In practical applications, frequency converters usually need to be equipped with reactors, filters, brake resistors and brake units to ensure the stability of their performance, extend the life of the equipment, and effectively avoid negative impacts on the power grid and equipment. The following are the functions of each component and their reasons:

1. Reactors
Reactors are usually added to the input or output of the frequency converter. Their main functions are:

Reduce harmonics and current fluctuations: frequency converters will generate harmonics, especially low-frequency harmonics (such as 5th and 7th harmonics). These harmonics will cause current fluctuations, affect motor operation and increase the burden on the power grid. Reactors can effectively suppress these harmonics and reduce the impact on the power grid and other equipment.

Smooth current fluctuations: reactors can reduce the impact of the frequency converter switching frequency on the current, make the current waveform smoother, and help reduce the current harmonics of the power grid.

Limit overvoltage and overcurrent: reactors can limit the occurrence of overvoltage or overcurrent in some cases, protecting frequency converters and motors from damage.

Reasons for installation: protect equipment, reduce the impact of harmonics on the power grid and electrical equipment, and avoid high-frequency fluctuations and overcurrent problems.

2. Filters
Filters are generally used at the output end of the inverter. Their functions are:

Eliminate high-frequency harmonics: The high-frequency switching noise generated by the inverter may interfere with the motor and other electrical equipment. The filter can improve the stability of the system by filtering out high-frequency noise.

Improve the operating environment of the motor: The filter can eliminate the impact of high-frequency harmonics on the motor, avoid problems such as overheating, vibration, and noise of the motor, and improve the stability of the motor operation.

Reducing electromagnetic interference (EMI): The filter can effectively reduce electromagnetic interference, ensure that the equipment meets the electromagnetic compatibility (EMC) standards, and avoid affecting the normal operation of other electronic equipment.

Reasons for installation: Reduce high-frequency interference and harmonics, improve the electrical environment of the system, and protect the motor and other equipment from interference.

3. Braking Resistor
Braking resistors are usually used in conjunction with brake units. Their main functions are:

Absorb regenerative energy: When the motor driven by the inverter stops, the rotational inertia of the motor will convert kinetic energy into electrical energy and feed it back to the inverter. If no measures are taken, excessive regenerative energy may cause the DC bus voltage to be too high and damage the inverter. The braking resistor can absorb this excess energy and convert it into heat energy, thereby preventing the DC bus voltage from being too high.
Improve the braking effect: In high-speed motor drive applications, the braking resistor can effectively help the motor decelerate quickly and prevent the motor from generating too high a reverse current due to inertia when it stops.
Reason for installation: Absorb the regenerative energy of the motor to ensure the safe operation of the inverter and the motor, especially in applications with frequent start/stop.

4. Braking Unit
The braking unit is used in conjunction with the braking resistor. It is mainly responsible for controlling and adjusting the work of the braking resistor:

Control the DC bus voltage: When the inverter is working, the inertia of the motor may feed too much energy back into the DC bus, causing the bus voltage to increase. The function of the braking unit is to monitor the DC bus voltage. When the voltage is too high, it automatically triggers the braking resistor to absorb excess energy to prevent the bus voltage from exceeding the standard.
Provide fast braking: The braking unit and the resistor work together to enable the inverter to quickly consume excess energy when the motor stops or reverses the brake, reduce the motor stop time, and improve the efficiency of the control system.
Reasons for installation: Control the backflow of regenerative energy, protect the inverter from excessive voltage, and ensure fast and safe motor braking.

Summary
In the actual application of the inverter, the installation of reactors, filters, brake resistors and brake units can:
Effectively suppress harmonics, reduce electromagnetic interference, and ensure the stability of equipment and power grids.
Improve the efficiency and life of motor operation, and reduce problems such as motor overheating, noise, and vibration caused by high-frequency noise.
Process the regenerative energy of the motor, prevent the inverter DC bus voltage from being too high, and ensure the safe and stable operation of the system.
Therefore, the reasonable configuration of these components can significantly improve the performance of the inverter, improve the safety of the system, and extend the service life of the equipment.
When using a variable frequency drive (VFD), not all applications require the installation of reactors, filters, brake resistors, and brake units. Whether these components need to be installed depends on the specific application environment, system requirements, and equipment working conditions. Here are some common reasons and scenarios for adding these components:

