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Driving asynchronous motors with variable frequency drives is the most common application in the industry. PWM inverter speed control devices are widely used for their high energy efficiency, movement accuracy, high starting torque, and low starting current. However, the 6-pulse diode rectifier circuit located in the input stage of the driver can be considered as a harmonic current source that produces harmonics such as 5th, 7th, 11th, and 13th ( based on the formula h = np±1)

The capacitor in the DC bus charges and discharges based on the diode switching.

The graph below shows the typical load current flowing into the grid.

Simulation of 75kW Variable Frequency Drive :

The simplest and most cost-effective way to approximate the current waveform given in the graph to a sinusoidal waveform is to mount an line reactor in series to the input of Variable Frequency Drive

Simulation of 75kW Variable Frequency Drive with Line Reactor :

Line reactors have bidirectional effects, reducing the harmonics generated by variable

frequency drives on the network and protecting these devices from voltage spikes in the grid.

When mounting line reactors, it’s important to consider the load current since impedance is related to its derivative. Selecting a line reactor based on the load current ensures maximum benefit, and it should not operate in saturation at this level.

Mounting the line reactor in front of the speed control device causes a voltage drop relative to the reactor impedance value.

As the voltage drop value increases, the rate of reducing harmonics in the reactor also increases.

Based on extensive field experience accumulated over many years, it has been determined that a 4% impedance value effectively reduces harmonics. Additionally, due to the manageable voltage drop it generates, it does not hinder the operation of the speed control device, making it an optimal choice.

A 4% impedance reactor causes a 4% voltage drop at nominal current and does not harm devices as they are designed to tolerate a wide range of input voltages.

Below is the technical drawing of the 75kW line reactor. It is an optional device that needs to be mounted next to the variable frequency drive. However, it’s not feasible for a line reactor with an impedance value affecting harmonics to fit inside the body of the variable frequency drives.

The line reactor offers several benefits:

  • It reduces harmonic currents supplied to the grid by the variable frequency drive.
  • It filters out harmonics from other harmonic-generating loads.
  • It protects the variable frequency drive from voltage spikes coming from the grid.
  • It diminishes notches caused by commutation events, reducing their depth.
  • Its presence prevents transient effects triggered by power factor correction systems, thereby averting production losses.

The benefits of line reactors far outweigh their costs and they are recommended for use with every variable frequency drive as essential accessories for optimal performance.

Ideal Type of Connection:

Didem Ergun Sezer
Ergun Elektrik A.Ş.

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