contacts over time.

Find more practical advice on working with frequency converters in our article Top questions about VFDs and soft starters, and real-world application examples in Frequency converters: application examples.

Frequently Asked Questions

Can a VFD be used without chokes?

Yes, a VFD can operate without external chokes under favourable conditions: short cables (under 20 m), a stable supply network, and a modern motor with reinforced insulation. However, even under such conditions, a line reactor improves power quality and increases system reliability. With long cables or unfavourable network conditions, chokes are practically mandatory.

What is the difference between a choke and a sine-wave filter, and when should each be used?

A motor choke reduces the voltage rise rate (dU/dt) and partially smooths the current, but the output remains a PWM signal. A sine-wave filter (LC filter) fully converts PWM into a sinusoid. A choke is suitable for cables up to 100 m and standard industrial motors. A sine-wave filter is needed for very long cables, explosion-proof motors, or to reduce acoustic noise to a minimum.

How do I determine whether my VFD needs a line reactor?

Check three factors: supply transformer capacity (if it exceeds VFD power by 10 or more times, a reactor is needed), presence of other VFDs or high-power non-linear loads on the same bus, and supply quality (frequent voltage spikes, light flickering). If any one factor is present — install a line reactor.

What impedance percentage should I choose for the choke: 2% or 4%?

The standard value for most applications is 3-4% for line reactors and 2% for motor chokes. A 4% impedance choke provides better harmonic suppression but causes greater voltage drop (up to 4% of nominal). For voltage-sensitive processes (precision machining, textiles), choose 2%; for pumps and fans, 4%.

Is it necessary to install both a line reactor and a motor choke simultaneously?

In most industrial systems, one choke is sufficient. A line reactor is preferred when the main concern is supply quality and equipment interaction. A motor choke is preferred for long cables or weak motor insulation. Both chokes are installed simultaneously for high-power drives (from 55 kW), long cables (from 50 m), and strict electromagnetic compatibility (EMC) requirements. Also read about VFD configuration specifics for compressor equipment.