1. Situations where reactors are needed
High grid harmonic pollution: When the inverter is used in an environment where the grid power supply conditions are unstable or the grid has strong harmonic pollution, the reactor can help reduce the harmonics generated by the inverter switching frequency to avoid causing greater pollution to the grid.
High inverter power: In the application of high-power inverters, especially inverters above 50kW, reactors can effectively reduce current fluctuations and reduce the impact on the grid and equipment.
Large grid voltage fluctuations: Reactors can suppress grid voltage fluctuations to ensure the normal operation of the inverter, especially in areas where the grid voltage is unstable or fragile.
Typical applications: inverters with high-power loads such as power plants, heavy machinery, and mines; strict industrial grid environments are required.

2. Situations where filters are needed
High-frequency noise problems in motor drives: The high-frequency switching noise generated by the inverter may cause electromagnetic interference (EMI) to the motor and surrounding electronic equipment. If your application needs to reduce electromagnetic interference, or if sensitive electronic equipment (such as PLCs, sensors, etc.) works nearby, filters are very necessary.
Comply with electromagnetic compatibility (EMC) requirements: If the equipment needs to meet strict EMC standards, the filter can effectively reduce the interference of electromagnetic radiation and conduction to ensure that the equipment meets national or international electromagnetic compatibility standards.
Improve motor operation: If the inverter drives the motor and there are problems such as motor overheating, increased noise or vibration, the filter can reduce the impact caused by high-frequency harmonics.
Typical applications: Applications with strict requirements on electromagnetic interference, such as high-precision manufacturing, laboratory equipment, communication equipment, medical equipment, etc.

3. Situations where brake resistors are required
Frequent start/stop or braking requirements: In situations where frequent starting and stopping are required, the regenerative energy generated by the motor due to inertia may cause the DC bus voltage to rise sharply. At this time, a brake resistor is needed to absorb this part of the energy to prevent the voltage from exceeding the standard and ensure the normal operation of the inverter.
High-load applications with long-term operation: If the motor load is large and runs for a long time, especially when the motor is decelerating or stopping, it may generate a large reverse energy. The brake resistor can prevent the motor from generating excessive voltage due to inertia.
Applications that require rapid shutdown or load deceleration: For example, in applications such as belt conveyors and elevators that require rapid shutdown, brake resistors can accelerate motor deceleration and shorten stopping time.
Typical applications: cranes, conveyor belts, textile machinery, elevators, fans and pumps that start and stop quickly, etc.
4. Situations where brake units are required
Occasions where regenerative energy needs to be controlled: When the motor needs to be used in the case of rapid stopping or reverse braking, the DC bus voltage may be too high. The brake unit can monitor and control this voltage to ensure that it does not cause damage to the inverter.
The regenerative energy fed back by the motor is large: For high-power inverters, especially on large inertia loads such as fans, pumps, heavy machinery, etc., the regenerative energy generated by the motor inertia is large. The brake unit is used together with the brake resistor to ensure that the regenerative energy is effectively absorbed and avoid failures caused by excessive voltage.
Operating under high load and high dynamic conditions: For example, in situations where frequent speed changes are required (such as elevators and cranes), the brake unit can help quickly consume the feedback energy and protect the inverter and motor.
Typical applications: high dynamic response motor drive systems, such as elevators, cranes, belt conveyors, automated production lines, etc.

Summary:
These components are usually required in the following cases:

When the grid quality is poor, the harmonics are large, or the voltage fluctuations are large, install a reactor to protect the inverter and the grid.

When there are strict requirements for electromagnetic interference (EMI) or the smoothness of motor operation needs to be improved, install a filter.
For applications with frequent start/stop or rapid deceleration, it is necessary to install a brake resistor and a brake unit to help control the feedback regenerative energy and ensure the safe operation of the inverter and the motor.
Whether these components need to be installed depends on the specific needs of the system, the load type, and the working environment. For applications with high power, frequent start/stop, or strict electrical environment requirements, these additional components are usually considered.